Tropical Pacific Sea Surface Temperature Anomalies Research Articles

Statistical relationships between the Interdecadal Pacific Oscillation and El Niño–Southern Oscillation

AbstractThe climate of the Pacific Ocean varies on interannual, decadal, and longer timescales. This variability is dominated by the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), both of which have profound impacts on countries within and well beyond the Pacific. To date, previous studies have only examined a small subset of the possible links between ENSO, its diversity, and the IPO. Here we focus on the statistical relationship between decadal variability in ENSO properties and the IPO, testing the null hypothesis that the IPO arises from random decadal changes in ENSO activity, including ENSO diversity. We use observed sea surface temperature (SST) records since 1920 to investigate how the timing, structure, frequency, duration, and magnitude of El Niño and La Niña events differ between IPO phases. We find that using the relative frequency of El Niño and La Niña events and either the mean event duration or SST magnitude can reproduce up to 60% of the IPO Tripole Index timeseries. While the spatial SST patterns that represent the IPO and ENSO are similar, the IPO is meridionally broader in the central to eastern Pacific, which may be caused by a lagged relationship with low-frequency SST variability in the equatorial Pacific. In addition, North Pacific SST anomalies of opposite sign to the tropical Pacific SST anomalies is a unique feature of the IPO that cannot be explained by decadal ENSO variability. This suggests a clear IPO and ENSO relationship, but also independence in some of the IPO’s characteristics.

A SST-constructed Ocean Heat Content index in crossing ENSO spring persistence barrier

The El Niño-Southern Oscillation (ENSO) spring persistence barrier (SPB) describes the feature in which the predictive skills of ENSO decrease significantly in the boreal spring. This paper investigates an index constructed using sea surface temperature (SST), namely SSTH, which is based on tropical Pacific Ocean Heat Content (OHC) in crossing ENSO SPB. Inspired by the dynamical relationship between the tropical Pacific OHC and eastern Pacific SST anomalies, SSTH is constructed by SST anomalies (i.e., Niño3.4 index) to represent OHC. We show that this index leads ENSO SST anomalies by about 10 months, making it effective in crossing ENSO SPB. Particularly, among the 50 ENSO events from 1950 to 2022, 27 years were identified to be caused by SSTH signals. Compared with warm water volume (WWV) or the west of WWV (WWVw), this index is more stable and effective after the 21st century because the effective region of subsurface OHC changed dramatically afterward. However, SSTH avoids this problem as it is constructed by SST anomalies alone. Finally, as SST data is reliable before 1980, SSTH is utilized to study the interdecadal lead-lag relationship between subsurface OHC and ENSO SST.

A Zonal See‐Saw Variation of Tropical Cyclogenesis Over the Arabian Sea and Bay of Bengal‐South China Sea

AbstractThe variation of tropical cyclone (TC) genesis over the North Indian Ocean (NIO) and South China Sea (SCS) during the post‐monsoon season is explored during 1979–2020. A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and Bay of Bengal (BOB) –SCS (BOBSCS for brevity). More (less) TCs over the AS tend to concur with fewer (more) TCs over the BOBSCS. Analyses of both the observations and the Coupled Model Intercomparison Project Phase 6 outputs show that tripole sea surface temperature (SST) anomalies in the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesis. Positive western Indian (WI) Ocean and tropical central and eastern Pacific (CEP) Ocean SST anomalies and negative western Pacific (WP) SST anomalies induce anomalous ascending and descending motions and high and low humidity transportation to the middle troposphere over the AS and BOBSCS, respectively. Meantime, anticyclonic vorticity anomalies appear in the BOBSCS region and negative anomalies of vertical wind shear are induced over the NIO and SCS. The combined positive contributions of upward motion, mid‐level humidity increase, and low vertical wind shear favor higher TC genesis over the AS, whereas the joint negative effects of downward motion, mid‐level water vapor decrease, and lower‐level anticyclonic vorticity anomalies result in less TC genesis over the BOBSCS. Numerical experiments show a critical role of the WI SST anomalies in the dipole pattern of vertical motion, which is crucial for the zonal dipole variation of TC genesis.

Revisiting the Impacts of Tropical Pacific SST Anomalies on the Pacific Meridional Mode during the Decay of Strong Eastern Pacific El Niño Events

Abstract The Pacific meridional mode (PMM) can modulate El Niño–Southern Oscillation (ENSO) and is also affected by ENSO-related tropical Pacific sea surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: a positive one of central tropical Pacific SSTAs and a negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like those in 1982/83 and 1997/98. However, we find that no strong, negative PMM-like SSTAs appeared, although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfavorable for establishing the wind–evaporation–SST feedback for a negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combined using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Niño events even without strong, negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing a negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events. Significance Statement This paper aims to reevaluate the previously proposed effect of strong eastern Pacific El Niño events, like those in 1982/83 and 1997/98, on exciting a negative Pacific meridional mode (PMM). We find that although the PMM indices were strongly negative during the decay of strong eastern Pacific El Niño events, the large negative PMM sea surface temperature anomalies (SSTAs) could not be observed in the subtropical Pacific. Further diagnosis indicates that the PMM index can be large if strong SSTAs occur in eastern tropical Pacific even without subtropical SSTAs, implying that one should be careful when using the PMM index.

Connection between the Tropical Pacific and Indian Ocean and Temperature Anomaly across West Antarctic

West Antarctic and the Antarctic Peninsula have experienced dramatic warming in austral spring since the 1970s. Using observations and the Community Atmosphere Model version 4 (CAM4), this study explores the physical mechanism by which the tropical Pacific and Indian Ocean temperature anomaly mode (PIM) affects the dipolar surface air temperature (SAT) anomalies across the West Antarctic in austral spring. The positive phase of the PIM, characterized by positive sea surface temperature anomalies (SSTAs) in the tropical central-eastern Pacific and western Indian Ocean and negative SSTAs in the Maritime Continent, can generate two branches of stationary Rossby wave trains propagating from the tropical central Pacific and southeastern Indian Ocean to the West Antarctic, with an anticyclonic anomaly appearing over the Amundsen Sea. The northerlies advect warmer air to the Ross–Amundsen Seas, but southerlies advect colder air to the Antarctic Peninsula–Weddell Sea, resulting in the dipole of SAT anomalies over the West Antarctic. In this process, the role of tropical central-eastern Pacific SSTAs dominate, and it is amplified by the SSTAs around the Maritime Continent. The SSTAs in the western Indian Ocean combined with the SSTAs over the Maritime Continent further contribute to the western pole of the SAT. Only simulation that includes a prescribed PIM forcing can exactly reproduce the observations of the dipolar SAT response across the West Antarctic, indicating the need to treat the tropical Pacific and Indian Oceans as a unified whole.

A Novel Mechanism for Extreme El Niño Events: Interactions between Tropical Cyclones in the Western North Pacific and Sea Surface Warming in the Eastern Tropical Pacific

Abstract This study presents a novel mechanism for the generation of extreme El Niño events by analyzing interactions between tropical cyclones (TCs) in the western North Pacific (WNP) in spring [March–May (MAM)] and summer [June–August (JJA)] and sea surface warming in the eastern tropical Pacific. It is suggested that anomalously strong TCs in the WNP in MAM and JJA are essential for the formation of extreme El Niño events. MAM TCs excite considerable westerly wind bursts (WWBs) and facilitate the generation of El Niño events in late spring. The sea surface temperature (SST) in the central-eastern tropical Pacific increases prominently during the following summer, which is due to the warm water carried by downwelling Kelvin waves induced by the anomalous westerlies in the western tropical Pacific associated with the WNP TCs, as well as the lessening cold water upwelling resulting from the deepening thermocline in the eastern tropical Pacific. The developing El Niño in turn contributes to the TC activities over the southeastern quadrant of the WNP in summer, characterized by a stronger intensity, higher frequency, and longer duration. The resulting JJA TC-induced westerlies could further enhance the eastern tropical Pacific warm SST anomalies, and thus an extreme El Niño event tends to appear in the following autumn and winter. These physical processes are verified by several sets of atmosphere–ocean coupled model experiments. Significance Statement Tropical cyclone activity is one of the most destructive phenomena in the world. Extreme El Niño events can also cause devastating climate disasters. Understanding the relationship between these two events can help with disaster forecasting and prevention. This study finds that TC activity in the western North Pacific contributes to the appearance of extreme El Niño events. Abnormally active TC activity in spring can cause strong near-equatorial westerly wind anomalies, eastward transport of warm water from the western tropical Pacific, and a deepening ocean thermocline in the east, resulting in the earlier onset of El Niño events. Influenced by the El Niño event, the TC activity in summer is strengthened, which in turn continues to promote the surface warming of the central-eastern tropical Pacific, eventually resulting in extreme event occurrence.

Variations of Summer Extreme and Total Precipitation over Southeast Asia and Associated Atmospheric and Oceanic Features

Abstract Southeast Asia lies at the heart of heavy precipitation on Earth, and a large amount of latent heat released here provides substantial energy for the global atmospheric circulation. Utilizing gauge-based daily precipitation and the self-organizing map technique, the summertime extreme and total precipitation over Southeast Asia during 1979–2019 are classified into three and five distinct patterns, respectively. The three extreme precipitation clusters are characterized by southern dry and northern wet (C1_extreme), overall wet (C2_extreme), and northern dry and southern wet (C3_extreme) structures. The frequencies of these patterns exhibit increasing trends during the analysis, although they are not statistically significant for C1_extreme. The C1_extreme pattern is accompanied by an anomalous cyclone over the South China Sea in response to negative Indian Ocean sea surface temperature anomalies (SSTAs). The C2_extreme and C3_extreme clusters are characterized by a westward extension of the western Pacific subtropical high, regulated by cool SSTAs over the tropical central-eastern Pacific that are induced by the tropical North Atlantic warming and the tropical Pacific and Atlantic SSTAs, respectively. For total precipitation, the first and second clusters show overall dry distributions, which are mainly composed of nonextreme precipitation. The spatial patterns and atmospheric and oceanic features associated with the other three clusters of total precipitation bear large resemblances to those of C1_extreme, C2_extreme, and C3_extreme, respectively, but their trends exhibit smaller similarities. Comparing the differences between extreme and total precipitation over Southeast Asia could improve our understanding of their regional variabilities and relationships, and potentially their global impacts. Significance Statement We explore the atmospheric and oceanic processes associated with extreme precipitation, and compare them with those for total precipitation over Southeast Asia. The three key modes of summertime extreme precipitation over Southeast Asia are characterized by southern drought and northern flood, overall flood, and northern drought and southern flood structures. Each pattern is closely linked to tropical sea surface temperature anomalies, but in different regions. Total precipitation can be classified into five distinct patterns. The first two modes are largely determined by nonextreme precipitation, while the other three bear large resemblances to the three extreme precipitation modes. These findings provide guidance on what is the key factor in driving each extreme precipitation mode over Southeast Asia, allowing better prediction.

The Role of Coupled Feedbacks in the Decadal Variability of the Southern Hemisphere Eddy‐Driven Jet

AbstractRecent work has suggested that tropical Pacific decadal variability and external forcings have had a comparable influence on the observed changes in the Southern Hemisphere summertime eddy‐driven jet over the satellite era. Here we contrast the zonally asymmetric response of the Southern Hemisphere eddy‐driven jet to tropical Pacific decadal variability by designing an atmosphere‐only PAC‐A experiment using the Community Atmosphere Model version 5 (CAM5) and comparing it with the fully coupled Community Earth System Model Version 1 (CESM1) tropical Pacific pacemaker (PAC‐C) experiments. In both frameworks, the tropical Pacific sea surface temperature (SST) anomalies are identical (model climatology plus observed anomalies), which allows the PAC‐C and PAC‐A experiments to be used to estimate the impact of coupling on teleconnections from the tropical Pacific to the Southern Hemisphere extratropics. The observed summertime South Pacific jet intensification is reproduced in both coupled and uncoupled experiments, indicating that the central and eastern tropical Pacific (hereafter, tropical Pacific) SST impacts the South Pacific jet mainly via direct atmospheric teleconnections. By contrast, only the coupled PAC‐C captures the summertime poleward shift of the South Atlantic‐Indian jet, suggesting that air‐sea coupling is essential in driving the teleconnections between tropical Pacific SST anomalies and South Atlantic‐Indian jet variations.

The Second Rainy Stage Onset in the Central Highlands of Vietnam

AbstractTwo distinct rainfall stages over the Central Highlands (CH) of Vietnam during the rainy season have been objectively defined using the high‐resolution Vietnam Gridded Precipitation data set for 1983–2010 (28 years): a second rainy stage (SRS) embedded in the conventional rainy season. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until mid‐August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Niño developing phases, particularly during the Niño3.4 sea surface temperature (SST) increase from January through December. Water vapor budget analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset.

Global Impact of ENSO on Dust Activities with Emphasis on the Key Region from the Arabian Peninsula to Central Asia

AbstractThe El Niño‐Southern Oscillation (ENSO) has been found to be closely related to dust aerosol activities in many parts of the world. However, previous studies have rarely explored the ENSO’s impacts in the global context. In this study, the responses of the interannual variability of global atmospheric dust content to the ENSO cycle (including the eastern Pacific and central Pacific types) are examined using the MERRA‐2 atmospheric aerosol reanalysis data. The ENSO‐related tropical eastern Pacific sea surface temperature (SST) anomalies in the antecedent winter (November to January) have the most significant effect on the airborne dust content in a region from the Arabian Peninsula to Central Asia during the following spring and summer (April to July), which is identified as the key region (KR) of the ENSO’s impacts. The positive (negative) tropical eastern Pacific SST anomalies in the preceding winter during the El Niño (La Niña) years correspond to significantly reduced (increased) dust column content in the following spring and summer over the KR, which was confirmed using singular value decomposition. With the above (below)‐normal winter SST anomalies in the tropical eastern Pacific, the Walker Circulation weakens (strengthens), and the divergence (convergence) over the western tropical Pacific and the convergence (divergence) over the southern tropical Indian Ocean in the lower troposphere are strengthened, which facilitate (restrain) the southward diffusion of airborne dust in the KR. Meanwhile, the increased (decreased) precipitation and soil moisture and the corresponding vegetation conditions help limit (promote) the dust emission in the KR during the El Niño (La Niña) years.

What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?

AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.

Inter-basin and Multi-time Scale Interactions in generating the 2019 Extreme Indian Ocean Dipole

AbstractAn unprecedented extreme positive Indian Ocean Dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD is triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and sustained by the local atmosphere-ocean interaction thereafter. During September-November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific further enhance the Indian Ocean’s easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA is strengthened by two consecutive Madden Julian Oscillation (MJO) events that originate from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-timescale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.

Effect of the El Niño Decaying Pace on the East Asian Summer Monsoon Circulation Pattern during Post-El Niño Summers

El Niño events vary from case to case with different decaying paces. In this study, we demonstrate that the different El Niño decaying paces have distinct impacts on the East Asian monsoon circulation pattern during post-El Niño summers. For fast decaying (FD) El Niño summers, a large-scale anomalous anticyclone dominates over East Asia and the North Pacific from subtropical to mid-latitude; whereas, the East Asian monsoon circulation display a dipole pattern with anomalous northern cyclone and southern anticyclone for slow decaying (SD) El Niño summers. The difference in anomalous East Asian monsoon circulation patterns was closely associated with the sea surface temperature (SST) anomaly patterns in the tropics. In FD El Niño summers, the cold SST anomalies in the tropical central-eastern Pacific and warm SST anomalies in the Maritime Continent induce the anticyclone anomalies over the Northwest Pacific. In contrast, the warm Kelvin wave anchored over the tropical Indian Ocean during SD El Niño summers plays a crucial role in sustaining the anticyclone anomalies over the Northwest Pacific. In particular, the opposite atmospheric circulation anomaly patterns over Northeast Asia and the mid-latitude North Pacific are mainly modulated by the stationary Rossby wave trains triggered by the opposite SST anomalies in the tropical eastern Pacific during FD and SD El Niño summers. Finally, the effect of distinct summer monsoon circulation patterns associated with the El Niño decay pace on the summer climate over East Asia are also discussed.

Interannual to Decadal Variability of Tropical Indian Ocean Sea Surface Temperature: Pacific Influence versus Local Internal Variability

AbstractThe Indian Ocean has received increasing attention for its large impacts on regional and global climate. However, sea surface temperature (SST) variability arising from Indian Ocean internal processes has not been well understood particularly on decadal and longer time scales, and the external influence from the tropical Pacific has not been quantified. This paper analyzes the interannual-to-decadal SST variability in the tropical Indian Ocean in observations and explores the external influence from the Pacific versus internal processes within the Indian Ocean using a linear inverse model (LIM). Coupling between Indian Ocean and tropical Pacific SST anomalies (SSTAs) is assessed both within the LIM dynamical operator and the unpredictable stochastic noise that forces the system. Results show that the observed Indian Ocean basin (IOB)-wide SSTA pattern is largely a response to the Pacific ENSO forcing, although it in turn has a damping effect on ENSO especially on annual and decadal time scales. On the other hand, the Indian Ocean dipole (IOD) is an Indian Ocean internal mode that can actively affect ENSO; ENSO also has a returning effect on the IOD, which is rather weak on decadal time scale. The third mode is partly associated with the subtropical Indian Ocean dipole (SIOD), and it is primarily generated by Indian Ocean internal processes, although a small component of it is coupled with ENSO. Overall, the amplitude of Indian Ocean internally generated SST variability is comparable to that forced by ENSO, and the Indian Ocean tends to actively influence the tropical Pacific. These results suggest that the Indian–Pacific Ocean interaction is a two-way process.

Analysis of Record-High Temperature over Southeast Coastal China in Winter 2018/19: The Combined Effect of Mid- to High-Latitude Circulation Systems and SST Forcing over the North Atlantic and Tropical Western Pacific

AbstractIn winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China.

Revisiting remote drivers of the 2014 drought in South-Eastern Brazil

South-Eastern Brazil experienced a devastating drought associated with significant agricultural losses in austral summer 2014. The drought was linked to the development of a quasi-stationary anticyclone in the South Atlantic in early 2014 that affected local precipitation patterns over South-East Brazil. Previous studies have suggested that the unusual blocking was triggered by tropical Pacific sea surface temperature (SST) anomalies and, more recently, by convection over the Indian Ocean related to the Madden–Julian Oscillation. Further investigation of the proposed teleconnections appears crucial for anticipating future economic impacts. In this study, we use numerical experiments with an idealized atmospheric general circulation model forced with the observed 2013/2014 SST anomalies in different ocean basins to understand the dominant mechanism that initiated the 2014 South Atlantic anticyclonic anomaly. We show that a forcing with global 2013/2014 SST anomalies enhances the chance for the occurrence of positive geopotential height anomalies in the South Atlantic. However, further sensitivity experiments with SST forcings in separate ocean basins suggest that neither the Indian Ocean nor tropical Pacific SST anomalies alone have contributed significantly to the anomalous atmospheric circulation that led to the 2014 South-East Brazil drought. The model study rather points to an important role of remote forcing from the South Pacific, local South Atlantic SSTs, and internal atmospheric variability in driving the persistent blocking over the South Atlantic.

Distinct tropical Pacific sea surface temperature anomaly regimes enhanced under recent global warming

AbstractEl Niño and Southern Oscillation (ENSO) events are usually monitored by tropical Pacific sea surface temperature anomaly (SSTA) patterns with dramatic impacts on the global climate. To explore the diversity of the tropical Pacific SSTA, a novel method combined empirical orthogonal function analysis and the K‐means clustering algorithm is carried out to classify SSTA patterns during 1950–2016. Meanwhile, the total distance variance and total silhouette value are introduced to determine an optimal number of distinguishable representative SSTA patterns. Ten SSTA patterns are obtained, which shows frequent basin‐wide warming and extreme cold ENSO events in recent decades. It may be attributed to the changes in composition of the intrinsic modes along with the background mode of slowly increasing east–west SST gradient. The comparative analysis between periods 1950–1969 and 1997–2016 suggests that the two regimes of tropical Pacific SSTA, featured as extreme warm and moderate warm/extreme cold patterns respectively, become more distinct under recent global warming.

Interdecadal variability of South–Southeast Asian rainfall and cross‐equatorial flows during April–May

AbstractFrom April to May, South–Southeast Asian rainfall (SSEAR) and cross‐equatorial flows (CEFs) off the East African coast have exhibited interdecadal variability over the past four decades. Both SSEAR and CEF strengthened based on comparative analysis for the periods 1979–1998 and 1999–2016. Associated modulating processes feature warm sea surface temperature (SST) anomalies in the tropical western North Pacific (WNP) and cold SST anomalies in the tropical eastern Pacific. These features lead to a low‐level clockwise circulation anomaly across the North and South Indian Ocean. This intensifies CEFs via anomalous southerly flows along the circulation's western section and transports moisture via anomalous westerly flows on its northern side that enhances SSEAR. After 1998, SSEAR and CEFs vary coherently from 1999 to 2005, but incoherently from 2006 to 2016. For coherent cases (1999–2005), warm SST anomalies in the tropical WNP invoke a clockwise circulation anomaly over the tropical Indian Ocean. Meanwhile, cold anomalies in the tropical Indian Ocean and strong warm anomalies over Asian landmasses cause a meridional temperature difference (MTD) forming a strong cyclonic anomaly in the north that constrains the clockwise circulation anomaly south of 15°N. The circulation's western section enhances CEFs, while anomalous westerly flows on its northern side enhance moisture supply over southern parts of the Asian landmasses thereby increasing SSEAR. For incoherent cases (2006–2016), SST anomalies are warm in both the tropical WNP and Indian Ocean, while anomalous warming over Asian landmasses is more moderate. A weakened MTD yields an isolated weakened cyclonic anomaly in the north allowing for the development of a northward‐displacing clockwise circulation anomaly with anomalous westerly flows north of 20°N. Rainfall increases a little in northern parts of South–Southeast Asia but decreases noticeably in southern parts. Intensified CEFs thus occur concurrently with decreased SSEAR.

The mechanism of EAP-EU combined impact on summer rainfall over North Asia

This article uses wave activity flux and sea surface temperature (SST) to explore the mechanism of EAP-EU combined impact on summer rainfall in North Asia (NA). EAP and EU teleconnections can be divided into four categories of configuration according to their anomaly phase features. The category I (II) configurations cause rainfall in the Urals and the Okhotsk Sea and the region from Northeast China to Japan being less (more), the Lake Baikal to the Okhotsk Sea being more (less), which leads to the summer rainfall in NA mainly presenting the “− + −” (“+ − +”) a tripole wavelike structure. Category I (II) configurations also show the West-East reverse anomalies of summer rainfall in northern NA and the North-South opposite anomalies in eastern NA. The EAP and EU teleconnections have an obvious correlation with the ENSO-like pattern of SST anomalies over the tropical Pacific. When warm ENSO is in the development phase, it is conducive to stimulate Rossby waves propagating eastward from the North Atlantic Ocean via the PNA teleconnection, forming the EU teleconnection across the Eurasia continent. Meanwhile, the warm summer SSTA anomalies over tropical east Pacific and IMC are conducive to the local thermal convective over Indo-Pacific warm pool and motivate the EAP teleconnection, propagating northward to mid-high latitude region. EAP and the EU teleconnections overlapped with each other, form the tripole atmospheric structure in NA, and produce relevant rainfall anomaly patterns. For the category IV of EAP-EU configuration, the warm SST anomalies in the equatorial region gradually weaken from previous winter to summer. Weak tropical east Pacific SST anomalies are conducive to excite the EU teleconnection with different wave train features against the category II, presenting a strong positive anomaly center over the Ural mountain and Baikal lake. This positive center connected with the EAP caused positive anomalies over FER resulting in the dipole pattern of atmospheric and rainfall anomalies in the summer season. The condition is opposite for the category III (I) configuration against that of category IV (II). It is also found that the influence on EAP-EU configuration caused by ENSO-like pattern of SST anomalies over the tropical Pacific has an obvious asymmetric feature that category II (I) configuration relevant SST forcing is much significant than that of category IV(III) configuration.

Influence of Eastern Tibetan Plateau Spring Snow Cover on North American Air Temperature and Its Interdecadal Change

AbstractPrevious studies revealed the influence of the autumn–winter Tibetan Plateau snow cover on atmospheric circulation and climate in the North American region. The present study documents the relationship between the eastern Tibetan Plateau snow cover and the North American air temperature in spring and the associated physical processes using satellite-based snow cover, reanalysis atmospheric and surface variables, observation-based surface air temperature (SAT), and sea surface temperature (SST). A stable relationship is identified between the eastern Tibetan Plateau snow cover and the North American SAT in spring before the mid-2000s. Positive snow-cover anomalies over the eastern Tibetan Plateau induce cooling in the local atmospheric column. The atmospheric cooling stimulates a large-scale atmospheric wave pattern at the upper level that extends northeastward from the eastern Tibetan Plateau via northeast Asia and the North Pacific to North America. An anomalous high forms over North America, accompanied by anomalous descent. In the northwestern part, the horizontal advection by anomalous southerly winds along the west flank of anomalous anticyclone induces SAT increase. In the central part, the enhanced surface sensible heat flux following anomalous descent-induced downward shortwave radiation increase leads to SAT increase. The relationship between the eastern Tibetan Plateau snow cover and the North American SAT is weakened after the mid-2000s. The weakened relationship is attributed to an intensified impact of tropical central Pacific SST anomalies on the North American SAT variations through a Pacific–North America-like atmospheric circulation pattern, which overcomes the influence of the Tibetan Plateau snow-cover anomalies.