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Near‐equatorial zonal currents along 80.5°E in the upper Indian Ocean are key to tropical inter‐basin hydrographic exchanges. However, their meridional‐width variability and underlying causes have remained elusive. In this study, we use satellite‐derived ocean current data to reexamine the latitudinal structure of Wyrtki jets (WJs), Monsoon Circulation (MC), and South Equatorial Countercurrent (SECC) along 80.5°E on seasonal to interannual timescales. The WJs are characterized by a transient semiannual feature within the latitudinal range of 2.5°S–3°N, whereas the MC and the SECC have a dominant annual period within the latitudinal range of 3°–5.5°N and 2.5°–6°S, respectively. Empirical orthogonal function (EOF) analysis separates the seasonal latitudinal boundaries of the WJs, the MC, and the SECC, and reveals their interannual variability in response to anomalous wind forcing. EOFs1–3 represent the WJ, MC, and SECC modes accounting for 54%, 19%, and 14% of the total variance, respectively. The near‐equatorial zonal currents are asymmetrically responsive to the Indian Ocean Dipole (IOD), with an abnormally strong westward current resulting from the latitudinal superposition of the WJ and SECC modes in the autumn of 1997, 2006, and 2015 during strong positive IOD events. However, the abnormally enhanced eastward WJs occurred only during weak IOD events in the autumn of 1995, 1999, and 2005, accompanied by latitudinal destructive interference of strong westward SECC. Model sensitivity experiments show that the asymmetric response of near‐equatorial zonal currents to climate modes is closely related to the spatial differences in large‐scale wind anomalies associated with wind‐driven and eastern‐boundary‐generated waves. Plain Language Summary: This study examines and quantifies the meridional‐width variability of near‐equatorial zonal currents on seasonal to interannual timescales and diagnoses the associated forcing scenarios by using remote observations, ocean reanalysis, and a continuously stratified linear longwave ocean model. The frequency and phase decompositions reveal the seasonal latitudinal boundaries of the Wyrtki jets, Monsoon Circulation, and South Equatorial Countercurrent along 80.5°E. The interannual variability of the near‐equatorial zonal currents show an asymmetric response to the mature phases of strong positive and negative Indian Ocean Dipole events. The underlying mechanisms of these asymmetric processes are related to the spatial differences in the large‐scale wind anomalies and the associated wind‐driven and eastern‐boundary‐generated waves. Key Points: Frequency and phase decompositions of near‐equatorial zonal currents show the seasonal latitudinal boundaries of WJs, MC, and SECC along 80.5°EMeridional‐width variability of the near‐equatorial zonal currents shows an asymmetric response to strong pIOD and nIOD on interannual timescalesUnderlying mechanisms are related to spatial differences in large‐scale wind anomalies associated with wind/eastern‐boundary generated waves [ABSTRACT FROM AUTHOR]

A large Chlorophyll‐a (Chla) bloom developed in the southeastern Bay of Bengal (BoB) (4°N–10°N, 88°E−95°E) during winter 2019 and was observed from Moderate Resolution Imaging Spectroradiometer‐Aqua Chla data. By analyzing various observational and reanalysis datasets, we found that remote Equatorial Indian Ocean (EIO) upwelling waves and local Ekman pumping associated with the extreme Indian Ocean Dipole (IOD) event in 2019 were responsible for the Chla bloom. The extreme IOD 2019 favored strong upwelling Kelvin waves propagating from the eastern EIO into the BoB along the eastern boundary. These Kelvin waves were reflected into the central BoB as upwelling Rossby waves manifested by eddylike sea surface height propagating westward approximately along 5°N. In addition, local wind stress curl generated strong Ekman pumping in the southeastern BoB. As a result, upwelling processes associated with westward‐propagating Rossby waves and local Ekman pumping together uplifted the thermocline, breaking the stratification so that subsurface nutrient‐rich water was transported to the surface to cause a large Chla bloom in the southeastern BoB during winter 2019. The Chla bloom had a ring structure and its spatial shape was modulated by westward upwelling Rossby waves and cyclonic eddies. This study highlights the important role of IOD events in the formation of Chla bloom in the southeastern BoB during winter. Plain Language Summary: The Bay of Bengal (BoB) receives a large amount of freshwater influx from river discharges and precipitation. The freshwater causes strong stratification that inhibits the nutrient transport from the subsurface to the euphotic zone, leading to the BoB being a biologically low productive region. A large Chlorophyll‐a (Chla) bloom developed in the southeastern BoB during winter 2019 and was observed from satellite Chla data. We found that anomalous oceanic processes were responsible for the winter Chla bloom. During the fall to winter of 2019, strong easterly wind generated Equatorial Indian Ocean waves that propagated into the BoB along the eastern coast. These coastal waves were reflected into the central southern BoB approximately along 5°N. Both the remote Equatorial Indian Ocean waves and local wind favored intensified upwelling in the southeastern BoB. As a result, strong stratification in the southeastern BoB was broken and subsurface nutrient‐rich water was transported to the surface, contributing to a large winter Chla bloom in 2019. Key Points: A large Chlorophyll‐a (Chla) bloom developed in the southeastern Bay of Bengal during winter 2019 and was observed from Moderate Resolution Imaging Spectroradiometer Aqua Chla dataThe bloom was caused by remote Equatorial Indian Ocean waves and local Ekman pumping associated with the extreme Indian Ocean Dipole event in 2019The Chla bloom had a ring structure and its spatial shape was modulated by westward upwelling Rossby waves and cyclonic eddies [ABSTRACT FROM AUTHOR]

Oceanic temperature fluctuations are one of the leading factors affecting marine fish populations. Indian Ocean Dipole (IOD), characterized as the sea surface temperature (SST) anomaly change, is an ocean–atmosphere interactive process causing interannual climate variability in the Indian Ocean. Influences of the IOD on the tuna catch rates are supported by previous research. Yet, there remains limited information about the impacts on the abundance of tuna stocks. In this study, we used the standardized Catch Per Unit Effort (CPUE) index to present the stock abundance and compared the effects of the IOD on the bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) among different management areas of the Indian Ocean Tuna Commission (IOTC). Results show significant correlations between IOD events on both species' abundance in the tropical western Indian Ocean. However, in the tropical eastern Indian Ocean and the southern Indian Ocean, neither bigeye nor yellowfin tuna abundances were significantly correlated by the IOD. For the whole Indian Ocean, IOD was significantly correlated uniquely with the yellowfin tuna abundance. Our results emphasized the importance of evaluating the climate variability effects over fisheries abundance species by species and per fishing areas analyses. [ABSTRACT FROM AUTHOR]

The impact of Indian Ocean Dipole (IOD) events on the generation and intensity of tropical cyclones under the influence of monsoons is explored. The standardized sea surface temperature (SST) anomalies are computed for the pre-monsoon and post-monsoon months for the Bay of Bengal (BOB) and Arabian Sea (AS) from 1971 to 2020 and relationships are analyzed with the frequency of tropical cyclones for the neutral, positive and negative IOD years. Ocean states are sensitive to cyclonic conditions exhibiting a complex spectral distribution of the wave energy. Due to a tropical cyclone, the surface waves remain under high wind forcing conditions for prolonged periods generating a huge amount of energy. The spectral wave model SWAN (Simulating WAves Nearshore) is used to generate the energy density spectra during FANI (26 April–5 May 2019), which was a pre-monsoon extreme severe cyclonic storm, and BULBUL (5–12 November 2019), which was a post-monsoon very severe cyclonic storm in the BOB region. This study aims to estimate the intensity of wave energy during tropical cyclones in the pre- and post-monsoon months for 2019 (an extremely positive IOD year). [ABSTRACT FROM AUTHOR]

In recent years, and particularly from 2000 onwards, the North Indian Ocean (NIO) has been acting as a major sink of ocean heat that is clearly visible in the sub-surface warming trend. Interestingly, a part of the NIO—the Arabian Sea (AS) sector—witnessed dramatic variations in recent sub-surface warming that has direct repercussion on intense Tropical Cyclone (TC) activity. This study investigated the possible causative factors and physical mechanisms towards the multi-decadal warming trends in surface and sub-surface waters over the AS region. Responsible factors towards warming are examined using altimetric observations and reanalysis products. This study used ORAS5 OHC (Ocean Heat Content), derived meridional and zonal heat transport, currents, temperature, salinity, Outgoing Longwave Radiation (OLR), and air-sea fluxes to quantify the OHC build-up and its variability at water depths of 700 m (D700) and 300 m (D300) during the past four decades. The highest variability in deeper and upper OHC is noticed for the western and southern regions of the Indian Ocean. The warming trend is significantly higher in the deeper regions of AS compared to the upper waters, and relatively higher compared to the Bay of Bengal (BoB). Increased OHC in AS show good correlation with decreased OLR in the past 20 years. An analysis of altimetric observations revealed strengthening of downwelling Kelvin wave propagation leading to warming in eastern AS, mainly attributed due to intrusion of low saline water from BoB leading to stratification. Rossby wave associated with deepening of thermocline warmed the southern AS during its propagation. Heat budget analysis reveals that surface heat fluxes play a dominant role in warming AS during the pre-monsoon season. Increasing (decreasing) trend of surface heat fluxes (vertical entrainment) during 2000–2018 played a significant role in warming the southeastern sector of AS. [ABSTRACT FROM AUTHOR]

This study focuses on the regional wind variability that controls the intensity of cold-water upwelling off Sumatra – a key feature of the Indian Ocean Dipole (IOD). Our analysis of daily atmospheric data reveals the existence of convectively triggered synoptic-scale atmospheric cyclones in the South-East Tropical Indian Ocean (SETIO). The northern branch of the cyclones corresponds to westerly equatorial wind events, whereas the eastern branch involves north-westerly winds that operate to suppress cold-water upwelling off Sumatra’s west coast. Data for the period 1988–2022 show that 5–9 SETIO cyclones normally form each year during the boreal summer–autumn season, effectively suppressing upwelling in the region. In contrast, there are only few (1–2) cyclone events in years identified as positive phases of the IOD, when the absence of cyclones concurs with the development of strong coastal upwelling off Sumatra. Our findings suggest that the absence or presence of SETIO cyclones contributes to IOD variability.

Most climate forecast agencies failed to make successful predictions of the strong 2020/2021 La Niña event before May 2020. The western equatorial Pacific warm water volume (WWV) before the 2020 spring failed to predict this La Niña event because of the near neutral state of the equatorial Pacific Ocean in the year before. A strong Indian Ocean Dipole (IOD) event took place in the fall of 2019, which is used as a precursor for the La Niña prediction in this study. We used observational data to construct the precursory relationship between negative sea level anomalies (SLA) in the southeastern tropical Indian Ocean (SETIO) in boreal fall and negative Niño 3.4 sea surface temperature anomalies index one year later. The application of the above relation to the prediction of the 2020/2021 La Niña was a great success. The dynamics behind are the Indo-Pacific “oceanic channel” connection via the Indian Ocean Kelvin wave propagation through the Indonesian seas, with the atmospheric bridge playing a secondary role. The high predictability of La Niña across the spring barrier if a positive IOD should occur in the previous year suggests that the negative SETIO SLA in fall is a much better and longer predictor for this type of La Niña prediction than the WWV. In comparison, positive SETIO SLA lead either El Niño or La Niña by one year, suggesting uncertainty of El Niño predictions.

Positive/negative Indian Ocean Dipole (IOD) tends to co‐occur with El Niño/La Niña. Five positive IODs, however, are observed under La Niña condition in 1961, 1966, 1967, 2007, and 2011. This study uses the monthly ocean temperature and atmospheric reanalysis datasets from ECMWF to investigate how these positive IOD events occurred. It is found that the five positive IODs are consistently accompanied by a moderately strong positive subsurface IOD (Sub‐IOD) event that initiated earlier. The Sub‐IOD‐related oceanic dynamical processes including oceanic Kelvin and Rossby waves help to promote the development of the positive IODs. More specifically, the equatorial easterly anomalies in the Tropical Indian Ocean (TIO) that are crucial for the development of the positive Sub‐IODs and IODs in 1961/2011 are triggered by an anticyclonic curl over South Indian Ocean that is induced by a Southern Hemispheric Indian Ocean Subtropical Dipole mode. The crucial equatorial easterly anomalies in 1967 are triggered by a thermocline warming in the southwestern TIO induced by westward downwelling oceanic Rossby waves, while in 1966/2007, the equatorial easterly anomalies are connected with the anomalous Walker circulation resulting from the certain sea surface temperature anomaly pattern over the equatorial Pacific. On the other hand, the remote effects of La Niña on TIO are relatively weaker, being overwhelmed by the above triggering processes. [ABSTRACT FROM AUTHOR]

As two pronounced anomaly patterns over the tropical Indian Ocean, the Indian Ocean basin mode (IOBM) and the Indian Ocean Dipole (IOD) feature phase‐locking with seasonal cycle. We find that IOBM and IOD exhibit significant connection without the influence of the preceding El Niño–Southern Oscillation (ENSO). The tropical IOBM cooling in preceding winter, which is accompanied by anomalous westerlies in the lower troposphere of the tropical eastern Indian Ocean, can transit to a positive IOD in subsequent seasons. In boreal spring following a IOBM cooling, the equatorial wind anomalies over the Indian Ocean turn to easterlies due to the Rossby wave response to the decreased precipitation over the Maritime Continent region. The increased zonal gradient of the sea surface temperature anomalies in the tropical Indian Ocean favours the intrinsic Bjerknes feedback to build up a positive IOD. With the reinforcing process of the weakened Walker circulation in the tropical Indian Ocean, the positive IOD events develop during boreal summer and then reach the peak in following autumn. These results demonstrate that the IOD is transited from the preceding IOBM, which is independent of the influence of the preceding ENSO in boreal winter. [ABSTRACT FROM AUTHOR]

Wind climate along the southwest coast of India (Kerala coast) has been analysed using 41 years of Climate Forecast System Reanalysis (CFSR) winds to delineate long‐term trends and variability. The study reveals significant decreasing trends in annual mean wind speeds, of the order of −2.0 to −2.5 cm·s−1·year−1, for the period 1979–2019. Southwest monsoon has contributed the highest weakening trends (−3.0 to −4.0 cm·s−1·year−1) due to significant decrease in the southwesterly/westerly wind speeds. The wind climate and trends during the northeast monsoon and premonsoon seasons have been characterized largely by the prevalence of sea breeze and shamal–makran wind systems. We find that the intensity of both sea breeze and shamal–makran wind systems reduces from north to south along the Kerala coast, which is in consistent with the earlier studies. Decadal oscillations in wind speeds, driven by the decadal variability in large‐scale atmosphere–ocean circulations, are evident. Recent changes in the frequency of occurrence of Indian Ocean Dipole in the Indian Ocean have determined the overturning trends since 2010, and resulted in increasing trends in the current decade along the central and northern coasts of Kerala. Furthermore, interannual variability in wind speeds has been linked to the El Niño–Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD). [ABSTRACT FROM AUTHOR]

The interactions between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter‐basin connection is time‐varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections, including the La Niña‐like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are two most important interannual climate signals exciting world‐wide climate and socioeconomic impacts. Their interactions are known to have great implications for global climate variability and seasonal climate predictions. However, the ENSO–IOD relationship is time‐varying and show prominent decadal fluctuations. Such changes in ENSO–IOD relationship are associated with the decadal variation in IOD teleconnections and predictability. The prediction skill of IOD is high when the ENSO–IOD relationship is strong, and vice versa. But it remains unknown what controls the decadal modulation of Indo‐Pacific inter‐basin connection. Our findings suggest that the Atlantic Multidecadal Oscillation (AMO) may modulate the ENSO–IOD inter‐basin connection. The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections. Those mean state changes further influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, thereby modulating the ENSO–IOD relationship. This study advances our understandings on inter‐basin and cross‐scale interactions in the climate system. Key Points: The inter‐basin connection between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole is time‐varying and controlled by the remote Atlantic Multidecadal Oscillation (AMO)The AMO sea surface temperature anomalies drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnectionsThe AMO induced mean state changes influence both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing [ABSTRACT FROM AUTHOR]

In June‐July 2020, the Middle and Lower Yangtze River Region (MLYR) encountered the most severe precipitation event over the past 60 years. The Indian Ocean Dipole (IOD) in the previous autumn of the unusual precipitation event was the main contributing factor. Nonetheless, not every strong IOD event could bring anomalously high precipitation in the MLYR. This study showed significantly enhanced influence of IOD on East Asia climate since 2000. The IOD's effect on MLYR summer precipitation has strengthened significantly with the correlation coefficient up to 0.55 during 2000–2020. The northward movement of East Asia–Pacific pattern associated with IOD since 2000 (as compared with before) was primarily responsible for the enhanced effect. Since 2000, the MLYR has been controlled by a deep baroclinic instability system with anomalous low pressure in the lower troposphere and anomalous high pressure (completely different from before). Furthermore, the western Pacific subtropical high shifted northward and westerly jets were significantly enhanced with eastward expansion. All of these patterns contributed to the water vapor convergence and precipitation generation in the MLYR, which was not the case in the past. The record‐breaking IOD in 2019 autumn and the enhanced IOD's climatic effect in 2000–2020 contribute to the extreme Meiyu in 2020 jointly. The significant change in the influence of IOD and the enhanced climate effect of IOD highlight exciting new avenues for improving climate predictions in the MLYR. Plain Language Summary: The excessive Meiyu rainfall over the Middle and Lower Yangtze River Region (MLYR) during summer 2020 was beyond expectations of meteorologists, because in the 2019/20 winter, the Niño3.4 index was marginal at only 0.5°C as compared to 2.4°C and 2.6°C in 1997/98 and 2015/16 winters, respectively. Researches pointed out that the severely positive Indian Ocean Dipole (IOD) in the previous season was the main contributing factor on 2020 extreme Meiyu event in the MLYR. Nonetheless, not every strong IOD event could bring anomalously high precipitation in the MLYR. Our study shows significantly enhanced influence of IOD on East Asia climate since 2000. The IOD's effect on MLYR summer precipitation has strengthened significantly with the correlation coefficient up to 0.55 during 2000–2020. The IOD‐induced circulation patterns, including East Asia–Pacific pattern, the western Pacific subtropical high, and westerly jets, shifted northward with stronger intensity since 2000 (as compared with before). These results help us to reconsider the influence of tropical Indian Ocean sea surface temperature when forecasting weather or the possible climate changes under global warming. Key Points: The impact of Indian Ocean Dipole (IOD) on the Yangtze precipitation has enhanced in recent decadesThe IOD‐induced East Asia–Pacific pattern moves northward since 2000The interdecadal change in IOD's climatic effect largely contributes to the extreme Meiyu in 2020 [ABSTRACT FROM AUTHOR]

Indian Ocean Dipole (IOD) prediction is a challenging problem, largely relying on the relationship between IOD and El Niño‐Southern Oscillation (ENSO). This study demonstrates that heat content internal to the Indian Ocean can be an effective predictor providing extra IOD predictability, through constructing statistical prediction models with and without heat content as a predictor. Two recently proposed heat content predictors, equatorial heat content (EQHC) and heat content in the eastern pole of the Dipole (SEHC) are compared in this study. Results show that EQHC is more effective partly because it is relatively independent of ENSO and partly because it does not rely on IOD persistence, as does SEHC. The efficacy of EQHC as an IOD predictor is seasonally dependent, being most effective at 5–8‐month lead times beginning in the preceding late boreal winter and spring. Plain Language Summary: The Indian Ocean Dipole (IOD), a major climate mode in the tropical Indian Ocean, can lead to severe floods and droughts over surrounding continental areas. Prediction of the IOD is very challenging due to the apparent lack of effective predictors besides the El Niño‐Southern Oscillation. This study compares two recently proposed predictors for the IOD based on upper ocean heat content, one along the equator and one in the southeastern tropical Indian Ocean. Equatorial heat content proves to be more effective, especially for predictions starting in the late boreal winter and spring. This study helps to improve our understanding of IOD dynamics and its predictability. Key Points: Heat content internal to the Indian Ocean provides IOD predictability in addition to El Niño‐Southern Oscillation, especially at 5–8‐month lead timesHeat content as a predictor is seasonally dependent, being most effective as an initial condition in the boreal winter and springEquatorial heat content is more useful than heat content in the eastern pole of the Dipole as a predictor when accounting for persistence [ABSTRACT FROM AUTHOR]

Many parts of East Africa experienced extremely dry conditions during the short rains season of October–December 2021. It was predicted a few months before by the ensemble seasonal prediction system based on the SINTEX‐F climate model. The analysis of co‐variability of inter‐member anomalies has revealed that the 2021 negative Indian Ocean Dipole was responsible for these unusually dry conditions over East Africa. We also show that a hybrid statistical‐dynamical framework is more skillful than the SINTEX‐F model at predicting drought in East Africa on longer lead time, which may help people to take necessary mitigation measures to reduce the devastating impact of the drought. Plain Language Summary: Many parts of East Africa experienced extremely dry conditions during the short rains season (October–December) of 2021. Such a devastating drought in East Africa leads to unsafe drinking water, food insecurity, and other socio‐economic issues. We found the 2021 negative Indian Ocean Dipole (IOD) was responsible for these unusually dry conditions over East Africa. The IOD is an intrinsic ocean–atmosphere coupled climate phenomenon in the tropical Indian Ocean with cold (warm) sea surface temperature anomalies and reduced (enhanced) rainfall in the western (eastern) tropical Indian Ocean during its negative phase. Some of the devastating droughts (severe floods) in East Africa are observed during negative (positive) IOD years, respectively. Improved predictions of the IOD and its impacts may help us develop necessary mitigation measures to reduce the devastating impacts. Key Points: East Africa suffered from an extreme drought during the short rains season of October–December 2021The 2021 negative Indian Ocean Dipole was responsible for the devastating drought and played a key role in its seasonal predictabilityA hybrid statistical‐dynamical prediction of drought in East Africa on longer lead time may help people to take mitigation measures [ABSTRACT FROM AUTHOR]

The 2019 extreme positive Indian Ocean dipole drove climate extremes over Indian Ocean rim countries with unclear carbon‐cycle responses. We investigated its impact on net biome productivity (NBP) and its constituent fluxes, using the Global Carbon Assimilation System (GCASv2) product, process‐based model simulations from TRENDYv9, and satellite‐based gross primary productivity (GPP). By distinguishing two separate regions, the India‐Africa and Asia‐Pacific, GCASv2 indicated enhanced terrestrial carbon uptake of 0.23 ± 0.20 PgC and release of 0.38 ± 0.15 PgC, respectively, during September–December (SOND) 2019. These NBP anomalies had comparable magnitudes to those following the 2015 extreme El Niño which, however, caused the consistent carbon release in both regions. The TRENDYv9 model ensemble confirmed these NBP responses, albeit with smaller magnitudes. These regional NBP anomalies were related to soil moisture variations with a dominant role of GPP. Understanding the impact of IOD provides new insights into mechanisms driving interannual variations in regional carbon cycling. Plain Language Summary: The extreme Indian Ocean Dipole (IOD) can drive climate extremes, such as floods, heatwaves, droughts, and wildfires, over the Indian Ocean rim countries. However, responses of regional terrestrial carbon cycling to IOD remained unclear. We used the net biome productivity (NBP) from an atmospheric inversion and multiple terrestrial biosphere models to demonstrate an enhanced terrestrial carbon uptake and release over the India‐Africa and Asia‐Pacific regions, respectively, during the extreme positive IOD (September–December) in 2019. These IOD‐induced regional NBP anomalies showed comparable magnitudes but different patterns to those following the 2015 extreme El Niño. Along with the more frequent extreme IOD under future greenhouse warming, IOD will be an important mechanism driving interannual variations in regional carbon cycling. Key Points: The 2019 extreme positive Indian Ocean Dipole caused the enhanced land carbon uptake over India‐Africa and release over Asia‐Pacific during September–DecemberThese regional net biome productivity (NBP) anomalies were closely related to soil moisture variations with a dominant role of gross primary productivityThese Indian Ocean Dipole‐induced regional NBP anomalies showed comparable magnitudes but different patterns to those following the 2015 extreme El Niño [ABSTRACT FROM AUTHOR]

The evolution of an extreme positive Indian Ocean Dipole (pIOD) that took place in the tropical Indian Ocean during the late boreal summer to early winter of 2019 is examined in terms of coupled ocean–atmosphere dynamics. The patterns of anomalous sea-surface temperature (SST) revealed a typical pIOD characteristic: cooling (warming) in the southeastern (western) tropical Indian Ocean. Based on the Dipole Mode Index (DMI), the evolution of the event started in mid-July and gradually strengthened with an abrupt weakening in early September before coming to its peak in late October/early November. It quickly weakened in November, and then it terminated in mid-December. During the peak phase of the event, the SST anomaly in the southeastern (western) tropical Indian Ocean reached about −2 °C (+1 °C). The pattern of anomalous SST was followed by an anomalous pattern in precipitation, in which deficit precipitation was observed over the eastern Indian Ocean, particularly over the Indonesia region. Earlier study has shown that dry conditions associated with the pIOD event created a favorable condition for a forest-peat fire in southern Sumatra. The number of fire hotspots has increased significantly during the peak phase of the 2019 pIOD event. In addition, anomalously strong upwelling forced by strong southeasterly wind anomalies along the southern coast of Java and Sumatra had induced a surface chlorophyll-a (Chl-a) bloom in this region. High surface Chl-a concentration was collocated with the negative SST anomalies observed off the southwest Sumatra coast and south Java. [ABSTRACT FROM AUTHOR]

Interannual variability of the Indian Summer Monsoon Rainfall (ISMR) is modulated by Sea Surface Temperature (SST) anomalies over Indo-Pacific Oceans, especially by the El Niño Southern Oscillation (ENSO). In general, coupled models used for seasonal prediction overestimate the correlation between ENSO and ISMR compared to observations. By analysing the observational data from 1982 to 2017, this study shows that the relationship between ENSO and ISMR is weak during August compared to the other months of the summer monsoon season (June, July, and September). This weak association between ENSO and ISMR during August is due to an increase in the synoptic variability. Thus, the effect of large-scale flow dominated by ENSO is suppressed by the formation of a synoptic system in the Bay of Bengal (BoB), making ENSO-ISMR relation feeble in August. The data analysis of various coupled models shows that all models underestimate synoptic variability, due to which simulated ENSO-ISMR relation is overestimated during August. Coupled model exhibit strong biases in relative humidity and cyclonic circulation over the northern BoB hence underestimating the synoptic variability. [ABSTRACT FROM AUTHOR]

Based on the Argo and multi‐satellite observations, this study investigates the asymmetric response of sea surface salinity (SSS) to two extreme positive and negative Indian Ocean Dipole (IOD) events in the southern tropical Indian Ocean (TIO). During positive IOD events, the easterly wind anomalies trigger zonal freshwater advection along the equator, decreasing SSS, while cold sea surface temperature (SST) depress the convective precipitation in the southeastern TIO (SETIO), increasing the SSS. Contrast SSS anomalies along the equator and in the SETIO show opposite phases during positive and negative IOD events. These SSS anomalies have enhanced in recent years, reaching the strongest in the extreme negative IOD in 2016 and positive IOD in 2019. The case study shows that positive SSS anomalies occurred in the southern TIO during both positive and negative IOD events in 2016 and 2019, suggesting significant asymmetry in SSS. Analysis of the mixed layer salinity budget shows that the local precipitation and the entrainment associated with the upwelling Rossby waves played an essential role in the SSS increase in 2016, while the decreased precipitation and horizontal advection caused by the westward South Equatorial Current contributed to the SSS increase in 2019. Further analyses suggest that SSS response is more sensitive to thermocline shoaling than deepening, and SSS response to precipitation is greater at shallower mixed layer than at deeper ones, which is the main reason for the asymmetric response of the SSS to negative and positive IOD events. Plain Language Summary: Indian Ocean Dipole (IOD) is one of the leading climate modes in the Indian Ocean, which impacts changes in temperature, precipitation, and salinity. This study reveals significant asymmetric responses in SSS during the decay phase of positive and negative IOD events. During the mature phase of positive and negative IOD events, there are opposite changes in sea surface temperature, atmospheric circulation, rainfall, and SSS in the equatorial and southeastern tropical Indian Ocean (SETIO). However, SSS increases in the SETIO during the decay phase of both positive and negative IOD events, revealing an asymmetric feature. This asymmetric structure of SSS anomalies has strengthened recently and reached extremes in the 2016 negative IOD event and the 2019 positive IOD event. The upwelling (downwelling) Rossby waves play an important role in the asymmetric response of SSS anomalies in the southern TIO since the SSS response is more sensitive to thermocline shoaling than deepening, and the shallower mixed layer may imply the SSS response to precipitation anomaly. Key Points: Contrast sea surface salinity (SSS) anomalies occur in the equatorial and southeastern tropical Indian Ocean (TIO) during the mature phases of Indian Ocean Dipole (IOD)SSS anomalies are out of phase during positive and negative IOD events but show an asymmetric change in the southern TIOThe asymmetry is due to different SSS responses to precipitation and thermocline changes associated with the ocean Rossby waves [ABSTRACT FROM AUTHOR]

In 2018-2021, several natural phenomena occurred that caused climate change in Indonesia. This climate change is estimated to have affected the dynamics of rainfall and temperature in Indonesia. This study aims to analyze the incidence of climate change, rainfall dynamics, and temperature dynamics, and find the correlation between rainfall versus temperature during the 2018-2021 dry season. The location of this research is on peatlands in Central Kalimantan and West Kalimantan where both locations have installed automatic measurement stations for rainfall and temperature. Peatlands were chosen because the dynamics of rainfall and temperature greatly affect the condition of peatlands that are prone to fires and floods. The results of this study indicate that very minimal rainfall occurs in the dry season in 2019, especially from July to September 2019. Rainfall in the 2019 dry season is much lower when compared to the dry season in 2018, 2020, and 2021. This happens because in 2019 two natural phenomena occurred simultaneously, namely: moderate IOD+ and weak El Niño. These two phenomena reinforce each other in reducing rainfall in Indonesia. The temperature during the dry season for 4 years did not show a significant difference, but in general, the temperature in Central Kalimantan was lower than the temperature in West Kalimantan. It was also found that there was a trend in the relationship between rainfall and temperature where the higher the rainfall, the lower the temperature. Based on the results of the statistical method of linear regression and t-test, it has been found that the correlation between rainfall and temperature is significant. [ABSTRACT FROM AUTHOR]

Using mooring observations and reanalysis, we show that anomalously strong westward Equatorial Undercurrent (wEUC) developed in June–July in 2016 and 1998 in the Indian Ocean, which coincided with extreme Indian Ocean Dipole (IOD) and El Niño events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and positive IOD events during 2015 and 1997, and their negative phases during 2016 and 1998. The constructive relationship between the delayed‐time contributions of eastern‐boundary‐reflected‐waves that excited by the easterlies in 2015 and 1997 and the direct contributions of wind‐forced‐waves that excited by the westerlies in 2016 and 1998 resulted in the intensified wEUC. Slow intermediate‐order baroclinic‐modes, rather than fast low‐order baroclinic‐modes, dominated the strong wEUC. The eastern‐boundary‐reflected‐waves dominated in 1997–1998 and directly wind‐forced‐waves dominated in 2015–2016. Our results emphasize the importance of constructive interactions of the directly‐wind‐forced and boundary‐reflected waves in driving the interannual variability of Indian Ocean wEUC. Plain Language Summary: This study investigated the unique westward Equatorial Undercurrent (wEUC) and its associated dynamics in the Indian Ocean. Mooring observations at (80°E, 2.5°N) reveal the occurrence of an anomalously strong wEUC between 100 and 200 m depth in June‐July (JJ) 2016. Simulations show that the constructive relationship between the directly forced and boundary‐reflected waves resulted in the unprecedentedly strong wEUC in JJ 2016. Equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and a positive Indian Ocean Dipole (IOD) during 2015, and their negative phases during 2016. Waves excited in 2015 hit the meridional boundary, and boundary‐generated waves propagated into the interior Indian Ocean in 2016, which were superposed onto the directly forced waves. A similar wave‐induced enhanced wEUC also occurred during the 1997–1998 strong El Niño and IOD events but with the dominant contribution coming from the eastern‐boundary‐reflected waves. Key Points: An anomalously strong westward Equatorial Undercurrent (wEUC) was observed in June–July (JJ) 1998 and 2016The intensified wEUC in JJ 1998 and 2016 was driven by the co‐occurrence of strong IOD and ENSO events during 1997–1998 and 2015–2016The dynamics are related to both directly wind‐driven and eastern‐boundary‐reflected intermediate‐order baroclinic waves [ABSTRACT FROM AUTHOR]

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large‐scale tropical climate processes such as the El Niño–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow‐up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build‐up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience 'false onsets' with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co‐occur with a flash drought. [ABSTRACT FROM AUTHOR]

Although climate models are important for making projections of future climate, little attention has been devoted to model simulation of the complex climate of Central Africa (CA). This study investigates rainfall biases through processes in three versions of the Met Office Unified Model over CA with both coupled and atmosphere‐only formulations for each version. The study shows that the models depict a wet (dry) bias over the eastern (coastal western) CA in the September–November season with the wet (dry) bias stronger in coupled (atmosphere‐only) models. Here, we explore potential regional to large‐scale causes of these biases. Results reveal that the overestimation of the simulated sinking branch of the Atlantic‐Congo zonal overturning cell is associated with a strong near‐surface temperature and pressure gradient between CA and the eastern Atlantic Ocean. This leads to strong low‐level westerlies (LLWs) which dry the coastal western CA and strengthen the intensity of the Congo basin cell. Over eastern CA, the wet bias is partially due to the misrepresentation in the strength of both African easterly jet components that shift the mid‐tropospheric moisture flux convergence southward favouring more convection south of the equator. Furthermore, the overestimation in the simulated width and intensity of the Congo basin cell is associated with a strong low‐level moisture convergence over eastern CA which contributes to more precipitation. Remote contributions to the wet bias come from both the Atlantic and the Indian Oceans. The simulated Atlantic‐Congo zonal overturning circulation dries the coast through its overestimated lower branch (LLWs) which moves further into the continent and advects more moisture to the eastern CA. In the meantime, during positive Indian Ocean Dipole years, the advected moisture from the Indian Ocean to the CA region is overestimated in models, much more in coupled models and contributes to the eastern CA wet bias. [ABSTRACT FROM AUTHOR]

The three strongest positive Indian Ocean dipole (pIOD) events over the past four decades occurred in 1994, 1997, and 2019 and caused severe social-economic impacts in the Indian Ocean (IO) rim countries. These three strongest positive pIOD events exhibited similar oceanic and atmospheric anomaly patterns during their mature phase, though with different physical origins. The extreme amplitudes of these pIOD events were closely related to concurrent El Niño or Madden–Julian oscillation (MJO) activity. For the 1997 extreme pIOD case, the concurrent extreme El Niño in the Pacific triggered strong easterly wind anomalies over the equatorial IO and therefore favored the rapid growth of the pIOD via intense upwelling along the Sumatra-Java coast. In contrast, the long stalled MJO convective anomalies over the western IO exerted an important role in the 2019 extreme pIOD event especially during September–October by causing easterly anomalies over the equatorial IO. During that year, the Pacific moderate warming had likely a small impact since no strong associated convection was present over the western and central equatorial Pacific due to the small amplitude of the Pacific sea surface temperature anomalies. Different from the 1997 and 2019 cases, both El Niño conditions in the Pacific as well as MJO activity over the western IO contributed to the 1994 extreme pIOD event. Our results emphasize that MJO activity should be considered as one physical drivers for the extreme pIOD event in addition to the traditional El Niño-associated forcing. [ABSTRACT FROM AUTHOR]

Abstract The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

Abstract The termination of Indian Ocean Dipole (IOD) events is examined in terms of equatorial wave dynamics. In situ and satellite observations combined with an output from a linear wave model are used in this study. Our emphasis is on the 1997 IOD event but our results apply to other positive IOD events as well. We find that the termination of anomalously cold sea surface temperature (SST) in the eastern pole of the dipole is associated with a warming tendency caused by the net surface heat fluxes. However, net surface heat fluxes alone cannot explain the total change in the SST. We show that during the peak phase of an IOD event, the weakening of zonal heat advection caused by eastern boundary-generated Rossby waves combined with the reduction of vertical entrainment and diffusion creates favorable conditions for surface heat fluxes to warm the SST in the eastern basin.

The lag correlations between the observed sea surface temperature anomalies (SSTA) in the southeastern tropical Indian Ocean in fall and those in the Pacific cold tongue at the one-year time lag are calculated in running windows of 7–11 years and are shown to have decadal variability. Similar decadal variability has also been identified in the historical simulations of the Community Climate System Model version 4 (CCSM4) that participates in the Coupled Model Intercomparison Project phase-5 (CMIP5). During the positive phases of the decadal variability, the significant lag correlations are diagnosed to be dominated by the oceanic channel dynamics, i.e., upwelling Kelvin waves associated with the Indian Ocean Dipole (IOD) force enhanced Indonesian Throughflow transport anomalies and western Pacific thermocline anomalies to propagate eastward, resulting in SSTA in the central-eastern equatorial Pacific Ocean. During the negative phases of the decadal variability, the subsurface lag correlations suggest that the western Pacific Ocean are still dominated by the oceanic channel dynamics, but do not correlate well with the SSTA in the cold tongue due to a deeper thermocline in the eastern equatorial Pacific. The IOD-ENSO teleconnection is not affected significantly by the anthropogenic forcing during either the positive or the negative decadal phases. The CCSM4 model is found to underestimate Indonesian Throughflow transport variability but overestimate the westerly wind anomalies in the western-central equatorial Pacific, which forces unrealistic anomalies in the equatorial Pacific Ocean associated with the IOD.

El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) can reduce the amount of rainfall in Indonesia. The previous study found that ENSO and IOD derived from the OISST dataset have an association with hotspots in Indonesia, especially in southern Sumatra dan Kalimantan. But the correlation results are still too small, and the correlation strength between regions has not been analyzed. Therefore, this study quantifies the association of the estimated ENSO and IOD derived from the ERA5 dataset on hotspots in Indonesia based on a Heterogeneous Correlation Map (HCM) and analyzes the correlation strength between regions in Indonesia. We use a singular value decomposition method to quantify this HCM. Besides OISST, ERA5 is an estimation data often used for weather forecast analysis. Therefore, this study quantifies the association of the estimated ENSO and IOD derived from the ERA5 dataset on hotspots in Indonesia based on a Heterogeneous Correlation Map (HCM) and analyzes the correlation strength between regions in Indonesia. Based on variance explained and correlation strength, the hotspot in Indonesia is more sensitive to ENSO and IOD derived from ERA5 than OISST. Consequently, the ERA5 data more useful to statistical analysis that requiring a substantial correlation.

This study provides comprehensive analysis on the contrasting effects caused by the 1997 and 2015 historical El Niño events linked with the Indian Summer Monsoon (ISM) rainfall. The presence of strong southeast-northwest oriented cold Sea Surface Temperature (SST) anomalies during 1997, that spatially extended from southwest Pacific to Southeast Indian Ocean (SEIO) in comparison to the 2015 event is the robust feature. Evidently, these anomalies are closely related to interaction between cyclonic (anticyclonic) circulation over South Pacific Convergence Zone (south Australia). During 2015, the conjunction of Modoki II and classical El Niño triggered an asymmetrical equatorial circulation throughout the Indo-western Pacific (IWP) and thereby stimulated the Southern Annular Mode (SAM) through troposphere and stratospheric pathway mechanism. In addition, in 2015, SAM impacted the Indian Ocean, which intern affected ISM rainfall. Positive SAM associated with westward shift of anticyclone over south of Australia alters the circulation by inducing westerly winds over the South Indian Ocean, thereby suppressing Indian Ocean Dipole (IOD), and inducing drought conditions over India during 2015. Moreover, the AUS index, an indicator for IOD strength in boreal summer, is a bridging factor prevalent over mid-latitude regions in the southern hemisphere. Results from this study indicate the complex interaction of southern hemisphere atmospheric flow and its role in modulating the Indian Ocean region thereby ISM rainfall. A better understanding of these underlying mechanism can significantly enhance the predictive skills and projections of monsoon variability and extremes. • The 1997 and 2015 El Niño events differently impacted Indian Summer Monsoon rainfall patterns. • Shifts in Indo-Pacific SST and circulation anomalies noticed with notable changes in air-sea interaction among these events. • SAM shifts anticyclone westward south of Australia, enhancing SIO westerlies, suppressing IOD, causing India's 2015 drought. • AUS index, indicating IOD strength in boreal summer, a key bridging factor across the Southern Hemisphere's mid-latitudes. [ABSTRACT FROM AUTHOR]

Based on first scientific principles, this study shows that both equatorial and coastal wind anomalies influence the development of positive phases of the Indian Ocean Dipole (IOD) in the south-east tropical Indian Ocean (SETIO). While southeasterly winds are favorable for upwelling along Sumatra's southwest coast, zonal equatorial wind anomalies are the main control of IOD variability given that the resultant Kelvin wave enhances or suppresses the Sumatran upwelling. Many previous studies have argued that easterly equatorial wind anomalies in the SETIO are essential for triggering the positive Bjerknes feedback, and thereby, the development of positive IOD (pIOD) events. With a focus on the particularly strong pIOD event of 2019, here we show that pIOD events can also evolve in the absence of zonal equatorial wind anomalies. Hence, it is possible that the Bjerknes feedback along the equator is less involved in IOD variability than previously thought. • Explores the ocean's response to wind forcing in the eastern tropical Indian Ocean. • Zonal equatorial wind anomalies are the main control of IOD variability. • Positive IOD events can evolve in the absence of zonal equatorial wind anomalies. [ABSTRACT FROM AUTHOR]

• Internal waves are regularly observed at a shallow tropical seamount ecosystem. • The internal wave regime is controlled by background stratification and forcing. • Thermocline depth relative to summit depth modulates the internal wave regime. • Basin scale processes drive stratification variation over interannual timescales. Shallow seamounts are becoming increasingly recognised as key habitats for conservation due to their role as biological refuges, particularly throughout oligotrophic oceans. Traditionally, Taylor caps have been invoked as the mechanism driving biomass aggregation over seamounts but emerging evidence based on higher resolution measurements highlights the importance of internal waves (IW) to the local ecosystem. These waves can flush the benthic habitat with cool water from depth and impact on nutrient supply over short time scales through turbulent mixing that may also influence fish behaviour. They are dependent on the regional stratification, however, and thus influenced by planetary-scale variability in oceanographic conditions. We present here detailed observations of the internal wave regime over a shallow seamount, called Sandes, in the central Indian Ocean throughout different phases of the Indian Ocean Dipole (IOD) that modulated the regional stratification. A deep thermocline, caused by the 2019 IOD event precluded internal wave activity over the summit, whereas a thermocline collocated with the summit during 2020 when the IOD reversed polarity resulted in a 30 m amplitude internal tide signal (t ∼ 12.5 h). A shallow thermocline, observed during 2022, resulted in propagation of IWs over the summit with less visible internal tide. Harmonic analysis shows the presence of high frequency waves (t ∼ 15 min) on both flanks of the seamount during 2020 & 2022, which are likely a result of local shear instability, whereas 2019 shows an asymmetric response, potentially due to the strong background current and suppression of the thermocline beneath the depth of the summit. The potential importance of the waves over the summit to the local ecosystem may be attributed to the elevated turbulence measured at the thermocline during internal wave propagation, with ε > 10-5 W kg-1 routinely observed. Our results highlight the ability of thermocline depth to act as a gating condition for internal wave evolution over the summit. These results show that, whilst the water column exhibits variability at short spatiotemporal scales compared to the frequently cited Taylor cap dynamics, it is also regulated by the wider basin scale processes. Thus, a more integrated approach is needed when assessing these dynamic and environmentally critical habitats to include the effects of physical oceanographic controls across multiple spatiotemporal scales. [ABSTRACT FROM AUTHOR]

Indian Ocean (IO) warming, the frequency of extreme positive Indian Ocean Dipole (EPIOD) events, and its impact on Indian monsoon rainfall (IMR) are more prominent in the recent era. A proper understanding of the relationship between the EPIOD and IMR is very important because two thirds of the people in India are dependent on agriculture and related economy. The primary focus of this study is on three main aspects: (i) analysing the monthly trend of sea surface temperature (SST) in the IO; (ii) the spatial variability of SST anomalies (SSTA) linked to the EPIOD and its impact on the IMR; and (iii) the factor responsible for the intensification of the EPIOD. These aspects are thoroughly assessed using 12 NMME models, comparing them with observations and reanalysis datasets from 1990 to 2020. The analysis reveals a warming trend in the central IO during June and July, with significant increases observed from August to November. Over the past 31 years, the central IO has experienced an approximate rise of 0.9 to 1 °C from June to November. Among the 12 models scrutinised, GFDL_FLOR_A, FLOR_B, and RSMAS_CCSM4 closely align with observed spatial trends. In contrast, models such as GFDL_SPEAR and NASA_GEOSS tend to overestimate these spatial trends. An increase in easterlies over the equatorial eastern IO was identified as a possible driver for the rising SST in the central IO during October and November. In response to EPIOD on the monthly spatial rainfall distribution over India, all NMME models often underestimate rainfall over the west coast and exhibit wet biases over the leeward side of the Western Ghats. Grid-wise skill scores for accumulated monthly rainfall demonstrated variations among NMME models, with GFDL_SPEAR, RSMAS_CCSM4, and other IC3 models showing higher skill. The interhemispheric pressure gradient (IHPG) has been identified as a precursor to EPIOD events. A positive IHPG from March to June is correlated with EPIOD occurrences from October to December. It is observed that half of the NMME models are capable of simulating a positive relationship between IOD and IHPG, consistent with the observed data. • The study highlights IO warming mechanism, EPIODE formation, and their relation to the IMR in observations and NMME models. • The cause of IO warming is attributed to an increase in low-level Easterlies wind strength over the central equatorial IO. • EPIODE formation, driven by interhemispheric pressure gradient, serves as a precursor. • The GFDL_SPEAR and RSMAS_CCSM4 models effectively capture monthly rainfall during EPIODE. [ABSTRACT FROM AUTHOR]

The open ocean region of southern Bay of Bengal (BoB) (~ 5° N–10° N; 87° E–97° E) is characterized as the region of low chlorophyll-a concentration (< 0.1 mg m−3) during winter (December-February). However, the analysis of satellite ocean colour data between 2003 and 2020 shows anomalous high surface chlorophyll concentration (~ 0.35–0.5 mg m−3) in the southern BoB during the winters, followed by the extreme positive Indian Ocean Dipole (pIOD) events in 2006 (W06-2007) and 2019 (W19–2020). It is found that anomalous upwelling Rossby wave generated remotely through the anomalous easterly wind in the equatorial Indian Ocean (EIO) during October–November leads to a thin barrier layer (BL), shallow thermocline and nutricline in the southern BoB during W06–2007 and W19–2020. Thin BL, and shallow nutricline, provide a conducive environment for surface chlorophyll enhancement through much easier vertical transport of nutrient-rich water to the near-surface layer by local near-climatological wind-induced vertical mixing during W06–2007 and W19–2020. It is also found that surface chlorophyll bloom was absent in the southern BoB during the winters, followed by the weak pIOD event in 2012, 2015, and 2018. During the winters followed by the weak pIOD events, the westward propagating downwelling Rossby wave generated through the westerly wind, similar to the climatological state, in the EIO during October–November leads to a thick BL, deeper thermocline and nutricline in the southern BoB. Thick BL, and deeper nutricline, provide an unfavourable environment for the vertical transport of nutrient-rich water to the near-surface layers to generate surface chlorophyll bloom during the winters followed by the weak pIOD event. [ABSTRACT FROM AUTHOR]

Coastal upwelling along the southern coast of Java brings cold and nutrient‐rich subsurface water to the surface. We explored whether the upwelling could trigger the onset of the Indian Ocean Dipole (IOD) by supplying cold water to the southeastern tropical Indian Ocean. We used satellite‐based daily chlorophyll‐a concentration (Chl‐a) data during 2003–2020 as a proxy of the coastal upwelling. We focused on first Chl‐a bloom that occurred in April‐June, the onset phase of the positive IOD (pIOD). We found that the timing and strength of the upwelling signals were significantly correlated with the subsequent IOD peaks. We diagnosed processes associated with the upwelling affecting sea surface temperature (SST) in the southeastern Indian Ocean. Results indicate that after the cold‐water upwelling south of Java, westward surface temperature advection plays a role in anomalously cooling the SST in the southeastern Indian Ocean and setting a favorable condition for the subsequent pIOD development. Plain Language Summary: Along the southwestern coastal waters of Java islands in Indonesia, coastal upwelling brings up cold water from below and can cool the ocean surface temperatures. Past studies reported that strong upwelling signals were observed together with the positive Indian Ocean Dipole (IOD) phenomenon. However, it was unclear if and how the coastal cold water spreads over large area of the Indian Ocean and affect the occurrence of IOD. We used sea surface chlorophyll‐a (Chl‐a) data estimated from satellite observations as an indicator of coastal upwelling. We focused on the date when the first Chl‐a bloom occurred south of Java in each year. On average, Chl‐a bloom occurs around June, but in years when it occurred earlier (around April‐May), Chl‐a bloom was followed by a positive IOD event in August‐October. We further investigated how the cold water in the coastal area spreads widely over the eastern Indian Ocean. It was found that stronger westward ocean currents expanded the cold water to a wider area of the eastern Indian Ocean and can set a favorable condition for the development of the positive IOD. Thus, accurate observation and understanding of the coastal upwelling may improve the predictability of the Indian Ocean climate. Key Points: Satellite‐based surface chlorophyll‐a (Chl‐a) can be a proxy of intraseasonal‐scale coastal upwelling along the southern coast of JavaThe timing and strength of the upwelling signals were significantly correlated with the subsequent peak of the Indian Ocean Dipole (IOD)The coastal upwelling may contribute to the onset and development of positive IOD through the process of anomalous cold‐water advection [ABSTRACT FROM AUTHOR]

The respective influences of the Indian Ocean dipole (IOD) and El Niño‐Southern Oscillation (ENSO) on the thermocline depth (represented by the 20°C isotherm depth, D20) variability in the south‐central Indian Ocean (SCIO) are investigated for the period of 1960–2020. The results show that pronounced interannual variations in D20 mainly occur in the SCIO and display a close relationship with the two tropical climate modes, with positive (negative) phase being associated with SCIO D20 deepening (shoaling). The D20 anomalies (D20A) induced by IOD and ENSO events have the same intensities but present different spatial–temporal features, as demonstrated by partial regression analysis. The IOD‐related D20A occurs during the mature phase of the event and behaves as a local response to anomalous wind stress curl induced by IOD, while the ENSO signature, dominated by the westward‐propagating oceanic Rossby waves, lasts much longer and extends further west. [ABSTRACT FROM AUTHOR]

The East African 'short rains' in October–December (OND) exhibit large interannual variability. Drought and flooding are not unusual, and long-range rainfall forecasts can guide planning and preparedness for such events. Although seasonal forecasts based on dynamical models are making inroads, statistical models based on sea surface temperature (SST) precursors are still widely used, making it important to better understand the strengths and weaknesses of such models. Here we define a simple statistical forecast model, which is used as a tool to shed light on the dynamics that link SSTs and rainfall across time and space, as well as on why such models sometimes fail. Our model is a linear regression, where the August states of El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) predict about 40% of the short rains variability in 1950–2020. The forecast errors are traced back to the initial SSTs: too-wet (too-dry) forecasts are linked linearly to positive (negative) initial ENSO and IOD states in August. The link to the initial IOD state is mediated by changes in the IOD between August and OND, highlighting a physical mechanism for prediction busts. We also identify asymmetry and nonlinearity: when ENSO and/or the IOD are positive in August, the range and variance of OND forecast errors are larger than when the SST indices are negative. Upfront adjustments of predictions conditional on initial SSTs would have helped in some years with large forecast busts, such as the dry 1987 season during a major El Niño, for which the model erroneously predicts copious rainfall, but it would have exacerbated the forecast in the wet 2019 season, when the IOD was strongly positive and the model predicts too-dry conditions. [ABSTRACT FROM AUTHOR]

It is well known that the Indian Ocean dipole (IOD) is closely related to El Niño-Southern Oscillation (ENSO). In this study, it is found that spring–summer north tropical Atlantic (NTA) sea surface temperature (SST) anomalies can contribute to the development of the IOD since the mid-1980s as well as ENSO. After the mid-1980s, an anticyclonic circulation over the subtropical northeastern Pacific could be excited by cool NTA SST anomalies in spring and summer due to a Gill-type Rossby-wave response. A low-level cyclonic circulation appears in the west Pacific in turn. The anomalous lower-level southwesterlies on the southeastern flank of the cyclonic circulation reduce the climatological wind speed, and thus, warm SST anomalies appear and extend to the central tropical Pacific via the wind-evaporation-SST (WES) feedback. As a result, Walker circulation over the Indo-Pacific region is changed, with anomalous descending in the eastern tropical Indian Ocean and ascending in the central tropical Pacific. The descending branch of anomalous Walker circulation generates surface southeasterly wind anomalies along the coast off Sumatra and lifts the thermocline in the eastern tropical Indian Ocean, which produces a positive IOD event in autumn. Such a cross-basin mechanism is supported by a coupled model experiment with warm SST perturbations over the NTA. In contrast, although cold NTA SST anomalies can induce warm SST anomalies in the eastern equatorial Pacific through interhemispheric meridional circulation before the mid-1980s, the responses in the tropical Indian Ocean are rather weak. In addition, the enhancement in the NTA-IOD relationship after the mid-1980s is suggested to result from the changes in the SST mean state under the context of global warming, which is confirmed by two coupled model experiments with different mean SST backgrounds. [ABSTRACT FROM AUTHOR]

To investigate the main modes of interannual variation of chlorophyll-a (Chla) with seasonal evolution and its variation cycle in the North Indian Ocean based on satellite-derived products during 1998–2016, a season-reliant empirical orthogonal function (S-EOF) analysis and power spectrum analysis based on Fourier transform are applied in the study. The first three dominate modes reveal distinct Chla variability, as the S-EOF1 features by one dipole pattern have a negative anomaly in the central western Indian Ocean and a positive anomaly off the Java–Sumatra coasts, which is mainly synchronously associated with the climate indices of the positive Indian Ocean dipole (IOD) and eastern Pacific El Nino (EP-El Niño). The S-EOF2 indicates a tripolar structure with positive anomalies located in the central Indian Ocean surrounded by two negative anomalies, which is one year behind a positive IOD and EP-El Niño event. The S-EOF3 exhibits a different dipole distribution, with a positive anomaly in the central west and a negative anomaly in the southeast, synchronized or lagging behind the central Pacific El Nino (CP-El Niño). Moreover, regarding the correlation between the main modes of interannual variation and the IOD and El Nino events, the dynamic parameters (such as SST, SLA, rain, and wind) of the tropical Indo-Pacific Ocean are discussed using time-delay correlation and linear regression analysis to explain the key factors and possible influencing mechanism of the joint seasonal and interannual variations of Chla in the northern Indian Ocean. [ABSTRACT FROM AUTHOR]

The IOD can be measured using the Dipole Mode Index (DMI) which is calculated based on the sea surface temperature in the Indian Ocean. Therefore, DMI can be predicted using sea surface temperature forecasting data, such as data provided by the European Center for Medium-Range Weather Forecasts (ECMWF). However, the data still has a bias as compared to the actual data, so to get a more accurate prediction, corrected data is needed. Therefore, the aim of this study is to predict DMI based on sea surface temperature forecasting data that has been corrected for bias using the quantile mapping method, a method that connects the distribution of forecasting and actual data. The results showed that the DMI prediction using corrected data was more accurate than the DMI prediction using ECMWF data. DMI predictions using corrected data have high accuracy to predict IOD events in October-April.

The coastal area of Java has become a centre of new economic growth. The southern coast of Java, which is directly adjacent to the tropical Indian Ocean, is very vulnerable to sea level rise caused by climate change. Information on variability and trends in sea level are therefore very important for adaptation and disaster mitigation efforts. This research was conducted to determine the variability and trend of sea level in the southern sea of Java. Data used were from satellite altimeter from 1993 to 2018 and tide gauges from 2007 to 2015. The rate of sea level rise was analysed using linear regression. The results showed that the sea level variability in the southern waters of Java was influenced by the Asian–Australian monsoon, eddy currents and the Indian Ocean Dipole (IOD). During June–November, there was a very significant decrease in sea level, especially in the south of East Java and Central Java, which was caused by upwelling and eddy currents. When there was a positive phase of IOD and an El Niño event, sea level decreased; conversely, when the IOD was in a negative phase, sea level increased. For the period 1993–2018, the sea level in the southern waters of Java increased by about 4.7 mm/year.

Coastal hypoxia—harmfully low levels of oxygen—is a mounting problem that jeopardizes coastal ecosystems and economies. The northern Indian Ocean is particularly susceptible due to human‐induced impacts, vast naturally occurring oxygen minimum zones, and strong variability associated with the seasonal monsoons and interannual Indian Ocean Dipole (IOD). We assess how natural factors influence the risk of coastal hypoxia by combining a large set of oxygen measurements with satellite observations to examine how the IOD amplifies or suppresses seasonal hypoxia tied to the Asian Monsoon. We show that on both seasonal and interannual timescales hypoxia is controlled by wind‐ and coastal Kelvin wave‐driven upwelling of oxygen‐poor waters onto the continental shelf and reinforcing biological feedbacks (increased subsurface oxygen demand). Seasonally, the risk of hypoxia is highest in the western Arabian Sea in summer/fall (71% probability of hypoxia). Major year‐to‐year impacts attributed to the IOD occur during positive phases along the eastern Bay of Bengal (EBoB), where the risk of coastal hypoxia increases from moderate to high in summer/fall (21%–46%) and winter/spring (31%–42%), and along the eastern Arabian Sea (i.e., India, Pakistan) where the risk drops from high to moderate in summer/fall (53%–34%). Strong effects are also seen in the EBoB during negative IOD phases, when the risk reduces from moderate to low year‐round (∼25% to ∼5%). This basin‐scale mapping of hypoxic risk is key to aid national and international efforts that monitor, forecast, and mitigate the impacts of hypoxia on coastal ecosystems and ecosystem services. Plain Language Summary: Coastal "dead zones," with vanishingly low oxygen levels, stress marine organisms, compress their habitats, threaten the sustainability of fisheries, and can lead to mass mortality of marine life. The northern Indian Ocean is particularly vulnerable due to natural physical processes that bring oxygen‐poor water onto the continental shelf from vast offshore regions of low‐oxygen, and biological processes that can intensify coastal dead zones. Alongside these natural processes, coastal dead zones in the Indian Ocean are also influenced by human activity, such as fertilizer use and waste water management. In this study we use a large set of observations in the northern Indian Ocean to quantify how natural processes trigger coastal dead zones seasonally, and how they increase or decrease the risk of coastal dead zones from year‐to‐year. This information, on where and when coastal dead zones are most likely to occur, is key to anticipate and mitigate impacts on ecosystems and economies. Key Points: Regions at higher risk of coastal hypoxia in the northern Indian Ocean are identified using coastal oxygen observationsThe Indian Ocean Dipole (IOD) can amplify or suppress the seasonal risk of hypoxia by modulating coastal upwelling/downwellingThe risk is most amplified during positive IOD phases along the coast of the eastern Bay of Bengal [ABSTRACT FROM AUTHOR]

A prominent subsurface upwelling over the Seychelles–Chagos Thermocline Ridge (SCTR) in the southwestern tropical Indian Ocean is suppressed when downwelling Rossby waves propagate from the eastern Indian Ocean during the positive phase of the Indian Ocean Dipole (IOD) or El Niño periods. Recent studies have suggested that the upwelling can be further suppressed during the co-occurrence years of positive IOD and El Niño, associated with the strong easterly wind anomalies in the eastern Indian Ocean. This study examines the temporal variations in the SCTR upwelling strength during 1968–2017, identifies strong upwelling suppression events, and analyzes their characteristics by focusing on the role of both remote and local wind forcing, not limited to the remote influence linked to the IOD and the El Niño Southern Oscillation. A total of nine events were identified for the 50-year period, with seven of them occurring 3–6 months after the co-occurring peaks of positive IOD and El Niño in the eastern Indian Ocean. However, only IOD exhibited a positive phase before the 2011–2012 suppression event associated with the downwelling-favorable anticlockwise local wind anomalies in the SCTR. Moreover, the 1978–1979 suppression event was primarily caused by the strong anticlockwise wind anomalies in the SCTR, without any significant influence of the positive IOD or El Niño. The results suggest that the role of local winds, as well as the remote forcing, are significant in the upwelling suppression over the SCTR, which would contribute to understanding the potential predictability of the upwelling variations. [ABSTRACT FROM AUTHOR]

A new framework of complex networks based on event synchronization that can be applied to the development of temperature anomalies associated with climate variation phenomena is developed to reveal propagation characteristics of ocean temperature anomalies and their asymmetry between positive and negative phases of the Indian Ocean Dipole (IOD). When a complex network based on sea surface temperature (SST) is constructed, it is revealed that SST anomalies in the eastern pole associated with both phases of the IOD tend to be preceded by those in the eastern equatorial region and from the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas before extreme SST anomalies compared to the negative IOD. Plain Language Summary: The Indian Ocean Dipole (IOD) is an interannual climate phenomenon of the tropical Indian Ocean. During positive IOD events, anomalous cooling of sea surface temperature (SST) is observed in the southeastern tropical Indian Ocean, whereas anomalous warming occurs in the western tropical Indian Ocean. Anomalies with the opposite signs occur during negative IOD events. To investigate how such anomalous SST in the southeastern tropical Indian Ocean propagates during the IOD, a new framework of complex networks is proposed. When the complex network is constructed based only on SST, it is shown that both anomalously warm SST associated with the negative IOD and anomalously cool SST associated with the positive IOD tend to be preceded by those in the eastern equatorial region and to the south. When a complex network is constructed based on both surface and subsurface temperatures, it is found that the positive IOD tends to experience extreme subsurface temperature anomalies over more vast areas compared to the negative IOD. This suggests that the positive IOD may be more predictable than the negative IOD. Key Points: Complex network based on event synchronization is developed to examine temperature anomalies associated with the Indian Ocean Dipole (IOD)Sea surface temperature (SST) anomalies in the eastern pole tend to originate from the equator and to the south for both IOD phasesSST anomalies associated with the positive IOD tend to be preceded by subsurface temperature anomalies over more vast areas [ABSTRACT FROM AUTHOR]

With their complex narrow passages and vigorous mixing, the Indonesian seas provide the only low-latitude pathway between the Pacific and Indian Oceans and thus play an essential role in regulating Pacific-Indian Ocean exchange, regional air-sea interaction, and ultimately, global climate phenomena. While previous investigations using remote sensing and numerical simulations strongly suggest that this mixing is tidally driven, the impacts of monsoon and El Niño Southern Oscillation (ENSO) on tidal mixing in the Indonesian seas must play an important role. Here we use high-resolution sea surface temperature from June 2002 to June 2021 to reveal monsoon and ENSO modulations of mixing. The largest spring-neap (fortnightly) signals are found to be localized in the narrow passages/straits and sills, with more vigorous tidal mixing during the southeast (boreal summer) monsoon and El Niño than that during the northwest (boreal winter monsoon) and La Niña. Therefore, tidal mixing, which necessarily responds to seasonal and interannual changes in stratification, must also play a feedback role in regulating seasonal and interannual variability of water mass transformations and Indonesian throughflow. The findings have implications for longer-term variations and changes of Pacific–Indian ocean water mass transformation, circulation, and climate. [ABSTRACT FROM AUTHOR]

Twelve years of remotely sensed all-sat merged chlorophyll-a concentration unveils strong signatures of chlorophyll-a blooms along the south Java coast. An unprecedented three-times increase in chlorophyll-a concentration is significantly observed along the south Java coast during the southeast monsoon (June–October) than the northwest monsoon (December–April). The multiple regression analysis of dynamic factors evidently indicates that seasonal upwelling is predominantly controlled by the seasonally evolving coastal eddies associated with the seasonally reversing south Java coastal currents (SJCC) and Ekman mass transport (EMT), followed by the relative roles of sea surface temperature (SST) and wind stress curl. The eddy-induced upwelling and EMT-induced coastal upwelling lead to chlorophyll-a blooms during southeast monsoon, well-supported by the entrainment of cold and saline waters (thermocline doming) with low spiciness. On the other hand, the coastal eddies associated with SJCC and SST anomalies play a significant role in modulating the interannual surface chlorophyll-a variability in the domain. Intense chlorophyll-a blooms are observed during the positive IOD years, whereas the least chlorophyll-a concentration is observed during the negative IOD years. The unprecedentedly least chlorophyll-a concentrations during 2010 and 2016 are attributed to the intense and prolonged surface marine heatwaves. [ABSTRACT FROM AUTHOR]

Abstract Background Dengue is rapidly expanding climate-sensitive mosquito-borne disease worldwide. Outbreaks of dengue occur in various parts of India as well but there is no tool to provide early warning. The current study was, therefore, undertaken to find out the link between El Niño, precipitation, and dengue cases, which could help in early preparedness for control of dengue. Methods Data on Oceanic Niño Index (ONI) was extracted from CPC-IRI (USA) while the data on monthly rainfall was procured from India Meteorological Department. Data on annual dengue cases was taken from the website of National Vector Borne Disease Control Programme (NVBDCP). Correlation analysis was used to analyse the relationship between seasonal positive ONI, rainfall index and dengue case index based on past 20 years’ state-level data. The dengue case index representing ‘relative deviation from mean’ was correlated to the 3 months average ONI. The computed r values of dengue case index and positive ONI were further interpreted using generated spatial correlation map. The short-term prediction of dengue probability map has been prepared based on phase-wise (El Niño, La Niña, and Neutral) 20 years averaged ONI. Results A high correlation between positive ONI and dengue incidence was found, particularly in the states of Arunachal Pradesh, Chhattisgarh, Haryana, Uttarakhand, Andaman and Nicobar Islands, Delhi, Daman and Diu. The states like Assam, Himachal Pradesh, Meghalaya, Manipur, Mizoram, Jammu & Kashmir, Uttar Pradesh, Orissa, and Andhra Pradesh shown negative correlation between summer El Niño and dengue incidence. Two - three month lag was found between monthly ‘rainfall index’ and dengue cases at local-scale analysis. Conclusion The generated map signifies the spatial correlation between positive ONI and dengue case index, indicating positive correlation in the central part, while negative correlation in some coastal, northern, and north-eastern part of India. The findings offer a tool for early preparedness for undertaking intervention measures against dengue by the national programme at state level. For further improvement of results, study at micro-scale district level for finding month-wise association with Indian Ocean Dipole and local weather variables is desired for better explanation of dengue outbreaks in the states with ‘no association’.

Abstract The Indonesian sea is the only low-latitude pathway connecting two tropical oceans, which plays an important role in the coupled ocean-atmosphere mode in the Indo-Pacific sector. A small change in the sea surface temperature (SST) in the Indonesian sea has a significant influence on the precipitation and air-sea heat flux. During the past 33 years, the SST in the Indonesian sea has indicated a warming trend on the average of 0.19 ± 0.04 decade−1, which is larger than global SST warming trend. Moreover, the warming trend indicates seasonal variations, in which maximum trend occurred during boreal summer season. Investigation on the potential driver for this warming trend, namely, the net surface heat flux, resulted in an opposite trend (cooling trend), while the Ekman pumping and the wind mixing only play a minor role on the SST warming. Here, we proposed the upper layer process associated with an increasing trend in precipitation and decreasing trend in mixed layer depth (MLD) for the SST warming within the Indonesian seas. Shoaling of MLD gives a favorable condition for the surface heat flux to warm the surface ocean. However, shoaling of MLD could not solely explain the total SST warming within the Indonesian seas. The seasonal dependence in the warming trend, highest during boreal summer, was significantly related to the Indo-Pacific climate modes, namely the negative Indian Ocean Dipole (IOD) and La Niña events. Higher warming trend observed in the south of Makassar Strait and in the eastern Indonesian seas, in the vicinity of the Maluku Sea and the northeastern part of the Banda Sea, was significantly associated with the La Niña event. Meanwhile, strong warming trend observed in the Karimata Strait and Java Sea, and in the Flores Sea south of Sulawesi Island seems to be enhanced by the negative IOD event. Our rough quantitative estimate of the possible mechanism leading to the SST warming suggests that other mechanism might be at work in warming the SST within the Indonesian seas. Horizontal heat advection associated with an increasing trend of the heat transport from the Pacific into the Indian Ocean by the Indonesian Throughflow (ITF) might play a role in causing the warming trend within the Indonesian seas. However, to what extend this heat advection could modulate the SST warming is still unresolved in the present study. Further study based on realistic model output as well as long-term observational records is necessary to describe the dynamics underlying the warming trend within the Indonesian seas.

This study investigated the interannual variability of yellowfin tuna (Thunnus albacares) and bigeye tuna (Thunnus obesus) catches in the southwestern tropical Indian Ocean (SWTIO) over 25 years and its relationship to climate variability. The results indicate that the catch amount in the northern SWTIO exhibits a significant relationship with the temperature, salinity, and current variability in the upper ocean (< 400 m), associated with a significant subsurface upwelling variability, which is prominent only in the northern region. An increase of the tuna catches in the northern region is associated with the deepening of the thermocline depth and 20°C isotherm depth of the Seychelles–Chagos Thermocline Ridge, indicating suppression of the subsurface upwelling. Further analysis reveals that the catch amounts in the SWTIO tend to increase during the positive phase of the Indian Ocean Dipole. However, the catch variability in the northern SWTIO is more closely related to the El Niño–Southern Oscillation than the Indian Ocean Dipole. Favorable conditions for catches seem to develop in the northern region during El Niño years and continue throughout the following years. This relationship suggests the potential predictability of catch amounts in the northern SWTIO, an energetic region with strong subsurface upwelling variability.