Summer Precipitation Variability Research Articles

The influence of natural variability on extreme monsoons in Pakistan

The monsoons in Pakistan have been exceptionally harsh in recent decades, resulting in extraordinary drought conditions and record flooding events. The changing characteristics of extreme events are widely attributed to climate change. However, given this region’s long history of floods and droughts, the role of natural climate variability cannot be rejected without a careful diagnosis. Here, we examine how oceanic and atmospheric variability has contributed to unusual precipitation distributions in West South Asia. Variations in sea surface temperatures in the tropical Pacific and northern Arabian Sea, and internal atmospheric variability related to the circumglobal teleconnection pattern and the subtropical westerly jet stream, explain more than 70% of monthly summer precipitation variability in the 21st century. Several of these forcings have co-occurred with record strength during episodes of extreme monsoons, which have exacerbated the overall effect. Climate change may have contributed to increased variability and the in-phase co-occurrences of the identified mechanisms, but further research is required to confirm any such connection.

Summer precipitation variability in the Mongolian Plateau and its possible causes

Summer precipitation variability in the Mongolian Plateau and its possible causes

Multidecadal Oceanic Modulation of Summer Precipitation in North China in 1200-Year Global Climate Simulations

Abstract The multidecadal decrease of precipitation in North China since the 1960s has caused a significant impact on the ecological environment and social development. It has become a key concern for meteorologists and national decision-makers to understand what is driving this. To better understand the oceanic impact on the interdecadal variation of summer precipitation in North China, we analyze two 1200-yr global climate simulations from state-of-the-art coupled models with an explainable convolutional neural network (CNN). The accuracy of the CNN model in correctly diagnosing the increased and decreased summer precipitation anomaly in North China reaches 98% and 96.6% in two CMIP6 models, respectively. We find that a sea surface temperature anomaly (SSTA) in the northern Pacific, associated with the Pacific decadal oscillation (PDO), and an SSTA in the northern Atlantic, associated with the Atlantic multidecadal oscillation (AMO), play essential roles in the interdecadal variation of summer precipitation in North China. The modulations of the PDO and AMO on summer precipitation in North China are consistent with the results derived from limited observations and simulations, which means these mechanisms are robust. In particular, when the PDO is in a negative phase and the AMO is in a positive phase, the positive anomaly of precipitation and the associated circulation anomalies are intensified. This study also confirms that with the explainable deep learning method, a convolutional neural network can be used to identify multidecadal variability in North China summer precipitation, although limited datasets such as observational records make it more challenging to apply deep learning methods.

Causes of the Interannual Variation of Summer Precipitation in Eastern Southwest China

Using ERA5 reanalysis data, we conducted an EOF analysis of summer precipitation in the eastern part of southwestern China (ESWC) over the past 60 years. Our study aimed to investigate the spatial distribution characteristics and interannual variability of summer precipitation in the ESWC, as well as to reveal the possible physical mechanisms influencing its interannual variability. The results indicate that, at the interannual scale, the first two modes of summer precipitation in the ESWC exhibit a uniform pattern and a north–south dipole pattern, respectively. The maximum and minimum time coefficients of the first mode correspond to the severe flood and drought events in 1998 and 2006, respectively, indicating that their time coefficients reflect the typical events well. The tri-pole sea surface temperature anomaly in the North Atlantic, along with the sea ice anomaly near the Barents Sea, induces a quasi-zonal wave train in the upper troposphere. This wave train propagates from the tropical and mid-latitude Atlantic, traverses the mid-high latitudes of Eurasia, and reaches the eastern part of China. The wave train induces cyclonic (anticyclonic) anomalies in the north (south) of the ESWC, facilitating the convergence of high-latitude dry and cold air with warm and moist southwesterly winds from the tropical ocean. This convergence promotes increased summer precipitation in the ESWC. We provided valuable insights into the interannual variability of summer precipitation in the ESWC, shedding light on the physical mechanisms responsible for these variations.

The combined effect of multiple water vapor transport channels can better reflect the variability in summer precipitation over North China

The combined effect of multiple water vapor transport channels can better reflect the variability in summer precipitation over North China

The role of external and local atmospheric moisture fluxes in the interannual variability of peak summer precipitation over the northern fringe of the East Asian summer monsoon

On the northern fringe of the East Asian summer monsoon (NFEASM), both external atmospheric moisture transport and local evaporation are important moisture sources of moisture that feed peak summer precipitation, but their relative importance in regulating the precipitation variability remains unclear. This study uses the atmospheric water budget analysis to investigate the contributions of external and local atmospheric moisture fluxes to the interannual variability of peak summer (July and August) precipitation over the NFEASM between 1980 and 2015. The results show that, compared with the net atmospheric moisture flux (i.e., moisture flux convergence, MFC), although climatologically evaporation is of the same magnitude as precipitation, its standard deviation is much smaller than that of precipitation and MFC. With respect to moisture conservation, the atmospheric moisture budget indicates that the interannual variability in summer precipitation is derived primarily from changes in MFC. Moreover, the closer correlation between MFC and precipitation than between MFC and evaporation also implies that the variability of summer precipitation over the NFEASM is controlled mainly by fluctuations in the external atmospheric moisture fluxes, particularly by the moisture transport from the southwest, which is closely linked to the anticyclonic anomalous circulation over eastern China. In addition, although the westerly does not bring substantial moisture, its meridional movement can change the upper-level zonal wind divergence pattern, thereby affecting peak summer precipitation over the NFEASM.

Moisture source anomalies connected to flood‐drought changes over the three‐rivers headwater region of Tibetan Plateau

AbstractThe Flood‐Drought condition over the Three‐River Headwater region (TRHR) of the Tibetan Plateau (TP) exerts vital impacts on regional climate and downstream livelihoods. However, the reasons behind precipitation variability over this region remain elusive. In this study, the multi‐year moisture sources of water vapour reaching the TRHR were identified to reveal their connection to the interannual variability of summer precipitation by utilizing a Lagrangian diagnosis analysis. The results show that the moisture sources could be tracked backward covering vast areas, with the peak values located mostly at the northern Indian continent and adjacent regions of TRH itself. The variability of integrated moisture sources' contribution agrees well with the summer precipitation on the interannual scale; However, the role of different regions varies in line with their geographical location. More specifically, summer precipitation is positively correlated with the moisture source over the adjacent areas of TRH and the remote terrestrial‐oceanic regions, in contrast to a negative correlation to the water vapour departing from the Northeastern Asian and western regions to the TP. The Indian summer monsoon circulation and local recycling process play a dominant role in modulating the summer precipitation on the interannual scale, whereas the role of the westerlies is almost negligible. The abnormal atmosphere circulations triggered by mid‐high latitude wave trains and anomalies in SST in the central and eastern Pacific Ocean and Indian Ocean could serve as possible drivers for the precipitation variability over TRHR.

Sea surface temperature anomalies in different ocean basins affecting the interannual variations of summer precipitation in low-latitude highlands of Southeast Asia

Sea surface temperature anomalies in different ocean basins affecting the interannual variations of summer precipitation in low-latitude highlands of Southeast Asia

Has Arctic sea ice loss affected summer precipitation in North China?

AbstractArctic sea ice loss is one of the most striking features in changing of the climate system and affects weather and climate in the midlatitudes through complex interactions and feedback processes. However, it is not clear whether Arctic sea ice loss has affected summer precipitation in East Asia. This study shows that summer regional precipitation in North China exhibits obvious features of phasic variations. Before 1998, positive and negative precipitation anomalies appeared alternately. While from 1999 to 2011, negative precipitation anomalies were dominant. Since 2012, increased summer precipitation has been observed frequently. A direct cause of increases of summer precipitation in North China is an enhancement of water vapour transport from South China and China's marginal seas, which can be attributed to the strengthening and northward shifting of the northwestern Pacific subtropical high and the spatial configuration with west low–east high and north low–south high in East Asian height fields. Although summer atmospheric circulation anomalies in high latitudes are dominant during the sea ice melting phase, their contributions to summer precipitation variations in North China are little. Simulation experiments show that Arctic sea ice loss can significantly affect the atmospheric circulation in high latitudes; however, it cannot be regarded as a major cause of observed recent increases in precipitation in North China. Additionally, Arctic sea ice loss has little impact on summer precipitation in the Yangtze River valley and South China.

The Guiding Role of Rossby Wave Energy Dispersion Theory for Studying East Asian Monsoon System Dynamics

This paper is written to commemorate the 10th anniversary of academician Ye Duzheng (Yeh T.C.) pass away and his great contributions to the development of atmospheric dynamics. Under the inspiration and guidance of the theory of Rossby wave energy dispersion, remarkable progresses have been made in research on planetary wave dynamics and teleconnections of atmospheric circulation anomalies. This paper aims to make a brief review of the studies on the propagating characteristics of quasi-stationary planetary waves in a three-dimensional spherical atmosphere and the dynamic processes of the interannual and interdecadal variabilities of the East Asian summer and winter monsoon systems. Especially, this paper systematically reviews the progresses of the studies on the impacts of the interannual and interdecadal variabilities of the East Asia/Pacific (EAP) pattern teleconnection wave train propagating along the meridional direction over East Asia and the “Silk Road” pattern teleconnection wave train propagating along the zonal direction within the subtropical jet from West Asia to East Asia on the East Asian summer monsoon system and the summer precipitation variability in China, under the guidance of the theory of Rossby wave energy dispersion. Moreover, this paper reviews the dynamic processes of the impact of the interannual and interdecadal oscillations of the propagating waveguides of boreal quasi-stationary planetary waves on the variability of the East Asian winter monsoon system.

ENSO Impact on Summer Precipitation and Moisture Fluxes over the Mexican Altiplano

In the warm season, El Niño/Southern Oscillation (ENSO) causes periods with more rain in Northern Mexico during its positive phase, while less rainfall is recorded in the southern regions during the negative phase. This research study evaluates the variability of summer (July–September) precipitation and moisture fluxes under different ENSO scenarios in the Mexican Altiplano and coast of the state of Nayarit. The catchment of Rio San Pedro-Mezquital (SPM-RB) connects both regions. Using the Oceanic Niño Index (ONI), the years that signal change from El Niño to La Niña (1998), neutral conditions (2005), and strong (moderate) La Niña (1999) were selected to get an insight of ENSO impact on summer precipitation. For anomalies in the Altiplano, two additional contrasting years were analyzed—2006 (mostly dry) and 2010 (wet)—to determine moisture sources. Summer rainfall conditions in 1998 and 1999 had an opposite behavior between coastal Nayarit (wet) and the Altiplano (dry), while in 2005, rainfall deficits were observed in both regions. The moisture fluxes showed large divergence areas over central Mexico and the Southeastern United States in years of intense drought (1998 and 1999) caused by two high-pressure cells at middle levels of the troposphere (500 hPa). The moisture transport mechanisms into the Altiplano were related to atmospheric circulation at the upper level (200 hPa). The variations of the moisture fluxes from 2006 to 2010 are less strong at middle levels. The Eastern Pacific moisture convergence along the western coast of Mexico favors above-average rainfall anomalies in the coastal region but below-average anomalies in the Altiplano.

Increase in summer precipitation over the Sichuan Basin in recent decades and possible causes

AbstractA wet‐to‐dry shift in summer over the Sichuan Basin (SCB) during the late 20th century was noticed by previous studies. However, this study found that summer precipitation over SCB significantly increases during the past two decades, with an abrupt change point around 2011/2012. An anomalous cyclone over Lake Baikal, accompanied by an anticyclone over southern China, is responsible for the increase in SCB precipitation by inducing anomalous upper‐level divergence, mid‐level ascending motion and moisture convergence. Such atmospheric circulation anomalies are closely associated with the sea surface temperature (SST) warming over the Barents–Kara Sea (BKS). Observational analyses and model experiments indicate that the SST warming over the BKS warms local atmosphere and weakens meridional air temperature gradients over northern Asia and induces local anomalous easterlies, which further induce the anomalous cyclone over Lake Baikal that favours SCB precipitation. Additionally, the inhomogeneous warming over Europe increases local meridional air temperature gradients, enhances atmospheric baroclinicity and intensifies local transient eddy activities over central Europe. The intensified transient eddy activities generate negative geopotential height tendency anomalies over northern Europe, which can further excite a Rossby wave train that directly influences SCB precipitation, and can warm the atmosphere above the Kara Sea that indirectly influences SCB precipitation. This study can deepen our understanding of the interdecadal variability of summer precipitation over the SCB.

The Enhancement of the Summer Precipitation Teleconnection between India and the Northern Part of Eastern China after the Late 1990s

Abstract As subsystems of the Asian summer monsoon, summer precipitation variations in India and the northern part of eastern China (NEC) are physically connected. This study noted that the connection has been significantly enhanced after 1999 compared to 1979–98, which is due to the strengthened water vapor transportation connection between the two regions. That is associated with interdecadal variations of the combined effects of El Niño–Southern Oscillation (ENSO) and sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean (TIO) on the northwest Pacific subtropical high (NWPSH) and the Indo-Pacific Walker cell. Against the background of La Niña, the strengthened NWPSH and Indo-Pacific Walker cell favor water vapor transport to India and the NEC since 1999. Accordingly, summer precipitation in the two regions increases simultaneously, leading to the enhancement of the summer precipitation teleconnection between them. In addition, the influence of TIO SSTAs and the Indian Ocean dipole (IOD) on Indo-Pacific circulations decreases, which further enhances the relative importance of ENSO on the summer precipitation in the two regions. However, during 1979–98, La Niña SSTAs has weak effects on the NWPSH and Indo-Pacific Walker cell, the negative TIO SSTAs significantly weaken NWPSH, and the negative IOD also obstructs the westward extension of the Indo-Pacific Walker cell. Circulations and water vapor transportation related to the Indian Ocean and NEC summer precipitation are inconsistent, resulting in a weak precipitation teleconnection between them. The above conclusions are also validated by extreme case analysis and CMIP6 models. Significance Statement This paper mainly studies the influences of different types of ENSO and Indian Ocean SSTAs on the interdecadal variations of the summer precipitation relationship between India and the northern part of eastern China (NEC). We find that the summer precipitation relationship between them is strengthened again after 1999, which deepens the understanding of summer precipitation in Asia and has great significance for improving dynamic models’ prediction skills. The interdecadal variations of the combined effects of the Indian and Pacific Oceans are the fundamental reasons for the interdecadal variations of precipitation relationships, which promotes the understanding of interactions of different oceans and their impacts on Asian climate.

Diagnosing Potential Impacts of Tibetan Plateau Spring Soil Moisture Anomalies on Summer Precipitation and Floods in the Yangtze River Basin

AbstractSoil moisture as a key variable of land processes greatly influences the weather and climate. This study investigates the observed linkage between Tibetan Plateau (TP) spring soil moisture and summer precipitation and floods in the Yangtze River basin during 1988–2008 using satellite and in‐situ observations. A significant (p = 1%) proportion of interannual variations of summer precipitation (about 25%) in the Yangtze River basin can be attributed to spring TP soil moisture anomalies which show a dipole pattern. When spring soil moisture anomalies are positive (negative) over eastern (western) TP, there is more summer precipitation and consequently river discharge in the Yangtze River basin, or vice versa. The possible mechanisms can be explained from the perspectives of surface energy balance and atmospheric thermodynamics. More (less) spring soil moisture over the eastern (western) plateau enhances summer diabatic heating, which may be related to soil moisture memory. The enhanced summer diabatic heating stimulates vigorous ascending motions over TP, which diverge in the upper troposphere (200 hPa) and descend over the western Pacific. This is conducive to the enhancement and mutual proximity of the South Asian High (SAH) and the Western Pacific Subtropical High (WPSH). As a result, the warm and humid air flows from the Bay of Bengal and the western Pacific and the cold and dry air flows from the boreal continent converge in the Yangtze River basin, causing excessive summer precipitation. Therefore, TP spring soil moisture can be considered a seasonal predictor of summer precipitation and possible subsequent floods in the Yangtze River basin.

Decadal Shift in Summer Precipitation Variability over East Asia in the Mid-2000s and Wave Propagation toward North America

Abstract The East Asian summer climate displays a marked change after the late 1990s. This is principally due to a weakening of the Pacific–Japan (PJ) teleconnection pattern that was a dominant driver of precipitation variability over East Asia. Nevertheless, western Japan has frequently experienced heavy rainfall events over the past several years. Atmospheric reanalysis and observational datasets are used to investigate summer precipitation variability over East Asia in a view of interdecadal changes from 1979 to 2020. East Asian summer precipitation has increased the most in Japan, especially over and around the Southwest Islands in southern Japan, where cumulus convection has been more dominant for the characteristic of precipitation variability including mei-yu–baiu rainfall than frontal structure since the mid-2000s. Atmospheric analysis in vertical structures and convective instability indicates that moist instability is neutralized by cumulus convection over the southern East China Sea and off the Pacific coast of Japan, where recent warming in sea surface temperature (SST) along the Kuroshio exceeds the SST threshold for convection. This decadal shift in precipitation variability has a close relationship with the second precipitation mode over East Asia, which has taken the place of the PJ pattern as a leading driver of precipitation variability over western Japan in the past decade. The enhanced cumulus convection is enabled to act as a forcing mechanism for the Rossby wave train from East Asia toward North America along the westerly jet or for the eastward extension of the Silk Road pattern, which possibly favors a heatwave in the Pacific Northwest areas of North America.

Hardwoods influence effect of climate and intraspecific competition on growth of woodland longleaf pine trees

AbstractLongleaf pine woodlands of the North American Coastal Plain are proposed to be resilient to climate change impacts, but little is known about changes in limiting factors to longleaf pine growth as climate has changed in late 20th century and early 21st century. Moreover, the role that neighborhood trees play in the context of climate change remains largely unexplored. We used static and moving window tree ring and climatic analyses to measure the effects of climate on longleaf pine growth at a site in southwest Georgia, USA. We then performed maximum likelihood analysis to examine the influence of neighboring hardwoods on the response of longleaf pine growth to the joint effects of competition and climate. Analysis of climate data from local stations in southwest Georgia over six decades indicated that mean air temperature decreased until the late 20th century then began to rise, and that the variability of spring and summer precipitation has increased. Tree ring and climate analyses indicated longleaf pine radial growth is sensitive to precipitation and air temperature, and that the strength of correlation of longleaf pine growth to summer air temperature and summer precipitation increased since the 1950s. Likelihood models, which were applied over a shorter (23‐year) period and explicitly incorporated competition, did not support a link between summer temperature and growth but did indicate summer precipitation increased growth. Furthermore, basal area (BA) of neighboring hardwoods was correlated with greater pine growth per millimeter of precipitation. BA of neighboring longleaf pine negatively affected growth of conspecific trees; the presence of hardwoods increased the competitive effect when BA of neighboring pine trees was low (<10 m2 ha−1) but decreased the competitive effect when BA of neighboring pine trees was high (≥10 m2 ha−1). These results suggest that retention or recruitment of hardwood trees when restoring longleaf pine woodlands may contribute to increased ability to withstand dry summers and may help to allay concerns of managers that retention of hardwoods will unduly affect growth of residual mature longleaf pines.

Soil surface grass phytolith morphotypes across bioclimatic gradients and biomes at about latitude 29°S in South Africa

This study examines the distribution patterns of grass subfamilies and their diagnostic silica phytoliths along a transect running approximately at latitude 28° S in South Africa. The transect encompasses 77 surface samples collected along moisture and temperature gradients, across 6 biomes and 39 ecozones, and spanning elevations between near-sea level and over 2253 m. This study tests the relation between the distribution and density of grass subfamilies and the proportions of their diagnostic grass silica short cell phytoliths (GSSCP) representing them, and how these proportions correlate with five climatic variables (mean temperature of the wettest quarter, annual precipitation, precipitation of the wettest quarter, precipitation variability, and the aridity index). The results suggest that C3-grass diagnostic GSSCP morphotypes increase with a decline in temperature in the wettest quarter, occurring in areas receiving mostly winter rain or at high elevations in areas of summer rain. Of the C4 grass subfamilies, the Panicoideae increase with summer precipitation and low precipitation variability, while the Aristidoideae-Stipagrostis the opposite. The distribution of Chloridoideae depends on complex changes in all climatic variables, suggesting aridity gradients only in the eastern half of the transect. In view of the data, this study reviews the usefulness of the phytolith humidity-aridity index (Iph) and the water stress index (Fs) and proposes the Panicoideae index (PI) and Stipagrostis index (SI) as indicator of aridity and high precipitation variability in southern Africa.

Evaluation of multi‐RCM ensembles for simulating spatiotemporal variability of Asian summer monsoon precipitation in the CORDEX‐East Asia Phase 2 domain

AbstractFive regional climate models (RCMs)—CCLM, RegCM, HadGEM3‐RA, SNURCM and WRF—participating in the Coordinated Regional Climate Downscaling Experiment‐East Asia (CORDEX‐EA) Phase 2 project are evaluated for their ability to simulate spatiotemporal variability in Asian summer precipitation. For this purpose, two dynamical downscaling sets, experiments forced by ERA‐Interim reanalysis data (reproduction experiment) and historical data from three Coupled Model Intercomparison Project 5 (CMIP5) general circulation models (GCMs) (historical experiment) are analysed. The horizontal resolution of the downscaled results is 25 km, and the analysis period is from 1981 to 2005 (25 years). The RCMs show reasonable performance in simulating the spatial and temporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain. To assess spatiotemporal patterns in Asian precipitation, cyclostationary empirical orthogonal function (CSEOF) analysis is used. In the first mode representing seasonal variations, the model ensemble results of both the reproduction experiment (Rep_ENS) and the historical experiment (His_ENS) simulate the peak times, location of precipitation and progression of the monsoons in Asia reasonably. Their features are greatly influenced by the moisture flux, indicating that the relation between precipitation and 850 hPa moisture flux is depicted well by RCM ensembles. In the second mode, which represents intraseasonal variations, the Rep_ENS depicts the western North Pacific summer monsoon break (WNPSM break) phenomenon similar to the observation. Although the His_ENS simulates the WNPSM break later than the observation, it does present the intraseasonal variation in the East Asian summer monsoon region. Based on these results, the five RCM ensemble results show the ability to capture spatiotemporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain, as well as added value from dynamical downscaling.

Disentangling the Driving Mechanisms of the Tripole Mode of Summer Rainfall over Eastern China

Abstract The spatial distribution of summer rainfall anomalies over eastern China often shows a tripole pattern with rainfall anomalies over the Yangtze River basin varies in opposite phase with North China and South China. It is not clear whether this tripole pattern is an intrinsic atmospheric mode or it is remotely forced. Using two sets of model outputs from 20 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), this paper investigates the driving mechanisms of this leading rainfall mode and its major influencing factors. One set (piControl) is fully coupled atmosphere–ocean simulations under constant preindustrial forcing and the other (sstClim) is atmosphere-alone models forced by prescribed climatological sea surface temperatures (SSTs). By comparing results from these two different sets of simulations, it is found that the tripole pattern is the leading mode of summer precipitation variability over eastern China with or without oceanic forcing. It can be regarded as an intrinsic atmospheric mode although air–sea interaction can modify its temporal variability. The cyclonic–anticyclonic atmospheric circulation anomaly over the northern North Pacific is identified as a key factor in both experiments. As atmospheric internal variability, it is related to a circumglobal zonal wave train propagating along the westerly jet stream. When air–sea interactions involved, modulation from SST anomalies is exerted through the meridional Pacific–Japan/East Asia–Pacific wave train propagating along the East Asian coast. Our results suggest that the North Pacific could be another key region providing potential predictability to the East Asian monsoon in addition to the Indo-Pacific.

Interannual Variability of Extreme Precipitation during the Boreal Summer over Northwest China

Herein, we investigated the characteristics and mechanisms of interannual variability of extreme summer precipitation in northwest China (NWC). The four high-resolution precipitation predicting products under assessment indicated that both the accumulation of summer daily precipitation ≥95th percentile, and the summer maxima of daily precipitation generally decreased in a southeast—northwest direction, while relatively high values were observed in the Tienshan and Qilian Mountain areas. In turn, the Tropical Rainfall Measuring Mission (TRMM) satellite dataset underestimated extreme precipitation in mountainous areas, while Asian precipitation highly—resolved observational data integration towards evaluation (APHRODITE) and Climate Prediction Center (CPC) captured the characteristics of extreme precipitation in NWC. AMIP-type simulations of the interannual variability of extreme summer precipitation in NWC were quite unsuccessful. However, all of them can capture the increasing trends. Therefore, we further found that the interannual increase in extreme precipitation in NWC is strongly correlated with the weakened South Asian high, strengthened Western Pacific Subtropical high, the enhanced westerly jet, and the mid- to high-latitude Rossby wave trains, whose formation and sustenance can be traced back to sea surface temperature-anomalies in the western Pacific in May, June, and July. An increased sea surface temperature promotes convection and induces diabatic heating, which stimulates anticyclonic anomalies that disturb the enhanced westerly jet, resulting in a barotropic Rossby wave train via the Gill-type response. Additionally, it guides more water vapor convergence to NWC and enhances upward motion via anticyclonic anomalies over western Europe and Eastern Asia, and cyclonic anomalies over Central Asia.