Atmospheric Rossby Wave Research Articles

Increased projected changes in quasi-resonant amplification and persistent summer weather extremes in the latest multimodel climate projections

High-amplitude quasi-stationary atmospheric Rossby waves with zonal wave numbers 6–8 associated with the phenomenon of quasi-resonant amplification (QRA) have been linked to persistent summer extreme weather events in the Northern Hemisphere. QRA is not well-resolved in current generation climate models, therefore, necessitating an alternative approach to assessing their behavior. Using a previously-developed fingerprint-based semi-empirical approach, we project future occurrence of QRA events based on a QRA index derived from the zonally averaged surface temperature field, comparing results from CMIP 5 and 6 (Coupled Model Intercomparison Project). There is a general agreement among models, with most simulations projecting substantial increase in QRA index. Larger increases are found among CMIP6-SSP5-8.5 (42 models, 46 realizations), with 85% of models displaying a positive trend, as compared with 60% of CMIP5-RCP8.5 (33 models, 75 realizations), with a reduced spread among CMIP6-SSP5-8.5 models. CMIP6-SSP3-7.0 (23 models, 26 realizations) simulations display qualitatively similar behavior to CMIP6-SSP5-8.5, indicating a substantial increase in QRA events under business-as-usual emissions scenarios, and the results hold regardless of the increase in climate sensitivity in CMIP6. Projected aerosol reductions in CMIP6-SSP3-7.0-lowNTCF (5 models, 16 realizations) lead to halting effect in QRA index and Arctic Amplification during the 1st half of the twenty-first century. Our analysis suggests that anthropogenic warming will likely lead to an even more substantial increase in QRA events (and associated summer weather extremes) than indicated by past analyses.

Relationships between amplified quasi‐stationary waves and humid‐heat extremes in the Yangtze River Delta region

AbstractHigh humidity combined with high temperature decreases the human body's capability to dissipate heat, causing serious health issues. While increased attention has been paid to changes in humid‐heat extremes under the current climate and in the future, the causes of near‐surface humid‐heat extremes over the Yangtze River Delta region are still unclear. Here we investigate the relationships between quasi‐stationary waves (QSWs), which are atmospheric Rossby waves with phase speed close to zero, and humid‐heat extremes over the Yangtze River Delta region during the summer from 1959 to 2021. Additionally, the potential physical processes that link them are also explored. We find that the QSWs with wavenumber 1–8 present different vertical structures: Wave 1–2 are baroclinic, while Wave 3–8 are barotropic. Wave 1, 3, 5, 6, 7 and 8 show phase‐locking behaviour. When the amplitudes of Wave 1, 3, 5 and 6 are higher, the frequency of humid‐heat extremes near the surface at specific locations tends to be higher than the norm. The high‐amplitude waves in different wavenumbers modulate near‐surface temperature and/or humidity in various ways, ultimately resulting in anomalously intensive humid‐heat extremes.

Multiple serial correlations in global air temperature anomaly time series.

Serial correlations within temperature time series serve as indicators of the temporal consistency of climate events. This study delves into the serial correlations embedded in global surface air temperature (SAT) data. Initially, we preprocess the SAT time series to eradicate seasonal patterns and linear trends, resulting in the SAT anomaly time series, which encapsulates the inherent variability of Earth's climate system. Employing diverse statistical techniques, we identify three distinct types of serial correlations: short-term, long-term, and nonlinear. To identify short-term correlations, we utilize the first-order autoregressive model, AR(1), revealing a global pattern that can be partially attributed to atmospheric Rossby waves in extratropical regions and the Eastern Pacific warm pool. For long-term correlations, we adopt the standard detrended fluctuation analysis, finding that the global pattern aligns with long-term climate variability, such as the El Niño-Southern Oscillation (ENSO) over the Eastern Pacific. Furthermore, we apply the horizontal visibility graph (HVG) algorithm to transform the SAT anomaly time series into complex networks. The topological parameters of these networks aptly capture the long-term correlations present in the data. Additionally, we introduce a novel topological parameter, Δσ, to detect nonlinear correlations. The statistical significance of this parameter is rigorously tested using the Monte Carlo method, simulating fractional Brownian motion and fractional Gaussian noise processes with a predefined DFA exponent to estimate confidence intervals. In conclusion, serial correlations are universal in global SAT time series and the presence of these serial correlations should be considered carefully in climate sciences.

Identification of Rossby Atmosphere-Tropical Cyclone in Eastern Indonesian Waters

Recent research has revealed that tropical cyclones can develop over eastern Indonesian waters influenced by marine heatwaves and Rossby waves in the atmosphere. However, there is no study documenting tropical cyclones that occur in conjunction with atmospheric Rossby waves (Rossby Atmospheric-Tropical Cyclones) and their association with increased sea surface temperatures in eastern Indonesian waters. This study aims to document the influence of Rossby waves in the atmosphere on the formation of tropical cyclones around the Indonesian region using 5 case studies in 2017-2022, namely: December 2017, January 2020, December 2020, December 2021, and April 2022. This study uses wind data, sea surface temperature, specific humidity, and temperature (2m) obtained from the European Re-Analysis (ERA5) with a temporal and spatial resolution of one hour and 0.25°×0.25°. The identification of Rossby waves is based on the Rossby index issued by the North Carolina Institute for Climate Studies (NCICS). In this study, the Rossby Atmosphere-Tropical Cyclone is grouped into three phases, namely: early phase, mature phase, and late phase, using composite and statistical methods to calculate anomalies. The results showed that in the early phase, the existence of Rossby waves was shown by two twin vortices over eastern Indonesia, which was supported by high specific humidity, warming sea surface temperature (>+0.4°C), and higher surface temperature (>+0.3°C) over Timor. In the mature phase, the twin vortices over eastern waters transformed into a tropical cyclone over the Philippines. In the final phase, specific humidity decreases, sea surface temperature cools (<-0.3°C), and surface temperature decreases (<-0.3°C). The results also prove the crucial role of Timor waters in forming Rossby waves that can grow into tropical cyclones around Indonesia.

Southern Hemisphere Circumpolar Wavenumber‐4 Pattern Simulated in SINTEX‐F2 Coupled Model

AbstractInterannual sea surface temperature (SST) variations in the subtropical‐midlatitude Southern Hemisphere are often associated with a circumpolar wavenumber‐4 (W4) pattern. This study is the first attempt to successfully simulate the SST‐W4 pattern using a state‐of‐the‐art coupled model called SINTEX‐F2 and clarify the underlying physical processes. It is found that the SST variability in the southwestern subtropical Pacific (SWSP) plays a key role in triggering atmospheric variability and generating the SST‐W4 pattern during austral summer (December‐February). In contrast, the tropical SST variability has a very limited effect. The anomalous convection and associated divergence over the SWSP induce atmospheric Rossby waves confined in the westerly jet. Then, the synoptic disturbances circumnavigate the subtropical Southern Hemisphere, establishing an atmospheric W4 pattern. The atmospheric W4 pattern has an equivalent barotropic structure in the troposphere, and it interacts with the upper ocean, causing variations in mixed layer depth due to latent heat flux (LHF) anomalies. As incoming climatological solar radiation goes into a thinner (thicker) mixed layer, the shallower (deeper) mixed layer promotes surface warming (cooling). This leads to positive (negative) SST anomalies, developing the SST‐W4 pattern during austral summer. Subsequently, anomalous entrainment due to the temperature difference between the mixed layer and the water below the mixed layer, anomalous LHF, and disappearance of the overlying atmospheric W4 pattern cause the decay of the SST‐W4 pattern during austral autumn. These results indicate that accurate simulation of the atmospheric forcing and the associated atmosphere‐ocean interaction is essential to capture the SST‐W4 pattern in coupled models.

Resonant Forcing by Solar Declination of Rossby Waves at the Tropopause and Implications in Extreme Events, Precipitation, and Heat Waves—Part 1: Theory

The purpose of this first article is to provide a physical basis for atmospheric Rossby waves at the tropopause to clarify their properties and improve our knowledge of their role in the genesis of extreme precipitation and heat waves. By analogy with the oceanic Rossby waves, the role played by the pycnocline in ocean Rossby waves is replaced here by the interface between the polar jet and the ascending air column at the meeting of the polar and Ferrel cell circulation or between the subtropical jet and the descending air column at the meeting of the Ferrel and Hadley cell circulation. In both cases, the Rossby waves are suitable for being resonantly forced in harmonic modes by tuning their natural period to the forcing period. Here, the forcing period is one year as a result of the variation in insolation due to solar declination. A search for cause-and-effect relationships is performed from the joint representation of the amplitude and phase of (1) the velocity of the cold or warm modulated airflows at 250 mb resulting from Rossby waves, (2) the geopotential height at 500 mb, and (3) the precipitation rate or ground air temperature. This is for the dominant harmonic mode whose period can be 1/16, 1/32, or 1/64 year, which reflects the intra-seasonal variations in the rising and falling air columns at the meeting of the polar, Ferrel, and Hadley cell circulation. Harmonics determine the duration of blocking. Two case studies referring to extreme cold and heat waves are presented. Dual cyclone–anticyclone systems seem to favor extreme events. They are formed by two joint vortices of opposite signs reversing over a period, concomitantly with the involved modulated airflows at the tropopause. A second article will be oriented toward (1) the examination of different case studies in order to ascertain the common characteristics of Rossby wave patterns leading to extreme events and (2) a map of the globe revealing future trends in the occurrence of extreme events.

Origins of Biweekly Sea Surface Temperature Variability in the Eastern Equatorial Pacific and Atlantic

AbstractBiweekly sea surface temperature (SST) variability significantly contributes to over 50% of the intraseasonal variability in the eastern equatorial Pacific (EEP) and Atlantic (EEA). Our study investigates this biweekly variability, employing a blend of in–situ and reanalysis data sets. The research identifies biweekly signals in SST, meridional wind, and ocean currents, notably in September–November in EEP and June–August in EEA. Biweekly southerly (northerly) winds drive instantaneous northward (southward) ocean currents in EEP, but with a 1–2‐day phase delay in EEA. Consequently, these currents lead to SST anomalies with a 3–4‐day lag in both EEP and EEA due to the presence of the cold tongue. The study reveals the origin of biweekly wind fluctuations in the western Pacific for EEP and the subpolar Pacific for EEA, connected by atmospheric Rossby waves validated through a linearized non‐divergent barotropic model. This research affirms the influence of subtropical and subpolar atmospheric forcing on equatorial SST.

Influence of the Quasi‐Biennial Oscillation on tropical convection and its teleconnection to the midlatitudes in boreal winter

AbstractRecently, a connection between the quasi‐biennial oscillation (QBO) and the Madden–Julian oscillation (MJO) was found in observations. The boreal winter seasonal mean amplitude of the daily MJO is anticorrelated with the QBO winds measured at 50 hPa. We investigate whether this relationship is captured in reforecasts of past winters in a seasonal prediction system, and find the ensemble mean response does not show this association, with essentially zero correlation. By repeated subsampling of the ensemble reforecast data, we also show that the observed correlation is very unlikely to arise merely as the result of sampling in the relatively short observational record. Instead, we conclude that the reforecasts genuinely cannot capture the observed QBO–MJO seasonal relationship. We also consider the boreal winter seasonal mean response of tropical convection to the QBO and its influence on the subtropics. We find that the reforecasts are able to capture the stationary pattern found in the observed outgoing long‐wave radiation (OLR) over the tropical west Pacific in response to the QBO. We also find that the associated upper tropospheric divergence in observations interacts with the west Pacific subtropical jet to produce atmospheric Rossby wave patterns that propagate across the midlatitude Pacific Ocean. These extratropical waves have the potential to interfere with the climatological waves in midlatitudes, modifying the strength of the midlatitude annular mode through interactions with the extratropical stratosphere. However, the observed influence on wavenumber 1 is not captured by the reforecasts, suggesting a weak extratropical influence and a partial explanation for the weaker‐than‐observed QBO teleconnection to the Arctic Oscillation and North Atlantic Oscillation.

Unusual subseasonal variability of Indian summer monsoon rainfall in 2020

AbstractAbove‐normal all‐India summer monsoon (ISM) rainfall was received in 2020 with 109% of its long‐period average. According to the India Meteorological Department (IMD), ISM 2020 experienced robust month‐to‐month rainfall variations; in particular, the peak monsoon month of July 2020 witnessed deficit rainfall. The present study investigated the possible mechanisms that are responsible for the July 2020 deficit rainfall. Analysis revealed that the strong low‐level moisture divergence caused by the westward‐propagating atmospheric Rossby wave, induced by suppressed western North Pacific (WNP) convection, is primarily responsible for the July rainfall deficit. The WNP suppressed convection is linked to anomalous low‐level anticyclonic circulation. It is noted that the intense WNP anticyclone is maintained by the tropical Indian Ocean (TIO) warming‐induced atmospheric Kelvin waves and strong low‐level convergence over the Meiyu‐Baiu rainband. Unlike July, the strength of the WNP anticyclone and TIO warming are weaker in June and August. In addition to the observational analysis, the skill of the Climate Forecast System version‐2 (CFSv2) model in predicting the July rainfall at a lead of about two months is examined. Strong positive rainfall anomalies over India are predicted by the model in July, which is in contrast to the observations. Despite predicting the TIO warming, the model failed to capture the westward extension of the WNP anticyclone, which impacted the July rainfall prediction. To understand the importance of the WNP anticyclone on ISM rainfall, a CFSv2 sensitivity experiment is carried out by imposing strong negative sea surface temperature (SST) anomalies over the WNP region to excite the anticyclone. Both the westward extension of the WNP anticyclone and suppressed rainfall over the core monsoon region in July are captured by the model when low‐level circulation is imposed. The model sensitivity experiment strongly supports the role of the WNP anticyclone and its westward extension in inducing reduced rainfall in July 2020 over the monsoon trough region.

Snow Accumulation Variability at the South Pole From 1983 to 2020, Associated With Central Tropical Pacific Forcing

AbstractDespite a variety of efforts made to measure snow accumulation at the South Pole (SP), snow accumulation changes and their mechanism have not yet been fully explained. Here, SP stake farm measurements, global sea surface temperature observations, and atmospheric circulation data from European Centre for Medium‐Range Weather Forecasts Reanalysis version 5 were used to investigate the annual and seasonal snow accumulation changes at the SP during 1983–2020, and their association with central tropical Pacific Sea surface temperature variations. SP annual snow accumulation decreased significantly for the 1983–2007 period at a rate of −39.7 ± 1.4 mm decade−1, but switched to a dramatically positive trend during 2008–2020 (108.7 ± 2.7 mm decade−1), with the strongest increase in the austral autumn. The switch to a dramatically upward trend can largely be attributed to a cyclonic anomaly over the South Atlantic and an anticyclonic anomaly over the Drake Passage, causing the enhanced advection of warm and wet air into the SP. These circulation patterns were generated by an atmospheric Rossby wave train forced by rapid warming in the central tropical Pacific during 2008–2020.

Coupled Feedbacks From the Tropical Pacific to the Atlantic Meridional Overturning Circulation

AbstractThe tropical Pacific Ocean is a key regulator of Earth's climate, with teleconnections that influence remote locations all around the world. Here we use partially coupled climate model experiments to show that tropical Pacific cooling related to an abrupt Atlantic Meridional Overturning Circulation (AMOC) slowdown can strengthen the AMOC by ∼25%. This tropical‐extratropical teleconnection occurs initially via atmospheric Rossby waves propagating from the tropical Pacific to the North Atlantic which alter surface climate conditions locally. These changes facilitate ocean heat loss from the subpolar gyre, favoring enhanced oceanic convection. The AMOC strengthening is subsequently enhanced by anomalous northward salt advection in the Atlantic, with a potential contribution from oceanic wave adjustment triggered by increased Southern Ocean westerly winds. These results highlight the influence of the tropical Pacific on the AMOC on multidecadal timescales and suggest that cold phases of tropical Pacific decadal variability could drive temporary strengthening of the AMOC.

Meteorology of ‘ridging high’ rainfall over the KwaZulu‐Natal coastal plains

AbstractThe meteorology of ridging highs on the coast of KwaZulu‐Natal (KZN) South Africa was studied using Principal Component (pc) analysis of daily ERA5 sea level air pressure (SLP) fields 1980–2021. The second pc mode time score representing ridging highs, was associated with rain‐showers moving northeast‐ward along the KZN coast. Temporal analysis revealed an Oct‐Dec peak occurrence and 12‐day pulse intervals induced by the eastward migration of atmospheric Rossby waves in early summer. From a ranking of SLP pc2 and associated daily time series, a case study emerged: 14–18 Nov. 2009. This ridging high weather scenario had cold southerly winds subsiding over the warm Agulhas Current. Surface moisture fluxes >350 W/m2 create a buoyant airmass that slows and lifts over the coastal plains, inducing shallow clouds that reduce net solar radiation and potential evaporation. Interannual variability of ridging highs was studied by filtering the daily SLP pc2 time score to retain seasonal fluctuations and correlating with regional fields of SST. This revealed warming in the SW Indian Ocean associated with positive phase Indian Ocean Dipole (IOD). The atmospheric circulation forms a NW trough over Madagascar which deforms eastward moving mid‐latitude anticyclones. Thus, ridging highs that bring beneficial Oct‐Dec rains to the coast of KZN depend on downstream climate coupling with the IOD. Long‐term trends from a coupled model simulation 1980–2100 suggest a poleward retreat of the anticyclonic ridge, consistent with declining Durban‐area river flow in early summer.

Atmospheric Modeling Study on Convection-Triggered Teleconnections Driving the Summer North American Dipole

Abstract The summer North American dipole (NAD) is a pattern of climate variability linked to variations in boreal forest seed production and migration of seed-eating birds. This is a modeling investigation of two teleconnections identified as drivers of the NAD in prior observational work: 1) tropically sourced atmospheric Rossby waves associated with anomalies in the phase distribution of the Madden–Julian oscillation (MJO) (i.e., phases 1 and 6 are anomalously prominent), and 2) a pan-Pacific atmospheric Rossby wave linked to East Asian monsoonal (EAM) convection. Sea surface temperature (SST) boundary forcing experiments were conducted with the Community Earth System Model 2 (CESM2) to trigger convection patterns that align with those observed during EAM and nonuniform phase distributions of MJO. For the EAM case, an El Niño–like SST dipole pattern combined with cool southern Japan SST forcing produced a convection and jet stream shift anomaly over East Asia and the northern Pacific with a positive NAD pattern downstream over North America, similar to the observed pattern when precipitation over East Asia (PEA) is relatively high. A companion experiment with only ENSO-like SST forcing also produced the NAD but featured a different structure over the Eurasian continent with a response resembling the summer east Atlantic (SEA) pattern over eastern North America and the eastern Atlantic. Simulation results suggest that the southern Japan SST forcing region has a secondary importance in triggering the NAD, producing only a somewhat NAD-like pattern by itself and only slightly improving the NAD produced by ENSO-like forcing. Simulations using SST forcing to induce seasonal convection anomalies with spatial patterns similar to anomalously frequent occurrence of MJO phase 1 (phase 6) produced circulation response patterns resembling the positive NAD (negative NAD).

Coherent Bimodal Events in Ensemble Forecasts of 2-m Temperature

Abstract A previous study has shown that a large portion of subseasonal-to-seasonal European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts for 2-m temperature exhibit properties of univariate bimodality, in some locations occurring in over 30% of forecasts. This study introduces a novel methodology to identify “bimodal events,” meteorological events that trigger the development of spatially and temporally correlated bimodality in forecasts. Understanding such events not only provides insight into the dynamics of the meteorological phenomena causing bimodal events, but also indicates when Gaussian interpretations of forecasts are detrimental. The methodology that is developed allows one to systematically characterize the spatial and temporal scales of the derived bimodal events, and thus uncover the flow states that lead to them. Three distinct regions that exhibit high occurrence rates of bimodality are studied: one in South America, one in the Southern Ocean, and one in the North Atlantic. It is found that bimodal events in each region appear to be triggered by synoptic processes interacting with geographically specific processes: in South America, bimodality is often related to Andes blocking events; in the Southern Ocean, bimodality is often related to an atmospheric Rossby wave interacting with sea ice; and in the North Atlantic, bimodality is often connected to the displacement of a persistent subtropical high. This common pattern of large-scale circulation anomalies interacting with local boundary conditions suggests that any deeper dynamical understanding of these events should incorporate such interactions. Significance Statement Repeatedly running weather forecasts with slightly different initial conditions provides some information on the confidence of a forecast. Occasionally, these sets of forecasts spread into two distinct groups or modes, making the “typical” interpretation of confidence inappropriate. What leads to such a behavior has yet to be fully understood. This study contributes to our understanding of this process by presenting a methodology that identifies coherent bimodal events in forecasts of near-surface air temperature. Applying this methodology to a database of such forecasts reveals several key dynamical features that can lead to bimodal events. Exploring and understanding these features is crucial for saving forecasters’ resources, creating more skillful forecasts for the public, and improving our understanding of the weather.

Impacts of April Stratosphere–Troposphere Coupling on the South Asian Premonsoon Circulation

Abstract The premonsoon circulation over South Asia in May shows remarkable interannual variations, and it can modulate the onset of the Asian summer monsoon. The present study derives two dominant stratosphere–troposphere modes over the North Atlantic in April via applying principal component (PC) analysis to regional geopotential height during 1979–2015. Both of the modes reflect the North Atlantic Oscillation (NAO)-like circulation at the midtroposphere, but the circulation patterns and planetary wave activity between the two modes are quite different. The first mode represents a stratosphere–troposphere-coupled mode. Further analysis indicates that during the years with a positive phase of the first mode, the low-level anomalous southerlies over the northern Barents Sea (BS) lead to a reduction in local sea ice, which could persist into May through the ice-albedo feedback. The BS sea ice anomalies in May could generate a southeastward propagating Rossby wave train, producing an anomalous anticyclonic circulation to the west of Qinghai–Tibet Plateau. This anomalous anticyclone induces increases in local air temperature and the meridional temperature gradient, resulting in a strengthened premonsoon circulation. Given that prediction of the premonsoon circulation over South Asia remains a challenging issue, the stratosphere–troposphere-coupling modes in April provide another potential predictability source. On the other hand, ENSO has a significant effect on the South Asian premonsoon circulations in May, and can also lead to the NAO-like circulation by influencing atmospheric Rossby waves. Such an NAO-like circulation is closely related to the second mode.

A strong high-temperature event in late-spring 2023 in Yunnan province, Southwest China: Characteristics and possible causes

In April–May 2023, Yunnan province of Southwest China experienced a strong high-temperature event with relatively large return period and small occurrence probability, causing adverse impacts on the regional development. The possible causes for the 2023 high-temperature event are mainly involved with an upper-level high-pressure system induced by a Rossby wave train and the precipitation shortage associated with a lower-level anticyclonic circulation over the Bay of Bengal. On the one hand, the atmospheric Rossby wave train is attributed to the decreased snow depth in the North America. On the other hand, the lower-level anticyclone over the Bay of Bengal may be caused by the warming SSTs in the tropical western Indian ocean and in the tropical eastern Pacific via the excited Matsuno-Gill response and the atmospheric signals of Southern Oscillation, respectively. The mechanisms are validated by the pacemaker experiments run with CESM1 climate model. These three factors that cause the 2023 high-temperature event are highly correlated to the surface temperature anomalies in Yunnan. Hence, a physical-based multiple regression model with the leave-five-out cross validation can be constructed to perform a skilful seasonal prediction.

Intensified modulation of the Pacific north equatorial current bifurcation by the southern annular mode since the early 1990s

Long-term reanalysis data were used to assess inter-decadal to decadal modulations of the North Equatorial Current (NEC) bifurcation in the Pacific after the early 1990s. The wind stress curl anomaly (WSCA) in the region of 10° N-15° N and 160° E-170° E (C-BOX) had been found to excite Rossby waves and control NEC bifurcation along the Philippine coast. Our analysis revealed that the WSCA in the C-BOX has been remotely modulated by the Southern Annular Mode (SAM) since the early 1990s. It is shown that the SAM shifted to its positive phase at this transition and began strongly impacting the WSCA in the C-BOX and the NEC bifurcation. During the positive SAM phase after the early 1990s, strong climate variability occurred in the tropical to subtropical area of the North Pacific, with a clear footprint connected to the Antarctic region. Consistent with that finding, we determined that during the positive SAM phase, a dipole sea surface temperature pattern was generated in the South Pacific; this induced an atmospheric Rossby wave train in upper-level wind shear that propagated northward to the North Pacific. Such effects further enhanced downward motion and divergence at the surface, intensifying the easterlies in the equatorial area and the anticyclonic WSCA in the C-BOX. The anticyclonic WSCA in the C-BOX substantially excited downwelling oceanic Rossby waves at the surface, inducing an equatorward trend of NEC bifurcation after the early 1990s.

Surface Heating Over the Tibetan Plateau Associated With the Antarctic Oscillation

AbstractThe 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.

Impacts of the Interannual Variability of the Kuroshio Extension on the East Asian Trough in Winter

The responses of the East Asian Trough (EAT) to the Kuroshio Extension (KE) interannual fluctuation and the underlying mechanisms in the boreal winter are investigated through the lag regression approach in this study. When the KE is in the stable state, the sea surface temperature (SST) front is strengthened, with cold (warm) SST anomaly in the western (eastern) region of the KE, releasing less (more) heat into the atmosphere. The opposite patterns hold for the KE unstable periods. The analysis of the observations shows that the stable KE corresponds to a deeper EAT, accompanied with a stronger winter monsoon over Mongolia and northeastern China. The atmospheric Rossby waves, transient eddies, and thermal winds are found to be responsible for this relationship between the KE and EAT. The SST warming in the lower reaches of the KE excites the Rossby wave activity that propagates toward East Asia, leading to 25% of the EAT amplification. Meanwhile, influenced by the KE-induced Rossby waves, the background baroclinicity is intensified over Japan, which enhances the transient eddy activity, contributing to another 42% magnitude of the EAT deepening. In addition, as depicted by the thermal wind theory, the strong SST cooling in the upper branch of the KE forces an anomalous cyclonic circulation through modifying the meridional temperature gradient, facilitating the EAT development. The finding points to the better understandings of the EAT and associated East Asian winter climate variability, which are crucial for their major economic and social impacts on the large populations in the region.

Tropical origins of the record-breaking 2020 summer rainfall extremes in East Asia

The East Asian countries have experienced heavy rainfalls in boreal summer 2020. Here, we investigate the dynamical processes driving the rainfall extremes in East Asia during July and August. The Indian Ocean basin warming in June can be responsible for the anticyclonic anomalies in the western North Pacific (WNP), which modulate the zonally-elongated rainfalls in East Asia during July through an atmospheric Rossby wave train. In August, the East Asian rainfall increase is also related to the anticyclonic anomalies in the subtropical WNP, although it is located further north. The north tropical Atlantic warming in June partly contributes to the subtropical WNP rainfall decrease in August through a subtropical teleconnection. Then the subtropical WNP rainfall decrease drives the local anticyclonic anomalies that cause the rainfall increase in East Asia during August. The tropical Indian Ocean anomalously warmed in June and the subtropical WNP rainfall decreased in August 2020, which played a role in modulating the WNP anticyclonic anomalies. Therefore, the record-breaking rainfall extremes in East Asia that occurred during summer 2020 can be explained by the teleconnections associated with the tropical origins among the Indian, Pacific, and Atlantic Oceans and their interbasin interactions.