Walker Circulation Research Articles

Changes in ENSO Modulation of the Distribution of Rapidly Intensifying Tropical Cyclones over the Western North Pacific in Boreal Autumn

Abstract This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.

Interannual impact of tropical southern Atlantic SST on surface air temperature over East Asia during boreal spring

Using reanalysis data and simulations, this study revealed a pronounced negative interannual relationship between tropical southern Atlantic (TSA) sea surface temperature (SST) and East Asian surface air temperature (SAT) during boreal spring (March–May). Results confirm that the March–May TSA–SST anomaly can be considered an independent tropical driver unrelated to El Niño–Southern Oscillation. A possible mechanism linking TSA–SST and East Asian spring SAT involves an atmospheric wave train, energy conversion, and potential vorticity (PV)–θ dynamics. The anomalous TSA–SST induces an anomalous Walker circulation, which initiates a wave train that extracts energy from a westerly jet and propagates toward East Asia. Subject to PV–θ dynamics, the East Asian PV anomaly embedded within this wave train leads to bowed isentropes and resultant notable anomalous East Asian SAT. In particular, the bootstrapping results suggest that TSA–SST anomaly can cause an approximately sevenfold increase in the occurrence probability of extreme East Asian spring SAT.

Identifying the Drivers of Caribbean Severe Weather Impacts

Severe weather impacts in the central Caribbean are quantified by an objective index of daily maximum wind and rainfall (W•R) in the area 16–19°N, 63–69°W over the period 1970–2021. The index, based on ERA5 hindcast assimilation of satellite and in situ data, peaks from the July to October season as high sea temperatures and weak wind shear promote tropical cyclogenesis. Climate forcing is studied by reducing the W•R index to seasonal values and regressing the time series onto reanalysis fields 10°S–25°N, 180°W–20°E. The outcome reflects Jul–Oct warming in the tropical Atlantic, cooling in the tropical east Pacific (cold tongue), decreased/increased convection over the Pacific/Atlantic, and tropical upper easterly winds. New findings emerge in the Mar–Jun season preceding higher W•R: reduced SW-cloud bands in the northeast Pacific, a convective trough over the equatorial Atlantic, and Caribbean cold-air outbreaks. The multivariate El Niño Southern Oscillation index correlates with Jul–Oct Caribbean W•R at 2-month lead time and shows growing influence. Composite analysis of the top-10 years identifies an anomalous Pacific–Atlantic Walker Circulation favoring higher Caribbean W•R. Salinity is below normal and heat flux is downward across the Atlantic. Anomalous low-level airflow inhibits upwelling in the SW Caribbean, deepening atmospheric moisture. A leading case (TC Fiona 2022) demonstrates the environmental conditions underpinning storm intensification. The key drivers of severe weather impacts yield guidance in strategic planning, risk management and disaster preparedness. New insights are gained from a localized index of severe weather.

The variability of winter North Atlantic blocking and cold extremes connected to North Atlantic Sea surface temperature modes and ENSO: The mediated role of North Atlantic jet

In this paper, the linkage of the sub-seasonal variability of North Atlantic blocking (NAB) to the interannual-to-decadal winter midlatitude North Atlantic jet (NAJ) is examined. The variability of NAJ in meridional position (strength) is characterized by the first (second) empirical orthogonal function (EOF) mode of the winter-mean 46-days low-pass filtered North Atlantic 500-hPa zonal wind. The composite reveals that NAB exhibits weak (strong) westward movement and small (large) zonal width during the north- (south-) shifted NAJ winters, which result in severe cold anomalies over the southern Europe (northern Eurasia and eastern North America) and are characterized by a precursor of the positive (negative) North Atlantic Oscillation. In contrast, the NAB event during the strong (weak) NAJ winters mainly originates from the intensification of a ridge over North Europe (Atlantic), which has short (long) lifetime, small (large) intensity, small (large) zonal width, and leads to weather influences similar to those during south (north-) shifted NAJ winters. The underlying mechanism of the sub-seasonal variability of NAB can be explained by the nonlinear multi-scale interaction model of atmospheric blocking from the perspective of the meridional potential vorticity gradient.It is further shown that the meridional position of NAJ exhibits significant interannual variability, which is associated with the El Nino/Southern Oscillation (ENSO) via the changes in the Walker Cell and the Atlantic Hadley circulation as well as the second EOF of surface sea temperature (SST) over the North Atlantic. The strength of NAJ shows significant interannual-to-decadal variability, which is mainly regulated by the first EOF mode of North Atlantic SST.

The strength, position, and width changes of the intertropical convergence zone since the Last Glacial Maximum.

The intertropical convergence zone (ITCZ) plays a key role in regulating tropical hydroclimate and global water cycle through changes in its convection strength, latitudinal position, and width. The long-term variability of the ITCZ, along with the corresponding driving mechanisms, however, remains obscure, mainly because it is difficult to separate different ITCZ variables in paleoclimate proxy records. Here, we report a speleothem oxygen isotope (δ18O) record from southwestern Sulawesi, Indonesia, and compile it with other speleothem records from the Maritime Continent. Using the spatial gradient of speleothem δ18O along a transect across the ITCZ, we constrain ITCZ variabilities over the Maritime Continent during the past 30,000 y. We find that ITCZ convection strength overall intensified from the last glacial period to the Holocene, following changes in climate boundary conditions. The mean position of the regional ITCZ has moved latitudinally no more than 3° in the past 30,000 y, consistent with the deduction from the atmospheric energy framework. However, different from modern observations and model simulations for future warming, the ITCZ appeared narrower during both the late Holocene and most part of the last glacial period, and its expansion occurred during Heinrich stadials and the early-to-mid Holocene. We also find that during the last glacial and deglacial period, prominent millennial-scale ITCZ changes were closely tied to the variability of the Atlantic meridional overturning circulation (AMOC), whereas during the Holocene, they were predominantly modulated by the long-term variability of the Walker circulation.

Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering

Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

Zonal Indian Ocean Variability Drives Millennial‐Scale Precipitation Changes in Northern Madagascar

AbstractThe low latitude Indian Ocean is warming faster than other tropical basins, and its interannual climate variability is projected to become more extreme under future emissions scenarios with substantial impacts on developing Indian Ocean rim countries. Therefore, it has become increasingly important to understand the drivers of regional precipitation in a changing climate. Here we present a new speleothem record from Anjohibe, a cave in northwest (NW) Madagascar well situated to record past changes in the Intertropical Convergence Zone (ITCZ). U‐Th ages date speleothem growth from 27 to 14 ka. δ18O, δ13C, and trace metal proxies reconstruct drier conditions during Heinrich Stadials 1 and 2, and wetter conditions during the Last Glacial Maximum and Bølling–Allerød. This is surprising considering hypotheses arguing for southward (northward) ITCZ shifts during North Atlantic cooling (warming) events, which would be expected to result in wetter (drier) conditions at Anjohibe in the Southern Hemisphere tropics. The reconstructed Indian Ocean zonal (west‐east) sea surface temperature (SST) gradient is in close agreement with hydroclimate proxies in NW Madagascar, with periods of increased precipitation correlating with relatively warmer conditions in the western Indian Ocean and cooler conditions in the eastern Indian Ocean. Such gradients could drive long‐term shifts in the strength of the Walker circulation with widespread effects on hydroclimate across East Africa. These results suggest that during abrupt millennial‐scale climate changes, it is not meridional ITCZ shifts, but the tropical Indian Ocean SST gradient and Walker circulation driving East African hydroclimate variability.

Frequent but Predictable Droughts in East Africa Driven by a Walker Circulation Intensification

AbstractDuring and after recent La Niña events, the decline of the eastern East African (EA) March‐April‐May (MAM) rains has set the stage for life‐threatening sequential October‐November‐December (OND) and MAM droughts. The MAM 2022 drought was the driest on record, preceded by three poor rainy seasons, and followed by widespread starvation. Connecting these dry seasons is an interaction between La Niña and climate change. This interaction provides important opportunities for long‐lead prediction and proactive disaster risk management, but needs exploration. Here, for the first time, we use observations, reanalyses, and climate change simulations to show that post‐1997 OND La Niña events are robust precursors of: (a) strong MAM “Western V sea surface temperature Gradients” in the Pacific, which (b) help produce large increases in moisture convergence and atmospheric heating near Indonesia, which in turn produce (c) regional shifts in moisture transports and vertical velocities, which (d) help explain the increased frequency of dry EA MAM rainy seasons. We also show that, at 20‐year time scales, increases in atmospheric heating in the Indo‐Pacific Warm Pool region are attributable to warming Western V SST, which is in turn largely attributable to climate change. As energy builds up in the oceans and atmosphere, during and after La Niña events, we see stronger heating and heat convergence over warm tropical waters near Indonesia. The result of this causal chain is that increased Warm Pool atmospheric heating and moisture convergence sets the stage for dangerous sequential droughts in EA. These factors link EA drying to a stronger Walker Circulation and explain the predictable risks associated with recent La Niña events.

Decadal Modulation of the Relationship Between Tropical Southern Atlantic SST and Subsequent ENSO by Pacific Decadal Oscillation

AbstractThis study identifies the relationship between tropical southern Atlantic (TSA) sea surface temperature anomaly (SSTA) and the El Niño‐Southern Oscillation (ENSO) and focuses on how the Pacific Decadal Oscillation (PDO) modulates this relationship. Results suggest a significant but non‐stationary interannual TSA‐ENSO relationship which undergoes a significant decadal shift. A strong TSA‐ENSO relationship is observed during the positive PDO phase, while this relationship is weak during the negative PDO phase. Two processes, involving the anomalous Pacific Walker circulation (PWC) and the intensity of air‐sea interactions over the Pacific, are proposed for this decadal shift. During the positive PDO phase, the weak and variable PWC and strong air‐sea interaction facilitate the development of SSTA in the tropical Pacific triggered by TSA SSTA, resulting in a strong TSA‐ENSO relationship and vice versa. These findings emphasize the important role of the modulation of PDO on the TSA‐ENSO relationship.

Spatiotemporal Intertropical Convergence Zone dynamics during the last 3 millennia in northeastern Brazil and related impacts in modern human history

Abstract. Changes in tropical precipitation over the past millennia have usually been associated with latitudinal displacements of the Intertropical Convergence Zone (ITCZ). Recent studies provide new evidence that contraction and expansion of the tropical rain belt may also have contributed to ITCZ variability on centennial timescales. Over tropical South America few records point to a similar interpretation, which prevents a clear diagnosis of ITCZ changes in the region. In order to improve our understanding of equatorial rain belt variability, our study presents a reconstruction of precipitation for the last 3200 years from the northeastern Brazil (NEB) region, an area solely influenced by ITCZ precipitation. We analyze oxygen isotopes in speleothems that serve as a faithful proxy for the past location of the southern margin of the ITCZ. Our results, in comparison with other ITCZ proxies, indicate that the range of seasonal migration, contraction, and expansion of the ITCZ was not symmetrical around the Equator on secular and multidecadal timescales. A new NEB ITCZ pattern emerges based on the comparison between two distinct proxies that characterize the ITCZ behavior during the last 2500 years, with an ITCZ zonal pattern between NEB and the eastern Amazon. In NEB, the period related to the Medieval Climate Anomaly (MCA – 950 to 1250 CE) was characterized by an abrupt transition from wet to dry conditions. These drier conditions persisted until the onset of the period corresponding to the Little Ice Age (LIA) in 1560 CE, representing the longest dry period over the last 3200 years in NEB. The ITCZ was apparently forced by teleconnections between Atlantic and Pacific that controlled the position, intensity, and extent of the Walker cell over South America, changing the zonal ITCZ characteristics, while sea surface temperature changes in both the Pacific and Atlantic stretched or weakened the ITCZ-related rainfall meridionally over NEB. Wetter conditions started around 1500 CE in NEB. During the last 500 years, our speleothems document the occurrence of some of the strongest drought events over the last centuries, which drastically affected population and environment of NEB during the Portuguese colonial period. The historical droughts were able to affect the karst system and led to significant impacts over the entire NEB region.

Causes of the Unprecedented Month‐To‐Month Persistent Extreme Heat Event Over South China in Early Summer 2020: Role of Sea Surface Temperature Anomalies in the Tropical Indo‐Pacific Region

AbstractAn unprecedented month‐to‐month persistent extreme heat event attacked South China in early summer (May–July) 2020, with an extremely and persistently stronger‐than‐normal intensity and higher‐than‐normal number of extreme heat days. The underlying mechanisms were investigated from the month‐to‐month perspective using observations, reanalysis data, and numerical experiments. The results showed that a local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea directly contributed to this persistent extreme heat event via anomalous warm advection heating and diabatic warming at near‐surface level. Further investigation revealed that the rapid transition from El Niño to La Niña with subsequent development in the central and eastern tropical Pacific, along with persistent warming in the tropical Indian Ocean in the early summer of 2020, were crucial to the maintenance and reinforcement of the local anomalous anticyclone, with greater importance of the latter. On the one hand, rapid cooling in the central and eastern tropical Pacific enhanced the Walker circulation and further triggered an abnormal local Hadley cell, leading to anomalous subsidence over South China and the South China Sea. On the other hand, the persistent warming in the tropical Indian Ocean resulted in an enhanced and westward‐extended western North Pacific subtropical high via eastward‐propagating Kelvin waves and an intensified and eastward‐extended South Asian high by heating the troposphere. These two aspects of atmospheric circulation configuration favored the maintenance and reinforcement of the local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea.

Drivers of δ18O Variability Preserved in Ice Cores From Earth's Highest Tropical Mountain

AbstractIn 2019, four ice cores were recovered from the world's highest tropical mountain, Nevado Huascarán (Cordillera Blanca, Peru; 9.11°S, 77.61°W). Composite hydroclimate records of the two Col cores (6,050 masl) and the two Summit cores (6,768 masl) are compared to gridded gauge‐analysis and reanalysis climate data for the most recent 60‐year. Spatiotemporal correlation analyses suggest that the ice core oxygen stable isotope (δ18O) record largely reflects tropical Pacific climate variability, particularly in the NINO3.4 region. By extension, the δ18O record is strongly related to rainfall over the Amazon Basin, as teleconnections between the El Niño Southern Oscillation and hydrological behavior are the main drivers of the fractionation of water isotopes. However, on a local scale, modulation of the stable water isotopes appears to be more closely governed by upper atmospheric temperatures than by rainfall amount. Over the last 60 years, the statistical significance of the climate/δ18O relationship has been increasing contemporaneously with the atmospheric and oceanic warming rates and shifts in the Walker circulation. Isotopic records from the Summit appear to be more sensitive to large‐scale temperature changes than the records from the Col. These results may have substantial implications for modeling studies of the behavior of water isotopes at high elevations in the tropical Andes.

Exploring dominant processes for multi-month predictability of western Pacific precipitation using deep learning

Over the past half-century, there has been an increasing trend in the magnitude and duration of the Madden-Julian Oscillation (MJO) attributable to the significant warming trend in the Western Pacific (WP). The MJO, bridging weather and climate, influences global and regional climate through atmospheric teleconnections, and climate models can predict it for up to 4–5 weeks. In this study, we use deep learning (DL) methods to investigate the predictability of the MJO-related western Pacific precipitation on a multi-month time scale (5–9 weeks). We examine numerous potential predictors across the tropics, selected based on major MJO theories and mechanisms, to identify key factors for long-term MJO prediction. Our results show that DL-based useful potential predictability of the WP precipitation can be extended up to 6–7 weeks, with a correlation coefficient skill ranging from 0.60 to 0.65. Observational and heat map analysis suggest that cooling anomalies in the central Pacific play a crucial role in enhancing westerly anomalies over the Indian Ocean and warming in the WP, thereby strengthening the Walker circulation in the equatorial Pacific. In addition, the predictability of WP precipitation is higher in La Nina years than in El Nino or normal years, suggesting that mean cooling in the central Pacific may contribute to increased predictability of the MJO-related WP precipitation on the multi-month time scale. Additional model experiments using observed sea surface temperature (SST) anomalies over the central Pacific confirmed that these anomalies contribute to enhanced MJO-related convective anomalies over the WP. The study highlights that DL is a valuable tool not only for improving MJO-related WP prediction but also for efficiently exploring potential mechanisms linked to long-term predictability.

Aridification of Northwest Australia and Nutrient Decline in the Timor Sea During the 40 Kyr World

AbstractStudying tropical hydroclimate and productivity change in the past is critical for understanding global climate dynamics. Northwest Australia is an ideal location for investigating Australian monsoon dynamics, the variability of the Indonesian Throughflow (ITF), and their impact on past productivity and Pacific warm pool evolution, which remain poorly understood during the 40 kyr world in the mid‐early Pleistocene. In this study, we present multi‐proxy records from International Ocean Discovery Program (IODP) Site U1483 in the Timor Sea spanning the last 2,000 ka, including orbitally‐resolved records from the 40 kyr world between 2,000 and 1,300 ka. Our results suggest that northwest Australia underwent a step of increased aridification and that productivity in the Timor Sea declined during the transition from ∼1,700 to ∼1,400 ka. We attribute this aridification to the reduced moisture supply to this region caused by the ITF restriction and warm pool contraction. We ascribe the declined productivity to a decrease in the nutrient supply of the Pacific source water associated with global nutrient redistribution. At orbital timescale, multiple mechanisms, including sea level changes, monsoon, and the Intertropical Convergence Zone (ITCZ) dynamics, and variations in the ITF and Walker circulation could have controlled variations of productivity and terrigenous input in the Timor Sea during the 40 kyr world. Our bulk nitrogen and benthic carbon isotope records suggest a strong coupling to biogeochemical changes in the Pacific during this period. This research contributes to a better understanding of tropical hydroclimate and productivity changes during the 40 kyr world.

Impacts of the North Atlantic biases on the upper troposphere/lower stratosphere over the extratropical North Pacific

The winter upper troposphere/lower stratosphere temperature/vertical motion response over the extratropical North Pacific induced by North Atlantic changes is not well understood. Here, using robust diagnostic calculations conducted in a fully coupled high-resolution climate model, we correct the North Atlantic ocean circulation biases and show that during wintertime, the North Atlantic cold surface temperature biases lead to a warmer upper troposphere/lower stratosphere over the extratropical North Pacific. In the upper troposphere/lower stratosphere over the extratropical North Pacific, this winter warming temperature response is linked to the vertical motion response through a simple leading order thermodynamic relationship between changes in the horizontal advection and adiabatic heating. The upper troposphere/lower stratosphere vertical motion response, which is also associated with the North Atlantic induced Walker circulation response over the tropical North Pacific, can provide a rough estimation of the upper troposphere/lower stratosphere warming response over the extratropical North Pacific.

Glacial sea level low-stands regulated the upper ocean hydrology in the southern South China Sea over the past ∼ 280 kyr

The southern South China Sea (SCS) is an ideal region for investigating the low-latitude climatic and oceanic dynamics in the context of local and global effects. However, records of upper-water hydrography beyond the last two glacial cycles are still limited from the southern SCS. In this study, we present shell δ18O, Mg/Ca and Nd/Ca records of three planktonic foraminiferal species (Globigerinoides ruber, Globigerinoides sacculifer and Pulleniatina obliquiloculata) from Core TX05 to explore changes in upper-water hydrography in the southern SCS over the past ∼280 kyr. The results show that shell δ18O of the three species exhibit more positive values in glacials than interglacials. The δ18O residual (i.e. δ18Osw-ice) calculated by shell δ18O subtracting the signals of ice volume and temperature suggests fresher upper seawaters (lower salinity) during glacials than interglacials. This may be caused by terrestrial freshwater discharge from the paleo-Sunda continent that exposed during glacial sea level low-stands. The freshwater discharge may further be ascribed to relatively abundant precipitation over the paleo-Sunda continent during glacials, possibly pointing to the collective effects of the southward shift of the Intertropical Convergence Zone and the enhanced Pacific Walker circulation. The vertical thermal gradients (ΔTrub-obl and ΔTsac-obl) at Site TX05 show lower values during glacials than interglacials. The SST differences of the southern SCS from the northern SCS (ΔSSTS-N) and from the western tropical Pacific (ΔSSTSCS-WTP) correlate well with indicators from the Chinese Loess section, indicating relatively strong East Asian winter monsoon during glacials. The good correlation of ΔT, ΔSSTS-N and ΔSSTSCS-WTP to the relative sea-level suggests a remarkable effect of sea level low-stands and the strengthened East Asian winter monsoon during glacials on the upper ocean hydrography in the southern SCS.

Pleistocene Sunda Shelf submersion-exposure cycles initiate vegetation Walker Circulation feedback

Abstract Recent research has found that the subsiding Sunda Shelf (Southeast Asia) was permanently exposed prior to ca. 400 ka with initial submersion-exposure cyclicity, associated with interglacial-glacial sea-level cycles, beginning between 400 and 240 ka. We analyzed the impact submersion-exposure cycles on regional environment and climate through a 640 k.y. leaf-wax carbon isotope (δ13Cwax) reconstruction at Andaman Sea Site U1448, representing relative changes in C3/C4 plant abundances. Prior to ca. 250 ka, the Sunda region was inhabited by a stable C3 (forest) biome, after which submersion-exposure cycles initiated with the deglacial sea-level rise at ca. 250 ka. During subsequent glacial-age sea-level drops, the newly exposed shelf was rapidly colonized by C4 grasses, followed by slow transitions back to C3 forests, representing a tenfold increase in the variability of C3/C4 vegetation in the Sunda region. The C3/C4 regime shift since 250 ka is coherent across the Southeast (SE) Asia peninsula and Sunda Shelf and is coincident with a shift in the east-west sea-surface temperature gradient in the equatorial Pacific Ocean. We hypothesize that the expansion of C4 grasslands promoted and sustained drier glacial-age climates over SE Asia via a feedback mechanism that contributed to weakening the ascending branch of the east-west atmospheric circulation in the equatorial Pacific region known as the Walker Circulation. Our results indicate that the Sunda Shelf region has a larger influence on Walker Circulation than is seen in current paleoclimate simulations.

Forced changes in the Pacific Walker circulation over the past millennium

The Pacific Walker circulation (PWC) has an outsized influence on weather and climate worldwide. Yet the PWC response to external forcings is unclear1,2, with empirical data and model simulations often disagreeing on the magnitude and sign of these responses3. Most climate models predict that the PWC will ultimately weaken in response to global warming4. However, the PWC strengthened from 1992 to 2011, suggesting a significant role for anthropogenic and/or volcanic aerosol forcing5, or internal variability. Here we use a new annually resolved, multi-method, palaeoproxy-derived PWC reconstruction ensemble (1200–2000) to show that the 1992–2011 PWC strengthening is anomalous but not unprecedented in the context of the past 800 years. The 1992–2011 PWC strengthening was unlikely to have been a consequence of volcanic forcing and may therefore have resulted from anthropogenic aerosol forcing or natural variability. We find no significant industrial-era (1850–2000) PWC trend, contrasting the PWC weakening simulated by most climate models3. However, an industrial-era shift to lower-frequency variability suggests a subtle anthropogenic influence. The reconstruction also suggests that volcanic eruptions trigger El Niño-like PWC weakening, similar to the response simulated by climate models.

Influence of ENSO on Tropospheric Ozone Variability in East Asia

AbstractAbundance of tropospheric ozone is determined by the interplay among emissions, chemistry, and climate dependent transport. It is important to identify the essential factors that modulate ozone in China and Korea because of increasing ozone trends in this area. In this study, we investigated the correlations of tropospheric ozone in East Asia with El Niño Southern Oscillations (ENSO). The model simulated ozone from 2000 to 2020 with Community Atmosphere Model version 6 with chemistry (CAM6‐chem), ECMWF Atmospheric Composition Reanalysis data, and ozone profiles measured in South Korea, Japan, and Hong Kong. These data were utilized to understand the ozone–ENSO relationship in this region. We found that tropospheric ozone in East Asia is significantly correlated with the wintertime Niño 3.4 index in El Niño/La Niña developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Niño 3.4 index. Ozone concentrations in China and the Korean Peninsula are ∼5 ppb higher during El Niño developing summer compared to La Niña in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Niño/La Niña decaying spring, the opposite occurs in the same region. In the decaying spring, stronger variability was observed in the subtropical regions of the lower to mid troposphere, indicating higher ozone concentrations during El Niño compared to La Niña. This variability is associated with the fluctuations in the Walker circulation, large‐scale sinking/rising motions, and the accompanying biomass burning events. The model reproduced the ozone variability associated with ENSO in the ozonesonde observations.

Early onset of heavy rainfall on the northern coast of Ecuador in the aftermath of El Niño 2015/2016

In January 2016, a high-precipitation event (HPE) affected northwestern Ecuador, leading to devastating flooding in the Esmeraldas River Basin. The HPE appeared in the aftermath of the 2015/16 El Niño as an early onset of heavy rainfalls, normally expected in the peak rainy season between March and April. We investigate the local HPE atmospheric setting and the regional “weather-within-climate” characteristics of the growing-season rainfall between December and January using gauge data, satellite imagery, and reanalysis. The unusual convective environment in late January 2016 involved local and synoptic drivers leading the development of a mesoscale convective complex (MCC) during the nighttime of 24th January. The genesis of the MCC was related to an early-arriving thermal weather state and orographic lifting; the Andean ranges acted as both a channel boosting upslope flow and convective updrafts and as a heavy rain divide for inner valleys. The synoptic controls were associated with 1) a southern boundary of the inter-tropical convergence zone, abnormally displaced to 4°N as response to the 2015/16 El Niño where eastward air surges merge upward vertical mass fluxes; 2) the arrival to the Ecuadorian coast of an equatorially propagating Kelvin wave; and 3) a low-mid level moisture influx coming from the Amazon associated with ascent due to changes in the Walker circulation. Lastly, we suggest that the convective environment in late January was also favored by cross-time-scale interference of the very strong El Niño event and a strong and persistent Madden-Julian oscillation (MJO) in the central Pacific.