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Journal articles on the topic 'Midlatitude'
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1
Orbe, Clara, Paul A. Newman, Darryn W. Waugh, Mark Holzer, Luke D. Oman, Feng Li, and Lorenzo M. Polvani. "Airmass Origin in the Arctic. Part I: Seasonality." Journal of Climate 28, no. 12 (June 11, 2015): 4997–5014. http://dx.doi.org/10.1175/jcli-d-14-00720.1.
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Abstract The first climatology of airmass origin in the Arctic is presented in terms of rigorously defined airmass fractions that partition air according to where it last contacted the planetary boundary layer (PBL). Results from a present-day climate integration of the Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) reveal that the majority of air in the Arctic below 700 mb last contacted the PBL poleward of 60°N. By comparison, 62% (±0.8%) of the air above 700 mb originates over Northern Hemisphere midlatitudes (i.e., “midlatitude air”). Seasonal variations in the airmass fractions above 700 mb reveal that during boreal winter air from midlatitudes originates primarily over the oceans, with 26% (±1.9%) last contacting the PBL over the eastern Pacific, 21% (±0.87%) over the Atlantic, and 16% (±1.2%) over the western Pacific. During summer, by comparison, midlatitude air originates primarily over land, overwhelmingly so over Asia [41% (±1.0%)] and, to a lesser extent, over North America [24% (±1.5%)]. Seasonal variations in the airmass fractions are interpreted in terms of changes in the large-scale ventilation of the midlatitude boundary layer and the midlatitude tropospheric jet.
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Wissmeier, Ulrike, and Robert Goler. "A Comparison of Tropical and Midlatitude Thunderstorm Evolution in Response to Wind Shear." Journal of the Atmospheric Sciences 66, no. 8 (August 1, 2009): 2385–401. http://dx.doi.org/10.1175/2009jas2963.1.
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Abstract The influence of vertical wind shear on storm development within a tropical environment is studied with the aid of two numerical models and compared with that in simulations of midlatitude storms. The simulations show that larger wind shears are required in a tropical environment than in a midlatitude environment for a storm of given updraft velocity to split. This finding is supported by the experience of forecasters at the Australian Bureau of Meteorology Regional Forecasting Centre in Darwin that the operational storm forecasting tools developed for midlatitude storms overforecast supercells within the tropics. That tropical storms require higher shears to split can be attributed either to the larger gust front speed or to the earlier gust front occurrence compared to those in the midlatitudes. A fast gust front cuts off the storm from the warm moist inflow and the updraft has little or no time to split. In the cases where the midtropospheric relative humidity is larger in the tropics or comparable with that in the midlatitudes, the total liquid water and ice content within the deeper tropical storms is larger than in the midlatitude storms, causing a stronger downdraft. In other words, the main contribution to the negative buoyancy of the downdraft is the water loading rather than the evaporative cooling. When a tropical storm is simulated in an environment with smaller midtropospheric relative humidity than in the midlatitudes, the amount of liquid water and ice within the storm is comparable to that within the midlatitude storm. Intense evaporation within the tropical storm then leads to a stronger negative buoyancy than in the midlatitude storm, causing a stronger downdraft and thus an earlier or a faster-spreading gust front. At higher shears in the tropics, entrainment reduces the storm depth and thus water loading, resulting in a delayed gust front initiation and/or reduction of the gust front speed, which then allows storm splitting to occur.
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Peings, Y., J. Cattiaux, S. Vavrus, and Gudrun Magnusdottir. "Late Twenty-First-Century Changes in the Midlatitude Atmospheric Circulation in the CESM Large Ensemble." Journal of Climate 30, no. 15 (August 2017): 5943–60. http://dx.doi.org/10.1175/jcli-d-16-0340.1.
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Projected changes in the midlatitude atmospheric circulation at the end of the twenty-first century are investigated using coupled ocean–atmosphere simulations from the Community Earth System Model Large Ensemble (CESM-LENS). Different metrics are used to describe the response of the midlatitude atmospheric dynamics in 40 ensemble members covering the 1920–2100 period. Contrasted responses are identified depending on the season and longitudinal sector that are considered. In winter, a slowdown of the zonal flow and an increase in waviness is found over North America, while the European sector exhibits a reinforced westerly flow and decreased waviness. Extreme temperature events in midlatitudes are more sensitive to thermodynamical than dynamical changes, and a general decrease in the intensity of wintertime cold spells is found. Analyses of individual ensemble members reveal a large spread in circulation changes due to internal variability. Causes for this spread are found to be tied to the Arctic amplification in the Pacific–North American sector and to the polar stratosphere in the North Atlantic. A competition mechanism is also discussed between the midlatitude response to polar versus tropical changes. While the upper-tropospheric tropical warming pushes the jet stream poleward, in winter, Arctic amplification and the weaker polar vortex exert an opposite effect. This competition results in a narrowing of the jet path in the midlatitudes, leading to decreased/unchanged waviness/blockings. This interpretation somewhat reconciles conflicting results between the hypothesized effect of Arctic amplification and projected changes in midlatitude flow characteristics. This study also illustrates that further understanding of regional processes is critical for anticipating changes in the midlatitude dynamics.
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Keller, Julia H. "Amplification of the Downstream Wave Train during Extratropical Transition: Sensitivity Studies." Monthly Weather Review 145, no. 4 (April 1, 2017): 1529–48. http://dx.doi.org/10.1175/mwr-d-16-0193.1.
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Abstract A tropical cyclone (TC) undergoing extratropical transition (ET) may support the amplification of a Rossby wave train in the downstream midlatitudes. Within the context of downstream baroclinic development, the TC acts as an additional source of eddy kinetic energy (). Previous studies concluded that the impact depends, in particular, on the phasing between the TC and the midlatitude flow and the continuation of the generation during ET. These studies did not quantify the impact of ET on the within a downstream Rossby wave train. The present study uses ensemble sensitivity analysis to examine the sensitivity of downstream Rossby wave train amplification to the budget of the transitioning TC and of the upstream midlatitude features for Typhoon Choi-Wan (2009) and Hurricane Hanna (2008) in ECMWF ensemble forecasts. The amplification of the downstream wave train is measured using the amplitude of its associated maxima. The sensitivity of the maximum’s intensity at a particular forecast time to the budget terms of the TC and the upstream midlatitudes at earlier forecast times is determined. The results show that increasing the budget terms within Choi-Wan (Hanna) by one standard deviation can result in an up to 36% (23%) more intense downstream maximum. This is favored by the phasing between Choi-Wan and the midlatitude trough, and the reintensification of Hanna, respectively. By contrast, weaker contributions to downstream Rossby wave amplification arise from budget terms associated with flow features in the upstream midlatitudes.
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Fajber, Robert, Paul J. Kushner, and Frédéric Laliberté. "Influence of Midlatitude Surface Thermal Anomalies on the Polar Midtroposphere in an Idealized Moist Model." Journal of the Atmospheric Sciences 75, no. 4 (April 1, 2018): 1089–104. http://dx.doi.org/10.1175/jas-d-17-0283.1.
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Abstract Evidence from models and observations suggests that the vertical distribution of entropy in the extratropical troposphere reflects the horizontal distribution of entropy at the surface. This isentropic linkage, which is accomplished through moist isentropic mass transport driven by extratropical waves, becomes more apparent when the effect of latent heat release by condensing moist parcels is accounted for. This study focuses on the stratification of the Arctic troposphere, which is connected by moist isentropes to the midlatitude surface. A relatively simple moist general circulation model without radiative feedbacks involving water vapor or clouds is used to study the linkage between the midlatitude surface and the Arctic midtroposphere. Zonally symmetric midlatitude thermal perturbations switched on at the surface drive a moist potential temperature response in the Arctic midtroposphere with a lag of about 2 weeks. This response increases the gross moist vertical stability in the Arctic while generally decreasing it, or increasing it only weakly, in the midlatitudes. The moist isentropic streamfunction is shifted poleward owing to the poleward entropy flux response and is shifted upward (i.e., to higher entropy) owing to the zonal-mean entropy response. The results suggest a potential novel mechanism by which the midlatitudes might influence polar lapse rates and their associated radiative feedbacks.
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Chen, Tsing-Chang, Wan-Ru Huang, and Eugene S. Takle. "Annual Variation of Midlatitude Precipitation." Journal of Climate 17, no. 21 (November 1, 2004): 4291–98. http://dx.doi.org/10.1175/jcli3201.1.
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Abstract Annual variation of midlatitude precipitation and its maintenance through divergent water vapor flux were explored by the use of hydrological variables from three reanalyses [(NCEP–NCAR, ECMWF Re-Analysis (ERA), and Goddard Earth Observing System (GEOS-1)] and two global precipitation datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and Global Precipitation Climatology Project (GPCP)]. Two annual variation patterns of midlatitude precipitation were identified:Tropical–midlatitude precipitation contrast: Midlatitude precipitation along storm tracks over the oceans attains its maximum in winter and its minimum in summer opposite to that over the tropical continents.Land–ocean precipitation contrast: The annual precipitation variation between the oceans and the continent masses exhibits a pronounced seesaw.The annual variation of precipitation along storm tracks of both hemispheres follows that of the convergence of transient water vapor flux. On the other hand, the land–ocean precipitation contrast in the Northern Hemisphere midlatitudes is primarily maintained by the annual seesaw between the divergence of stationary water vapor flux over the western oceans and the convergence of this water vapor flux over the eastern oceans during winter. The pattern is reversed during the summer. This divergence–convergence exchange of stationary water vapor flux is coupled with the annual evolution of upper-level ridges over continents and troughs over the oceans.
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Garfinkel, Chaim I., and Darryn W. Waugh. "Tropospheric Rossby Wave Breaking and Variability of the Latitude of the Eddy-Driven Jet." Journal of Climate 27, no. 18 (September 10, 2014): 7069–85. http://dx.doi.org/10.1175/jcli-d-14-00081.1.
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Abstract A dry general circulation model is used to investigate the connections between Rossby wave breaking and the latitude of the midlatitude tropospheric eddy-driven jet. An ensemble of experiments is constructed in which the jet latitude is influenced by a midlatitude tropospheric temperature anomaly that resembles observed climate change and by the imposition of a stratospheric polar vortex, and the distribution of Rossby wave breaking frequency is examined for each experiment. The shift in wave breaking per degree latitude of jet shift is then compared for three different sources of jet movement: the tropospheric baroclinic forcing imposed in midlatitudes, the imposition of a stratospheric polar vortex, and the internal variability of the midlatitude eddy-driven jet. It is demonstrated that all three sources of jet movement produce a similar change in Rossby wave breaking frequency per degree of jet shift. Hence, it is difficult (if not impossible) to isolate the ultimate cause behind the shift in Rossby wave breaking in response to the two external forcings.
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Harr, Patrick A., and Jonathan M. Dea. "Downstream Development Associated with the Extratropical Transition of Tropical Cyclones over the Western North Pacific." Monthly Weather Review 137, no. 4 (April 1, 2009): 1295–319. http://dx.doi.org/10.1175/2008mwr2558.1.
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Abstract The movement of a tropical cyclone into the midlatitudes involves interactions among many complex physical processes over a variety of space and time scales. Furthermore, the extratropical transition (ET) of a tropical cyclone may also result in a high-amplitude Rossby wave response that can extend to near-hemispheric scales. After an ET event occurs over the western portion of a Northern Hemisphere ocean basin, the high-amplitude downstream response often forces anomalous midlatitude circulations for periods of days to a week. These circulations may then be related to high-impact weather events far downstream of the forcing by the ET event. In this study, downstream development following ET events over the western North Pacific Ocean is examined. Local eddy kinetic energy analyses are conducted on four cases of North Pacific tropical cyclones of varying characteristics during ET into varying midlatitude flow characteristics during 15 July–30 September 2005. The goal is to examine the impact of each case on downstream development across the North Pacific during a period in which these events might increase the midlatitude cyclogenesis across the North Pacific during a season in which cyclogenesis is typically weak. Four typhoon (TY) cases from the summer of 2005 are chosen to represent the wide spectrum of variability in ET. This includes a case (TY Nabi 14W) that directly resulted in an intense midlatitude cyclone, a case in which a weak midlatitude cyclone resulted (TY Banyan 07W), a case in which the decaying tropical cyclone was absorbed into the midlatitude flow (TY Guchol 12W), and a case (TY Saola 17W) in which the tropical cyclone decayed under the influence of strong vertical wind shear. The variability in downstream response to each ET case is related to specific physical characteristics associated with the evolution of the ET process and the phasing between the poleward-moving tropical cyclone and the midlatitude circulation into which it is moving. A case of downstream development that occurred during September 2005 without an ET event is compared with the four ET cases.
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Sampe, Takeaki, Hisashi Nakamura, Atsushi Goto, and Wataru Ohfuchi. "Significance of a Midlatitude SST Frontal Zone in the Formation of a Storm Track and an Eddy-Driven Westerly Jet*." Journal of Climate 23, no. 7 (April 1, 2010): 1793–814. http://dx.doi.org/10.1175/2009jcli3163.1.
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Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere–ocean interaction in shaping the tropospheric general circulation.
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Nikulin, G., and A. Karpechko. "The mean meridional circulation and midlatitude ozone buildup." Atmospheric Chemistry and Physics 5, no. 11 (November 24, 2005): 3159–72. http://dx.doi.org/10.5194/acp-5-3159-2005.
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Abstract. The wintertime ozone buildup over the Northern Hemisphere (NH) midlatitudes and its connection with the mean meridional circulation in the stratosphere are examined statistically on a monthly basis from October to March (1980–2002). The ozone buildup begins locally in October with positive total ozone tendencies over the North Pacific, which spread eastward and westward in November and finally cover all midlatitudes in December. The local onset of the buildup in October is not evident in zonal mean ozone tendency, which is close to zero. From November to March, zonal mean total ozone tendency (50°–60° N) shows a strong correlation (|r|=0.7) with several zonal mean parameters associated to the mean meridional circulation, namely: eddy heat flux, temperature tendency, the vertical residual velocity and the residual streamfunction. At the same time, on the latitude-altitude cross section, correlation patterns between ozone tendency and widely used eddy heat flux are not uniform during winter. The strongest correlations are located equatorward (almost throughout the stratosphere) or poleward (only in the lower stratosphere) of the edge of the polar vortex. Such distribution may depend on the existence of the midlatitude and polar waveguides which defined refraction of upward propagating waves from the troposphere either to the midlatitude stratosphere or to the polar stratosphere. As a consequence of the nonuniform correlation patterns, heat flux averaged over the common region 45°–75° N, 100 hPa is not always an optimum proxy for statistical models describing total ozone variability in midlatitudes. Other parameters approximating the strength of the mean meridional circulation have more uniform and stable correlation patterns with ozone tendency during winter. We show that the NH midlatitude ozone buildup has a stable statistical relationship with the mean meridional circulation in all months from October to March and half of the interannual variability in monthly ozone tendencies can be explained by applying different proxies of the mean meridional circulation.
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Quinting, Julian F., and Sarah C. Jones. "On the Impact of Tropical Cyclones on Rossby Wave Packets: A Climatological Perspective." Monthly Weather Review 144, no. 5 (May 2016): 2021–48. http://dx.doi.org/10.1175/mwr-d-14-00298.1.
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Many studies have highlighted the importance of recurving tropical cyclones (TCs) in triggering Rossby waves. This study investigates the impact of western North Pacific (WNP), south Indian Ocean, and North Atlantic recurving TCs on the amplitude and frequency of synoptic-scale Rossby wave packets (RWPs) over a 30-yr period. The results indicate a significant increase of RWP frequency downstream of WNP and south Indian Ocean TCs. A statistically significant RWP amplitude anomaly downstream of these TCs suggests that RWPs, which are associated with TCs, are stronger than those that generally occur in midlatitudes. North Atlantic TCs do not seem to be associated with a statistically significant increase in RWP frequency and amplitude downstream. Processes that contribute to Rossby wave amplification are identified by creating composites for WNP TCs with and without downstream development. Potential vorticity, eddy kinetic energy, and quasigeostrophic forcing diagnostics highlight dynamical mechanisms that contribute to the synergistic interaction between the TC and the midlatitude flow. The existence of an upstream Rossby wave favors a downstream development. Diabatically enhanced upper-level divergent flow that can be attributed to the nonlinear interaction between the TC and the midlatitude flow impedes the eastward propagation of the upstream trough, amplifies the downstream ridge, and intensifies the jet. The amplified midlatitude flow provides upper-level forcing, which helps to maintain the predominantly diabatically driven divergent flow. Forecast uncertainties that are related to these complex TC–midlatitude flow interactions may spread into downstream regions. A climatological analysis of ensemble reforecast data emphasizes the importance of TC–midlatitude flow interactions and Rossby wave amplification on downstream predictability.
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Ye, Hailun, Wen Yi, Baozhu Zhou, Jianfei Wu, Bingkun Yu, Penghao Tian, Jianyuan Wang, et al. "Multi-Instrumental Observations of Midlatitude Plasma Irregularities over Eastern Asia during a Moderate Magnetic Storm on 16 July 2003." Remote Sensing 15, no. 4 (February 20, 2023): 1160. http://dx.doi.org/10.3390/rs15041160.
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This study presents the observations of midlatitude plasma irregularities over Eastern Asia during a moderate magnetic storm on 16 July 2003. Multi-instrumental observations, including the ground-based ionosondes, the GNSS networks, and the CHAMP and ROCSAT-1 satellites, were utilized to investigate the occurrence and characteristics of midlatitude plasma irregularities. The midlatitude strong spread F (SSF) mainly occurred in the midnight–morning sector as observed by ionosondes over Japan during this storm. SSF was related to plasma depletions, which is also recorded by GNSS network in the form of the enhancement of the rate of total electron content (TEC) change index (ROTI). The possible mechanism for the generation of SSF is that the enhanced eastward electric fields, associated with the prompt penetration electric fields and disturbance dynamo electric fields, cause the uplift and latitudinal extension of equatorial plasma bubbles (EPBs) to generate the observed midlatitude SSF further. Meanwhile, plasma density increased significantly under the influence of this storm. In addition, other common type of spread F, frequency spread F (FSF), was observed over Japan on the non-storm day and/or at high latitude station WK545, which seems to be closely related to the coupling of medium-scale traveling ionospheric disturbances (MSTIDs) and sporadic E (Es) layer. The above results indicate that various types of midlatitude spread F can be produced by different physical mechanisms. It is found that SSF can significantly affect the performance of radio wave propagation compared with FSF. Our results show that space weather events have a significant influence on the day-to-day variability of the occurrence and characteristics of ionospheric F-region irregularities at midlatitudes.
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Grooß, Jens-Uwe, Paul Konopka, and Rolf Müller. "Ozone Chemistry during the 2002 Antarctic Vortex Split." Journal of the Atmospheric Sciences 62, no. 3 (March 1, 2005): 860–70. http://dx.doi.org/10.1175/jas-3330.1.
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Abstract In September 2002, the Antarctic polar vortex was disturbed, and it split into two parts caused by an unusually early stratospheric major warming. This study discusses the chemical consequences of this event using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The chemical initialization of the simulation is based on Halogen Occultation Experiment (HALOE) measurements. Because of its Lagrangian nature, CLaMS is well suited for simulating the small-scale filaments that evolve during this period. Filaments of vortex origin in the midlatitudes were observed by HALOE several times in October 2002. The results of the simulation agree well with these HALOE observations. The simulation further indicates a very rapid chlorine deactivation that is triggered by the warming associated with the split of the vortex. Correspondingly, the ozone depletion rates in the polar vortex parts rapidly decrease to zero. Outside the polar vortex, where air masses of midlatitude origin were transported to the polar region, the simulation shows high ozone depletion rates at the 700-K level caused mainly by NOx chemistry. Owing to the major warming in September 2002, ozone-poor air masses were transported into the midlatitudes and caused a decrease of midlatitude ozone by 5%–15%, depending on altitude. Besides this dilution effect, there was no significant additional chemical effect. The net chemical ozone depletion in air masses of vortex origin was low and did not differ significantly from that of midlatitude air, in spite of the different chemical composition of the two types of air masses.
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Guineva, Veneta, Rolf Werner, Irina Despirak, Natalya Kleimenova, Andris Lubchich, Pavel Setsko, Atanas Atanassov, Rumiana Bojilova, Lyubomira Raykova, and Dimitar Valev. "BASIC RESULTS FROM THE PROJECT “INVESTIGATION OF THE GEOMAGNETIC DISTURBANCES PROPAGATION TO MID-LATITUDES AND THEIR INTERPLANETARY DRIVERS IDENTIFICATION FOR THE DEVELOPMENT OF MID-LATITUDE SPACE WEATHER FORECAST”." PHYSICS OF AURORAL PHENOMENA 46, no. 1 (2023): 23–29. http://dx.doi.org/10.51981/2588-0039.2023.46.005.
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The project is directed to one of the topical tasks of the solar-terrestrial physics: study of the midlatitude effects of the magnetospheric substorms as a key element of the space weather. The goal of the project was to conduct a comprehensive analysis of the spatiotemporal characteristics of magnetospheric substorms and their effects at midlatitudes depending on space weather conditions. For this purpose, studies of various phenomena related to the development of substorm disturbances and their propagation to midlatitudes were carried out. For the first time, an original catalog of the variations of the magnetic field at the midlatitude Bulgarian station Panagjurishte (PAG) was created for the period 2007 - 2022. A methodology was developed and universal programs were created for processing data from European stations, for obtaining maps of the spatial distribution of magnetic variations, and for calculating the midlatitude positive bay (MPB) index. Analyses of events during quiet and disturbed geomagnetic conditions, during slow flows in the solar wind or high speed streams from coronal holes, were carried out. Some cases of supersubstorms have been studied in detail. The hypothesis of the development of an additional substorm current wedge during supersubstorms was confirmed. The morphological features of the polar substorms were also studied. Catalogs of supersubstorms and polar substorms for the past 20 years have been created. The relationships between the statistical distributions of the MPB index and widely used geomagnetic indices and solar wind parameters were established. Cases of occurrence of intense geomagnetically induced currents (GIC) during several strong magnetic storms were identified and analyzed.
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Andersen, S. B., and B. M. Knudsen. "The influence of polar vortex ozone depletion on NH mid-latitude ozone trends in spring." Atmospheric Chemistry and Physics Discussions 6, no. 2 (March 10, 2006): 1793–811. http://dx.doi.org/10.5194/acpd-6-1793-2006.
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Abstract. Reverse domain-filling trajectory calculations have been performed for the years 1993, 1995, 1996, 1997, and 2000 to calculate the spreading of ozone depleted air from the polar vortex to midlatitudes in spring. We find that for these years with massive Arctic ozone depletion the zonal mean total ozone column at midlatitudes is reduced with between 7 and 12 DU in the April-May period. The polar vortex and remnants have preferred locations which leads to longitudinal differences in the midlatitude ozone trends. Together with decadal variations in circulation the dilution of ozone depleted air may explain the major fraction of longitudinal differences in midlatitude ozone trends. The dilution also has a significant impact on the zonal mean ozone trends. With a multiple linear regression model we find that the dilution may explain 39% of the trend in the period 1979–1997 and 54% of the trend in the period 1979–2002. By calculting the effect of simply removing the dilutions we get the possibly more reliable estimates that that dilution may explain 29% of the trend in the period 1979–1997 and 33% of the trend in the period 1979–2002.
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Blanchard-Wrigglesworth, Edward, and Qinghua Ding. "Tropical and Midlatitude Impact on Seasonal Polar Predictability in the Community Earth System Model." Journal of Climate 32, no. 18 (August 20, 2019): 5997–6014. http://dx.doi.org/10.1175/jcli-d-19-0088.1.
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Abstract The impact on seasonal polar predictability from improved tropical and midlatitude forecasts is explored using a perfect-model experiment and applying a nudging approach in a GCM. We run three sets of 7-month long forecasts: a standard free-running forecast and two nudged forecasts in which atmospheric winds, temperature, and specific humidity (U, V, T, Q) are nudged toward one of the forecast runs from the free ensemble. The two nudged forecasts apply the nudging over different domains: the tropics (30°S–30°N) and the tropics and midlatitudes (55°S–55°N). We find that the tropics have modest impact on forecast skill in the Arctic or Antarctica both for sea ice and the atmosphere that is mainly confined to the North Pacific and Bellingshausen–Amundsen–Ross Seas, whereas the midlatitudes greatly improve Arctic winter and Antarctic year-round forecast skill. Arctic summer forecast skill from May initialization is not strongly improved in the nudged forecasts relative to the free forecast and is thus mostly a “local” problem. In the atmosphere, forecast skill improvement from midlatitude nudging tends to be largest in the polar stratospheres and decreases toward the surface.
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Field, Paul R., Andrew J. Heymsfield, and Aaron Bansemer. "Snow Size Distribution Parameterization for Midlatitude and Tropical Ice Clouds." Journal of the Atmospheric Sciences 64, no. 12 (December 1, 2007): 4346–65. http://dx.doi.org/10.1175/2007jas2344.1.
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Abstract Many microphysical process rates involving snow are proportional to moments of the snow particle size distribution (PSD), and in this study a moment estimation parameterization applicable to both midlatitude and tropical ice clouds is proposed. To this end aircraft snow PSD data were analyzed from tropical anvils [Tropical Rainfall Measuring Mission/Kwajelein Experiment (TRMM/KWAJEX), Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE)] and midlatitude stratiform cloud [First International Satellite Cloud Climatology Project Research Experiment (FIRE), Atmospheric Radiation Measurement Program (ARM)]. For half of the dataset, moments of the PSDs are computed and a parameterization is generated for estimating other PSD moments when the second moment (proportional to the ice water content when particle mass is proportional to size squared) and temperature are known. Subsequently the parameterization was tested with the other half of the dataset to facilitate an independent comparison. The parameterization for estimating moments can be applied to midlatitude or tropical clouds without requiring prior knowledge of the regime of interest. Rescaling of the tropical and midlatitude size distributions is presented along with fits to allow the user to recreate realistic PSDs given estimates of ice water content and temperature. The effects of using different time averaging were investigated and were found not to be adverse. Finally, the merits of a single-moment snow microphysics versus multimoment representations are discussed, and speculation on the physical differences between the rescaled size distributions from the Tropics and midlatitudes is presented.
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Frierson, Dargan M. W. "Midlatitude Static Stability in Simple and Comprehensive General Circulation Models." Journal of the Atmospheric Sciences 65, no. 3 (March 1, 2008): 1049–62. http://dx.doi.org/10.1175/2007jas2373.1.
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Abstract The static stability of the extratropical troposphere is examined in two atmospheric general circulation models (GCMs) over idealized boundary conditions, with emphasis on the role of moisture in determining the midlatitude stability. The determination of the static stability is compared within two models: an idealized moist model with simplified representations of radiative transfer and other physical processes, and a comprehensive GCM with full physics. The GCMs are run over a zonally symmetric, fixed sea surface temperature (SST) aquaplanet surface, with a multitude of SST distributions to study the response of the extratropical static stability over a wide parameter range. In both models, the dry static stability averaged over the midlatitudes increases both with increases in the meridional temperature gradients, and with increases in the mean SST. These changes in static stability are compared with both moist theories and dry theories. Dry baroclinic eddy theories are invalid for the entire parameter range in the idealized GCM, and for much of the parameter range considered in the comprehensive GCM. A moist theory, on the other hand, works remarkably well in predicting the midlatitude stability over the entire parameter range for both models. These simulations give strong support for the influence of moisture on the thermal structure of the midlatitudes.
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Rudeva, Irina, Ian Simmonds, David Crock, and Ghyslaine Boschat. "Midlatitude Fronts and Variability in the Southern Hemisphere Tropical Width." Journal of Climate 32, no. 23 (November 8, 2019): 8243–60. http://dx.doi.org/10.1175/jcli-d-18-0782.1.
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Abstract This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.
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Aquila, V., L. D. Oman, R. Stolarski, A. R. Douglass, and P. A. Newman. "The Response of Ozone and Nitrogen Dioxide to the Eruption of Mt. Pinatubo at Southern and Northern Midlatitudes." Journal of the Atmospheric Sciences 70, no. 3 (March 1, 2013): 894–900. http://dx.doi.org/10.1175/jas-d-12-0143.1.
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Abstract Observations have shown that the mass of nitrogen dioxide decreased at both southern and northern midlatitudes in the year following the eruption of Mt. Pinatubo, indicating that the volcanic aerosol had enhanced nitrogen dioxide depletion via heterogeneous chemistry. In contrast, the observed ozone response showed a northern midlatitude decrease and a small southern midlatitude increase. Previous simulations that included an enhancement of heterogeneous chemistry by the volcanic aerosol but no other effect of this aerosol produce ozone decreases in both hemispheres, contrary to observations. The authors’ simulations show that the heating due to the volcanic aerosol enhanced both the tropical upwelling and Southern Hemisphere extratropical downwelling. This enhanced extratropical downwelling, combined with the time of the eruption relative to the phase of the Brewer–Dobson circulation, increased Southern Hemisphere ozone via advection, counteracting the ozone depletion due to heterogeneous chemistry on the Pinatubo aerosol.
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Lu, Jian, Gang Chen, and Dargan M. W. Frierson. "Response of the Zonal Mean Atmospheric Circulation to El Niño versus Global Warming." Journal of Climate 21, no. 22 (November 15, 2008): 5835–51. http://dx.doi.org/10.1175/2008jcli2200.1.
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Abstract The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Niño forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley cell and the equatorward shift of the tropospheric zonal jets in response to El Niño, the Hadley cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the projected “El Niño–like” enhanced warming over the equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and midlatitudes in comparison to the El Niño. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: 1) The increase in static stability of the subtropical and midlatitude troposphere, a robust result of the quasi-moist adiabatic adjustment to the surface warming, may stabilize the baroclinic eddy growth on the equatorward side of the storm tracks and push the eddy activity and the associated eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley cell and the shift of midlatitude jets; 2) the strengthening of the midlatitude wind at the upper troposphere and lower stratosphere, arguably a consequence of increases in the meridional temperature gradient near the tropopause level due to the tropospheric warming and tropopause slope, may increase the eastward propagation of the eddies emanating from the midlatitudes, and thus the subtropical region of wave breaking displaces poleward together with the eddy-driven circulation. Both mechanisms are somewhat, if not completely, distinct from those in response to the El Niño condition.
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Kehoe, Ryan M., Mark A. Boothe, and Russell L. Elsberry. "Dynamical Tropical Cyclone 96- and 120-h Track Forecast Errors in the Western North Pacific." Weather and Forecasting 22, no. 3 (June 1, 2007): 520–38. http://dx.doi.org/10.1175/waf1002.1.
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Abstract The Joint Typhoon Warning Center has been issuing 96- and 120-h track forecasts since May 2003. It uses four dynamical models that provide guidance at these forecast intervals and relies heavily on a consensus of these four models in producing the official forecast. Whereas each of the models has skill, each occasionally has large errors. The objective of this study is to provide a characterization of these errors in the western North Pacific during 2004 for two of the four models: the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the U.S. Navy’s version of the Geophysical Fluid Dynamics Laboratory model (GFDN). All 96- and 120-h track errors greater than 400 and 500 n mi, respectively, are examined following the approach developed recently by Carr and Elsberry. All of these large-error cases can be attributed to the models not properly representing the physical processes known to control tropical cyclone motion, which were classified in a series of conceptual models by Carr and Elsberry for either tropical-related or midlatitude-related mechanisms. For those large-error cases where an error mechanism could be established, midlatitude influences caused 83% (85%) of the NOGAPS (GFDN) errors. The most common tropical influence is an excessive direct cyclone interaction in which the tropical cyclone track is erroneously affected by an adjacent cyclone. The most common midlatitude-related errors in the NOGAPS tracks arise from an erroneous prediction of the environmental flow dominated by a ridge in the midlatitudes. Errors in the GFDN tracks are caused by both ridge-dominated and trough-dominated environmental flows in the midlatitudes. Case studies illustrating the key error mechanisms are provided. An ability to confidently identify these error mechanisms and thereby eliminate likely erroneous tracks from the consensus would improve the accuracy of 96- and 120-h track forecasts.
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Blackport, Russell, and James A. Screen. "Observed Statistical Connections Overestimate the Causal Effects of Arctic Sea Ice Changes on Midlatitude Winter Climate." Journal of Climate 34, no. 8 (April 2021): 3021–38. http://dx.doi.org/10.1175/jcli-d-20-0293.1.
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AbstractDisentangling the contribution of changing Arctic sea ice to midlatitude winter climate variability remains challenging because of the large internal climate variability in midlatitudes, difficulties separating cause from effect, methodological differences, and uncertainty around whether models adequately simulate connections between Arctic sea ice and midlatitude climate. We use regression analysis to quantify the links between Arctic sea ice and midlatitude winter climate in observations and large initial-condition ensembles of multiple climate models, in both coupled configurations and so-called Atmospheric Model Intercomparison Project (AMIP) configurations, where observed sea ice and/or sea surface temperatures are prescribed. The coupled models capture the observed links in interannual variability between winter Barents–Kara sea ice and Eurasian surface temperature, and between winter Chukchi–Bering sea ice and North American surface temperature. The coupled models also capture the delayed connection between reduced November–December Barents–Kara sea ice, a weakened winter stratospheric polar vortex, and a shift toward the negative phase of the North Atlantic Oscillation in late winter, although this downward impact is weaker than observed. The strength and sign of the connections both vary considerably between individual 35-yr-long ensemble members, highlighting the need for large ensembles to separate robust connections from internal variability. All the aforementioned links are either absent or are substantially weaker in the AMIP experiments prescribed with only observed sea ice variability. We conclude that the causal effects of sea ice variability on midlatitude winter climate are much weaker than suggested by statistical associations, evident in observations and coupled models, because the statistics are inflated by the effects of atmospheric circulation variability on sea ice.
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Sang, Xiaozhuo, Xiu-Qun Yang, Lingfeng Tao, Jiabei Fang, and Xuguang Sun. "Evaluation of synoptic eddy activities and their feedback onto the midlatitude jet in five atmospheric reanalyses with coarse versus fine model resolutions." Climate Dynamics 58, no. 5-6 (September 27, 2021): 1363–81. http://dx.doi.org/10.1007/s00382-021-05965-9.
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AbstractInteraction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction.
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Wu, Xiaokang, Huang Yang, Darryn W. Waugh, Clara Orbe, Simone Tilmes, and Jean-Francois Lamarque. "Spatial and temporal variability of interhemispheric transport times." Atmospheric Chemistry and Physics 18, no. 10 (May 29, 2018): 7439–52. http://dx.doi.org/10.5194/acp-18-7439-2018.
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Abstract. The seasonal and interannual variability of transport times from the northern midlatitude surface into the Southern Hemisphere is examined using simulations of three idealized “age” tracers: an ideal age tracer that yields the mean transit time from northern midlatitudes and two tracers with uniform 50- and 5-day decay. For all tracers the largest seasonal and interannual variability occurs near the surface within the tropics and is generally closely coupled to movement of the Intertropical Convergence Zone (ITCZ). There are, however, notable differences in variability between the different tracers. The largest seasonal and interannual variability in the mean age is generally confined to latitudes spanning the ITCZ, with very weak variability in the southern extratropics. In contrast, for tracers subject to spatially uniform exponential loss the peak variability tends to be south of the ITCZ, and there is a smaller contrast between tropical and extratropical variability. These differences in variability occur because the distribution of transit times from northern midlatitudes is very broad and tracers with more rapid loss are more sensitive to changes in fast transit times than the mean age tracer. These simulations suggest that the seasonal–interannual variability in the southern extratropics of trace gases with predominantly NH midlatitude sources may differ depending on the gases' chemical lifetimes.
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Spall, Michael A., and David Nieves. "Wind-Forced Variability of the Remote Meridional Overturning Circulation." Journal of Physical Oceanography 50, no. 2 (February 2020): 455–69. http://dx.doi.org/10.1175/jpo-d-19-0190.1.
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AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.
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Wang, Lu, Tim Li, Tianjun Zhou, and Xinyao Rong. "Origin of the Intraseasonal Variability over the North Pacific in Boreal Summer*." Journal of Climate 26, no. 4 (February 15, 2013): 1211–29. http://dx.doi.org/10.1175/jcli-d-11-00704.1.
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Abstract The spatial structure and temporal evolution of the intraseasonal oscillation (ISO) in boreal summer over the midlatitude North Pacific Ocean are investigated, through the diagnosis of NCEP reanalysis data. It is found that the midlatitude ISO has an equivalent-barotropic structure, with maximum amplitude at 250 hPa. Initiated near 120°W, the ISO perturbation propagates westward at a phase speed of about 2.4 m s−1 and reaches a maximum amplitude at 150°W. A diagnosis of barotropic energy conversion shows that the ISO gains energy from the summer mean flow in the ISO activity region. A center-followed column-averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the ISO perturbation. There is a two-way interaction between ISO flows and synoptic eddies. While a cyclonic (anticyclonic) ISO flow causes synoptic-scale eddies to tilt toward the northwest–southeast (northeast–southwest) direction, the tilted synoptic eddies then exert a positive feedback to reinforce the ISO cyclonic (anticyclonic) flow through eddy vorticity transport. The reanalysis data and numerical simulations show that the midlatitude ISO is primarily driven by local processes and the tropical forcing accounts for about 20% of total intraseasonal variability in midlatitudes. However, 20% might be an underestimate given that the tropical intraseasonal forcing is not fully included in the current observational analysis and modeling experiment.
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Marcella, Marc P., and Elfatih A. B. Eltahir. "The Hydroclimatology of Kuwait: Explaining the Variability of Rainfall at Seasonal and Interannual Time Scales." Journal of Hydrometeorology 9, no. 5 (October 1, 2008): 1095–105. http://dx.doi.org/10.1175/2008jhm952.1.
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Abstract This paper presents an analysis of the spatial, seasonal, and interannual variabilities of Kuwaiti rainfall. Based on an analysis of rain gauge, as well as satellite, datasets, it is estimated that about 110–190 mm of rainfall occurs annually in Kuwait, depending on the dataset sampled. The corresponding estimates for the standard deviations of the annual rainfall are about 40–70 mm. Discrepancies between values arise from the different techniques used in constructing each dataset. Moreover, the spatial distribution of annual rainfall features a gradual increase from the southwest to the northeast. A distinct rainy season occurs from November to April, with double peaks in January and March. In addition, the seasonal variability of rainfall is associated with shifts in patterns of midlatitude storm tracks, which propagate southward toward the Middle East during the winter and spring season. These trends are characterized using estimates of the spatial correlations of rainfall in Kuwait with the surrounding region. At the interannual time scale, significant correlation is found between the tropical El Niño–Southern Oscillation (ENSO) and annual rainfall anomalies. Similar weak correlations are found between midlatitude rainfall in Europe and rainfall in Kuwait. The weak connections observed with both tropical and midlatitude atmospheric systems are consistent with the fact that Kuwait is located in the transitional zone between the tropics and midlatitudes.
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Liu, Chun, Yuefeng Li, and Wei Song. "Variation in Dipole Blocking Associated with Arctic Warming in Winter: Potential Contributions to Cold and Extremely Cold Events over Eurasia." Atmosphere 10, no. 5 (May 6, 2019): 249. http://dx.doi.org/10.3390/atmos10050249.
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In this study, the barotropic mode of thermal forcing responsible for the difference in temperature between the Arctic and midlatitude regions was simplified by the nonlinear Schrӧdinger equation with disturbance terms using multiscale perturbation methods. The impact of Arctic warming on dipole blocking, which results in temperature anomalies over the midlatitudes of Eurasia, was studied using the direct perturbation theory for solitons. The results showed: (1) if only nonlinear effects exist between waves and zonal flows, a dipole blocking structure can present in the westerly air flows; (2) the temperature gradient between midlatitude warming and Arctic cooling inhibits the development of dipole blocking structures; and (3) Arctic warming is theoretically more conducive to intensifying the strength of dipole blocking and meridional activities over Eurasia and is more likely to cause the southward invasion of cold air from the Arctic, thereby inducing regionally cold and even extremely cold events in the mid- and low latitudes of Eurasia, including eastern China.
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Zhang, Gan, Hiroyuki Murakami, Thomas R. Knutson, Ryo Mizuta, and Kohei Yoshida. "Tropical cyclone motion in a changing climate." Science Advances 6, no. 17 (April 2020): eaaz7610. http://dx.doi.org/10.1126/sciadv.aaz7610.
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The locally accumulated damage by tropical cyclones (TCs) can intensify substantially when these cyclones move more slowly. While some observational evidence suggests that TC motion might have slowed significantly since the mid-20th century (1), the robustness of the observed trend and its relation to anthropogenic warming have not been firmly established (2–4). Using large-ensemble simulations that directly simulate TC activity, we show that future anthropogenic warming can lead to a robust slowing of TC motion, particularly in the midlatitudes. The slowdown there is related to a poleward shift of the midlatitude westerlies, which has been projected by various climate models. Although the model’s simulation of historical TC motion trends suggests that the attribution of the observed trends of TC motion to anthropogenic forcings remains uncertain, our findings suggest that 21st-century anthropogenic warming could decelerate TC motion near populated midlatitude regions in Asia and North America, potentially compounding future TC-related damages.
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Pepler, Acacia, and Andrew Dowdy. "A Three-Dimensional Perspective on Extratropical Cyclone Impacts." Journal of Climate 33, no. 13 (July 1, 2020): 5635–49. http://dx.doi.org/10.1175/jcli-d-19-0445.1.
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AbstractCyclones can be identified from gridded pressure data at different levels of the troposphere, with vertical structure known to influence the temporal development and impacts of midlatitude cyclones. However, studies of midlatitude cyclones typically focus on cyclones identified at a single atmospheric level. This paper examines how the frequency of vertically organized or deep cyclones varies around the world, with a focus on southeastern Australia. About 50% of global cyclones identified from mean sea level pressure show a coherent vertical structure extending to at least 500 hPa, based on ERA-Interim reanalysis data, and shallow cyclones are most common in the global midlatitudes. Using a combination of reanalysis data and satellite-based rainfall and lightning, we show that in southeast Australia deep cyclones have higher intensities, longer durations, and more severe winds and rainfall than either shallow surface cyclones or upper-level cyclones with no surface low, motivating a three-dimensional approach for future cyclone analyses.
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Zhang, Jiankai, Fei Xie, Wenshou Tian, Yuanyuan Han, Kequan Zhang, Yulei Qi, Martyn Chipperfield, Wuhu Feng, Jinlong Huang, and Jianchuan Shu. "Influence of the Arctic Oscillation on the Vertical Distribution of Wintertime Ozone in the Stratosphere and Upper Troposphere over the Northern Hemisphere." Journal of Climate 30, no. 8 (April 2017): 2905–19. http://dx.doi.org/10.1175/jcli-d-16-0651.1.
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The influence of the Arctic Oscillation (AO) on the vertical distribution of stratospheric ozone in the Northern Hemisphere in winter is analyzed using observations and an offline chemical transport model. Positive ozone anomalies are found at low latitudes (0°–30°N) and there are three negative anomaly centers in the northern mid- and high latitudes during positive AO phases. The negative anomalies are located in the Arctic middle stratosphere (~30 hPa; 70°–90°N), Arctic upper troposphere–lower stratosphere (UTLS; 150–300 hPa, 70°–90°N), and midlatitude UTLS (70–300 hPa, 30°–60°N). Further analysis shows that anomalous dynamical transport related to AO variability primarily controls these ozone changes. During positive AO events, positive ozone anomalies between 0° and 30°N at 50–150 hPa are related to the weakened meridional transport of the Brewer–Dobson circulation (BDC) and enhanced eddy transport. The negative ozone anomalies in the Arctic middle stratosphere are also caused by the weakened BDC, while the negative ozone anomalies in the Arctic UTLS are caused by the increased tropopause height, weakened BDC vertical transport, weaker exchange between the midlatitudes and the Arctic, and enhanced ozone depletion via heterogeneous chemistry. The negative ozone anomalies in the midlatitude UTLS are mainly due to enhanced eddy transport from the midlatitudes to the latitudes equatorward of 30°N, while the transport of ozone-poor air from the Arctic to the midlatitudes makes a minor contribution. Interpreting AO-related variability of stratospheric ozone, especially in the UTLS, would be helpful for the prediction of tropospheric ozone variability caused by the AO.
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Andersen, S. B., and B. M. Knudsen. "The influence of polar vortex ozone depletion on NH mid-latitude ozone trends in spring." Atmospheric Chemistry and Physics 6, no. 10 (July 11, 2006): 2837–45. http://dx.doi.org/10.5194/acp-6-2837-2006.
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Abstract. Reverse domain-filling trajectory calculations have been performed for the years 1993, 1995, 1996, 1997, and 2000 to calculate the spreading of ozone depleted air from the polar vortex to midlatitudes in spring. We find that for these years with massive Arctic ozone depletion the zonal mean total ozone column at midlatitudes is reduced with between 7 and 12 DU in the April-May period. The polar vortex and remnants have preferred locations which leads to longitudinal differences in the midlatitude ozone trends. Together with decadal variations in circulation the dilution of ozone depleted air may explain the major fraction of longitudinal differences in midlatitude ozone trends. For the period 1979–1997 the dilution may explain 50% of the longitudinal differences in ozone trends and for the period 1979–2002 it may explain 45%. The dilution also has a significant impact on the zonal mean ozone trends in the April-May period. Although uncertainties are large due to uncertainties in the ozone depletion values and neglect of ozone depletion in other years than 1993, 1995, 1996, 1997, and 2000 we have tried to calculate the size of this effect. We estimate that dilution may explain 29% of the trend in the period 1979–1997 and 33% of the trend in the period 1979–2002 as a lower limit.
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Patterson, Matthew, Tim Woollings, Thomas J. Bracegirdle, and Neil T. Lewis. "Wintertime Southern Hemisphere Jet Streams Shaped by Interaction of Transient Eddies with Antarctic Orography." Journal of Climate 33, no. 24 (December 15, 2020): 10505–22. http://dx.doi.org/10.1175/jcli-d-20-0153.1.
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AbstractThe wintertime Southern Hemisphere extratropical circulation exhibits considerable zonal asymmetries. We investigate the roles of various surface boundary conditions in shaping the mean state using a semi-realistic, atmosphere-only climate model. We find, in agreement with previous literature, that tropical sea surface temperature (SST) patterns are an important contributor to the mean state, while midlatitude SSTs and sea ice extent play a smaller role. Our main finding is that Antarctic orography has a first-order effect on the structure of the midlatitude circulation. In the absence of Antarctic orography, equatorward eddy momentum fluxes associated with the orography are removed and hence convergence of eddy momentum in midlatitudes is reduced. This weakens the Indian Ocean jet, making Rossby wave propagation downstream to the South Pacific less favorable. Consequently, the flow stagnates over the mid- to high-latitude South Pacific and the characteristic split jet pattern is destroyed. Removing Antarctic orography also results in a substantial warming over East Antarctica partly because transient eddies are able to penetrate farther poleward, enhancing poleward heat transport. However, experiments in which a high-latitude cooling is applied indicate that these temperature changes are not the primary driver of circulation changes in the midlatitudes. Instead, we invoke a simple barotropic mechanism in which the orographic slope creates an effective potential vorticity gradient that alters the eddy momentum flux.
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McGraw, Marie C., and Elizabeth A. Barnes. "New Insights on Subseasonal Arctic–Midlatitude Causal Connections from a Regularized Regression Model." Journal of Climate 33, no. 1 (December 9, 2019): 213–28. http://dx.doi.org/10.1175/jcli-d-19-0142.1.
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ABSTRACT Arctic–midlatitude teleconnections are complex and multifaceted. By design, targeted modeling studies typically focus only on one direction of influence—usually, the midlatitude atmospheric response to a changing Arctic. The two-way, coupled feedbacks between the Arctic and the midlatitude circulation on submonthly time scales are explored using a regularized regression model formulated around Granger causality. The regularized regression model indicates that there are regions in which Arctic temperature drives a midlatitude circulation response, and regions in which the midlatitude circulation drives a response in the Arctic; however, these regions rarely overlap. In many regions, on submonthly time scales, the midlatitude circulation drives Arctic temperature variability, highlighting the important role the midlatitude circulation can play in impacting the Arctic. In particular, the regularized regression model results support recent work that indicates that the observed high pressure anomalies over Eurasia drive a significant response in the Arctic on submonthly time scales, rather than being driven by the Arctic.
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Purich, Ariaan, Tim Cowan, Seung-Ki Min, and Wenju Cai. "Autumn Precipitation Trends over Southern Hemisphere Midlatitudes as Simulated by CMIP5 Models." Journal of Climate 26, no. 21 (October 16, 2013): 8341–56. http://dx.doi.org/10.1175/jcli-d-13-00007.1.
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Abstract In recent decades, Southern Hemisphere midlatitude regions such as southern Africa, southeastern Australia, and southern Chile have experienced a reduction in austral autumn precipitation; the cause of which is poorly understood. This study focuses on the ability of global climate models that form part of the Coupled Model Intercomparison Project phase 5 to simulate these trends, their relationship with extratropical and subtropical processes, and implications for future precipitation changes. Models underestimate both the historical autumn poleward expansion of the subtropical dry zone and the positive southern annular mode (SAM) trend. The multimodel ensemble (MME) is also unable to capture the spatial pattern of observed precipitation trends across semiarid midlatitude regions. However, in temperate regions that are located farther poleward such as southern Chile, the MME simulates observed precipitation declines. The MME shows a strong consensus in twenty-first-century declines in autumn precipitation across southern Chile in both the medium–low and high representative concentration pathway (RCP) scenarios and across southern Africa in the high RCP scenario, but little change across southeastern Australia. Projecting a strong positive SAM trend and continued subtropical dry-zone expansion, the models converge on large SAM and dry-zone-expansion-induced precipitation declines across southern midlatitudes. In these regions, the strength of future precipitation trends is proportional to the strength of modeled trends in these phenomena, suggesting that unabated greenhouse gas–induced climate change will have a large impact on austral autumn precipitation in such midlatitude regions.
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Bandoro, Justin, Susan Solomon, Aaron Donohoe, David W. J. Thompson, and Benjamin D. Santer. "Influences of the Antarctic Ozone Hole on Southern Hemispheric Summer Climate Change." Journal of Climate 27, no. 16 (August 7, 2014): 6245–64. http://dx.doi.org/10.1175/jcli-d-13-00698.1.
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Abstract Over the past three decades, Antarctic surface climate has undergone pronounced changes. Many of these changes have been linked to stratospheric ozone depletion. Here linkages between Antarctic ozone loss, the accompanying circulation changes, and summertime Southern Hemisphere (SH) midlatitude surface temperatures are explored. Long-term surface climate changes associated with ozone-driven changes in the southern annular mode (SAM) at SH midlatitudes in summer are not annular in appearance owing to differences in regional circulation and precipitation impacts. Both station and reanalysis data indicate a trend toward cooler summer temperatures over southeast and south-central Australia and inland areas of the southern tip of Africa. It is also found that since the onset of the ozone hole, there have been significant shifts in the distributions of both the seasonal mean and daily maximum summertime temperatures in the SH midlatitude regions between high and low ozone years. Unusually hot summer extremes are associated with anomalously high ozone in the previous November, including the recent very hot austral summer of 2012/13. If the relationship found in the past three decades continues to hold, the level of late springtime ozone over Antarctica has the potential to be part of a useful predictor set for the following summer’s conditions. The results herein suggest that skillful predictions may be feasible for both the mean seasonal temperature and the frequency of extreme hot events in some SH midlatitude regions of Australia, Africa, and South America.
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Barber, Katelyn A., and Gretchen L. Mullendore. "The Importance of Convective Stage on Out-of-Cloud Convectively Induced Turbulence from High-Resolution Simulations." Monthly Weather Review 148, no. 11 (November 2020): 4587–605. http://dx.doi.org/10.1175/mwr-d-20-0065.1.
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AbstractTurbulence (clear-air, mountain wave, convectively induced) is an aviation hazard that is a challenge to forecast due to the coarse resolution ultilized in operational weather models. Turbulence indices are commonly used to aid pilots in avoiding turbulence, but these indices have been designed and calibrated for midlatitude clear-air turbulence prediction (e.g., the Ellrod index). A significant limitation with current convectively induced turbulence (CIT) prediction is the lack of storm stage dependency. In this study, six high-resolution simulations of tropical oceanic and midlatitude continental convection are performed to characterize the turbulent environment near various convective types during the developing and mature stages. Second-order structure functions, a diagnostic commonly used to identify turbulence in turbulence prediction systems, are used to characterize the probability of turbulence for various convective types. Turbulence likelihood was found to be independent of region (i.e., tropical vs midlatitude) but dependent on convective stage. The probability of turbulence increased near developing convection for the majority of cases. Additional analysis of static stability and vertical wind shear, indicators of turbulence potential, showed that the convective environment near developing convection was more favorable for turbulence production than mature convection. Near developing convection, static stability decreased and vertical wind shear increased. Vertical wind shear near mature and developing convection was found to be weakly correlated to turbulence intensity in both the tropics and the midlatitudes. This study emphasizes the need for turbulence avoidance guidelines for the aviation community that are dependent on convective stage.
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Kretschmer, Marlene, Dim Coumou, Laurie Agel, Mathew Barlow, Eli Tziperman, and Judah Cohen. "More-Persistent Weak Stratospheric Polar Vortex States Linked to Cold Extremes." Bulletin of the American Meteorological Society 99, no. 1 (January 1, 2018): 49–60. http://dx.doi.org/10.1175/bams-d-16-0259.1.
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Abstract The extratropical stratosphere in boreal winter is characterized by a strong circumpolar westerly jet, confining the coldest temperatures at high latitudes. The jet, referred to as the stratospheric polar vortex, is predominantly zonal and centered around the pole; however, it does exhibit large variability in wind speed and location. Previous studies showed that a weak stratospheric polar vortex can lead to cold-air outbreaks in the midlatitudes, but the exact relationships and mechanisms are unclear. Particularly, it is unclear whether stratospheric variability has contributed to the observed anomalous cooling trends in midlatitude Eurasia. Using hierarchical clustering, we show that over the last 37 years, the frequency of weak vortex states in mid- to late winter (January and February) has increased, which was accompanied by subsequent cold extremes in midlatitude Eurasia. For this region, 60% of the observed cooling in the era of Arctic amplification, that is, since 1990, can be explained by the increased frequency of weak stratospheric polar vortex states, a number that increases to almost 80% when El Niño–Southern Oscillation (ENSO) variability is included as well.
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Torn, Ryan D., and Gregory J. Hakim. "Comparison of Wave Packets Associated with Extratropical Transition and Winter Cyclones." Monthly Weather Review 143, no. 5 (May 1, 2015): 1782–803. http://dx.doi.org/10.1175/mwr-d-14-00006.1.
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Abstract Differences in the development of wave packets associated with midlatitude cyclones and the extratropical transition (ET) of tropical cyclones in the western North Pacific (WNP) and Atlantic basins are diagnosed observationally by compositing Climate Forecast System Reanalysis (CFSR) data over a 32-yr period and applying the null hypothesis of no difference in the development, structure, propagation, or downstream extent. While the development of midlatitude cyclones during the fall (August–October) and winter (November–March) amplifies a preexisting wave packet moving through the midlatitude storm track, ET is associated with the quasi-stationary amplification of the midlatitude flow. The ET cases involving the interaction of the decaying TC with a preexisting midlatitude trough are associated with a greater downstream amplitude and longer-lasting downstream response than ET cases that do not involve the interaction with a trough. In the WNP, ET wave packets have greater amplitude than those associated with winter midlatitude cyclones, but are similar to those associated with fall midlatitude cyclones. Moreover, ET events are associated with larger wavelengths and a statistically significant meridional wind anomaly farther downstream. By contrast, ET wave packets in the Atlantic basin have less amplitude and do not reach as far downstream as wave packets associated with fall and winter cyclones. WNP ET is characterized by larger integrated moisture flux convergence and, thus, latent heat release relative to its midlatitude counterpart, while Atlantic basin ET has smaller moisture flux convergence compared to midlatitude cyclones, which could explain why Atlantic ET is associated with less-amplified wave packets.
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Vogel, B., R. Müller, A. Engel, J. U. Grooß, D. Toohey, T. Woyke, and F. Stroh. "Midlatitude ClO during the maximum atmospheric chlorine burden: in situ balloon measurements and model simulations." Atmospheric Chemistry and Physics 5, no. 6 (June 24, 2005): 1623–38. http://dx.doi.org/10.5194/acp-5-1623-2005.
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Abstract. Chlorine monoxide (ClO) plays a key role in stratospheric ozone loss processes at midlatitudes. We present two balloon-borne in situ measurements of ClO conducted in northern hemisphere midlatitudes during the period of the maximum of total inorganic chlorine loading in the atmosphere. Both ClO measurements were conducted on board the TRIPLE balloon payload, launched in November 1996 in León, Spain, and in May 1999 in Aire sur l'Adour, France. For both flights a ClO daylight and night-time vertical profile was derived over an altitude range of approximately 15-35 km. ClO mixing ratios are compared to model simulations performed with the photochemical box model version of the Chemical Lagrangian Model of the Stratosphere (CLaMS). Simulations along 24-hour backward trajectories were performed to study the diurnal variation of ClO in the midlatitude lower stratosphere. Model simulations for the flight launched in Aire sur l'Adour 1999 show an excellent agreement with the ClO measurements. For the flight launched in León 1996, an overall good agreement is found, whereas the flight is characterized by a more complex dynamical situation due to a possible mixture of vortex and non-vortex air. We note that for both flights at solar zenith angles greater than 86°-87° simulated ClO mixing ratios are higher than observed ClO mixing ratios. However, the present findings indicate that no substantial uncertainties exist in midlatitude chlorine chemistry of the stratosphere.
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Hajkowicz, L. A. "Morphology of quantified ionospheric range spread-F over a wide range of midlatitudes in the Australian longitudinal sector." Annales Geophysicae 25, no. 5 (June 4, 2007): 1125–30. http://dx.doi.org/10.5194/angeo-25-1125-2007.
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Abstract. Ionograms from a standard vertical-incidence ionosonde chain (nine stations), obtained over a wide range of southern latitudes (in geom.lat. range: 23°–52° S), were digitally scanned at 5-min intervals at nighttime (18:00–06:00 LT) for 13 months (January 2004–January 2005). An important parameter of the F-region, so-called range spread-F (Sr), was for the first time quantified in km. Maximum in Sr was recorded at a sounding frequency of 1.8 MHz for each night and for each ionosonde station. A distinct pattern in the magnitude (in km) and in the percentage occurrence of the range spread-F was present in southern winter only (the June solstice). The sub-auroral region (geom. lat. ≥52° S) is characterised by consistently high spread-F (average Sr≈100 km) on 80–100 per cent of the observed nights. There is a sharp equatorward boundary in the spread-F activity in a latitudinal range: 52°–48° S followed by a midlatitude region (44°–48° S) which exhibits a peak in Sr (≈50 km) in winter only, observed on half of the nights. The midlatitude activity reaches its minimum at 42°–43° S, with Sr less than 20 km on one third of the nights. The low midlatitudes (23°–36° S) are characterised by a strong peak in Sr again in winter, centred at about 30° S (average Sr≈70 km) on 80 per cent of the nights. The pattern becomes largely absent during other seasons particularly in southern summer (the December solstice) when spread-F activity shifts to sub-auroral latitudes. The pattern in the occurrence of spread-F appears to have a global character as the enhanced spread-F activity is observed in the Japanese sector in local summer (i.e. the June solstice). It appears that the midlatitude spread-F minimum is only apparent but not real. It delineates the boundary between aurorally generated spread-F (due to travelling ionospheric disturbances, TIDs) and low midlatitude spread-F whose origin is not known.
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Gaetani, Marco, Benjamin Pohl, Maria del Carmen Alvarez Castro, Cyrille Flamant, and Paola Formenti. "A weather regime characterisation of winter biomass aerosol transport from southern Africa." Atmospheric Chemistry and Physics 21, no. 21 (November 12, 2021): 16575–91. http://dx.doi.org/10.5194/acp-21-16575-2021.
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Abstract. During austral winter, a compact low cloud deck over the South Atlantic contrasts with clear sky over southern Africa, where forest fires triggered by dry conditions emit large amounts of biomass burning aerosols (BBAs) in the free troposphere. Most of the BBA burden crosses the South Atlantic embedded in the tropical easterly flow. However, midlatitude synoptic disturbances can deflect part of the aerosol from the main transport path towards southern extratropics. In this study, the first objective classification of the synoptic variability controlling the spatial distribution of BBA in southern Africa and the South Atlantic during austral winter (August to October) is presented. By analysing atmospheric circulation data from reanalysis products, a six-class weather regime (WR) classification of the region is constructed. The classification reveals that the synoptic variability is composed of four WRs, representing disturbances travelling at midlatitudes, and two WRs accounting for pressure anomalies in the South Atlantic. The WR classification is then successfully used to characterise the aerosol spatial distribution in the region in the period 2003–2017, in both reanalysis products and station data. Results show that the BBA transport towards southern extratropics is controlled by weather regimes associated with midlatitude synoptic disturbances. In particular, depending on the relative position of the pressure anomalies along the midlatitude westerly flow, the BBA transport is deflected from the main tropical route towards southern Africa or the South Atlantic. Moreover, the WRs accounting for midlatitude disturbances show organised transition sequences, which allow one to illustrate the evolution of the BBA northerly transport across the region in the context of a wave pattern. The skill in characterising the BBA transport shown by the WR classification indicates the potential for using it as a diagnostic/predictive tool for the aerosol dynamics, which is a key component for the full understanding and modelling of the complex radiation–aerosol–cloud interactions controlling the atmospheric radiative budget in the region.
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Hinssen, Y. B. L., C. J. Bell, and P. C. Siegmund. "The influence of the stratosphere on the tropospheric zonal wind response to CO<sub>2</sub> doubling." Atmospheric Chemistry and Physics Discussions 10, no. 10 (October 12, 2010): 23895–925. http://dx.doi.org/10.5194/acpd-10-23895-2010.
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Abstract. The influence of a CO2 doubling on the stratospheric potential vorticity (PV) is examined in two climate models. Subsequently, the influence of changes in the stratosphere on the tropospheric zonal wind response is investigated, by inverting the stratospheric PV. Radiative effects dominate the stratospheric response to CO2 doubling in the Southern Hemisphere. These lead to a stratospheric PV increase at the edge of the polar vortex, resulting in an increased westerly influence of the stratosphere on the tropospheric midlatitude winds in late winter. In the Northern Hemisphere, dynamical effects are also important. Both models show a reduced polar PV and an enhanced midlatitude PV in the Northern Hemisphere winter stratosphere. These PV changes are related to an enhanced wave forcing of the winter stratosphere, as measured by an increase in the 100 hPa eddy heat flux, and result in a reduced westerly influence of the stratosphere on the high latitude tropospheric winds. In one model, the high latitude PV decreases are, however, restricted to higher altitudes, and the tropospheric response due to the stratospheric changes is dominated by an increased westerly influence in the midlatitudes, related to the increase in midlatitude PV in the lower stratosphere. The tropospheric response in zonal wind due to the stratospheric PV changes is of the order of 0.5 to 1 m s−1. The total tropospheric response has a somewhat different spatial structure, but is of similar magnitude. This indicates that the stratospheric influence is of importance in modifying the tropospheric zonal wind response to CO2 doubling.
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Hinssen, Y. B. L., C. J. Bell, and P. C. Siegmund. "The influence of the stratosphere on the tropospheric zonal wind response to CO<sub>2</sub> doubling." Atmospheric Chemistry and Physics 11, no. 10 (May 26, 2011): 4915–27. http://dx.doi.org/10.5194/acp-11-4915-2011.
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Abstract. The influence of a CO2 doubling on the stratospheric potential vorticity (PV) is examined in two climate models. Subsequently, the influence of changes in the stratosphere on the tropospheric zonal wind response is investigated, by inverting the stratospheric PV. Radiative effects seem to dominate the stratospheric response to CO2 doubling in the Southern Hemisphere. These lead to a stratospheric PV increase at the edge of the polar vortex, resulting in an increased westerly influence of the stratosphere on the troposphere, increasing the midlatitude tropospheric westerlies in late winter. In the Northern Hemisphere, dynamical effects are also important. Both models show a reduced polar PV and an enhanced midlatitude PV in the Northern Hemisphere winter stratosphere. These PV changes are likely related to an enhanced wave forcing of the winter stratosphere, as measured by an increase in the 100 hPa eddy heat flux, and result in a reduced westerly influence of the stratosphere on the high latitude tropospheric winds. In one model, the high latitude PV decreases are, however, restricted to higher altitudes, and the tropospheric response due to the stratospheric changes is dominated by an increased westerly influence in the midlatitudes, related to the increase in midlatitude PV in the lower stratosphere. The tropospheric response in zonal wind due to the stratospheric PV changes is of the order of 0.5 to 1 m s−1. The total tropospheric response has a somewhat different spatial structure, but is of similar magnitude. This indicates that the stratospheric influence is of importance in modifying the tropospheric zonal wind response to CO2 doubling.
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Branstator, Grant. "Long-Lived Response of the Midlatitude Circulation and Storm Tracks to Pulses of Tropical Heating." Journal of Climate 27, no. 23 (December 1, 2014): 8809–26. http://dx.doi.org/10.1175/jcli-d-14-00312.1.
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Abstract The midlatitude response to localized equatorial heating events that last 2 days is examined through experimentation with an atmospheric general circulation model. Such responses are argued to be important because many tropical rainfall events only last a short time and because the responses to such pulses serve as building blocks with which to study the impacts of more general heating fluctuations. The experiments indicate that short-lived heating produces responses in midlatitudes at locations far removed from the source and these responses persist much longer than the pulses themselves. Indeed pulse forcing, which is essentially white in time, produces upper-tropospheric responses that have an e-damping time of almost a week and that are detectable for more than two weeks in the experiments. Moreover the upper-tropospheric structure of the reaction to short pulses is remarkably similar to the reaction to steady tropical heating, including having a preference for occurring at special geographical locations and being composed of recurring patterns that resemble the leading patterns of responses to steady heating. This similarity is argued to be a consequence of the responses to pulses having little or no phase propagation in the extratropics. The impact of short-lived tropical heating also produces a persistent response in midlatitude surface fields and in the statistics of synoptic eddies. The implications these results have for subseasonal variability are discussed. These include 1) the potential for improving subseasonal prediction through improved assimilation and short-range forecasts of tropical precipitation and 2) the difficulties involved in attributing subseasonal midlatitude events to tropical heating.
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Deng, Difei, and Elizabeth A. Ritchie. "Rainfall Characteristics of Recurving Tropical Cyclones over the Western North Pacific." Journal of Climate 31, no. 2 (January 2018): 575–92. http://dx.doi.org/10.1175/jcli-d-17-0415.1.
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A dataset of 88 recurving western North Pacific tropical cyclones from 2004 to 2015 is investigated for rainfall characteristics during their period of recurvature. The TCs are categorized into two groups based on different large-scale patterns from empirical orthogonal function analysis. Group 1 is characterized by an intense midlatitude baroclinic zone and close distance between the zone and TC, while Group 2 is characterized by a weaker midlatitude baroclinic zone and more remote distance between the zone and TC at the time of recurvature. The results show the large-scale environment has substantial impact on TC rainfall patterns. In Group 1, as the TC approaches and is embedded into the baroclinic zone, a relatively strong interaction between the TC and midlatitudes occurs, which is reflected by a rapid increase of environmental vertical wind shear and TC translation speed, the alignment of the shear vector and motion vector, and a sharp contrast of temperature and moisture. Higher rainfall and wider coverage of rainfall tends to be produced along the track after recurvature, and the rainfall pattern turns from a right-of-track (ROT) to a left-of-track (LOT) preference. Conversely, in Group 2, a relatively weak interaction between the TC and midlatitude circulation occurs, which is reflected by weaker vertical wind shear and slower TC motion, a separation of the shear vector and motion vector, and a weak gradient of temperature and moisture. The corresponding rainfall swath for Group 2 exhibits a narrower rainfall swath after recurvature. The rain pattern changes from a LOT to ROT preference.
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Ferreira, Rosana Nieto, Linwood Hall, and Thomas M. Rickenbach. "A Climatology of the Structure, Evolution, and Propagation of Midlatitude Cyclones in the Southeast United States." Journal of Climate 26, no. 21 (October 16, 2013): 8406–21. http://dx.doi.org/10.1175/jcli-d-12-00657.1.
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Abstract The seasonal and interannual variability of the structure, evolution, and propagation of midlatitude cyclones in the southeast United States are studied using a composite analysis. In the upper levels, the composites show that the axis of the wintertime upper-level trough remains north–south oriented and propagates eastward along 40°N, while the summertime upper-level trough has a much slower propagation at a farther north latitude and an axis that is tilted in the northeast–southwest direction. Upper-level circulation changes are consistent with a shift from wintertime “cyclonic behavior” to summertime “anticyclonic behavior” midlatitude cyclones. Significant changes in the low-level structure and precipitation patterns of midlatitude cyclones ensue from these upper-level changes. While the winter composite is characterized by eastward-propagating midlatitude cyclones that extend deep into the subtropics, the summer composite is characterized by semistationary midlatitude troughs that only briefly skirt the subtropics. Wintertime precipitation occurs only in and ahead of the surface low pressure center, whereas summertime precipitation occurs in all days of the composite. As a result, over 70% (30%) of wintertime (summertime) precipitation in the Carolinas occurs on days when midlatitude cyclones are present. The wintertime composites also show that midlatitude cyclones produce more precipitation on the windward side of the Appalachians than over the Carolinas, suggesting a rain shadow effect of the mountains. The ENSO-related variability of the structure, evolution, and propagation of midlatitude cyclones shows the presence of a more intense and southward-displaced upper-level jet, stronger midlatitude cyclones, and more intense precipitation over a larger area during El Niño than La Niña or normal years.
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Schneider, Tapio, Tobias Bischoff, and Hanna Płotka. "Physics of Changes in Synoptic Midlatitude Temperature Variability." Journal of Climate 28, no. 6 (March 13, 2015): 2312–31. http://dx.doi.org/10.1175/jcli-d-14-00632.1.
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Abstract This paper examines the physical processes controlling how synoptic midlatitude temperature variability near the surface changes with climate. Because synoptic temperature variability is primarily generated by advection, it can be related to mean potential temperature gradients and mixing lengths near the surface. Scaling arguments show that the reduction of meridional potential temperature gradients that accompanies polar amplification of global warming leads to a reduction of the synoptic temperature variance near the surface. This is confirmed in simulations of a wide range of climates with an idealized GCM. In comprehensive climate simulations (CMIP5), Arctic amplification of global warming similarly entails a large-scale reduction of the near-surface temperature variance in Northern Hemisphere midlatitudes, especially in winter. The probability density functions of synoptic near-surface temperature variations in midlatitudes are statistically indistinguishable from Gaussian, both in reanalysis data and in a range of climates simulated with idealized and comprehensive GCMs. This indicates that changes in mean values and variances suffice to account for changes even in extreme synoptic temperature variations. Taken together, the results indicate that Arctic amplification of global warming leads to even less frequent cold outbreaks in Northern Hemisphere winter than a shift toward a warmer mean climate implies by itself.
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Grise, Kevin M., and Lorenzo M. Polvani. "Southern Hemisphere Cloud–Dynamics Biases in CMIP5 Models and Their Implications for Climate Projections." Journal of Climate 27, no. 15 (July 29, 2014): 6074–92. http://dx.doi.org/10.1175/jcli-d-14-00113.1.
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Abstract This study quantifies cloud–radiative anomalies associated with interannual variability in the latitude of the Southern Hemisphere (SH) midlatitude eddy-driven jet, in 20 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two distinct model types are found. In the first class of models (type I models), total cloud fraction is reduced at SH midlatitudes as the jet moves poleward, contributing to enhanced shortwave radiative warming. In the second class of models (type II models), this dynamically induced cloud radiative warming effect is largely absent. Type I and type II models have distinct deficiencies in their representation of observed Southern Ocean clouds, but comparison with two independent satellite datasets indicates that the cloud–dynamics behavior of type II models is more realistic. Because the SH midlatitude jet shifts poleward in response to CO2 forcing, the cloud–dynamics biases uncovered from interannual variability are directly relevant for climate change projections. In CMIP5 model experiments with abruptly quadrupled atmospheric CO2 concentrations, the global-mean surface temperature initially warms more in type I models, even though their equilibrium climate sensitivity is not significantly larger. In type I models, this larger initial warming is linked to the rapid adjustment of the circulation and clouds to CO2 forcing in the SH, where a nearly instantaneous poleward shift of the midlatitude jet is accompanied by a reduction in the reflection of solar radiation by clouds. In type II models, the SH jet also shifts rapidly poleward with CO2 quadrupling, but it is not accompanied by cloud radiative warming anomalies, resulting in a smaller initial global-mean surface temperature warming.