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In this study, we applied the conditional nonlinear optimal perturbation (CNOP) method to generate nonlinear fast‐growing initial perturbations for ensemble forecasting, aiming to assess the effectiveness of the CNOP method in improving the forecast skill of climate events. Our findings reveal a significant improvement in the forecast skill of the Indian Ocean Dipole (IOD) within the CNOP ensemble forecast, particularly at long lead times, thereby extending the skilful forecast lead times. Notably, this improvement is more prominent for strong IOD events, with skilful forecast lead times exceeding 12 months, outperforming many current state‐of‐the‐art coupled models. The high forecast skill of the CNOP method is primarily attributed to its ability to capture the uncertainties in the wind anomaly field in the eastern Indian Ocean (EIO) closely associated with IOD evolution. Consequently, CNOP ensemble members exhibit significant deviations from the control forecast, resulting in a large ensemble spread encompassing IOD evolution. Furthermore, a comparison with the climate‐relevant singular vectors (CSV) method in terms of IOD and El Niño–Southern Oscillation (ENSO) predictions reveals the superior performance of the CNOP ensemble forecast. Despite the initial perturbations for ensemble forecasting being generated aimed at improving IOD forecast skill, the CNOP method significantly improves the forecast skill of both IOD and ENSO events, with a greater improvement for ENSO. Additionally, the CNOP ensemble forecast system provides more reliable estimates of forecast uncertainties and exhibits higher reliability with increasing lead times. In conclusion, the CNOP method effectively captures the nonlinear physical processes of climate events and improve the forecast skill. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) mode exerts distinct impacts on the climate in China and can further affect tropospheric ozone. Using long‐term GEOS‐Chem simulations, we found distinct changes in planetary boundary layer ozone throughout China during positive and negative phases of IOD. In summer, ozone shows synchronized increases except in southern China during positive IOD; the ozone increases are dominated by chemical production and transport in northern and western China, respectively. The increased precursor from biogenic emissions contributed to ozone chemical formation in the northern region, and the increased precipitation and decreased solar radiation hindered ozone production in southern China. Ozone changes show good symmetry over most regions during negative IOD. In autumn, the ozone reduction in southern China shares the same reason as summer, while the chemical increase over northern China is affected more by changes in solar radiation and relative humidity than in the precursor emission. Plain Language Summary: Indian Ocean Dipole (IOD) is an important climate variability in the Indian Ocean; here, we found that the variation of IOD from its neutral state to positive or negative phases can strongly modulate ozone within planetary boundary layer (PBL) in China. In summer, positive IOD generally increases ozone over the entire China; northern and western China experience ozone increases mainly due to chemical reactions and regional transport, while southern China sees hindered ozone production from factors like increased rainfall. During autumn for positive events, the ozone reduction in southern China shares the same reason as summer, while northern China is affected more by local weather conditions such as sunlight and humidity rather than increases in reactants during summer. Ozone exhibits opposite changes in negative IOD events due to reverse effects. This research indicates that the IOD's phases influence PBL ozone in China, with varying effects depending on the regions and seasons. Key Points: In summer, positive Indian Ocean Dipole (IOD) induces a synchronous increase in planetary boundary layer (PBL) ozone up to 5% throughout China, and vice versa for negative IODPBL ozone increases over northern and western China in summer for positive IOD is dominated by precursor increase and regional transportPBL ozone increases in autumn are primarily driven by heightened chemical production influenced by meteorological conditions [ABSTRACT FROM AUTHOR]

This study investigates the dominant modes of interannual variability of snowfall frequency over the Eurasian continent during autumn and winter, and explores the underlying physical mechanisms. The first EOF mode (EOF1) of snowfall frequency during autumn is mainly characterized by positive anomalies over the Central Siberian Plateau (CSP) and Europe, with opposite anomalies over Central Asia (CA). EOF1 during winter is characterized by positive anomalies in Siberia and negative anomalies in Europe and East Asia (EA). During autumn, EOF1 is associated with the anomalous sea ice in the Kara–Laptev seas (KLS) and sea surface temperature (SST) over the North Atlantic. Increased sea ice in the KLS may cause an increase in the meridional air temperature gradient, resulting in increased synoptic-scale wave activity, thereby inducing increased snowfall frequency over Europe and the CSP. Anomalous increases of both sea ice in the KLS and SST in the North Atlantic may stimulate downstream propagation of Rossby waves and induce an anomalous high in CA corresponding to decreased snowfall frequency. In contrast, EOF1 is mainly affected by the anomalous atmospheric circulation during winter. In the positive phase of the North Atlantic Oscillation (NAO), an anomalous deep cold low (warm high) occurs over Siberia (Europe) leading to increased (decreased) snowfall frequency over Siberia (Europe). The synoptic-scale wave activity excited by the positive NAO can induce downstream Rossby wave propagation and contribute to an anomalous high and descending motion over EA, which may inhibit snowfall. The NAO in winter may be modulated by the Indian Ocean dipole and sea ice in the Barents-Kara-Laptev Seas in autumn. [ABSTRACT FROM AUTHOR]

The equatorial Kelvin waves, remotely excited by basin‐scale climate modes, and subsequent coastal trapped waves significantly influence the intraseasonal variations, their low‐frequency modulations, and the frequency of extreme sea level events along the western coast of India. This study demonstrates that the frequency of extreme events are linked to the phase of the Indian Ocean Dipole mode. The temporal changes in the occurrence frequency of extremes are simulated in an eddy‐resolving ocean model consistently with observations. However, a non‐eddying model significantly underestimate the occurrence frequency of extreme sea level events, suggesting the importance of coastal trapped wave propagations regulated by the horizontal scale with the Rossby radius of deformation. This result implies that many state‐of‐the‐art climate models with a one‐degree ocean horizontal resolution may underestimate future coastal sea level variability and the frequency of extreme events under global warming and potential modulations of major internal climate modes. Plain Language Summary: Sea level variations in the northern Indian Ocean are influenced by ocean waves near the coast, typically in a horizontal scale less than 100 km. It remains unclear whether there is a relationship between extreme events associated with coastal waves and climate variability. Also, if such a relationship exists, it is uncertain how well it is represented in climate simulations, which often have relatively coarse horizontal resolution. To highlight the role of relatively small scale coastal waves, this study compared sea level variations along the western coast of India using two ocean models with coarse and fine horizontal resolutions. We found that the high‐resolution model adequately simulates the generation and propagation of coastal waves, and thus successfully simulate sea level variations along western India modulated by large scale climate variability with a 20–150‐day time scale. This result suggests that many recent climate simulations may have underestimated the frequency of extreme sea level events in coastal regions. Key Points: The eddy‐resolving model represents the intrasesonal sea level variability along the coast of India explained by coastal trapped wavesThe occurrence frequency of extreme intraseasonal sea level anomalies is significantly underestimated in the non‐eddying modelChanges in the probability distributions of sea level associated with the Indian Ocean Dipole are simulated in the eddy‐resolving model [ABSTRACT FROM AUTHOR]

This research investigates the origins of high-salinity water in the southern Bay of Bengal (SBHSW) in summer (June to August) and its interannual variabilities associated with El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). The two distinctive sources of the SBHSW, including the eastern Arabian Sea (AS) origin and the western equatorial Indian Ocean (EIO) origin mentioned in the previous studies are identified. The eastern AS origin has much higher salinity (35.5 PSU vs. 35.2 PSU), shallower isopycnal layers (1023.0–1023.5 kg m-3 vs. 1024.0–1024.5 kg m-3), and intrudes into the Bay of Bengal (BOB) approximately one month later compared to the western EIO origin, while accounting for 53% of the total salinity variability compared to 47% contribution from the western EIO origin. The interannual variabilities of SBHSW are closely linked with ENSO and IOD events, with contributions of approximately 47% and 41%, respectively. During El Niño decaying years, high-salinity water export from the western EIO origin into the southern bay (< 7°N) is enhanced due to the anomalous strong spring Equatorial Undercurrent (EUC) associated with El Niño; while in the north of 7°N, the SBHSW was freshened both by the increased rainfall and by the downwelling associate with the anomalous anticyclonic wind. Similar processes operate but in an opposite direction during La Niña decaying years. In contrast, IOD exerts its influence on the SBHSW primarily through modulating the strength of SMC. Positive (negative) IOD caused the anomalous freshening (salty) SBHSW by weakening (enhancing) SMC and consequently advecting less (more) saline water into the BOB. [ABSTRACT FROM AUTHOR]

Historically, El Niño events have consistently signalled below-average monsoon rainfall in India excluding years like 1997. Despite 2023 being an El Niño year, India experienced normal monsoon seasonal rainfall (-6% of the Long Period Average: LPA) with above-average rainfall (+ 13% of LPA) during July and September but unadorned deficit rainfall in August (-36% of LPA). Thus, the complex relationship of El Niño with the Indian summer monsoon rainfall (ISMR) is apparently evident during summer 2023. Monthly rainfall variations starkly challenge conventional hypotheses, necessity for a profound understanding of the dynamics behind them. During June 2023, El Niño triggered a robust midlatitude upper-level Rossby wave over the north central Pacific. The propagation of wave energy along the westerly jet is further evidenced by wave activity flux. Specifically in August, this waveguide induced upper-level cyclonic circulation cantered around north China with strong northerly wind anomalies in its western flank supports the low-level cyclonic circulation with a southward tilt to the south of Japan in response to the equivalent Barotropic structure. This coupling intensified the anomalous cyclonic circulation over the WNP, bolstered by potent El Niño influence and the phase of the Madden–Julian Oscillation (MJO). These anomalies facilitated moisture transfer from the monsoon region to the WNP, as a result excess (deficit) rainfall is evident over the WNP (ISMR) region due to strong large-scale ascending (descending) in August 2023. During July and September, in contrast, the absence of a midlatitude Rossby wave, the prevailing positive Indian Ocean Dipole (IOD) and MJO offset this El Niño induced rainfall deficiency. El Niño impact on monsoon rainfall is significant, however, this observational study highlights the pivotal roles of IOD, MJO, and midlatitude circulation patterns. Their interplay creates substantial rainfall variability, emphasizing the complexity of El Niño-monsoon teleconnections. [ABSTRACT FROM AUTHOR]

This study examines direct current measurements in the central equatorial Indian Ocean during the positive phase of the extreme Indian Ocean Dipole (pIOD) in 2019. Analysis of near-surface zonal current at 77°E and 83°E reveals a notable delay in the onset of fall Wyrtki Jets (WJs), typically occurring in October. The WJs formed in December but persisted for a shorter duration. On the other hand, the subsurface currents display a strong eastward flow persistent from October 2019 to June 2020 which is abnormal to a normal condition. The anomalous subsurface zonal currents exhibited magnitudes three times stronger during pIOD 2019 compared to pIOD composites. Due to the continued sub-surface flow, wavelet analysis revealed a dominant semi-annual cycle near the surface and an annual cycle in the subsurface layers. To explore the underlying processes behind these anomalies, we used the Modular Ocean Model in conjunction with a linear, continuously stratified model. Our experiments revealed that the weakening of fall Wyrtki Jets in 2019 was driven by anomalous easterlies associated with the extreme pIOD phase. However, the persistence of eastward subsurface currents was attributed to anomalous westerlies in the western Indian Ocean. These westerlies generated Kelvin waves propagating eastward, which upon impinging the eastern boundary, reflected and propagated downwards as Rossby waves, ultimately intensifying the subsurface currents. Overall, our study underscores the significance of equatorial wave dynamics in driving currents during the extreme IOD event. [ABSTRACT FROM AUTHOR]

Droughts and climate change are causing severe and persistent dust storms in the arid and hot regions. The potential existence of significant relationships between the Oceanic Niño Index (ONI), Indian Ocean Dipole (IOD), and the MERRA-2 dust column mass density data are thus investigated in the State of Kuwait during 1990–2021 as a study case in the hot and arid regions. The findings show that the correlation starts from ONI-DJF (December–January-February) with the Pearson test value of -0.36 and increasing to a value of -0.55 on ONI-AMJ (April-May-June), the same results were obtained when applying the Spearman correlation test. The correlation exists between the July dust column mass density and IOD-January with a value of -0.484 and − 0.544 over Pearson and ' 'Spearman's test, respectively, all estimated p-values are lower than 0.05. The results of the One-Way ANOVA show that the output is pretty straightforward and statistically significant at p > 0.05, which was confirmed by the nonparametric Kruskal–Walli's test and Mann-Whitney Test. This considerable correlation means that active dust storms are linked with La Niña, while light dust storms occur during the El Niño event. A positive correlation occurs between precipitation and ONI - SON (September-October-November), while a negative correlation is found between precipitation and IOD-July. Our preliminary findings in this paper indicate that an appearance of -0.7°C in ONI-DJF could serve as a definitive marker of the La Nina phase and an indicator for at-risk communities to prepare for the expected activity dust storm. [ABSTRACT FROM AUTHOR]

This study investigated relationships between year‐to‐year variability in precipitation in the rainy season in Mozambique and major modes of climate variability in the Tropics and subtropics. The Niño3.4 index was strongly negatively correlated with precipitation in Mozambique's southern and central regions. We suggest that Rossby wave propagation reaching Southern Africa from the tropical Pacific is key to the relationship between precipitation in Mozambique and El Niño–Southern Oscillation. Subtropical Indian Ocean Dipole did not lead rainy‐season precipitation, but showed a simultaneous correlation with precipitation in southern, central and northeastern regions. Benguela Niño was found to have a significant positive lead correlation by 6 months with precipitation in the southern, central and northwestern regions. In contrast, Indian Ocean Dipole led precipitation in the southern, central and northeastern regions by 3 months. Overall, the modes of climate variability exerted stronger control over precipitation variability in southern and central Mozambique, and weaker control in northern Mozambique, particularly in the northwestern region. [ABSTRACT FROM AUTHOR]

The phytoplankton size structure exerts a significant influence on ecological processes and biogeochemical cycles. In this study, the interannual variations in remotely sensed phytoplankton size structure in the southern Tropical Indian Ocean (TIO) and the underlying physical mechanisms were investigated. Significant interannual fluctuations in phytoplankton size structure occur in the southeastern TIO and central southern TIO and are very sensitive to Indian Ocean Dipole (IOD) events. During positive IOD events, the southeast wind anomalies reinforce coastal upwelling off of Java and Sumatra, leading to a shift toward a larger phytoplankton structure in the southeastern TIO. The anomalous anticyclonic circulation deepened the thermocline and triggered the oceanic downwelling Rossby waves, resulting in a smaller phytoplankton structure in the southwestern TIO. During the decay phase of the strong positive IOD events, the sustained warming in the southwestern TIO induced basin-wide warming, thereby maintaining such an anomalous phytoplankton size structure into the following spring. The response of phytoplankton size structure and ocean dynamics displayed inverse patterns during the negative IOD events, with an anomalous larger phytoplankton structure in the central southern TIO. These findings enhance our understanding of phytoplankton responses to climate events, with serious implications for ecosystem changes in a warming climate. [ABSTRACT FROM AUTHOR]

The time-varying September-November relationship between the Indian Ocean Dipole (IOD) and Central African (CA) rainfall has strengthened since the 1990s, implying an increasing IOD influence over CA rainfall. Using observational and reanalysis datasets covering the 1980–2016 period, this study examines the CA circulation response associated with the Indian Ocean dynamics during the September-December IOD events, since this circulation constitutes a key moisture transport feature for CA rainfall variability. The results show that during positive IOD events (pIOD), the moisture transport drivers over CA and the Indian Ocean (IO) are synchronous, leading to an increase in CA rainfall, whereas the reverse pattern is observed during negative IOD events (nIOD). The equatorial easterly (westerly) moisture transport driven by the anticyclonic (cyclonic) circulation in the northern tropical IO and the weakening (intensification) of the African Easterly Jet's northern component (AEJ-N), leads to an increase (decrease) in CA rainfall during pIOD (nIOD). Warm (cold) SST anomalies in the eastern Indian Ocean during nIOD (pIOD) event, intensify (weaken) the large-scale upward motion, strengthening (weakening) the cyclonic circulation in the mid-troposphere, thus favoring a significant westerly (easterly) circulation. The AEJ-N weakening during pIOD events is associated with a strengthening of the meridional pressure gradient and a westward shift in the Saharan high location at the AEJ-N's northern edge. The results also reveal a significant influence of the Atlantic during pIOD events, induced by its teleconnection with the IO, whose effects are more modulated by the IOD's western pole warming than by the IOD-related SST gradient. [ABSTRACT FROM AUTHOR]

Understanding how models represent sub-seasonal rainfall variations and what influences model skill is essential for improving sub-seasonal forecasts and their applications. Here, empirical orthogonal function (EOF) analysis is employed to investigate weekly Eastern Africa short rains variability from October to December. The observed leading EOF modes are identified as (i) a monopole-like rainfall pattern with anomalies impacting southern Ethiopia, Kenya, and northern Tanzania; and (ii) a dipole-like rainfall pattern with contrasting anomalies between Tanzania and the northeastern sector of Eastern Africa. An examination of the links between the leading modes and specific climate drivers, namely, the Madden–Julian Oscillation (MJO), El Niño–Southern Oscillation, and Indian Ocean Dipole (IOD), shows that the MJO and IOD have the highest correlations with the two rainfall modes and indicates that the monopole (dipole)-like rainfall pattern is associated with MJO convective anomalies in the tropical Indian Ocean and western Pacific (Maritime Continent and Western Hemisphere). Assessments of model ability to capture and predict the leading modes show that the European Centre for Medium-Range Weather Forecasts (ECMWF) and the UK Met Office models outperform the National Centers for Environmental Prediction model at forecast horizons from one to four weeks ahead. Amongst the drivers examined, the MJO has the largest impact on the forecast skill of rainfall modes within the ECMWF model. If MJO-related variability is reliably represented, the ECMWF model is more skilful at predicting the main modes of weekly rainfall variability over the region. Our findings can support model developments and enhance anticipatory planning efforts in several sectors, such as agriculture, food security, and energy. [ABSTRACT FROM AUTHOR]

The teleconnections of the two types of the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on autumn rainfall over Southeast Asia and its 20 subregions are investigated during 1979–2019. Under El Niño, autumn rainfall reduces over the West Philippines and most of the Maritime Continent, while Indochina experiences alternating dry and wet conditions. Under El Niño Modoki, more rainfall reduction is evident in Indochina, East Philippines, Malay Peninsula, and North Sumatra. Conversely, El Niño Modoki causes less dry conditions than El Niño in southern Southeast Asia. La Niña and La Niña Modoki tend to increase autumn rainfall, except in Sumatra and some specific areas in Indochina. However, La Niña Modoki causes notably more rainfall than La Niña in northern Southeast Asia, including Indochina (except Myanmar), the Malay Peninsula, and Philippines. Although insignificant, La Niña Modoki also reproduces more rainfall in eastern Indonesia, whereas displaying less rain in the central and western Maritime Continent. These distinctions of ENSO Modoki compared to ENSO are driven by different sea surface temperature anomalies patterns, leading to a more northward Walker circulation and the presence/strength of an anomalous Philippine Sea cyclone. Positive IOD (negative IOD) also generally results in drier (wetter) autumn, with the rainfall anomaly patterns of positive IOD/negative IOD exhibiting similarities to El Niño/La Niña. However, compared to both types of ENSO, IOD impacts are weaker over Pacific-facing subregions while only stronger in specific equatorial Indian Ocean-facing subregions. This stronger impact comes from reduced (intensified) moisture transport from the equatorial Indian Ocean to southwestern Southeast Asia under positive IOD (negative IOD). [ABSTRACT FROM AUTHOR]

The study employs a hybrid Employing a hybrid ensemble empirical mode decomposition-temporal convolutional network (EEMD-TCN) machine learning approach to predict the IOD index and sea surface temperatures for various time spans (3, 6, 9, and 12 months ahead), and explores changes in the relationship between IOD and Indian summer monsoon rainfall (ISMR) across India from 1960 to 2020, considering risk and uncertainty. The methodology involves decomposing the IOD index and SST into consistent subcomponents using EEMD, followed by TCN modeling to forecast these subcomponents, resulting in predictions affected by risk and uncertainty. The IOD index's forecast precision surpasses that of SSTs, attributed to the higher number of high-frequency elements in the SST data, posing a greater challenge in prediction risk and uncertainty. The research also examines anomalies in rainfall, SST, and low-level wind circulation, highlighting the impact of IOD events. ISMR across India shows sensitivity to IOD events, with positive IOD (pIOD) events linked to increased rainfall and negative IOD (nIOD) events to decreased rainfall, except during the initial phase of the Indian summer monsoon, emphasizing the role of risk and uncertainty. Variations in SST, wind circulation, and moisture transport mechanisms in the Indian Ocean lead to notable precipitation changes during different IOD stages, especially from 1991 to 2020. Notably, a recent rise in the frequency of low-level equatorial jets (LEJs) in the Indian Ocean equatorial region and the Arabian Sea during pIOD events is observed, in contrast to the earlier decades of 1960–1990. [ABSTRACT FROM AUTHOR]

Studies have shown that the springtime Arctic Oscillation (AO) has a strong influence on the subsequent Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO). Nevertheless, not every AO is followed by IOD and ENSO. This study divides the spring AO into three types: followed by both IOD and ENSO (AO-Both), followed by ENSO only (AO-ENSO), and followed by IOD only (AO-IOD). In the AO-Both type, the Pacific component of the AO, which is characterized by a dipole pattern, induces sea surface temperature (SST) anomalies in the subtropical North Pacific that extend southward to the tropical Pacific via a wind-evaporation-SST (WES) feedback. The generated tropical SST anomalies influence subsequent ENSO via tropical air-sea interaction and impact autumn IOD via modulation of the tropical Walker circulation. In the AO-ENSO type, the Pacific component of the AO is featured by a mono-pole anomaly pattern with a center over the mid-latitude northeast Pacific. The northeast Pacific atmospheric anomalies propagate southward and lead to surface zonal wind anomalies in the tropical central Pacific via the WES feedback that influence subsequent ENSO via tropical air-sea interaction. In the AO-IOD type, the Pacific component of the AO is weak. Spring AO-related atmospheric heating anomalies over the North Atlantic excite a wave train extending from the North Atlantic to the western Pacific. The associated low-level wind anomalies induce SST anomalies around the Maritime Continent, which play a crucial role in influencing subsequent IOD. This study suggests that the spatial pattern of the AO plays a critical role in determining its impact on tropical climate variability. [ABSTRACT FROM AUTHOR]

This study reveals a strong relationship between the North Pacific Meridional Mode (PMM) in boreal spring and the Indian Ocean Dipole (IOD) in following autumn. A positive spring PMM tends to be followed by a positive IOD and vice versa. The mechanism for the influence of the PMM on the IOD is then investigated. Positive spring PMM-related cyclonic and SST warming anomalies over the subtropical North Pacific propagate southward to the equatorial central Pacific in the following summer via wind-evaporation-SST feedback. SST warming and enhanced atmospheric heating in summer in the equatorial central Pacific induce an anomalous Walker circulation with ascending anomalies over the tropical central Pacific and descending anomalies over the Maritime Continent. The descending anomalies over the Maritime Continent result in southeasterly wind anomalies off the west coast of Sumatra, which lead to cold SST anomalies in the southeastern tropical Indian Ocean via modulating surface heat flux and upwelling of cold water. The associated increase in zonal SST gradient in the tropical Indian Ocean leads to low-level easterly wind anomalies and contributes to warm SST anomalies in the western tropical Indian Ocean via oceanic dynamic process. The warm and cold SST anomalies in the tropical western and southeastern Indian Ocean further develop to an IOD event in the following autumn via a positive air-sea interaction. The above process for the PMM influence on IOD can be simulated in the long historical simulations of coupled climate models. This study suggests that the spring PMM is a potential precursor for the occurrence of IOD event in the following autumn. [ABSTRACT FROM AUTHOR]

In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection. [ABSTRACT FROM AUTHOR]

Heavy rainfall due to tropical cyclones (TCs) in the North Indian Ocean (NIO) adversely impacts nations frequently. Though extensive research has focused on TCs in the NIO, less attention has been given to the connection between TCs and extreme events in Sri Lanka. This study examined atmospheric characteristics during sixteen extreme events, focusing on linkages between TCs, the Indian Ocean Dipole (IOD), and mechanisms behind heavy rainfall associated with TCs over Sri Lanka. The results showed that in the pre-monsoon period, TCs move northward with high water vapor (WV) content accumulated in the Southern Hemisphere. This main WV flow over the equatorial Indian Ocean (EIO) is connected with TCs, causing considerable damage in the southwestern part of Sri Lanka. During negative IOD years, strong westerly winds create cyclonic circulations on either side of the equator. Conversely, during the post-monsoon period, the IOD phase has no significant effect. TCs generally followed westward tracks, supported by winds from the Northern Hemisphere, and caused heavy rainfall in the Eastern, Northern, and Northcentral provinces in Sri Lanka. These TCs are isolated from the main WV flow over EIO. Such observed common characteristics during pre-monsoon and post-monsoon seasons are key factors contributing to extreme rainfall in Sri Lanka. [ABSTRACT FROM AUTHOR]

Atmospheric Rivers (ARs), corridors of intensive water vapor transport, have received much attention recently due to their potentially large economic and societal impacts. However, a comprehensive understanding of the interannual variability of ARs over East Asia is still lacking. This study examines the frequency of AR occurrence over East Asia based on the EAR5 reanalysis. It is shown that the frequency of AR occurrence over East Asia displays obvious seasonality, with its peak in early boreal summer. ARs contribute to up to 50% of total precipitation and largely modulate the interannual variability of precipitation in East Asia during early boreal summer. The interannual variability of ARs over East Asia during early boreal summer is largely controlled by the Indo-western Pacific Ocean Capacitor (IPOC) effect, which is mainly triggered by El Niño-Southern Oscillation and sometimes by Indian Ocean dipole. During early boreal summer, a higher frequency of AR occurrence over East Asia is associated with anomalous anticyclonic circulation (AAC) over the Indo-western Pacific region. The AAC, the atmospheric signal of the IPOC mode, indicates an enhanced East Asian summer monsoon and a northwestward shift of the western North Pacific subtropical high, favoring the AR occurrence over East Asia. This study illustrates that a strong positive Indian Ocean dipole event could impact AR activity over East Asia in the following summer by triggering the IPOC effect. The IPOC-related signals 2–4 months ahead could be useful to predict the AR occurrence over East Asia during early boreal summer. [ABSTRACT FROM AUTHOR]

In this study, the diverse features of four strongest negative Indian Ocean dipole (nIOD) (1996, 1998, 2010 and 2016) since 1980 are quantitatively examined. The contributions of air-sea feedback to La Niña-related sea surface temperature (SST) anomalies (SSTA) are identified and their association with surface winds as well as different time-scale variations is compared. The growth of SSTA in Dipole Mode Index (DMI) indicates one event (1996) being a dipolar pattern and the other three (1998, 2010 and 2016) being monopolar patterns during their mature phases. During 1996 case, SSTA in west (IOD-W) and east (IOD-E) poles are both significant and comparable though with different physical origins. Effective Bjerknes-feedback dominated in accumulating warm-water in IOD-E and Madden–Julian oscillation (MJO) activity constructively enhances this IOD-E warming, while the heat loss induced by the wind-evaporation-SST (WES) and cloud-radiation-SST feedback lead to a fast and profound cooling in IOD-W. Developing on a non-La Niña background, the IOD-E warming is initiated with positive heat flux anomalies that lead the linear temperature advection by 1-month(s), indicating this dipole pattern as a local independent event that possibly triggered by interior-basin disturbances in the Indian Ocean (IO). Three monopolar cases all exhibited above 3.0 standard deviations warm peaks over IOD-E but with extremely weak signals in IOD-W. Though the Bjerknes feedback and heat-flux play crucial roles over the three monopolar cases with the strongest in 2016, the initial zonal winds are first established through atmospheric teleconnection without the leading involvement of heat flux. The concurrent extreme La Niña in 1998 and 2010 triggered strong westerly wind anomalies over the IO and favored the rapid growth of these monopolar SSTA, but La Niña influence stronger in 2010. The damping effect of nonlinear advection and the triggering effect of long-stalled MJO convective anomalies played important roles in the 1998 and 2016 monopolar cases. [ABSTRACT FROM AUTHOR]

The formation mechanisms of the Indian Ocean Dipole (IOD) have been studied extensively. However, few studies have considered the influence of the South Pacific atmosphere and sea surface temperature (SST) variability on IOD formation. In this study, reanalysis data, CMIP6 model outputs, and linear baroclinic model experiments were used to investigate this influence. The results revealed a close connection between the preceding boreal winter–spring Southwest Pacific Dipole (SWPD) SST anomaly (SSTA) pattern and the subsequent IOD and associated physical mechanisms. Facilitated by the local air–sea positive feedback, the positive SWPD, coupled with low-level cyclonic circulations, persists from the preceding winter to early summer. During early summer, the below-normal precipitation and diabatic heating associated with the SWPD western pole cooling strengthens and induces an anticyclonic circulation on its western side as the climatological southeasterly winds encroach northward. This anticyclonic circulation extends from southern Australia into the tropical southeastern Indian Ocean, favoring enhanced southeasterly winds and cooling off Java-Sumatra. Subsequently, the IOD develops in summer–autumn with the aid of wind–evaporation–SST and wind–thermocline–SST positive feedback. The westward downwelling Rossby wave and eastward upwelling Kelvin wave associated with surface wind anomalies also contribute to the IOD western pole warming and eastern pole cooling, respectively. Furthermore, the IOD interacts with El Niño through Walker circulation and increases its amplitude. These findings are supported by the results obtained from the linear baroclinic model and CMIP6 historical simulations and can potentially improve the predictability of the IOD. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between circulation patterns and austral summer temperature anomalies in southern Africa. The results show that the formation of continental lows tends to increase the thickness of the lower atmosphere. Further, the distinct variabilities of high and low pressure under the circulation types, influence air mass advection from the adjacent oceans, as well as atmospheric stability over land. Stronger anticyclonic circulation at the western branch of the Mascarene high-pressure system enhances the low-level cold air advection by southeast winds, decreases the thickness, and lowers the temperature over a majority of the land in southern Africa. Conversely, a weaker Mascarene High, coupled with enhanced cyclonic activity in the southwest Indian Ocean increases low-level warm air advection and increases temperature anomalies over vast regions in southern Africa. The ridging of a closed South Atlantic anticyclone at the southern coast of southern Africa results in colder temperatures near the tip of southern Africa due to enhanced low-level cold air advection by southeast winds. However, when the ridge is weak and westerly winds dominate the southern coast of southern Africa, these areas experience temperature increases. The northward track of the Southern Hemisphere mid-latitude cyclone, which can be linked to the negative Southern Annular Mode, reduces the temperature in the southwestern part of southern Africa. Also, during the analysis period, El Niño was associated with temperature increases over the central parts of southern Africa; while the positive Indian Ocean dipole was linked to a temperature increase over the northeastern, northwestern, and southwestern parts of southern Africa. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) with worldwide socio-economic impacts has been presented to mature either in boreal summer or autumn, leading to the classification of summer IOD and autumn IOD. Investigating the climate dynamics to distinguish between these two types of IOD can improve our understanding and prediction of the surrounding weather and climate. This study demonstrates that the emergence of the summer IOD is mainly attributed to internal air-sea interactions in the western tropical Indian Ocean (WIO), while the autumn IOD is significantly related to ENSO development. For the summer IOD, broad-scaled warm sea surface temperature anomalies in the WIO are conducive to the enhancement of convective perturbations. Then local ocean–atmosphere feedback associated with changes in convection and surface heat flux into the upper ocean plays a key role in triggering the summer IOD. For the autumn IOD, strong easterly wind anomalies in the eastern Indian Ocean initiate oceanic Rossby waves and Bjerknes feedback, leading to the formation of both the western and eastern poles. It is recognized that these intensified easterly wind anomalies mostly benefit from ENSO variability. The distinctive features and air-sea interactions intrinsic to the summer IOD and the autumn IOD revealed in this study can further contribute to more credible predictive models of diverse IOD events. [ABSTRACT FROM AUTHOR]

Marine Heat Waves (MHWs) can cause significant distress to marine environment and modulate air-sea interaction, which in turn can have economic and societal impacts. This study aims to identify surface and subsurface MHWs in the Bay of Bengal using available sea-surface temperature data and buoy observations spanning four decades. The results show significant increase in the number, frequency, duration, and intensity of surface MHWs in recent years. To better understand the relationship between MHW occurrences and different phases of two climate phenomena, namely, the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation, multiple extreme MHW event years between 2008 and 2018 are analyzed. The findings show that the surface MHWs become more prominent during El Niño/positive IOD events due to weakened winds, increased net heat flux input towards the ocean, increased stratification and warming tendency through vertical processes in the presence of inversion. This, in turn, affects the surface biological productivity in the region. Additionally, surface MHWs were also found to be driven by surface currents and eddies. On the other hand, subsurface MHWs develop during negative IOD/La Niña conditions due to the deepening of the thermocline, which is forced by coastally trapped downwelling Kelvin waves and reflected downwelling Rossby waves, and is followed by substantial freshening of deeper layers. This study provides a deeper understanding of the causes of surface and subsurface MHWs in the Bay of Bengal, which is a crucial basin in influencing monsoon and cyclonic events affecting its surrounding landmasses. [ABSTRACT FROM AUTHOR]

The extreme wind climate and its trends and variability of the southwest coast of India, particularly the Kerala coast, are investigated using hourly reanalysis wind speeds for the period 1979–2021. The observed maximum wind speed and 99th percentile wind speed off Kerala coast are 21.0 and 11.3 m·s−1, respectively. The predominant direction of high wind speeds is in the SW–WNW sector, which accounts for 93.5% of the total high winds. Extreme wind speeds in this region are not due to tropical cyclones alone, but also due to seasonal high winds. Seasonally, the premonsoon and SW monsoon extreme winds are predominantly in the WSW–WNW directional sector, while postmonsoon strong winds are present in all directional sectors, but predominantly in the SSW–WNW sector, followed by the E–SSE sector. The 99th percentile wind speeds during premonsoon, SW monsoon and postmonsoon are 8.8, 12.3 and 8.9 m·s−1, respectively. A significant decreasing trend is observed in the 99th percentile wind speeds during 1979–2021, ranging from 3 to 6 cm·s−1·year−1 off Kerala coast. This is in contrast with global increasing extreme wind speeds, and it can be attributed to regional variability as well as weakening of the land–sea thermal gradients associated with rising surface temperature in the Arabian Sea in the light of global warming. In addition to the long‐term declining trend and monthly as well as seasonal features, distinct interannual variations are also noticed in the 99th percentile wind speeds. Analysis shows that the atmospheric teleconnections associated with El Niño–La Niña events and Indian Ocean Dipole have an important role in the variability of extreme wind speeds. The co‐occurrence of positive (negative) IOD with El Niño (La Niña) results in a sharp decline (increase) in extreme wind speeds. Significant decadal variations are also noticed in the 99th percentile wind speeds. [ABSTRACT FROM AUTHOR]

Global climatic circulation indices play a major role in determining regional and global climate conditions. These atmospheric circulation patterns exhibit substantial variability, covering a wide geographical area and affecting weather-related events. The primary goal of this study was to examine and characterize various global climatic variability indices during the 1950 to 2020 period (El Niño Southern Oscillation, ENSO; Southern Oscillation Index, SOI; North Atlantic Oscillation, NAO; Atlantic Meridional Mode, AMM; and Indian Ocean Dipole, IOD). Also, this article try to investigating the link between these global climatic indices. Trend analysis showed that the ENSO index exhibits the highest recurrence frequency of correlation relationships with the other yearly global indices with significance at the 95% and 99% levels, while the NAO index exhibits the lowest recurrence frequency. On a seasonal basis, most indices demonstrate more abrupt changes during the winter season than during the summer. An increase occurred in events of abrupt changes in these indices over the last two decades (2000 to 2020), especially annually and in summer. The SOI exhibits the largest number of abrupt changes throughout the entire study period, spanning from positive to negative significant trends, whereas the IOD did not exhibit abrupt changes annually. Increasing and decreasing trends in the global climatic circulation indices may be related to natural and anthropogenic causes of climate change. Regarding both the correlation coefficient (CC) and partial correlation results, there existed a highly negative association between the ENSO and SOI in the annual, winter and summer time series. On the other hand, there is no relationship between ENSO and NAO. Furthermore, on an annual basis, there existed a highly negative association between the NAO and AMM and a less negative but still statistically significant association between these indices during the winter and summer seasons, respectively. Therefore, through the Azore high, the NAO could promote AMM. Moreover, when the NAO, AMM, and SOI are held constant, a positive and robust correlation is reached between the ENSO and IOD in winter season. Therefore, a developing IOD is intensified and sustained during the onset of an El Niño event in winter season. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data of monthly mean sea surface temperature (SST) from British Hadley Center and ozone mass mixing ratio from National Aeronautics and Space Administration (NASA) during 1980 -2015, two indexes IOBI and IODI of the main modes characterizing SST changes in the tropical Indian Ocean--Indian Ocean Basin (IOB) and Indian Ocean Dipole (IOD) were calculated firstly, and then the correlation of SST anomaly (SSTA) in the tropical Indian Ocean and ozone mass mixing ratio in the stratosphere over East Asia from 1980 to 2015 was analyzed. Besides, the impact of SST changes in the tropical Indian Ocean on the distribution of ozone layer in East Asia was discussed. The results show that SST changes in the tropical Indian Ocean had significant effects on stratospheric ozone distribution in East Asia, and it was consistent with the temporal changes of IOB and IOD. IOBI and IODI had a certain correlation with stratospheric ozone changes in East Asia, with a particularly significant correlation in the lower stratosphere (70 hPa) and middle stratosphere (40 hPa) especially during spring and autumn. [ABSTRACT FROM AUTHOR]

We examined the Indian Ocean Dipole (IOD)'s predictive ability for El Niño‐Southern Oscillation (ENSO). Our findings indicate that positive IODs have a stronger impact on the subsequent year's ENSO compared to negative IODs. To explain this asymmetry, we proposed the "IOD‐Atlantic‐Pacific" pathway, which involves the Atlantic in boreal winter as an intermediary. The pathway comprises two stages: (a) The asymmetric "IOD‐Atlantic" connection, where positive IODs in autumn trigger winter Atlantic Niño while negative IODs cannot trigger Atlantic Niña. This asymmetry is due to the modulation of the climatological mean state on the IOD‐induced atmospheric anomalies. (b) The "Atlantic‐ENSO" connection, which is symmetric, where winter Atlantic Niño (Niña) promotes the development of La Niña (El Niño), functioning independently of ENSO's self‐oscillation behavior. The Atlantic intermediary mechanism explains the asymmetric connection between IOD and the subsequent year's ENSO and provides the IOD with independent predictive power for ENSO beyond its self‐oscillation. Plain Language Summary: The Indian Ocean Dipole (IOD) plays a role in predicting the El Niño‐Southern Oscillation (ENSO) for the next year, but there is no agreement on how exactly this works and if it is separate from ENSO‐cycle itself. This study found that only positive IODs can sufficiently predict the following year's ENSO. This is because, positive IOD can cause a winter Atlantic Niño, which then leads to the development of La Niña in the spring and summer. However, negative IOD cannot cause an Atlantic Niña. Thus, the asymmetry in the influence of IOD on winter Atlantic lead to the asymmetry in the relationship between IOD and the following year's ENSO. The asymmetry in the IOD's influence on the Atlantic is due to how the climatic conditions affect the distribution of IOD‐induced atmospheric anomalies. The "IOD‐Atlantic‐Pacific" pathway gives the IOD its own predictive power for the next year's ENSO. These results do not only advance the understanding of the physical mechanisms of the IOD on the following's ENSO but also provides a foundation for improving ENSO prediction. Key Points: Positive Indian Ocean Dipole (IOD) can effectively predict the subsequent year's El Niño‐Southern Oscillation (ENSO), while negative IOD has little predictive powerAn Atlantic intermediary mechanism is proposed to link IOD to next year's ENSO, and explain their relationship's asymmetryIOD's impact on winter Atlantic is asymmetric due to the modulation of climatological convective activity on atmospheric responses to IOD [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) plays a crucial role in shaping local and global environments, yet its effects on interannual variability of the Arabian Sea oxygen minimum zone (ASOMZ) remains poorly understood. Here, we used a coupled physical‐biogeochemical model to investigate the dynamical response of the ASOMZ to extreme negative (2016) and positive (2019) IOD events. Our findings revealed that the suboxic area of the ASOMZ reduced (expanded) by ∼27% (∼28%) after the negative (positive) IOD event. Compared to the 2019 pIOD event, approximately 2.5 times more oxygen‐rich water was delivered into the Arabian Sea during the 2016 nIOD event, replenishing dissolved oxygen (DO) consumed by intensified upwelling‐induced enhanced remineralization of particulate organic matter (POM), thereby increasing the DO concentration in the Gulf of Aden. Conversely, more POM from the western Arabian Sea was transported to the central Arabian Sea, leading to a subsequent decrease in DO concentration there. Plain Language Summary: The Indian Ocean Dipole (IOD) is a climate phenomenon that sea surface temperature in the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern Indian Ocean south of Indonesia. This variability significantly impacts global atmospheric circulation and environments. The Arabian Sea oxygen minimum zone (ASOMZ) is an area in the Arabian Sea characterized by low dissolved oxygen (DO) levels, which can have adverse effects on marine life. We used a model to examine how extreme IOD events influence the ASOMZ. The results suggested that during the negative IOD event in 2016, the suboxic area of the ASOMZ decreased by approximately 27%, while it expanded by approximately 28% during the positive IOD event in 2019. The response of the ASOMZ to IOD events in the Gulf of Aden was primarily modulated by physical factors, such as the Somali Coastal Current and local upwelling. On the other hand, the ASOMZ in the central Arabian Sea was regulated by a combination of biological and physical processes. These findings contributed to our understanding of the ASOMZ's response to IOD events, which is essential for studying the Arabian Sea's marine ecosystem. Key Points: The response of the Arabian Sea oxygen minimum zone to the Indian Ocean Dipole events in the Gulf of Aden was modulated by physical factorsThe Arabian Sea oxygen minimum zone in the central Arabian Sea was regulated by both biological and physical processesThe upper edge of the Arabian Sea oxygen minimum zone invaded the lower euphotic zone (100–200 m) under the impact of Indian Ocean Dipole events [ABSTRACT FROM AUTHOR]

The seasonal evolution and interannual variability of latent heat flux (LHF) over the tropical Indian Ocean (TIO) region is examined for the period 1980–2019. The seasonal distribution of LHF is dominated by the winter pattern of each hemisphere in the TIO. Climatologically high LHF is mostly confined to the Arabian Sea and Bay of Bengal regions in the north Indian Ocean and the zonal belt between 10oS and 25oS in the south Indian Ocean. Warm and cold phases of El‐Niño Southern Oscillation, Indian Ocean Dipole (IOD), and their co‐occurrence events leave distinct LHF signatures over the TIO. The LHF anomalies during these events are more profound than pure events with enhanced latent heat loss when the La Niña and negative IOD events coincide. The LHF anomalies associated with the wind‐evaporation‐SST feedback is prominent when positive IOD co‐occur with the El‐Niño event. Both the vertical humidity gradient and wind speed changes play substantial roles in driving LHF anomalies during the co‐occurring events of NIOD and La Niña. [ABSTRACT FROM AUTHOR]

The influence of strong Indian Ocean Dipole (IOD) events on the evolution of the following year's La Niña events is investigated using the National Marine Environmental Forecasting Center (NMEFC) operational seasonal forecasting system. The observation results show that when the strong IOD occurs, the tropical Pacific can be in different sea surface temperature states. As such prediction system can well reproduce the air-sea evolution of the 1998/1999 and 2020/2021 La Niña events, the ocean temperature initializations in December during above events were perturbed with the system to assess the role of the oceanic channel and atmospheric bridge across the maritime continent in the forecasting of the La Niña events 1 year later. In the case of the neutral state of the tropical Pacific at the peak of the 2019 positive IOD, pacemaker experiments have demonstrated that the Indian Ocean subsurface temperature initialization in December 2019 is critically important for the successful prediction of the 2020/2021 La Niña. Experiments of sea surface temperature initialization in December 2019, with only the Indo-Pacific atmospheric bridge at work, failed to predict the 2020/2021 La Niña 1 year in advance. The comparison underlined the dominant role of the oceanic channel dynamics in the evolution of the 2020/2021 La Niña. Forecasting experiments beginning from the 1997/98 El Niño with and without the IOD initializations suggest that the delayed feedback of the tropical Pacific coupled system is more important in the forecasting of the 1998/1999 La Niña, with the Indo-Pacific oceanic channel dynamics playing a secondary yet very important role. Our study has underscored the importance of the Indo-Pacific oceanic channel dynamics in ENSO prediction and suggested the effectiveness of IOD subsurface temperature initialization in La Niña predictions at the 1-year lead time. [ABSTRACT FROM AUTHOR]

Volcanic eruptions can have significant climate impacts and serve as useful natural experiments for better understanding the effects of abrupt, externally forced climate change. Here, we investigate the Indian Ocean Dipole's (IOD) response to the largest tropical volcanic eruptions of the last millennium. Post‐eruption composites show a strong negative IOD developing in the eruption year, and a positive IOD the following year. The IOD and El Niño‐Southern Oscillation (ENSO) show a long‐term damped oscillatory response that can take up to 8 years to return to pre‐eruptive baselines. Moreover, the Interdecadal Pacific Oscillation (IPO) phase at the time of eruption controls the IOD response to intense eruptions, with negative (positive) IPO phasing favoring more negative (positive) IOD values via modulation of the background state of the eastern Indian Ocean thermocline depth. These results have important implications for climate risk in low‐likelihood, high‐impact scenarios, particularly in vulnerable communities unprepared for IOD and ENSO extremes. Plain Language Summary: Volcanic eruptions were previously found to affect the El Niño‐Southern Oscillation in the Pacific, but little is known about their effect on the Indian Ocean. In this study, we use simulations of the strongest tropical eruptions of the last millennium to better understand the effects of volcanic activity on the Indian Ocean. We find a consistent response of sea surface temperatures and surface winds across the basin between different simulated eruptions. This response scales with eruption intensity and is long‐lived, lasting for multiple years. We also find that sea surface temperatures in the tropical Pacific at the time of eruption influence the strength of the initial tropical Indian Ocean response. Understanding these connections will allow for improved climate predictability and preparedness following large tropical eruptions. Key Points: Both Indian Ocean Dipole (IOD) and El Niño‐Southern Oscillation (ENSO) show oscillatory responses for up to 8 years following strong tropical volcanic eruptionsThese eruptions induce a negative IOD followed by a positive IOD, and this response scales with eruption intensityInitial Interdecadal Pacific Oscillation phasing at time of eruption influences the post‐eruption response via low‐frequency thermocline depth modulation [ABSTRACT FROM AUTHOR]

Previous studies suggest that the winter surface freshening in the southeastern Arabian Sea (SEAS) contributes to the development of very high Sea Surface Temperatures (SST) thereby influencing the following summer monsoon onset. Here, we use forced and coupled simulations with a regional ocean general circulation model to explore the SEAS Sea Surface Salinity (SSS) variability mechanisms and impact on the monsoon. Both configurations capture the main SEAS oceanographic features, and confirm that the winter SSS decrease results from horizontal advection of Bay of Bengal freshwater by the cyclonic circulation around India during fall. A coupled model sensitivity experiment where salinity has no effect on mixing indicates that the salinity stratification reduces the SEAS mixed layer cooling by vertical processes by 3 °C seasonally. Salinity however enhances mixed layer cooling by a similar amount through concentrating negative winter surface heat fluxes into a thinner mixed layer, resulting in no climatological impact on SST and summer monsoon rainfall. The Indian Ocean Dipole (IOD) is the main driver of the winter SEAS SSS interannual variability (r ~ 0.8). Salty anomalies generated in the western Bay of Bengal during fall by positive IOD events are indeed transported by the cyclonic climatological coastal circulation, reaching the SEAS in winter. By this time, warm IOD-induced SST anomalies in the SEAS are already decaying, and the SEAS SSS anomalies hence do not contribute to their development. Overall, our model results suggest a weak climatological and interannual impact of the SEAS winter freshening on local SST and following monsoon onset. [ABSTRACT FROM AUTHOR]

This study investigates the role of perturbation potential energy (PPE) in energetic connection between the South China Sea summer monsoon (SCSSM) and Indian Ocean dipole (IOD). When the SCSSM is strong during boreal summer, the higher and lower PPE anomalies controlled primarily by the diabatic heating correspond to negative and positive energy conversion, favoring the ascending and descending motions over western North Pacific (WNP) and southern Maritime Continent (SMC), respectively. This implies the existence of the regional Hadley circulation. This regional Hadley circulation-induced lower southeasterly wind anomalies reduce the local sea surface temperature (SST) anomalies over the tropical southeastern Indian Ocean via the wind–evaporation–SST and wind–thermocline–SST feedbacks, increasing the zonal SST gradient over the tropical Indian Ocean. Thus, a positive IOD event develops in boreal summer, and verse vice. Although the SCSSM decays during boreal autumn, the increased gradient of the PPE anomalies intensifies the anomalous Walker circulation over the tropical Indian Ocean, providing positive feedback that allows the IOD to mature. Consequently, the PPE dipole over WNP and SMC serves as an energetic bridge between the SCSSM and IOD. [ABSTRACT FROM AUTHOR]

The regional sea level variability and its projection amidst the global sea level rise is one of the major concerns for coastal communities. The dynamic sea level plays a major role in the observed spatial deviations in regional sea level rise from the global mean. The present study evaluates 27 climate model simulations from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) for their representation of the historical mean states, variability and future projections for the Indian Ocean. Most models reproduce the observed mean state of the dynamic sea level realistically; however, consistent positive bias is evident across the latitudinal range of the Indian Ocean. The strongest sea level bias is seen along the Antarctic Circumpolar Current (ACC) regime owing to the stronger than observed south Indian Ocean westerlies and its equatorward bias. This equatorward shift of the wind field also results in a stronger positive windstress curl across the southeasterly trade wind regime in the southern tropical basin and an easterly wind bias along the equatorial waveguide. Owing to the anomalous easterly equatorial winds, the thermocline in the eastern tropical basin is shallower in the models than observed, resulting in enhanced variability there. Such spurious variability in the eastern part of the basin causes models to become biased towards the dipole zonal mode or Indian Ocean dipole patterns in the tropics. In the north Indian Ocean, the summer monsoon winds are weak in the model leading to weaker coastal upwelling and positive sea level bias along the western Arabian Sea. Further, it is noted that the high-resolution models compare better in simulating the sea level variability, particularly in the eddy-dominated regions like the ACC regime in interannual timescale. However, these improved variabilities do not necessarily produce a better mean state likely due to the spurious enhanced mixing driven by parametrizations set in these high-resolution models. Finally, the overall pattern of the projected dynamic sea level rise is similar for the mid (SSP2-4.5) and high-end (SSP5-8.5) scenarios, except that the magnitude is higher under the high emission situation. Notably, the projected dynamic sea level change is milder when only the best-performing models are used compared to the complete ensemble. [ABSTRACT FROM AUTHOR]

This paper analyzes the effect of the Australian High (AH) on the seasonal phase locking of Indian Ocean Dipole (IOD) events. The anomalous strong AH associated with the positive phase of the Antarctic Oscillation can cause significant easterly wind anomalies and northward cross‐equatorial flow over the western Maritime Continent (MC) by strengthening the Australian winter monsoon during May–August. The AH‐associated easterly anomalies and northward cross‐equatorial flow can create thermodynamic air‐sea feedback and contribute to a significant cooling anomaly in the western MC and the tropical eastern Indian Ocean. Without considering the effect of ENSO, these processes contribute to the occurrence of positive IOD events, which begin in early summer, peak in late summer, and decay rapidly thereafter. The effect of ENSO can extend the peak period of IOD into the boreal autumn of that year. An anomalous weak AH corresponds to the occurrence and seasonal phase locking of negative IOD events. Through combined empirical orthogonal function analysis, we find that the effect of the AH can well explain the seasonal phase locking of 34 IOD events (40 in total), which provides an important theoretical basis for the prediction of IOD events. Plain Language Summary: Seasonal phase locking is a critical characteristic of typical Indian Ocean Dipole (IOD) events. The IOD usually develops in boreal summer, peaks in autumn, and decays rapidly in winter. Note that the Australian High (AH) plays a key role on the seasonal phase locking of IOD events. It is found that The anomalous strong AH associated with the positive phase of the Antarctic Oscillation (AAO) can cause significant easterly wind anomalies and northward cross‐equatorial flow over the western Maritime Continent by strengthening the Australian winter monsoon during May–August. Key Points: Investigate the effect of Australian High (AH) the on the seasonal phase locking of Indian Ocean Dipole eventsThe Antarctic Oscillation can strengthen AH and Australian winter monsoonThe AH induced easterlies and cross‐equatorial flow contribute to a significant cooling anomaly in the tropical eastern Indian Ocean [ABSTRACT FROM AUTHOR]

La Niña was forecasted to be one of the predominant climatic modes in the 2020 boreal autumn, taking into consideration its impact on the short rain (October to November rainfall) in eastern Africa. Parallel to La Niña and just 2 months before the start of the 2020 short rain, the Greater Horn of Africa Regional Outlook Forum published extremely strong projections of below-normal rainfall in Burundi, Rwanda, most parts of Uganda, Kenya's southern region, and Tanzania. On the other hand, several regions of East Africa (EA) saw above average rainfall, especially in November. The current study examines the core and potential drivers causing the 2020 November rainfall to deviate from its historical response to the La Niña event. The driving force responsible for the wetter-than-anticipated 2020 November rainy conditions was identified. Although the ocean surface favors a negative phase of the Indian Ocean Dipole mode, it normally acts to suppress the short rain in EA, the mode decayed sooner than its life cycle in October. Lastly, the Madden–Julian Oscillation (MJO) in the Indian Ocean acted to reverse or weaken the Walker circulation (WC), the surface westerly (northeasterly) wind flow from the Congo Basin (central Indian Ocean) which conveys moist (warm) unstable air to EA, thereby promoting convective activity and rainfall. The study further identifies the reasons for dry conditions in October and December. In October, the MJO strength in the Maritime Continent and Western Pacific was associated with Somali low-level jet-like and WC across the Indian Ocean. The latter caused subsidence in EA, whereas the former triggered a strong friction-induced sinking motion along the eastern coast of Africa. To create a dry condition, they both collaborated. In December, La Niña strengthened WC across the Indian Ocean which induced subsidence in EA and resulted in poor rainfall. The diversity of mechanisms of MJO influence is relevant to problems of sub-seasonal weather forecasts for EA. [ABSTRACT FROM AUTHOR]

The societies of the Greater Horn of Africa (GHA) are vulnerable to variability in two distinct rainy seasons, the March–May 'long' rains and the October–December 'short' rains. Recent trends in both rainy seasons, possibly related to patterns of low-frequency variability, have increased interest in future climate projections from General Circulation Models (GCMs). However, previous generations of GCMs historically have poorly simulated the regional hydroclimate. This study conducts a process-based evaluation of simulations of the long and short rains in CMIP6, the latest generation of GCMs. Key biases in CMIP5 remain or are worsened, including long rains that are too short and weak and short rains that are too long and strong. Model biases are driven by a complex set of related oceanic and atmospheric factors, including simulations of the Walker Circulation. Biased wet short rains in models are connected with Indian Ocean zonal sea surface temperature (SST) gradients that are too warm in the west and convection that is too deep. Models connect equatorial African winds with the strength of the short rains, though in observations a robust connection is primarily found in the long rains. Model mean state biases in the timing of the western Indian Ocean SST seasonal cycle are associated with certain rainfall timing biases, though both biases may be due to a common source. Simulations driven by historical SSTs (AMIP runs) often have larger biases than fully coupled runs. A path towards using biases to better understand uncertainty in projections of GHA rainfall is suggested. [ABSTRACT FROM AUTHOR]

We investigated the relationship between the North Atlantic Oscillation (NAO) and Indian Ocean Dipole (IOD), which has remained unknown to date. Reanalysis data and linear baroclinic model experiments were employed in our study. The results showed significant correlation between the March NAO and the boreal summer and autumn IOD, independent of the El Niño–Southern Oscillation signal, verified by partial correlation analysis. Air–sea interaction over the western North Pacific (WNP) is a significant aspect of the physical mechanism through which the March NAO affects the subsequent IOD. A strong positive March NAO induces equivalent barotropic cyclonic circulation over the WNP through a steady Rossby wave, accompanied by a local tripole sea surface temperature (SST) anomaly pattern. Facilitated by local air–sea positive feedback, the low-level cyclonic circulation and associated precipitation anomalies over the WNP persist from early spring to summer and shift equatorward. During May–June, the WNP anomalous cyclone strengthens the southeasterly wind and enhances cooling off Sumatra–Java through local meridional circulation. Such circulation ascends over the WNP and descends over the tropical southeastern Indian Ocean and Maritime Continent. Subsequently, wind–evaporation–SST and wind–thermocline–SST positive feedback in the tropical Indian Ocean contribute to IOD development. A diagnosis of ocean mixed-layer heat budget indicated that the ocean dynamic process associated with the NAO contributes more to IOD development than does atmospheric thermal forcing. Determining the influence mechanism of the March NAO on the subsequent IOD is considered useful in advancing the seasonal prediction of IOD. [ABSTRACT FROM AUTHOR]

This study reveals that the Arctic Oscillation (AO) in boreal spring has a marked impact on the Indian Ocean Dipole (IOD) in the following autumn. When the spring AO is in its positive (negative) phase, a positive (negative) IOD tends to occur in the following autumn. Possible physical processes for the impact of the spring AO on the autumn IOD are further examined. Positive spring AO is accompanied by a dipole precipitation anomaly pattern over North Atlantic, with positive anomalies over high latitude and negative anomalies over mid-latitude. The associated atmospheric heating anomalies over mid-high latitudes North Atlantic further induces an atmospheric wave train from the North Atlantic to the Indian Ocean (IO), leading to pronounced easterly wind anomalies over the tropical northern IO. These easterly wind anomalies can cause warm sea surface temperature (SST) anomalies in the western tropical Indian Ocean (WTIO) by modulating surface heat fluxes and oceanic heat transport. The warm SST anomalies in the WTIO persist into the following autumn, which increase the zonal gradient of SST anomalies in the equatorial IO and lead to easterly wind anomalies over there. Moreover, the equatorial IO easterly wind anomalies can induce cold SST anomalies in the southeastern tropical Indian Ocean (SETIO) via increasing upwelling of cold water. In addition, previous studies have indicated that a positive spring AO could lead to significant positive precipitation anomalies in the tropical central Pacific in the following summer. Our results show that the associated atmospheric heating over the tropical central Pacific can enhance the southeasterly wind anomalies off the west coast of Sumatra via anomalous Walker circulation, which also play a role in contributing to cold SST anomalies in the SETIO. Therefore, the spring AO may exert a significant impact on the subsequent autumn IOD through the above processes and can be used as a potential predictor of the IOD event. [ABSTRACT FROM AUTHOR]

The present study focuses on the variability of subsurface ocean temperature and associated planetary waves (oceanic Kelvin and Rossby waves) in the Indian Ocean during the boreal summer intraseasonal oscillation (BSISO) phases in the Indian Ocean Dipole (IOD) years. To accomplish this, multi-sensor datasets have been considered for the period 1980–2021. During negative IOD (nIOD) years northward propagation of BSISO convection is consistent with the normal years, whereas incoherent patterns are evident in the positive (pIOD) years. Enhanced meridional gradient of sea surface temperature (SST) anomalies is evident during the nIOD years, which favours the coherent northward propagation of BSISO. Significant intraseasonal variations of subsurface ocean temperature anomalies are observed in the Equatorial Indian Ocean (EIO). Pronounced subsurface (50–150 m) temperature anomalies are evident during the different phases of BSISO during the nIOD years. Symmetric formation of equatorial downwelling Kelvin wave followed by upwelling Kelvin wave packet were observed during the nIOD BSISO phases. This study further suggests that the symmetric patterns of subsurface waves are evident during the nIOD years and led to clear variations of the northward propagation of BSISO. [ABSTRACT FROM AUTHOR]

The interactions between the tropical Indian Ocean and Pacific Ocean via the Walker circulation and the Indonesian Throughflow (ITF) play an important role in the inter-basin exchange and climate variations. The effects of the eastern Indian Ocean subsurface temperature conditions during the 2019 Indian Ocean dipole (IOD) event on the development of the 2020/2021 La Niña event are investigated of the National Marine Environmental Forecasting Center of China. Results of pacemaker experiments comparing the effects of the atmospheric bridge and the oceanic channel suggest that the subsurface temperature anomalies in the tropical Indian Ocean dominates the inter-basin forcing leading to the outburst of the 2020/2021 La Niña event. The strong subsurface cooling is found to propagate to the eastern equatorial Pacific Ocean through the Indonesian seas and induce a strong La Niña event at the end of 2020, showing the long memory of the oceanic channel dynamics and the amplification by the ocean-atmospheric coupling. In comparison, experiments forced by surface temperature anomalies over the tropical Indian Ocean show that the atmospheric bridge alone is unable to induce the onset of the 2020 La Niña. The results suggest the importance of the Indo-Pacific oceanic channel dynamics in ENSO forecasts. [ABSTRACT FROM AUTHOR]

Tibetan Plateau (TP) precipitation is affected by anomalous circulation systems in both the tropics and mid-latitudes, due to the TP's unique geographical location. By using observational, reanalysis, and CMIP6 model datasets, this study reveals the individual and joint effects of the Indian Ocean Dipole (IOD) and Silk Road pattern (SRP) on the interannual variability of TP precipitation in September. In the positive IOD phase, the zonal gradient of the sea-surface temperature anomalies (SSTAs) drives a Gill-type response with an anticyclonic anomaly over the Indian subcontinent and Bay of Bengal. To the north, anomalous westerlies induce a shallow trough, and the associated anomalous southwesterlies transport moisture to the southeastern TP, resulting in surplus precipitation there. Meanwhile, the westerly jet disturbances over the North Atlantic excite an SRP-like pattern, resulting in a baroclinic structure in northern India with the upper (lower) tropospheric anomalous anticyclone (cyclone) over west-central Asia (northern India). The anomalous southwesterlies to the east of the low-level cyclone transport abundant moisture to the southeastern TP, which results in increased precipitation there. The joint effects of IOD and SRP can explain nearly 52% of the TP precipitation anomaly, exceeding the contribution of IOD (19%) and SRP (27%) alone. Our results highlight the necessity of considering the joint effects of drivers in the tropics and mid-latitudes, providing a basis for more accurate simulations and predictions of TP precipitation. [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]

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]

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]

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]

• 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]