Easterly Anomalies Research Articles

Dominant Synoptic Systems for Summer Precipitation over the Complex Terrain of Southwestern China

Abstract Understanding precipitation over complex terrain, such as southwestern China, requires the consideration of both multiscale circulation and topography. First, the dominant synoptic system must be clarified, as it determines how multiscale topography affects precipitation. Here, based on a self-organizing map, large-scale winds are categorized into anomalous-westerly types, anomalous-easterly types, and transitional types. Four synoptic-scale systems (vortex type, cold-front type, tropical-depression type, and weak-synoptic-forcing type) dominate the summer precipitation. The vortex type occurs with strengthened large-scale westerlies, and its precipitation is distributed within the moisture convergence region. The cold-front type, tropical-depression type, and weak-synoptic-forcing type exhibit large-scale easterly anomalies. For the cold-front type, a low-level northeasterly blocked by topography shapes the northwest–southeast-oriented front zone at the upper highland slope. The precipitation frequency and intensity are high within the frontal zone, while the intensity is weak on both sides. For the tropical-depression type, moist low-level easterlies uplifted by westward-rising topography anchor precipitation at the lower slope. Large precipitation for the tropical-depression type is attributed to a high frequency. Large-scale horizontal winds are the weakest for the weak-synoptic-forcing type, and the local topography influences the scattered precipitation distribution. Both the frequency and intensity are high for the weak-synoptic-forcing type. Overall, long-lasting nocturnal events dominate the precipitation of the four synoptic types, while large-scale easterlies favor precipitation events with shorter durations and earlier peaks. For obvious synoptic systems, large-scale topography influences precipitation via a dynamic blocking effect, while the thermodynamic role of local topography is important with a weak-synoptic-forcing. Significance Statement Clarifying dominant synoptic systems is highly important for understanding precipitation over complex terrain, as the effect of topography on precipitation varies with different synoptic backgrounds. Taking southwestern China as a representative of complex terrain, this study objectively identified the dominant synoptic systems associated with summer precipitation. The distribution and fine-scale characteristics of precipitation have been further analyzed considering the combined influence of multiscale circulation and topography. In addition to advancing our understanding of precipitation in southwestern China, this study provides a reference for analyzing precipitation in other regions with complex terrains.

Equatorial Dynamics Significantly Weakened the Southward Somali Current Circulation during 1997–98

Abstract Based on reanalysis data and numerical model, this study documented an unusually weak southward Somali Current (SSC) circulation, an anticlockwise circulation in the western Arabian Sea, during the winter of 1997 to spring 1998, throughout the two-decade period from 1994 to 2017. Theoretical analysis and model simulations suggest that the exceptionally weak SSC circulation was primarily caused by abnormal wind forcing around the equator, associated with extreme climate events. The strong equatorial easterly anomalies are mainly responsible for the weakening of the SSC circulation north of ∼6°N and contribute to about half of the weakening south of ∼6°N, by regulating the layer thickness along the eastern boundary, following the time-dependent Sverdrup relation. The large negative wind stress curl anomalies near the equator contribute to another half of the weakening south of ∼6°N. This research underscores the dynamic connection between the circulation in the western basin and equatorial dynamics during extreme climate modes. Significance Statement The dynamic linkage between the western basin circulation and the equatorial forcing remains elusive. Based on reanalysis data, theoretical analysis, and numerical simulations, this study has confirmed that the exceptionally weak southward Somali Current circulation during the winter of 1997 to spring 1998 was significantly influenced by abnormal wind patterns around the equator, triggered by extreme climate modes.

The contribution of Arabian Sea warming to decreasing summer precipitation in the northern Greater Mekong Subregion

The Greater Mekong Subregion (GMS) is one of the world's most important agricultural regions. Over recent decades, the declining trend in precipitation has caused more frequent droughts over the northern plateau of the GMS, and this has led to a significant reduction in agricultural productivity. These drought events can also affect agriculture within the middle and lower parts of the Mekong River basin. To reveal the main causes of the decline in precipitation, this study investigates interdecadal variations in summer precipitation over the northern GMS during 1979–2022. Results indicate that summer conditions over the northern GMS entered a relatively dry period after the late 1990s. The interdecadal decrease in summer precipitation is closely associated with the warming of the northern Arabian Sea (AS), which induces a cyclonic anomaly that enhances local precipitation and associated diabatic heating. The anomalous diabatic heating induces a downstream Rossby wave train that intensifies the circumglobal teleconnection (CGT) pattern and causes the South Asian high (SAH) to extend farther southeastwards. The southeastward-displaced SAH induces easterly anomalies over the northern Bay of Bengal (BOB) that weaken the Indian summer monsoon (ISM) southwesterly flows and reduce moisture transport from the northern BOB to the northern GMS. The southeastward displacement of the SAH also causes anomalous descent over the northern GMS. Both conditions result in reduced precipitation over the northern GMS. Numerical experiments substantiate the proposed modulating role of AS warming in the interdecadal decrease in summer precipitation over the northern GMS.

Negative Surface Chlorophyll Concentration Anomalies in the Southeast Arabian Sea During Summer in 2015 and 2019

AbstractSatellite observations revealed two extremely low surface chlorophyll concentration (SCC) events with a warm sea surface temperature anomaly in the southeastern Arabian Sea (SEAS, 6°–15°N, 72°–77°E) during the summer (July–August–September) in 2015 and 2019. We find that the physical processes leading to these two similar low SCC events are remarkably different. The low SCC in the SEAS during summer 2019 is mainly related to the weakened upwelling and deepening of the thermocline depth due to the combined effects of the local wind anomalies and the arrival of westward‐propagating downwelling coastal Kelvin wave driven by easterly anomalies near the eastern Sri Lanka during an extreme positive Indian Ocean Dipole (IOD) event. In summer 2015, a weaker positive IOD‐induced easterly anomalies in the southern Bay of Bengal also drives downwelling coastal Kelvin waves westward, deepening the thermocline in the SEAS. But unlike that in summer 2019, the local wind stress curl anomalies in the SEAS during summer 2015 favors upwelling, which counteracts the downward motion of the coastal Kelvin waves, leading to weaker downward transport (one‐third of that in 2019). Meanwhile, the upper ocean layer in the SEAS experiences extreme warming during summer owing to the development of 2015/2016 super El Niño. This substantial warming enhances upper oceanic stratification, which results in weaker vertical mixing and reduces the SCC to an extremely low level despite the much weaker IOD strength in 2015.

Different Modulations of Arctic Oscillation on Wintertime Sea Surface Temperature Anomalies in the Northeast Pacific

AbstractPersistent positive sea surface temperature anomalies (SSTAs) in the mid‐latitude Northeast Pacific (NEP), also known as “warm blob” or “marine heatwave,” have substantial ecological and climate effects. This study delves into the spatiotemporal connection between Arctic Oscillation (AO) and SSTAs in the NEP. First, we conduct the lead‐lag correlation and maximum covariance analyses to disentangle the closest temporal relationship between October AO and wintertime SSTAs in the NEP. Then, we categorize the years in positive AO (pAO) phase into two groups: positive AO with warm anomaly (pAO&Blob) group and positive AO without warm anomaly (pAO&noBlob) group based on the October AO index and wintertime blob index. Results show that the positive phase of AO in October strongly influences the wintertime warm SSTAs in the NEP through local and remote pathways. The local pathway is contingent upon the longitudinal positioning of AO‐related high‐pressure anomaly (i.e., anomalous ridge) in the North Pacific. When easterly anomalies prevail at the southern flank of the anomalous ridge over the NEP, they foster warm SSTAs in the NEP. However, different locations of the high‐pressure anomalies may impede the NEP warming. Remote pathways indicate the teleconnections triggered by AO‐related precipitation increase in Greenland and decrease in East Asia, sustaining the high‐pressure anomaly and promoting the anomalous NEP warming. Hence, this study presents new evidence on polar and mid‐latitude climate connections, which may provide potential predictability for the warm SSTAs in the NEP.

Mechanism of Coupled Upper Troposphere and Boundary Layer Induces Two Types of Short‐Term Heavy Rainfall Along the Eastern Foothills of the Taihang Mountains

AbstractUsing 11 years of hourly merged rainfall records and ERA reanalysis data, this paper reveals two major circulation modes leading to two types of short‐term heavy rainfall (STHR) along the Taihang Mountains' eastern foothills, and further explains their mechanisms. One circulation mode has a distinct warm anomaly at 300 hPa covering most areas of North China, together with the boundary‐layer westerly anomaly occurring in North China and its southern region (UTWA‐BLWA). UTWA‐BLWA effectively contributes to the reinforcement of the upper‐level divergence and the low‐level moisture convergence by promoting the strengthening of upper anticyclonic and low‐level southwesterly anomalies. The combined effects of low‐level jet (LLJ) and topographic uplift form the central‐northern STHR pattern. The other circulation structure has a 300‐hPa warm anomaly located to the southeastern Russia and a prominent 300‐hPa cold anomaly covering the south, together with the boundary‐layer easterly anomaly occurring over the whole region of eastern China (UTCA‐BLEA). The southern STHR pattern is attributed to the exit of the boundary‐layer jet (BLJ) over lower elevations due to moist transport and dynamic uplift associated with the easterly anomaly. The results indicate that the locations of STHR are related to the direction, intensity, and height of the LLJ. The findings highlight that the upper‐tropospheric temperature anomalies (UTTA) and boundary‐layer easterly flow jointly modulate heavy rainfall. Analysis of the coupled upper troposphere and boundary layer could help understand and forecast heavy rainfall.

Relationship between the SST diurnal cycle over the Tropical Western Pacific Ocean and subseasonal/seasonal oscillations: Associations with wind speed and outgoing longwave radiation

In this study, the relationship between SST diurnal cycle,10 m wind speed (W10) and Outgoing Longwave Radiation (OLR) is investigated. A wavelet spectral analysis was applied to SST hourly data to identify the SST diurnal cycle over the Tropical Western Pacific Ocean (TWPO). The SST diurnal cycle was identified as a prominent spectrum peak in waves with an oscillation period of 1 day. An inverse energy cascade hypothesis suggests that the energy from the SST diurnal cycle propagates and gets absorbed by waves within the subseasonal timescale. Three windows were selected to represent the diurnal (0–2 days), subseasonal (15–60 days), and seasonal (80–200 days) timescales. A wavelet-filtered analysis was performed in these windows, revealing inverse SST/ wind speed and direct SST/ OLR correlations over TWPO. These findings are consistent with empirical parametric models. Additionally, this study demonstrates the rectification mechanism of SST through a wavelet-filtered approach, identifying statistically significant correlations at the 5 % level within the diurnal window (0–2 days), particularly in the central tropical Pacific. Wavelet-filtered anomalies of SST, W10, and OLR along 50–160°E reveal the alternating dry and wet phase propagation across the Indo-Pacific in the subseasonal window, which is associated with the Madden-Julian Oscillation (MJO). Furthermore, westward propagating anomalies in the Indian Ocean and eastward propagating anomalies east of the Maritime Continent (MC) and within the Pacific were identified in the seasonal window, resembling patterns of Rossby and equatorial Kelvin waves, respectively.

Formation Mechanisms of the Decadal Indian Ocean Dipole Driven by Remote Forcing from the Tropical Pacific

Abstract On decadal time scales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls. Significance Statement The decadal Indian Ocean dipole (D-IOD) is one of the leading decadal modes of the tropical Indian Ocean SST variations. Its formation mechanisms, especially those related to its spatiotemporal characteristics, are not well understood. Based on observations and reanalysis, we show that the tropical Pacific decadal variability (TPDV) mainly accounts for the distinctive spatial pattern and seasonality of the D-IOD via the seasonal migration of the anomalous Walker circulation and ensuing oceanic subsurface dynamics. Our results highlight that the TPDV is an important source of D-IOD’s predictability, and it might be beneficial for operational decadal predictions for the tropical Indian Ocean.

Mechanisms of early and late summer precipitation in Southwest China: dynamic and thermodynamic processes

This study investigates dynamic and thermodynamic components of moisture flux convergence in Southwest China (SW-MFC) and their underlying physical mechanisms during early and late summer. Using precipitation observation and CRA-40 reanalysis datasets from 1979 to 2023, the results show that both dynamic and thermodynamic processes modulate the SW-MFC in early summer (May-June), with dynamics playing a pivotal role. In contrast, the precipitation anomaly in late summer (July-August) is predominantly driven by the dynamic factors. Meanwhile, the large-scale circulation over the northern Indian Peninsula significantly modulates the SW-MFC. In early summer, anomalous convection around the Maritime Continent with the tripole sea surface temperature (SST) mode in the tropical Indo-Pacific can trigger the formation of “double ring” vertical zonal circulation cells. A large-scale westerly anomaly at the lower troposphere over the northern Arabian Sea foster cyclone strengthening over the northern Indian Peninsula, enhancing southerly moisture transport and increasing precipitation over Southwest China. During the late summer, large-scale dipole SST pattern between the subtropical central-eastern Pacific and the Indo-Pacific warm pool generates significant easterly anomalies towards the Maritime Continent. The SST gradient stimulates an extensive anticyclonic shear zone over the western equatorial Pacific, with an intensified low-pressure zone to its north. This atmospheric pattern over Southwest China and Indian Peninsula can form a vertical circulation circle that largely intensifies widespread precipitation. Numerical model experiments can reproduce the mechanisms of tropical Indo-Pacific joint effects on the Southwest precipitation in both early and late summer, providing a theoretical basis for understanding and forecasting summer precipitation over Southwest China.

Future changes in South Asian summer monsoon circulation under global warming: role of the Tibetan Plateau latent heating

The South Asian summer monsoon (SASM) is a significant monsoon system that exerts a profound impact on climate and human livelihoods. According to 38 models from the Coupled Model Intercomparison Project Phase 6, the SASM circulation is projected to weaken significantly under global warming as seen in the weakened low-level westerly wind over the northern tropical Indian Ocean; however, the associated climate dynamics is still under debate. Here, we identify that the weakened low-level westerly wind is closely related to the enhanced latent heating over the Tibetan Plateau (TP), which corresponds with increased summer precipitation in the future. The intensified TP latent heating triggers an anomalous meridional circulation with ascending motions over the plateau and descending motions to the south, leading to an anomalous low-level anticyclone over the northern tropical Indian Ocean. This anticyclone greatly weakens the prevailing low-level westerlies of the SASM through easterly anomalies at the anticyclone’s southern flank. Moisture budget analysis further reveals that increased atmospheric water vapor, rather than the vertical dynamic component, makes the largest contribution to the increased precipitation over the TP. This result confirms that the enhanced TP latent heating is a driver of atmospheric circulation change and contributes to weakening the SASM circulation.

Dominant contribution of atmospheric nonlinearities to ENSO asymmetry and extreme El Niño events

Extreme El Niño events have outsized impacts and strongly contribute to the El Niño Southern Oscillation (ENSO) warm/cold phase asymmetries. There is currently no consensus on the respective importance of oceanic and atmospheric nonlinearities for those asymmetries. Here, we use atmospheric and oceanic general circulation models that reproduce ENSO asymmetries well to quantify the atmospheric nonlinearities contribution. The linear and nonlinear components of the wind stress response to Sea Surface Temperature (SST) anomalies are isolated using ensemble atmospheric experiments, and used to force oceanic experiments. The wind stress-SST nonlinearity is dominated by the deep atmospheric convective response to SST. This wind-stress nonlinearity contributes to ~ 40% of the peak amplitude of extreme El Niño events and ~ 55% of the prolonged eastern Pacific warming they generate until the following summer. This large contribution arises because nonlinearities consistently drive equatorial westerly anomalies, while the larger linear component is made less efficient by easterly anomalies in the western Pacific during fall and winter. Overall, wind-stress nonlinearities fully account for the eastern Pacific positive ENSO skewness. Our findings underscore the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events and, more generally, ENSO asymmetry.

Influence of upper-tropospheric cold anomalies on heavy rainfall along the Eastern Foothills of Taihang Mountains

Using hourly rain records and reanalysis data, the climatic composite analysis and two case studies of heavy rainfall along the eastern foothills of Taihang Mountains are used to examine the influence of the upper troposphere cold anomalies (UTCA) on heavy rainfall. Three UTCAs, i.e., the cold anomaly located upstream of North China (upNC), the cold anomaly dominating the Japan Sea (JS), and the cold anomaly near the East China Sea (ES) are revealed to play positive roles in the heavy rainfall. The upNC cold anomaly and JS cold anomaly both stretch southeastward from the upper to the lower troposphere. The upNC cold anomaly can trigger a negative geopotential height anomaly accompanied by cyclonic circulation anomaly in the upper troposphere to strengthen upper-layer divergence, and a negative geopotential height anomaly in the middle and lower troposphere to enhance the low-level extratropical cyclone (ETC), resulting in heavy rainfall. JS cold anomaly can enhance the downstream trough to prevent the eastward propagation of the upstream convective systems, and further, promote significant southeasterly flows in the northeast of ETC in the middle and lower troposphere with more moist air transported to the eastern foothills of Taihang Mountains. The two heavy rainfall events indicate that ES cold anomaly stretches northwestward to the lower troposphere with a smaller horizontal scale. ES cold anomaly can increase the precipitation by enhancing the easterly anomaly to its north, which supplies a large amount of water vapor from the Northwest Pacific and amplifies the uplift of topography. The evolutions of upNC, JS, and ES cold anomalies 12-h in advance are found to be closely connected to hourly precipitation, which could be precursor signals to provide helpful information in forecasting heavy rainfall.

Observed Extreme Freshening in the Central Andaman Sea Induced by Strong Positive Indian Ocean Dipole

AbstractThe Indian Ocean Dipole (IOD) has been extensively studied for its significant impact on salinity distribution in tropical regions. However, its off‐equatorial influence in the northeastern Indian Ocean (IO) has received limited documentation thus far. In October 2019, a buoy in the central Andaman Sea (AS) observed an extreme freshening event that resulted in intensified upper‐ocean stratification and increased internal wave activities. The salt budget evaluation revealed the dominant role of horizontal advection. Interannual variability in autumn and winter sea surface salinity (SSS) showed a significant correlation with the IOD. Extreme freshening was observed exclusively during strong positive IOD (pIOD) years. This freshening primarily resulted from the outflow of low‐salinity water from the northeastern coast of the AS, driven by an anomalous anti‐cyclonic coastal‐trapped circulation exclusive to strong pIODs in the northeastern IO. This circulation tends to hinder southward freshwater transport in the western region while enhancing it in the eastern region. This circulation pattern is primarily influenced by Kelvin wave forcing, which is triggered by robust equatorial easterly anomalies that are typically more pronounced during strong pIOD events and weaker during weak pIOD events. The anticipated increase in the frequency of extreme freshening events due to the greenhouse warming has the potential to significantly modify the salinity distribution and freshwater transport in the future.

Arctic Sea Ice Loss Inducing Northwest Extension of Summer Precipitation over the Tibetan Plateau

Abstract Summer precipitation over the Tibetan Plateau (TP) has experienced obvious changes in recent decades, with significantly increased trends observed over the northern and western regions during 1961–2020. Results indicate that there are two pathways linking them to the reduced Arctic sea ice from late spring (April and May) to early summer (June and July). First, decreased sea ice in late spring favors wet and dry soil over the eastern Caspian Sea and northeastern TP by stimulating anomalous wavelike patterns and modulating snow melting. Furthermore, the soil moisture anomalies during late spring could maintain to July because of memory effects, strengthen the midlatitude Silk Road pattern, and contribute to the formation of a dipole mode around TP, characterized by a west cyclone and an east anticyclone. Second, through exciting an anomalous Rossby wave directly spreading from the Arctic to TP, a similar dipole pattern on 500-hPa geopotential height could be associated with the decline of sea ice in early summer. Consequently, enhanced southwesterly winds shown with the west cyclone cause more water vapor input to the western TP, while easterly anomalies along with the east anticyclone inhibit climatological westerly winds and prevent water vapor from outputting. This dipole pattern finally brings more moisture accumulated in the western and northern TP and increases the precipitation. This study links heterogeneous changes in summer precipitation over TP to Arctic sea ice changes and provides insights for further investigation into whether the TP could serve as a bridge connecting the Arctic and tropical climates.

Variability of spring ecosystem water use efficiency in Northeast Asia and its linkage to the Polar-Eurasia pattern

Given that water use efficiency (WUE) is an important indicator to measure the trade-off between carbon uptake and water consumption within the ecosystem, better understanding the variation of ecosystem WUE and related driving factors is of great interest. In this study, the variability of spring ecosystem WUE in Northeast Asia (NEA) was investigated. The results show that its primary mode exhibits a monosign variation. This mode is directly controlled by the variability of gross primary productivity. The climate conditions also play remarkable roles, featuring that warm surface air temperature (high soil moisture) favors enhanced ecosystem WUE in northern (southern) NEA. Further analysis reveals that the Polar-Eurasia (POL) pattern can significantly impact the variability of spring ecosystem WUE in NEA through changing surface air temperature and soil moisture. When the POL pattern lies in the positive phase during spring, anticyclonic circulation anomalies with an equivalent barotropic structure prevail over northern NEA, concurrent with anomalous easterlies over southern NEA and a weakening of the East Asian jet (EAJ). Accordingly, anomalous downward motion is introduced over northern NEA, resulting in higher surface air temperature which is beneficial for the increase of local ecosystem WUE. Meanwhile, the easterly anomalies help to increase water vapor transport into southern NEA and the weakened EAJ can induce anomalous ascending over southern NEA, favoring the increase of precipitation and hence soil moisture, which consequently enhances the ecosystem WUE in southern NEA.

Role of the Australian High in Seasonal Phase Locking of the Indian Ocean Dipole

AbstractThis paper analyzes the effect of the Australian High (AH) on the seasonal phase locking of Indian Ocean Dipole (IOD) events. The anomalous strong AH associated with the positive phase of the Antarctic Oscillation can cause significant easterly wind anomalies and northward cross‐equatorial flow over the western Maritime Continent (MC) by strengthening the Australian winter monsoon during May–August. The AH‐associated easterly anomalies and northward cross‐equatorial flow can create thermodynamic air‐sea feedback and contribute to a significant cooling anomaly in the western MC and the tropical eastern Indian Ocean. Without considering the effect of ENSO, these processes contribute to the occurrence of positive IOD events, which begin in early summer, peak in late summer, and decay rapidly thereafter. The effect of ENSO can extend the peak period of IOD into the boreal autumn of that year. An anomalous weak AH corresponds to the occurrence and seasonal phase locking of negative IOD events. Through combined empirical orthogonal function analysis, we find that the effect of the AH can well explain the seasonal phase locking of 34 IOD events (40 in total), which provides an important theoretical basis for the prediction of IOD events.

Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October

AbstractThis study examines the diurnal to interannual variations of the stratiform cloud cover in May–October (1971–2019) from a 3‐hourly station database and from ERA5 reanalyses over western equatorial Africa (WEA). The main diurnal variations of the local‐scale fraction and genus of stratiform clouds are synthesized into three canonical diurnal types (i.e., “clear,” “clear afternoon,” “cloudy” days). The interannual variations of frequencies of the three diurnal types during the cloudiest months (JJAS) are mostly associated with two main mechanisms: a meridional shallow overturning cell associating more “cloudy” and less “clear” and “clear afternoon” days to anomalous southerlies below 900 hPa over and around WEA, anomalous ascent around 5°–7°N, anomalous northerlies between 875 and 700 hPa, and anomalous subsidence over the equatorial Atlantic. This circulation is strongly related to interannual variations of the equatorial Atlantic upwelling (i.e., more clouds when the upwelling is strong) associated with a meridional shift of the Intertropical Convergence Zone over the Tropical Atlantic and adjacent continents. The second mechanism operates mostly in the zonal direction and involves again the coupled ocean–atmosphere system over the equatorial Atlantic, but also the remote El Niño–Southern Oscillation (ENSO). An anomalously cold equatorial Atlantic drives increased low‐level westerlies toward the Congo Basin. Warm ENSO events promote broad warm and easterly anomalies in the middle and upper troposphere, which increase the local static stability, and thus the local stratiform cloud cover over WEA. The present study suggests new mechanisms responsible for interannual variations of stratiform clouds in WEA, thus providing avenues of future research regarding the stability of the stratiform cloud deck under the ongoing differential warming of tropical ocean and land masses.

Increased occurrences of consecutive La Niña events under global warming

Most El Niño events occur sporadically and peak in a single winter1–3, whereas La Niña tends to develop after an El Niño and last for two years or longer4–7. Relative to single-year La Niña, consecutive La Niña features meridionally broader easterly winds and hence a slower heat recharge of the equatorial Pacific6,7, enabling the cold anomalies to persist, exerting prolonged impacts on global climate, ecosystems and agriculture8–13. Future changes to multi-year-long La Niña events remain unknown. Here, using climate models under future greenhouse-gas forcings14, we find an increased frequency of consecutive La Niña ranging from 19 ± 11% in a low-emission scenario to 33 ± 13% in a high-emission scenario, supported by an inter-model consensus stronger in higher-emission scenarios. Under greenhouse warming, a mean-state warming maximum in the subtropical northeastern Pacific enhances the regional thermodynamic response to perturbations, generating anomalous easterlies that are further northward than in the twentieth century in response to El Niño warm anomalies. The sensitivity of the northward-broadened anomaly pattern is further increased by a warming maximum in the equatorial eastern Pacific. The slower heat recharge associated with the northward-broadened easterly anomalies facilitates the cold anomalies of the first-year La Niña to persist into a second-year La Niña. Thus, climate extremes as seen during historical consecutive La Niña episodes probably occur more frequently in the twenty-first century.

Decadal changes in the relationship between Arctic stratospheric ozone and sea surface temperatures in the North Pacific

Using multi-source observations and ERA-Interim reanalysis data, this study found that the relationship between Arctic stratospheric ozone in March and North Pacific sea surface temperature (SST) in April shows dramatic decadal changes, and it significantly weakens since the 2000s. By enhancing the stratospheric circulations, a decreased Arctic stratospheric ozone favors tropospheric positive Arctic Oscillation (+AO)-like anomaly during PRE eras (1979–1999), with positive geopotential height (GH) anomalies over high-latitude Asia. The stratospheric ozone anomaly also favors the Asian positive GH anomalies via modulating high cloud and its related longwave radiation. According to geostrophic wind relationships, the Asian positive GH anomalies are accompanied by easterly anomalies over midlatitude Asia in late March, which further extend eastward and thereby induce easterly anomalies over the midlatitude North Pacific. The North Pacific easterly anomalies result in negative North Pacific Oscillation (–NPO)-like anomaly via vorticity forcing of zonal wind and the interactions between synoptic-scale eddies and mean flow, forcing significant SST anomalies. However, during POST eras (2000–2019), firstly, the +AO-related positive GH anomalies over high-latitude Asia are relatively weak, accompanied by weak easterly anomalies over midlatitude Asia according to geostrophic wind relations, which induce weak easterly anomalies over the North Pacific. Secondly, a westerly anomaly occurs over the midlatitude North Pacific, which partly offsets the easterly anomalies from Asia, weakening the North Pacific easterly anomalies. The weak easterly anomalies further induce weak –NPO and SST anomalies during the POST eras. Our analysis further indicates that the North Pacific westerly anomalies and weakening of the positive GH anomalies over high-latitude Asia associated with ozone are related to the interference of the westerly phase of quasi-biennial oscillation and the weakened stratospheric ozone anomaly over high-latitude Asia, respectively.

Formation Mechanism of the ENSO-Independent Summer Western North Pacific Anomalous Anticyclone

Abstract The western North Pacific anomalous anticyclone (WNPAC) is the key circulation modulating the East Asian summer climate. In this study, the formation mechanism of the summer WNPAC that is independent of El Niño–Southern Oscillation (ENSO) is investigated. Although ENSO has a significant relationship with WNPAC, except for the super El Niño years, the WNPAC index remains almost unchanged after removing ENSO’s impact, suggesting the possibility of other origins of the WNPAC apart from ENSO. An Atlantic-to-Pacific two-step mechanism is proposed for the formation of ENSO-independent summer WNPAC. In boreal spring, diabatic heating induced by the positive sea surface temperature anomalies (SSTAs) over the tropical Atlantic could stimulate a stationary equivalent barotropic Rossby wave train that travels across the Eurasian continent and ends in the tropical North Pacific. At the end of the Rossby wave train, the lower-level anomalous anticyclone advects negative moist enthalpy into the equator, which suppresses the local convection over the tropical North Pacific and equatorial central Pacific, and thus triggers the lower-level equatorial easterly anomaly to its west. During boreal summer, the lower-level easterly anomaly leads to the zonal dipole SSTA pattern with a negative center in the tropical central Pacific and a positive one in the Maritime Continent. Then, this dipole SSTA pattern over the Pacific exerts a relaying effect that further reinforces and westward shifts the dipole convection anomaly pattern, generating the WNPAC as a Gill-type response. This study underpins the independent role of Atlantic oceanic forcing through the extratropical route in the formation of ENSO-independent summer WNPAC. Significance Statement By linearly removing ENSO’s impact on summer monthly sea surface temperature anomalies (SSTAs), this study explores the sources of summer western North Pacific anomalous anticyclone (WNPAC) apart from ENSO. It is found that the high correlation between WNPAC and ENSO largely depends on the super El Niño events, and the origin of the ENSO-independent WNPAC is rooted in the tropical Atlantic oceanic forcing. The positive tropical Atlantic SSTAs in spring can independently induce atmospheric teleconnection from the North Atlantic to the North Pacific through the extratropical route, which triggers anomalous easterlies over the equatorial Pacific via suppressed local convection caused by the dry air advection over the North Pacific. The equatorial easterlies enhance the dipole anomalous SST and convection pattern over the Maritime Continent and tropical western/central Pacific in summer, which finally stimulates the WNPAC. The result emphasizes the independent role of tropical Atlantic SSTAs in the formation of summer WNPAC, thus providing a new perspective for the seasonal prediction of East Asian summer climate.