Sea Surface Temperature Dipole Research Articles

Diverse Responses of Strong Positive SST and Rainfall Indian Ocean Dipole Events under Greenhouse Warming

Abstract Recent research has shown that there is a projected increase in the frequency of strong positive Indian Ocean dipole (spIOD) events in terms of both sea surface temperature (SST) and precipitation. However, it is not clear whether the spIOD events defined by SST and precipitation, hereafter referred to as SST-spIOD and Pr-spIOD events, respectively, will increase at the same pace under greenhouse warming. Using climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and CMIP6 that can reasonably simulate the spIOD events, here we find that the occurrence of Pr-spIOD events increases much more than that of SST-spIOD events (+217% vs +77%) under the future high-emission scenario. The diverse response stems from a notably large increase in the spIOD events with strong rainfall but moderate SST anomalies (Pr-only spIOD), which is facilitated by a change in the local meridional Hadley circulation induced by land–sea thermal contrast in addition to the background mean-state SST warming. For the spIOD events with prominent anomalies in both rainfall and SST (concurrent spIOD), their occurrences are projected to increase, but their average SST amplitude is projected to weaken. The increased occurrence is associated with a westward-extended structural change induced by mean linear zonal advection feedbacks, while the weakened event-average SST amplitude results from a more stabilized atmosphere and coincides with the projected reduction in IOD SST skewness. Our results suggest that IOD-related mitigation strategies should consider the diverse responses of different kinds of spIOD events to greenhouse warming.

Contribution of internal variability to the Mongolian Plateau summer precipitation trends in MPI-ESM large-ensemble model

Summer precipitation over the Mongolian Plateau (MP) has experienced a consistent decline in recent decades. While the influence of atmospheric wave train on this reduction in precipitation has been recognized in prior studies, this study delves deeper into the physical mechanisms and quantifies the contributions of the internal atmosphere and oceanic variations to the diminishing precipitation utilizing a comprehensive 100-member ensemble simulations from the Max Planck Institute Earth System Model (MPI-ESM). Results show that the ensemble-mean precipitation in MP exhibits a positive trend and cannot explain the observed results. The precipitation trends vary significantly among individual ensemble members, highlighting the pivotal role of internal variability. The leading EOF mode of precipitation trends among ensemble members exhibits uniform variations. Further investigations reveal that the internal summer precipitation in MP is affected by the internal atmospheric circulation, the remote influence of the North Atlantic Dipole sea surface temperature (SST) anomalies, and Pacific Decadal Oscillation-like SST patterns. An eastward-propagating Rossby wave originating from the North Atlantic dipole SST anomalies provides the anomalous large-scale circulation that influences summer precipitation. The PDO contributes to reinforcing the anticyclonic anomaly over the MP. Additionally, the uncertainty of precipitation trends in MPI-ESM can be reduced by 13% through removing the internal atmospheric wave train-related precipitation variation, while oceanic factors only contribute about 7% uncertainty of precipitation variations. Our insights enhance the understanding of the physical drivers behind summer precipitation variability in the MP and effectively quantify the uncertainties stemming from internal variability.

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

AbstractThis 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.

Impact of Pacific blocking on the intraseasonal winter sea ice seesaw between the Bering and Okhotsk Seas

The winter sea ice cover (SIC) over the Bering Sea (BS) and Okhotsk Sea (OS) shows significant co-variability. In particular, a clear seesaw-like variability can be observed on an intraseasonal scale between the two seas. However, the mechanism by which the atmospheric circulation affects them is unclear and further insight is needed. In this study, based on reanalysis data from 1979 to 2019, we use composite analyses to examine the physical processes of atmospheric blocking that affect the intraseasonal seesaw SIC changes between OS and BS. The investigation reveals that Pacific blocking (PB) events can enhance the existing intraseasonal seesaw variation of the OS and BS SIC in winter. Based on the maximum core position of the anticyclonic center, we divide PB events into two subgroups: western PB and eastern PB. In addition, considering average zonal shifting speed, we further classify them into three subgroups: westward shifting PB, quasi-stationary PB and eastward shifting PB. Composite results show that the changes of SIC and surface air temperature (SAT) show different asymmetric characteristics for the different PB subgroups. Of the five PB regimes, the quasi-stationary PB is associated with the greatest reduction in SIC in the BS over its lifetime, both in magnitude and duration. Furthermore, we find that the local sea surface temperature (SST) anomaly upstream of the quasi-stationary PB, both during and prior to its onset, exhibits a significant dipole mode, along with a positive anomaly in the 300-hPa zonal wind. The SST dipole could serve as a precursor climatic background condition, likely triggering the occurrence of quasi-stationary PB and subsequently leading to intense SIC changes.

Impact of Sea Surface Temperature in the Extratropical Southern Indian Ocean on Antarctic Sea Ice in Austral Spring

Abstract The relationship between the seasonal Antarctic sea ice concentration (SIC) variability and the extratropical southern Indian Ocean (SIO) sea surface temperature (SST) is explored in this study. It is found that the Antarctic SIC in a wide band of the SIO, Ross Sea, and Weddell Sea is significantly related to an SIO dipole (SIOD) SST anomaly on the interannual time scale during austral spring. This relationship is linearly independent of the effects of El Niño–Southern Oscillation, the Indian Ocean dipole, and the Southern Hemisphere annular mode. The positive phase of the SIOD, with warm SST anomalies off of western Australia and cold SST anomalies centered around 60°E in high latitudes, stimulates a downstream wave train that induces large-scale cyclonic circulations over the SIO and the Ross and Weddell Seas. Subsequently, anomalous horizontal moisture advection causes water vapor divergence, changes the surface energy budget, and cools the underlying ocean, which leads to the increased SIC over the region in the SIO, Ross Sea, and Weddell Sea. This SIOD SST anomaly reached a record low during the austral spring of 2016 and promoted the prominent wave pattern at high latitudes, contributing to the dramatic decline of sea ice in the 2016 spring. In addition, the proportion of the SIC trend that is linearly congruent with the SIOD SST trend during austral spring is quantified. The results indicate that the trend in the SIOD SST may account for a significant component of the 1979–2014 SIC trend in the Ross Sea with the congruency peaking at 60%.

Exploring non-linear modes of the subtropical Indian Ocean Dipole using autoencoder neural networks

The subtropical Indian Ocean Dipole (SIOD) significantly influences climate variability, predominantly within parts of the Southern Hemisphere. This study applies an autoencoder—a type of artificial neural network (ANN)—known for its ability to capture intricate non-linear relationships in data through the process of encoding and decoding—to analyze the spatiotemporal characteristics of the SIOD. The encoded SIOD pattern(s) is compared to the conventional definition of the SIOD, calculated as the sea surface temperature (SST) anomaly difference between the western and eastern subtropical Indian Ocean. The analysis reveals two encoded patterns consistent with the conventional SIOD structure, predominantly represented by the SST dipole pattern south of Madagascar and off Australia’s west coast. During different analysis periods, distinct variability in the global SST patterns associated with the SIOD was observed. This variability underscores the SIOD’s dynamic nature and the challenges of accurately defining modes of variability with limited records. One of the ANN patterns has a substantial congruence match of 0.92 with the conventional SIOD pattern, while the other represents an alternate non-linear pattern within the SIOD. This implies the potential existence of additional non-linear SIOD patterns in the subtropical Indian Ocean, complementing the traditional model. When global temperature and precipitation are regressed onto the ANN temporal patterns and the conventional SIOD index, both appear to be associated with anomalous climate conditions over parts of Australia, with several other consistent global impacts. Nevertheless, due to the non-linear nature of the ANN patterns, their effects on local temperature and precipitation vary across different regions as compared to the conventional SIOD index. This study highlights that while the conventional SIOD pattern is consistent with the ANN-derived SIOD pattern, the climate system’s complexity and non-linearity might require ANN modeling to advance our comprehension of climatic modes.

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

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

Prevalent atmospheric and oceanic signals of the unprecedented heatwaves over the Yangtze River Valley in July–August 2022

Unprecedented heatwaves attacked the Yangtze River valley (YRV) in July–August of 2022. The heatwave-favorable circulation pattern for the YRV is identified by using a self-organizing map. Result shows that the dominant daily circulation pattern driving heatwaves in the YRV is the East Asian anticyclonic pattern (EAAP), characterized by an anomalous deep anticyclone over the East Asian mid-latitudes. Remarkably, the unprecedented frequent occurrence of EAAP in 2022 contributes about 84.6% of the temperature anomalies in the 35 YRV hot days, which is the key dynamical factor for the heatwave in 2022. Physical mechanisms for the occurrence of EAAP are revealed on interannual and interdecadal time scales. On interannual time scale, the dipole sea surface temperature (SST) anomalies over the North Atlantic, characterized by warm and cold SST anomalies in the midlatitudes and subtropics, respectively, exert an important influence on the EAAP days. This dipole SST pattern is unprecedentedly strong in 2022. It facilitates the EAAP through the maintenance of the coupled British–Okhotsk Corridor pattern and the Silk Road pattern in the mid-latitudes. Additionally, the persistent cold SST anomalies in tropical Indian ocean and the central-eastern Pacific from the preceding spring to summer contribute to cooling of the tropical troposphere and induce easterly anomalies in the East Asian subtropics, thereby further promoting the occurrence of EAAP. On interdecadal time scale, the negative Pacific Decadal Oscillation may establish favorable circulation conditions for the occurrence of the EAAP through modulating the interdecadal Silk Road pattern. These findings improve our understanding of the physical mechanisms for driving extreme heatwaves in the YRV and have an implication for enhancing regional heatwave prediction.

Varying Relationship between La Niña and SST Anomalies in the Kuroshio and Adjacent Regions during Boreal Winter: Role of the East Asian Winter Monsoon

Abstract The winter sea surface temperature (SST) anomalies in the Kuroshio and adjacent regions (KAR), which greatly influence the East Asian–North Pacific–North American climate, are closely related to El Niño–Southern Oscillation (ENSO). This SST relationship between the KAR and the equatorial eastern-central Pacific is widely assumed to be symmetric between El Niño and La Niña. Compared to previous studies indicating the significant and strong KAR warming during El Niño winters, this study indicates weakly negative KAR SST anomalies in the composite analysis for all La Niña events. Positive winter KAR SST anomalies unexpectedly appear in approximately half of La Niña events, which counteract negative SST anomalies in the rest of La Niña events. Further analysis suggests that the impact of La Niña on KAR SST anomalies is modulated by the East Asian winter monsoon (EAWM) during early winter. The weaker-than-normal EAWM offsets the anomalous northeasterly winds in the KAR induced by La Niña and then reinforces the KAR warming through warm oceanic advection. As for strong EAWM, it enhances the northeasterly winds to the west of an anomalous Philippine Sea cyclone associated with La Niña, leading to KAR cooling with more latent heat flux loss from the ocean and anomalous cold oceanic advection. Additionally, when the EAWM is independent of ENSO and is associated with the western Pacific pattern, it also can exhibit a pronounced influence on the KAR SST anomalies via the major processes of surface latent flux and horizontal heat advection in the ocean, accompanied by a change in Kuroshio transport. Significance Statement The interannual variability of SST from Philippine Sea to the Kuroshio and adjacent regions is considered an important forecast factor of the eastern China climate. Previous studies have reported a dipole SST pattern with Philippine Sea cooling and warming in the Kuroshio and adjacent regions during El Niño winter is dominant, and simply regarded that the effect of La Niña on western North Pacific SST anomalies was a mirror image of that of El Niño events. Here, we have found two distinctive SST patterns in the western North Pacific during La Niña winter. One type is the dipole SST pattern, characterized by Philippine Sea warming and by cooling in the Kuroshio and adjacent regions. The other type is the monopole SST pattern with uniform warming expanding from the Philippine Sea to the Kuroshio and adjacent regions. The dipole (monopole) SST pattern in western North Pacific is modulated by La Niña and a strong (weak) EAWM. Analysis shows that the above two SST patterns during La Niña winter are equally important during 1950–2021. This study has identified the influence of the EAWM independent of ENSO on the SST anomalies in the Kuroshio and adjacent regions.

Southern Annular Mode: A New Factor Impacts the East African Short Rains after the Early 1990s

Abstract It is well known that the Indian Ocean dipole (IOD) and El Niño–Southern Oscillation (ENSO) are closely related to rainfall variability over East Africa. This study found that the preceding August–October (ASO) Southern Annular Mode (SAM) can influence East African rainfall from October to December [i.e., East African short rains (EASR)], with a strong and significant relationship during the period of 1992–2019 than 1971–91. This is likely a result of the changes in SAM variations. It is further revealed that, during 1992–2019, the ASO SAM, via air–sea interactions, can excite meridional dipole sea surface temperature anomalies (SSTA) and warm SSTA over the southern Indian and Atlantic Oceans, respectively. In contrast, during 1971–91, the SAM induces warm SSTA over the southern Indian Ocean. Further analysis indicated that these ASO SAM-related SSTA patterns persist through the following October–December and exert a contrasting influence on the EASR. Based on the possible mechanism, the enhanced influence of SAM on EASR appears to be achieved through significant meridional circulation in the Indian Ocean basin, with anomalous sinking motion in the extratropics and the rising motion in the tropical Maritime Continent (MC) region. Correspondingly, strong convection is enhanced over the MC, altering zonal vertical circulation. This process results in anomalous westerlies in the tropical Indian Ocean, which reduces the atmospheric moisture, weakens convection, and suppresses rainfall over the central-western Indian Ocean, including the study region. Further analysis suggests that the prediction skill of the EASR related to the SAM, ENSO, and IOD was enhanced during 1992–2019. Significance Statement Several studies have investigated the influence of tropical climate anomalies, such as El Niño–Southern Oscillation and the Indian Ocean dipole, on the rainfall variability over East Africa. The region continues to be plagued by numerous climate-related disasters, including flooding and drought, and the associated varying impacts have been detrimental. As such, there is a need to understand further the factors and associated physical processes that cause such events and improve forecasting accuracy. This research emphasizes the role of the Southern Annular Mode (SAM), finding a strengthened lagged relationship with East African rainfall in recent decades. This implies that changes in the SAM should be considered in understanding present and future rainfall variability in the region.

Impact of the Eurasian Wave Train on the Interannual Variability of Autumn Precipitation in the Central Region of China

AbstractAutumn precipitation in the central region of China (APCC) can considerably influence on agricultural production and human livelihood. With reanalysis data from 1979 to 2020, it revealed that APCC is significantly affected by the second mode of the meridional wind field at 200 hPa over the Eurasian continent on the interannual scale, which featured a quasi‐barotropic “+‐+” wave‐like pattern from Europe to China (EC pattern) in extratropical zone. When the EC pattern is in a positive phase, central China lags behind the downstream positive geopotential height (anticyclone) anomaly. In this context, the transport of moisture from the south and east is enhanced and the advancement of the southerly monsoon is blocked, providing sufficient water vapor to support the occurrence of APCC. Further diagnosis revealed that local ascending motion is strengthened owing to the warm advection induced by anomalous southerly and positive vorticity advection associated with the anomalous anticyclone over East Asia. Additionally, wave flux analysis and model simulation suggest that the EC pattern results from the propagation of stationary Rossby wave, which could be excited by dipole sea surface temperature anomalies in the North Atlantic. These discoveries will contribute to improving the predictability of APCC.

Weakening of decadal variation of Northern Hemisphere land monsoon rainfall under global warming

Over the past century, Northern Hemisphere (NH) land monsoon rainfall (NHLMR) experienced significant decadal to multidecadal variations, mainly driven by an east–west sea surface temperature (SST) contrast over the Pacific (EWPC) and an interhemispheric North Atlantic–South Indian Ocean SST dipole (NAID). However, how the NHLMR’s decadal variation would vary and whether the oceanic forcing could continue to drive it in a warming world remain unexplored. Here, by analyzing 24 Coupled Model Intercomparison Project Phase 6 (CMIP6) models’ historical simulations and future projections, we show that the leading mode of decadal NHLMR will retain its nearly-uniform spatial pattern and representation of the NHLMR’s intensity. In the future, the significant periodicities of decadal NHLMR are shortened as emissions levels increase. The intensity of decadal NHLMR variation will experience a comprehensive decline under various emission scenarios, which may link to the weakened intensity of NAID and EWPC. Although the relationship between EWPC and decadal NHLMR is slightly weakened in the future, EWPC will remain a primary driver while NAID is no longer. The significant historical correlation between NAID and NHLMR is mainly attributed to the influence of increased anthropogenic aerosols emission. However, the NAID-NHLMR linkage would no longer exist owing to the decline of anthropogenic aerosol emission in the future.

Characterizing Atlantic interhemispheric teleconnection established by South American monsoon in austral summer

This study aims to characterize the interhemispheric teleconnection pattern, which is established by the South America (SA) summer monsoon over the Atlantic Ocean during January and February, and referred it as the Atlantic symmetric pattern (ASP). The ASP is characterized using the leading mode of interannual sea surface temperature (SST) variability of the Southwest Atlantic Ocean, where strong convection-SST coupling occurs. The pattern is manifested as two anomalous cyclonic-anticyclonic-cyclonic circulation trains aligned meridionally over the Atlantic Ocean, with a distinct SST dipole of the Southwest Atlantic Ocean and a tripole of the North Atlantic Ocean. The interhemispheric wave trains of the ASP are excited as a Gill-type response to convective activity in the SA summer monsoon, as confirmed in linear baroclinic model. Complementing previous studies on observed interhemispheric connection in the Atlantic, our findings highlight the importance of characterizing the ASP and its role in linking the South Atlantic SST, the SA summer monsoon, and North Atlantic climate. Further research is warranted to explore the impacts of the ASP on the Northern Hemisphere and its interactions with other climatic modes.

Enhanced Influence of Tropical Pacific Sea Surface Temperature Anomalies on Spring Extreme Heat Events Over Mid‐High Latitude Eurasia

AbstractThis study presents an interdecadal enhancement in the interannual relationship between spring extreme heat events (EHEs) over mid‐high latitude Eurasia and winter sea surface temperature (SST) over the tropical central‐eastern Pacific (TECP) after the late 1990s. Comparative analyses of the two subperiods before and after the late 1990s show that the interdecadal changes in the winter TECP SST could be responsible for such changeable relationship. After the late 1990s, the winter TECP SST anomalies (SSTA) feature a central Pacific (CP)‐type pattern. The CP‐type SSTA cause an anticyclone anomaly over the North Atlantic through the Pacific‐North American‐like teleconnection, further leading to a dipole SST pattern over the tropical to mid‐latitude North Atlantic. The CP‐type SSTA and North Atlantic dipole SSTA persist well from winter to spring, and the North Atlantic dipole SSTA favor the propagation of the CP‐type SST‐excited Rossby wave train to Eurasia in spring. Therefore, the TCEP SSTA have a significant interannual relationship with the spring Eurasian atmospheric circulations and EHEs after the late 1990s. The above physical processes are further confirmed by the SSTA sensitivity numerical experiments. In contrast, before the late 1990s, the winter TECP SSTA show an eastern Pacific‐type pattern, which is related to weak and insignificant North Atlantic SSTA and spring Eurasian atmospheric circulations; consequently, the TCEP SSTA have a weak relationship with the EHEs variations during this period. The TCEP SSTA with one season leading can provide valuable information for the prediction of spring Eurasian EHEs variations in the recent two decades.

Interannual relationship between South Pacific meridional sea surface temperature dipole and rainfall anomalies over South China in late-spring to early-summer without ENSO impact

Interannual relationship between South Pacific meridional sea surface temperature dipole and rainfall anomalies over South China in late-spring to early-summer without ENSO impact

The Tibetan Plateau’s Far-Reaching Impacts on Arctic and Antarctic Climate: Seasonality and Pathways

Abstract As the highest and most extensive plateau in the world, the Tibetan Plateau (TP) has remarkable effects on global climate. Through coupled model sensitivity experiments with and without the TP, we show that the TP can affect the Arctic directly via orography-forced stationary waves, and influence the Antarctic indirectly via stationary waves forced by sea surface temperature (SST) in the Indian Ocean. These far-reaching impacts occur mainly in wintertime. The fast atmospheric processes play an important role; particularly, the midlatitude westerly flow, which is stronger and closer to the equator in winter, provides a favorable condition for the eastward and poleward energy propagation of the forced waves. In the Northern Hemisphere, removing the TP causes a wave train traveling from the Asian continent to the North America–Atlantic Ocean region, resulting in intensified westerlies and thus an enhancement of stratospheric polar vortex and Arctic cooling. The pathways are northeastward directly in the upper level due to the background westerlies, while they are eastward and then northeastward in the lower level, modulated by the winter monsoon. To the south, the TP perturbation causes an anomalous cross-equatorial flow, leading to an anomalous SST dipole pattern in the Indian Ocean in the austral winter; this generates stationary waves propagating energy southeastward from the tropical Indian Ocean to the Antarctic, resulting in a Rossby wave train circulating around the Antarctic. Our study identifies the seasonality and pathways of the TP affecting the polar regions, which may help to understand the role of the TP in the future climate changes in polar regions.

Chlorophyll-a variability in different zones of the Indian Ocean around Sri Lanka concerning monsoon patterns and sea surface temperature

This study aims to investigate the CHL-a variability in the entire oceanic area of Sri Lanka concerning different monsoon patterns, maritime zones, and the influence of SST and Indian Ocean Dipole (IOD) using the chlorophyll-a (CHL-a) and sea surface temperature (SST) data obtained through the Moderate Resolution Imaging Spectroradiometer (MODIS). Obtained data were analyzed for three depth zones (continental shelf, 1000 m & 2000 m), two legalized zones (territorial Sea & Exclusive Economic Zone - EEZ), and five different climatic regions around the Island. There were significantly different (p < 0.05) mean CHL-a and SST values concerning various depth zones, legalized zones, and climatic regions with the four different monsoon periods. By considering the entire study area, the highest mean CHL-a value (3.878 mg m−3) with the lowest mean SST value (22.74 °C) was observed during the southwest monsoon (SWM) in the continental shelf (<100 m) off Southern Sri Lanka. The lowest mean CHL-a (0.12 mg m−3) was found within the EEZ in the first inter-monsoon (IM-1). Mean CHL-a values were always reduced towards the open ocean as the continental shelf, territorial sea, 1000 m zone, 2000 m zone, and EEZ consecutively in all climatic regions which strongly indicates due to nutrients drained from terrestrial runoff and induced upwelling during the activated monsoon patterns. Within the demarcated upwelling zone off the Southern coast of Sri Lanka, upwelling was prominent during the SWM, and CHL-a in the upwelling zone showed a significantly (p<0.05) negative correlation with SST and IOD, and a significantly positive (p<0.05) correlation with wind speed.

Persistent Mode of February-to-March Precipitation over Southern China: Variation, Mechanism, and Prediction

Abstract Precipitation variations over southern China in February and March have profound influences on local agricultural activities. Therefore, the leading intermonthly variation mode of February-to-March precipitation over southern China is investigated in this study using the extended empirical orthogonal function (EEOF) method. The first leading EEOF (EEOF1) pattern shows a persistent precipitation anomaly from February to March over southern China. Mechanistic analysis indicates that EEOF1’s different phases are related to different factors. A dipole sea surface temperature (SST) pattern with warm anomalies in the South China Sea (SCS) and cold anomalies in the western tropical Pacific (WTP) can lead to negative EEOF1 by exciting an anomalous Philippine Sea cyclone. In contrast, positive EEOF1 is influenced jointly by dipole patterns of SST in the southeast Indian Ocean (SEIO)–WTP area and atmospheric circulation over Europe and West Asia. The individual influence of the SST and atmospheric dipole patterns cannot induce persistent precipitation anomalies over southern China from February to March. Furthermore, we check the prediction skill of the National Center for Atmospheric Research (NCEP) Climate Forecast System (CFSv2) for positive and negative EEOF1. The evaluation results show that CFSv2 can predict the relationship between negative EEOF1 and the SCS–WTP dipole SST pattern, consequently showing skillful prediction for negative EEOF1 at the 1-month lead, with a correct rate of approximately 60%. However, CFSv2 cannot reproduce the combined effect of the aforementioned atmospheric dipole pattern and the SEIO–WTP dipole SST pattern and is therefore unskillful for predicting positive EEOF1, with a correct rate of only approximately 36%.

Secular Changes of the Decadal Relationship Between the Northern Hemisphere Land Monsoon Rainfall and Sea Surface Temperature Over the Past Millennium in Climate Model Simulations

AbstractUnderstanding the decadal variability of Northern Hemisphere land monsoon rainfall (NHLMR) is crucial to social‐economic development. However, due to the temporal limitation of instrumental data, the decadal relationship between NHLMR and sea surface temperature (SST) remains uncertain. The extended El Niño–Southern Oscillation (XEN) SST index and the North Atlantic–south Indian Ocean dipole (NAID) SST index can be the predictors to help us understand the decadal relationship between NHLMR and SST. In this study, using the Community Earth System Model‐Last Millennium Ensemble (CESM‐LME) simulations, we find a significant and stable decadal correlation between the NHLMR and XEN in the all‐forcing (AF) (with natural and anthropogenic forcings, AF), control (internal variability only), and other external forcings run over the past millennium, which means that this simulated NHLMR‐XEN relationship is caused by internal variability. The AF experiments show that the decadal relationship between NHLMR and NAID is practically non‐existent until an abrupt and significant increase in volcanic eruptions at about 1700, which is also indicated by the Paleoclimate Modeling Intercomparison Project Phase 3 (Paleoclimate Modeling Intercomparison Project Phase 3) simulations. Using the volcanic forcing sensitivity experiments, we find that the successive strong volcanic eruptions in the Northern Hemisphere (NH) significantly strengthen the synchronous changes of NHLMR and NAID after 1700, with a periodicity of 20–40‐year for NHLMR and NAID. Specifically, successive NH eruptions weaken the north‐south hemispheric thermal contrast, contributing to the weakened NAID index. The cross‐equator flow is thus weakened, decreasing the NHLMR, which cause a resonant behavior of NHLMR and NAID.

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

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