Anticyclonic Anomalies Research Articles

What Percentage of Silk-Road Pattern Triggers Pacific–Japan Pattern through Rossby Wave Breaking?

In this study, we investigate the rate at which the Silk Road pattern (SRP) with Rossby wave breaking (RWB) near the Asian jet exit causes the Pacific–Japan (PJ) pattern in boreal summer. In this study, the SRP case is detected using the two principal components of upper-tropospheric meridional winds over Eurasia and characterized by the presence of an upper-level anticyclonic anomaly over the Yellow Sea or near Japan. They are further classified into cases with and without RWBs.In the SRP case with RWB, the upper-level anticyclonic anomaly near the Asian jet exit has a more extended shape in the zonal direction and larger amplitude than in the case without RWB. In the composite, a wave train associated with the SRP appears over Eurasia, which is accompanied by the RWB near the Asian jet exit. The occurrence of RWB is associated with strong deceleration and diffluence in the basic state there. The RWB promotes enhanced convection on its southern side due to the intrusion of upper-level high potential vorticity toward the southwest, resulting in the formation of the PJ pattern. The excited PJ pattern in the composite has a dipole structure with cyclonic anomalies to the south and anticyclonic anomalies to the north. Approximately 60–70 % of the SRP case with RWB is accompanied by the PJ patterns.Conversely, in the case of the SRP without RWB, the composite represents a wave train structure over Eurasia but indicates neither enhanced convection south of the RWB nor PJ patterns. Approximately 40–50 % of the SRP case without RWBs is accompanied by the PJ patterns. Hence, the presence of RWBs increases the percentage of the formation of positive PJ patterns by a factor of 1.2–1.7, indicating that the RWB plays an important role in the excitation of PJ patterns.

Two Types of Wintertime Teleconnection Patterns over the Western North Pacific Associated with Regionally Different Heating Anomalies

The turnabout of air temperature anomalies over East Asia between the first and second halves of winter 2020/21 was examined from a teleconnection perspective with regionally different convective heating anomalies over the Indo-western Pacific sector. In the first half of winter 2020/21, the air temperature over East Asia was lower than normal, accompanied by a pair of anticyclonic and cyclonic anomalies in the upper troposphere southeast of the Tibetan Plateau and north of Japan, respectively. This dipole pattern is newly referred to as the Southeast Asia–Japan (SAJ) pattern in this study, indicating the propagation of Rossby waves caused by enhanced tropical convection over the eastern Indian Ocean toward the South China Sea. In the second half of winter 2020/21, the enhanced convection shifted eastward to the Philippine Sea. The subsequent anticyclonic anomaly changed its position to the south of Japan, which was similar to the western Pacific (WP)-like teleconnection pattern, causing warmer conditions over East Asia. The composite analysis indicated that the anomalous anticyclone over the southeastern Tibetan Plateau corresponding to the SAJ pattern emerged simultaneously with intensification of convection over the South China Sea. Half of the cases of the WP-like pattern have been accompanied by enhanced convection over the Philippine Sea. The different circulation patterns were reproduced by prescribing the heat source over the South China Sea and Philippine Sea to the linear baroclinic model. Moreover, the vorticity budget analysis suggested that the presence of upper-tropospheric convergence of winds to the southeast of the Tibetan Plateau seen in the climatology is conceivable for the in situ localized anomalous circulation constituting the SAJ pattern due to vortex stretching effects.

Influence of Madden–Julian Oscillation on Precipitation over the Tibetan Plateau in Boreal Summer

The influence of the Madden–Julian oscillation (MJO) on precipitation over the Tibetan Plateau (TP) during boreal summer is investigated using observational and reanalysis data during 1980–2020. The results show that summer precipitation over most areas of the eastern TP increases (decreases) in MJO Phases 1–2 (5–6), especially when the eastward-propagating MJO active convection is located over the Indian Ocean (Western Pacific) in Phase 2 (6). The most significant negative precipitation anomalies in Phase 4 (8) are located over the southern (northeastern) TP. Moreover, MJO has a relatively weakened effect on the TP summer precipitation in Phases 3 and 7 when its convection migrates to the eastern Indian Ocean and the western–central Pacific, respectively. The MJO-phase dependence of the TP summer precipitation anomalies is closely associated with the anomalous atmospheric circulation and evolution of the horizontal moisture flux convergence directly induced by MJO. When the MJO convection centers are located over the western Indian Ocean and the Pacific, high-level anticyclonic and low-level cyclonic anomalous circulations over the TP are excited. In contrast, when MJO locates over the Indian Ocean and the Maritime Continent, its diabatic heating can inspire high-level cyclonic and low-level anticyclonic circulation anomalies over the TP. The vertical motions and moisture transport from the Bay of Bengal caused by the MJO-excited large-scale circulation can modulate the TP summer precipitation. This study advances the understanding of the TP intraseasonal variability.

Anthropogenic and internal drivers of wind changes over the Amundsen Sea, West Antarctica, during the 20th and 21st centuries

Abstract. Ocean-driven ice loss from the West Antarctic Ice Sheet is a significant contributor to sea-level rise. Recent ocean variability in the Amundsen Sea is controlled by near-surface winds. We combine palaeoclimate reconstructions and climate model simulations to understand past and future influences on Amundsen Sea winds from anthropogenic forcing and internal climate variability. The reconstructions show strong historical wind trends. External forcing from greenhouse gases and stratospheric ozone depletion drove zonally uniform westerly wind trends centred over the deep Southern Ocean. Internally generated trends resemble a South Pacific Rossby wave train and were highly influential over the Amundsen Sea continental shelf. There was strong interannual and interdecadal variability over the Amundsen Sea, with periods of anticyclonic wind anomalies in the 1940s and 1990s, when rapid ice-sheet loss was initiated. Similar anticyclonic anomalies probably occurred prior to the 20th century but without causing the present ice loss. This suggests that ice loss may have been triggered naturally in the 1940s but failed to recover subsequently due to the increasing importance of anthropogenic forcing from greenhouse gases (since the 1960s) and ozone depletion (since the 1980s). Future projections also feature strong wind trends. Emissions mitigation influences wind trends over the deep Southern Ocean but has less influence on winds over the Amundsen Sea shelf, where internal variability creates a large and irreducible uncertainty. This suggests that strong emissions mitigation is needed to minimise ice loss this century but that the uncontrollable future influence of internal climate variability could be equally important.

The interannual variation of the first regional extreme hot events in southeastern China and the possible mechanism

The interannual variation of the first regional extreme hot events in southeastern China and the possible mechanism

Structures and Mechanisms of Heatwaves Related to Quasi-Biweekly Variability over Southern China

Abstract In the present study, the structures and mechanisms of the heatwaves (HWs) associated with the quasi-biweekly (QBW; 10–20-day period) variability (QBW-HW) over southern China (SC; 106°–120°E, 21°–30°N) are investigated by using observation data from surface stations in China and the related gridded dataset (CN05.1), and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis. We found that the strongest anticyclonic anomaly and subsidence appear over SC during the developing phase of QBW-HW, and then induced excess solar radiation at surface and significant diabatic heating lead to a positive surface air temperature change, thus favoring occurrence of QBW-HW over SC. In addition, we found a wet near-surface atmosphere in the QBW-HW events over SC, and further confirmed that near-surface moisture should play an important role in the occurrence of QBW-HW, via absorptions of longwave and shortwave radiation. This result is quite different from previous studies since they did not pay attention to the near-surface moisture. On the other hand, warmer SAT favors more water vapor evaporated from the moist soil when considering the Clausius–Clapeyron relationship. Then, the positive feedback processes promote the occurrence of QBW-HW over SC. In contrast, during the developing and warm phases of QBW-HW over SC, except for the near-surface level, the troposphere is in a dry condition, even at 850 and 700 hPa. In the QBW-HW events over SC, the factor responsible for the wet near-surface atmosphere is the enhanced surface evaporation, which is attributed to strengthened surface wind speed and background moist soil. Significance Statement Under the background of global warming, heatwaves over Southern China are experiencing an increasing trend. In this study, we want to understand the structures and mechanisms of the heatwaves related to 10–20-day (quasi-biweekly) variability. We that found some structures of heatwaves (e.g., anticyclonic anomalies along with subsidence) are consistent with previous studies. In addition, we also show that the moist soil and increased induced near-surface moisture play a key role in the occurrence of heatwaves over Southern China, via enhanced absorptions of longwave and shortwave radiation. This study is helpful for understanding the processes and prediction of heatwaves over Southern China. Future work should examine the findings by some numerical experiments with a climate model.

On the atmospheric background for the occurrence of three heat wave types in East China

On the atmospheric background for the occurrence of three heat wave types in East China

Roles of Atmospheric Variability and Arctic Sea Ice in the Asymmetric Arctic–Eurasia Temperature Connection on Subseasonal Time Scale

Abstract Despite the severe impacts on Eurasian extreme weather, the mechanisms and causes of the “warm Arctic–cold Eurasia” (WACE) pattern and its opposite phase “cold Arctic–warm Eurasia” (CAWE) remain a subject of active debate. With a focus on subseasonal time scale, this study investigates the roles of atmospheric variability and Arctic sea ice in the variation of asymmetric WACE and CAWE patterns in the cold season. WACE (CAWE) patterns are predominantly driven by the temperature advection by anticyclonic (cyclonic) wave activity anomaly over Ural region. Low-frequency processes from both eddy vorticity and heat fluxes are important for the formation of the Ural wave activity anomaly. The subseasonal Arctic sea ice anomaly plays an additional role in maintaining the persistence of WACE and CAWE anomalies through surface heat flux exchange and alteration of Ural wave activity anomaly. Both comprehensive and idealized numerical experiments suggest that sea ice anomalies or thermal forcing act to maintain the WACE pattern by increasing the persistence of Ural anticyclonic anomaly through reducing background flow. The net effect of subseasonal thermal forcing on the WACE and CAWE anomalies is dependent on the mean state on longer time scale. We argue that the dominance of WACE over CAWE is mainly attributed to stronger roles of internal low-frequency atmospheric variability in driving the Ural anticyclonic anomaly and sea ice anomaly or thermal forcing in extending the persistence of the Ural anticyclonic anomaly through modulation on the background flow. Significance Statement The purpose of this study is to better understand the subseasonal variability of the “warm Arctic–cold Eurasia” (WACE) and “cold Arctic–warm Eurasia” (CAWE) patterns, which have severe impacts on Eurasian extreme weather. We highlight a dominance of WACE over CAWE, and attribute it to stronger roles of atmospheric variability in driving the WACE pattern and Arctic sea ice in maintaining the WACE anomalies. These findings have important implications for improving the subseasonal prediction of regional extremes.

The Influence of the Madden–Julian Oscillation on the Wet Season Rainfall over Saudi Arabia

The influence of Madden–Julian oscillation (MJO) is examined on intraseasonal rainfall variability during the wet season (November–April) by using the real-time multivariate (RMM) MJO index, ERA5 reanalysis, and daily observed rainfall dataset from 26 stations in Saudi Arabia for the period 1985–2021. The MJO 8 phases are categorized into wet (phases 1, 2, 7 and 8) and dry (phases 3, 4, 5, and 6) based on the Saudi Arabian intraseasonal rainfall characteristics associated with MJO phases. It is observed that 41% (46%) of total (extreme) rainfall events occur during the MJO wet phases, while only 23% (18%) of such events occur during MJO dry phases. The intraseasonal variability signals are isolated from daily dataset by applying a 30- to 90-day period bandpass filter. The analyses are validated by constructing composites of daily filtered precipitation anomalies during MJO 8 phases. The physical mechanism indicates that the significant intraseasonal wetter conditions are linked with enhanced easterly and southeasterly moisture convergence over Saudi Arabia from the Arabian Sea. The atmospheric cyclonic circulation anomalies during the wet phases favor more moisture convergence and vertical moisture advection, which may lead to enhanced convection and rainfall. However, during the dry phases, anticyclonic circulation anomalies enhance moisture divergence and reduce vertical moisture advection and consequently suppress the convection and rainfall activity over Saudi Arabia. The analyses show that the intraseasonal rainfall variability over Saudi Arabia is significantly influenced by the MJO during the wet season. These findings have important implications for sub-seasonal rainfall forecasting in Saudi Arabia.

Marine Heatwaves in the South China Sea: Tempo-Spatial Pattern and Its Association with Large-Scale Circulation

A marine heatwave (MHW) can significantly harm marine ecosystems and fisheries. Based on a remotely sensed sea surface temperature (SST) product, this study investigated MHWs behaviors in the South China Sea (SCS) throughout the warm season (May to September) from 1982 to 2020. The distributions of the three MHW indices used in this study showed significant latitudinal variations: more frequent, longer, and more intense MHWs appear in the northern SCS, and less frequent, shorter, and weaker MHWs appear in the southern SCS. Using the empirical orthogonal function (EOF) method, we found that the first leading modes of the three MHW indices account for more than half of the total variance. The first leading modes reveal uniform anomalies throughout the SCS, with the maximum in the deep central portion and its surroundings. Their corresponding time series showed significant interdecadal variations, with a turning point around 2009. Since 2010, the SCS has seen an increase in the frequency, length, and severity of MHWs. The incidence of MHWs has been linked to the presence of stable near-surface anticyclonic anomalies, which reduced cloud cover and increased solar radiation. This abnormal pattern was usually accompanied by the intensification and westward shift of the western North Pacific subtropical high (WNPSH). The findings imply that MHWs in the SCS may be predictable on interannual and decadal scales.

Discrepancies between observations and climate models of large-scale wind-driven Greenland melt influence sea-level rise projections

While climate models project that Greenland ice sheet (GrIS) melt will continue to accelerate with climate change, models exhibit limitations in capturing observed connections between GrIS melt and changes in high-latitude atmospheric circulation. Here we impose observed Arctic winds in a fully-coupled climate model with fixed anthropogenic forcing to quantify the influence of the rotational component of large-scale atmospheric circulation variability over the Arctic on the temperature field and the surface mass/energy balances through adiabatic processes. We show that recent changes involving mid-to-upper-tropospheric anticyclonic wind anomalies – linked with tropical forcing – explain half of the observed Greenland surface warming and ice loss acceleration since 1990, suggesting a pathway for large-scale winds to potentially enhance sea-level rise by ~0.2 mm/year per decade. We further reveal fingerprints of this observed teleconnection in paleo-reanalyses spanning the past 400 years, which heightens concern about model limitations to capture wind-driven adiabatic processes associated with GrIS melt.

Modulation of sea surface temperature in three oceans on precipitation increase over Northwest China during the past 60 years: A review

Northwest China is a typical arid and semi-arid region that is part of Central Asia. However, during the past 60 years, the climate in Northwest China has shown a warm and humid trend, with both average and extreme precipitation continuing to increase. Humidification in Northwest China is mainly caused by anomalous westward water vapor transport. Change in the water vapor transport path is directly related to the Mongolian anticyclone anomaly and weakening of the Asian summer monsoon. Our research shows that interdecadal changes in sea surface temperature (SST) in the North Atlantic, North Pacific, and Indian oceans, play an important role in interdecadal adjustment of atmospheric circulation and the wetting climate over Northwest China. Since the 1980s, the Indian Ocean has been warming continuously, and the land–sea thermal gradient has weakened, resulting in a significant reduction in water vapor transport of the Asian summer monsoon. In contrast, anomalous northerly water vapor transport from the polar region increased. Concurrently, SST over the North Atlantic is also warming, and the Atlantic multidecadal oscillation (AMO) changes from the negative to positive phase, triggering anomalous anticyclones over Mongolia, which also leads to weakening of the Asian summer monsoon. Therefore, eastern China is affected by abnormal northeast winds. These northeast winds can continuously transport water vapor to the western region of China, leading to the prevalence of easterly winds in Northwest China. Moreover, Pacific decadal oscillation (PDO) changed from the positive to negative phase after the 1990s, which promoted the East Asian westerly jet to move to the Arctic and produce easterly anomalies in East Asia and Northwest China. Interdecadal changes in SST over the Indian, North Atlantic, and North Pacific Oceans all have reduced summer water vapor from the Indian Ocean. However, water vapor from the North Pacific and high latitudes (including the polar region) can be transported to Northwest China through easterly anomalies, resulting in increased precipitation and climate humidification.

Modelled realistic daily variation in low winter sea-ice concentration over the Barents Sea amplifies Asian cold events

The boreal wintertime atmospheric responses, especially cold events over central Asia, to low sea-ice concentration (SIC) with and without realistic daily variation over the Barents Sea are explored with the Community Atmosphere Model version 4.0 (CAM4.0). The results show that the general atmospheric responses to approximately equal winter-mean Arctic sea-ice loss with a similar pattern but with climatological versus realistic daily variation are different. With the forcing of low SIC with climatological daily variation, Asian cold events become a little longer and stronger than in the control experiment; this mainly results from the enhancement of a 500-hPa Ural anticyclonic anomaly. However, the low SIC forcing that includes realistic daily variability greatly intensifies central Asian cold events and the cyclonic anomaly downstream of the Ural anticyclone. Further analysis reveals that Asian cold events are closely associated with Arctic deep warming at an intraseasonal time scale, which is also the strongest in the perturbed experiment forced by low SIC with realistic daily variation. This work provides a better understanding of the linkage between sea-ice variation over the Barents Sea and central Asian cold events, which may improve extreme weather prediction. It also implies that it is necessary to force air–sea coupling models and atmospheric models with realistic daily SIC in the study of the relationship between Arctic sea ice and mid-latitude cold events.

PM10 aerosol enhancement in the anticyclonic anomalies caused by the East Asian spring warming of 2021

PM10 aerosol enhancement in the anticyclonic anomalies caused by the East Asian spring warming of 2021

Impact of early winter North Atlantic Oscillation on the dramatic alternation of seesaw haze intensity between late winter months in the North China Plain

Impact of early winter North Atlantic Oscillation on the dramatic alternation of seesaw haze intensity between late winter months in the North China Plain

Interdecadal change in autumn rainfall over Southeast China and its association with tropical Pacific SST

In this paper, Japan Meteorological Agency (JRA-55) reanalysis and observational rainfall datasets from the National Climate Center (NCC) of China, as well as satellite datasets from the Tropical Rainfall Measuring Mission (TRMM), Global Precipitation Climatology Project (GPCP), and International Satellite Cloud Climatology Project (ISCCP), are used. The correlation coefficient and fast Fourier transform (FFT) low-pass filter are also used, in order to reveal the interdecadal decrease in autumn rainfall in Southeast China (SEC) after 1990. The close and robust relationship between the interdecadal variation in autumn rainfall in SEC and sea surface temperature (SST) in the tropical Pacific is investigated. The most significant and stable region of correlation is located in 10° S–10° N, 160° E–160° W, in which there also exists interdecadal warming after 1990. Furthermore, the interdecadal warming of SST can induce Gill responses of the atmosphere: a cyclone anomaly is produced on each side of the equator in the lower troposphere, with a westerly anomaly to the west of the dateline, and an anticyclone anomaly is produced in the upper troposphere. In particular, the cyclone anomaly on the northern side of the equator is located in the Northwest Pacific (NWP), and its ambient northerly airflow weakens meridional water vapor transport, as well as the local descending motion and low-troposphere divergence, in favor of the interdecadal decrease in SEC rainfall after 1990. In addition, the sensitive experiments with ECHAM-5.4 model also confirm that the interdecadal warming in the region (10° S–10° N, 160° E–160° W) would motivate the atmospheric Gill response and thereby cause the sinking motion in SEC and support the interdecadal decrease in autumn rainfall in SEC.

The mechanisms of the subseasonal zonal oscillation of the western Pacific subtropical high in 10–25-day period and 25–50-day period

This study investigates the evolution characteristics and the mechanism, including the convection–circulation interaction and the air–sea interaction, of the subseasonal zonal oscillation of the western Pacific subtropical high (WPSH) in 10–25-day period and 25–50-day period. Accompanied with the westward extension of the WPSH, a suppressed convection propagates northwestward over the western Pacific with an anticyclonic circulation anomaly at its northwest. A warm sea surface temperature anomaly (SSTA) lags the anomalous convection, showing near-quadrature phase relationship in temporal, and the magnitude of SSTA in 25–50-day period is greater than that in 10–25-day period. Further analysis shows that in both two periods the westward extension of the WPSH is mainly attributed to the convection–circulation interaction. The suppressed convection-induced cooling stimulates the divergence anomaly at the northwest of the suppressed convection, causing anticyclonic anomaly through the horizontal divergence effect. In turn, the anticyclonic anomaly motivates the downward dry advection in planetary boundary layer (PBL) preceding the suppressed convection, and thus favors the propagation of the suppressed convection. Through such interaction processes, the convection and circulation anomalies are combined and propagate together, leading to the zonal oscillation of the WPSH. Moreover, it is also found that the significance of air–sea interaction in two periods is distinct due to different magnitudes of the SSTA. In 10–25-day period, the ocean has no obvious feedback on the atmospheric circulation, while in 25–50-day period, the oceanic anomaly can fully develop and impact on the atmosphere more effectively.

Surface Heating Over the Tibetan Plateau Associated With the Antarctic Oscillation

Abstract The surface heating on the Tibetan Plateau (TP) exerts large influence on the Asian summer monsoon. Drivers of TP surface heat include the tropical forcings and other climate modes in middle and high latitude of the Northern Hemisphere, however whether the climate modes in Southern Hemisphere influence the TP surface heat is rarely studied. Using multiple source data diagnosis and numerical experiment from an atmospheric general circulation model (AGCM), we found that the Antarctic Oscillation (AAO) in May can efficiently modulate the subsequent surface heat source over the TP in June. When the AAO is in positive phases, a northeastward propagating atmospheric Rossby wave train originates from the Amundsen Sea low. As a part of this wave train, a pair of anomalous cyclone and anticyclone in the Southern Hemisphere accelerates the surface southeasterlies between them, accompanied by cold sea surface temperature (SST) anomaly in the equatorial middle and eastern Indian Ocean induced by the wind‐evaporation‐SST feedback. Due to the large thermal inertia, in June, the cold SST anomaly stimulates anticyclone anomalies over the western TP and eastern Arabian Sea, which increase the moisture transportation toward the TP and are conducive to the formation and maintenance of the precipitation over the middle and eastern TP. In contrast, the surface heat source, which is dominated by the upward sensible heat flux, is reduced substantially. This result indicates that the AAO can be one of the precursors of the TP heat source, and may help improve the prediction of the Asian summer monsoon.

Evaluation of the interannual variability in the East Asian summer monsoon in AMIP and historical experiments of CAS FGOALS-f3-L

Evaluation of the interannual variability in the East Asian summer monsoon in AMIP and historical experiments of CAS FGOALS-f3-L

Atmospheric forcing dominates winter Barents-Kara sea ice variability on interannual to decadal time scales

The last two decades have seen a dramatic decline and strong year-to-year variability in Arctic winter sea ice, especially in the Barents-Kara Sea (BKS), changes that have been linked to extreme midlatitude weather and climate. It has been suggested that these changes in winter sea ice arise largely from a combined effect of oceanic and atmospheric processes, but the relative importance of these processes is not well established. Here, we explore the role of atmospheric circulation patterns on BKS winter sea ice variability and trends using observations and climate model simulations. We find that BKS winter sea ice variability is primarily driven by a strong anticyclonic anomaly over the region, which explains more than 50% of the interannual variability in BKS sea-ice concentration (SIC). Recent intensification of the anticyclonic anomaly has warmed and moistened the lower atmosphere in the BKS by poleward transport of moist-static energy and local processes, resulting in an increase in downwelling longwave radiation. Our results demonstrate that the observed BKS winter sea-ice variability is primarily driven by atmospheric, rather than oceanic, processes and suggest a persistent role of atmospheric forcing in future Arctic winter sea ice loss.