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In the past few decades, satellite altimetry has surpassed tremendous achievements in examining the mesoscale of ocean dynamics. Recently, new potential of the satellite in observing the variability of climate phenomena through atmospheric medium has been unlocked. As it has been ascertained that satellite altimetry is not only outstanding in monitoring ocean dynamics but also in observing atmospheric variability, we intuitively propose a hypothesis that satellite altimetry is reliable in monitoring Bjerknes feedback: a feedback loop that involves interactions between atmosphere and oceans. Thus, the aim of this study is to determine the capability of satellite altimetry in observing Bjerknes feedback through Precipitable Water Vapour (PWV) and Sea Level Anomaly (SLA) during the anomalous climate mode of Indian Ocean Dipole (IOD). The results signify convincing arguments as both PWV and SLA indexes are highly correlated with Dipole Mode Index (DMI), particularly in the western region of the Indian Ocean, which have correlations of 0.67 and 0.62, respectively. The correlations of PWV and SLA indexes are also significantly high: 0.73 and 0.69 in western and eastern regions, respectively. The Principal Component Analysis (PCA) results are also convincing as the spatial pattern of primary (PC1) and secondary (PC2) components of both PWV and SLA are associated with positive and negative IOD, respectively. The temporal patterns of PC1 for PWV and SLA have relatively high correlation with positive IOD. The substantially high temporal correlation (0.81) between PWV and SLA PC1 has reinforced the confidence in the capability of satellite altimetry in observing the Bjerknes feedback. [ABSTRACT FROM AUTHOR]

Coral bleaching occurs when symbiotic dinoflagellate algae leave corals due to warming surface waters. Since the early 1980s, the number and intensity of coral bleaching events have significantly risen globally, and many coral reefs are concerned to undergo irreversible phase shifts due to global warming. In this study, we examine spatiotemporal variations and relationships of high-temperature events, high-temperature anomaly events, and coral bleaching around the Ryukyu Islands, Japan. We also explore the correlation between these occurrences and the large-scale climate variability, El Niño–Southern Oscillation and Indian Ocean Dipole. More than 99% of the high-temperature events occurred from June to October, and more than 80% occurred from July to September, whereas high-temperature anomaly events occurred all year round. Corresponding with high-temperature events, more than 10% of the data reported overall bleaching, and more than 25% reported partial bleaching from June to October. Over 40% of data reported bleaching in August, of which 50% of these reports were categorized as overall bleaching and the remainder as partial. The spatial distribution of coral bleaching also corresponded better with high-temperature events than high-temperature anomaly events. In 2016, extensive and severe coral bleaching was reported around the Okinawa, Miyako, and Yaeyama Islands, where intense high-temperature events had occurred. The central-to-eastern equatorial Pacific displayed a negative sea surface temperature anomaly, and the eastern Indian Ocean did a positive anomaly following high-temperature events near the Ryukyu Islands. These findings indicated that the high-temperature events around the Ryukyu Islands show correlation with La Niña and a negative Indian Ocean Dipole. If the cumulative temperature of high-temperature events exceeds 56 °C-days every three years in the late 2030s, then the corals around the Ryukyu Islands will suffer high mortality within 20 years. [ABSTRACT FROM AUTHOR]

During boreal summer, the Southwest Monsoon Current (SMC) turns northeastward, transporting highly saline water into the Bay of Bengal (BOB) and significantly influencing the dynamics of the upper ocean. Previous studies have shown that an anticyclonic semi-geostrophic (SG) eddy forms on the eastern flank of the SMC, this formation associated with the kinetic energy transfer via the barotropic instability (BTI). The presence of such an eddy can attenuate the meridional salinity flux, potentially affecting the development of the circulation within the BOB. Acknowledging the importance of this phenomenon, this study revisits the SG eddy using satellite altimetry data, reanalysis datasets and in-situ observations from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) project. Our results show that a cyclonic eddy-like (CE-like) negative Sea Level Anomaly (SLA), generated in the eastern BOB due to regional anomalous wind stress curl, also contributes to the formation of the SG eddy. During the formation, mean flows on the northern edge of the SG eddy are strengthened, while southeastward currents on the eastern edge are structured influenced by CE-like SLA. Further instability analyses indicate that the anisotropic component of BTI is significantly larger than the isotropic component, which is attributed to the weak nonlinear planetary geostrophic convergence of the SG eddy and the strong horizontal shear in mean flow field induced by CE-like SLA. Additionally, our results point out that anomalies in wind stress curl over the eastern BOB and subsequent formation of negative SLA are likely influenced by the Indian Ocean Dipole. These findings suggest that the coupling between SMC instability and regional wind stress curl may play a pivotal role in the generation of SG eddy on interannual timescale, with important implications for regional ocean dynamics. [ABSTRACT FROM AUTHOR]

The impact of the positive and negative phase of the Indian Ocean dipole (IOD) on the Pacific blocking frequency (PBF) during boreal autumn is investigated based on the ERA5 daily reanalysis data from 1979 to 2021. It is found that the IOD index are characterized by an obvious interannual variation, and the most obvious dipole feature appears in boreal autumn, which is agree with previous studies. Strong 8 positive and 8 negative phases in boreal autumn are selected to further reveal the regulation of different IOD phases on PBF. The results show that the PBF exhibits negative (positive) anomaly in the North Pacific (East Siberia and Central Siberia) during the positive (negative) IOD phase. During the positive (negative) phase, the cold sea surface temperature anomaly (SSTA) in the tropical southeastern (western) Indian Ocean forces teleconnection wave trains, which are likely results of the northeastward propagation of Rossby wave energy. With such a wave train, a geopotential height anomaly of ‘north negative–south positive’ (‘north positive–south negative’) appears around the Bering Strait (Central Siberia) during the positive (negative) phase, which causes negative (positive) PBF anomalies, based on the ERA5 daily reanalysis data and ECHAM4.6 climate model output from 1979 to 2021. [ABSTRACT FROM AUTHOR]

Signatures of Indian Ocean Dipole (IOD) over the southeastern Arabian Sea (SEAS) during two representative IOD years: negative IOD 2016 (nIOD16) and positive IOD 2019 (pIOD19) are studied. Sea Surface Temperature (SST) over the SEAS reveals negative anomalies during the Fall transition months of nIOD16, whereas positive SST anomalies are observed during both southwest monsoon season and Fall transition months in pIOD19. In contrast, during the first half of the year, noticeable positive SST anomalies are observed in nIOD16, whereas SST anomalies are weak in pIOD19. The cooling rate during the SWM season of nIOD16 was twice that during pIOD19 and lasted one additional month. To understand the aforementioned anomalous features in the SST evolution, a heat budget analysis is performed based on a combination of in-situ data, satellite measurements and ocean reanalysis. The enhanced cooling during the SWM season of nIOD16 was driven largely by the net heat flux. In addition, the penetrative component of shortwave radiation also favoured enhanced cooling during both the SWM season and Fall transition months of nIOD16, driven by a relatively small mixed layer depth (MLD). The negative anomaly in MLD during the SWM and Fall transition months of nIOD16 is then shown to be in line with corresponding positive anomalies in the Ekman pumping. In contrast, relatively less intense cooling during the SWM season of pIOD19 is attributed to small resultant net heat flux and warming by the residual term. [ABSTRACT FROM AUTHOR]

ABSTRACT Flash drought events can be characterised by the quick depletion of crop root zone soil moisture (rapid intensification) and hence can be termed as agricultural flash droughts. These events can have devastating impacts, such as increasing the risk of agricultural yield loss, heatwaves and increased wildfire risk, which further have cascading impacts on the socio‐economic conditions. The regional hotspots of flash droughts are analysed for winter, pre‐monsoon, monsoon and post‐monsoon seasons over India from 1981 to 2020. We assess the impact of the El Niño‐Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on the flash drought frequency (FDF: number of flash drought events). The causal connection of the FDF with the ENSO and IOD is analysed using the PCMCI (Peter and Clark's algorithm combined with the Momentary Conditional Independence) algorithm. The monsoon season (June–September) is found to be more prone to flash droughts with higher spatial/regional average values of total per pixel FDF during the 40‐year period over the Central Northeast (~54) and West Central (~41) regions. It is observed that the fraction of the total number of flash droughts during the El Niño years (38.8%) is higher as compared with that in La Niña years (25.7%). It is also found that the co‐occurrence of positive/negative IOD with the El Niño phase can alter the seasonal fraction of FDF over India, highlighting the high complexity in the ENSO–IOD interactions. The causal analysis shows that only the Southern Peninsula and West Central regions have significant direct and lagged causal links of average per pixel FDF with IOD. Whereas, similar (direct and lagged) causal connections are observed between the ENSO and IOD. This study reveals that flash droughts and their teleconnections vary greatly among the seasons and regions in India, limiting its accurate predictions and increasing the risk to agricultural communities. [ABSTRACT FROM AUTHOR]

Abstract Coral bleaching occurs when symbiotic dinoflagellate algae leave corals due to warming surface waters. Since the early 1980s, the number and intensity of coral bleaching events have significantly risen globally, and many coral reefs are concerned to undergo irreversible phase shifts due to global warming. In this study, we examine spatiotemporal variations and relationships of high-temperature events, high-temperature anomaly events, and coral bleaching around the Ryukyu Islands, Japan. We also explore the correlation between these occurrences and the large-scale climate variability, El Niño–Southern Oscillation and Indian Ocean Dipole. More than 99% of the high-temperature events occurred from June to October, and more than 80% occurred from July to September, whereas high-temperature anomaly events occurred all year round. Corresponding with high-temperature events, more than 10% of the data reported overall bleaching, and more than 25% reported partial bleaching from June to October. Over 40% of data reported bleaching in August, of which 50% of these reports were categorized as overall bleaching and the remainder as partial. The spatial distribution of coral bleaching also corresponded better with high-temperature events than high-temperature anomaly events. In 2016, extensive and severe coral bleaching was reported around the Okinawa, Miyako, and Yaeyama Islands, where intense high-temperature events had occurred. The central-to-eastern equatorial Pacific displayed a negative sea surface temperature anomaly, and the eastern Indian Ocean did a positive anomaly following high-temperature events near the Ryukyu Islands. These findings indicated that the high-temperature events around the Ryukyu Islands show correlation with La Niña and a negative Indian Ocean Dipole. If the cumulative temperature of high-temperature events exceeds 56 °C-days every three years in the late 2030s, then the corals around the Ryukyu Islands will suffer high mortality within 20 years.

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]

During boreal summer, the Southwest Monsoon Current (SMC) turns northeastward, transporting highly saline water into the Bay of Bengal (BOB) and significantly influencing the dynamics of the upper ocean. Previous studies have shown that an anticyclonic semi-geostrophic (SG) eddy forms on the eastern flank of the SMC, this formation associated with the kinetic energy transfer via the barotropic instability (BTI). The presence of such an eddy can attenuate the meridional salinity flux, potentially affecting the development of the circulation within the BOB. Acknowledging the importance of this phenomenon, this study revisits the SG eddy using satellite altimetry data, reanalysis datasets and in-situ observations from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) project. Our results show that a cyclonic eddy-like (CE-like) negative Sea Level Anomaly (SLA), generated in the eastern BOB due to regional anomalous wind stress curl, also contributes to the formation of the SG eddy. During the formation, mean flows on the northern edge of the SG eddy are strengthened, while southeastward currents on the eastern edge are structured influenced by CE-like SLA. Further instability analyses indicate that the anisotropic component of BTI is significantly larger than the isotropic component, which is attributed to the weak nonlinear planetary geostrophic convergence of the SG eddy and the strong horizontal shear in mean flow field induced by CE-like SLA. Additionally, our results point out that anomalies in wind stress curl over the eastern BOB and subsequent formation of negative SLA are likely influenced by the Indian Ocean Dipole. These findings suggest that the coupling between SMC instability and regional wind stress curl may play a pivotal role in the generation of SG eddy on interannual timescale, with important implications for regional ocean dynamics.

Abstract The interannual variability of the Indonesian Throughflow (ITF) in the Makassar Strait in 2015–2017 is analyzed using observations and ORAS5 reanalysis data. Strong northward and ensuing strong southward current anomalies in the sub‐thermocline Makassar Strait are identified during and after the 2015/2016 El Niño, respectively. However, the upper layer current anomalies are weakly southward in September–October 2015 and strongly northward in April 2016 through April 2017. These anomalies of the Makassar Strait throughflow in the upper layer are found to be induced mainly by Indian Ocean Kelvin waves forcing sea surface height anomalies off the south Java coasts. The sub‐thermocline current anomalies are found to be generated by the westward and downward propagation of interannual Rossby waves from the central equatorial Pacific during and after the 2015/2016 El Niño. The results underline the importance of the planetary wave propagation into the Indonesian seas in forcing the ITF anomalies.

The Bay of Bengal summer monsoon (BOBSM) is the most prominent branch of the Asian summer monsoon system, which exhibits complex interannual variability. While previous studies have focused on the onset conditions of the BOBSM, less attention has been paid to the retreat of the BOBSM. In this study, we propose an index to measure BOBSM retreat, based on the mean zonal wind field at 850 hPa during the summer‐to‐winter monsoon transitions. By analyzing the climatic characteristics and interannual variability of the BOBSM retreat using this index, we find that BOBSM retreat exhibits significant interannual variability, which is closely related to the occurrence of Indian Ocean Dipole (IOD) events. Statistically, when a positive IOD event takes place in the boreal autumn season, the retreat of the summer monsoon occurs earlier correspondingly. Conversely, the retreat is delayed when a negative IOD event occurs. Plain Language Summary: The South Asian summer monsoon is the most important monsoon system in the world. One of its component branches is the Bay of Bengal summer monsoon (BOBSM), which has a strong impact on local social activities due to its heavy rainfall and associated disasters. While the onset of the BOBSM has received considerable attention, the retreat of the BOBSM has been relatively overlooked, leading to an poor understanding of the BOBSM system. In this study, we focus on the retreat process of the BOBSM, examining its climatological features and interannual variability by introducing a retreat index. We also show the important role played by the Indian Ocean Dipole event in modulating the retreat date on the interannual timescale. Key Points: A Bay of Bengal summer monsoon (BOBSM) retreat index is introduced to reveal the features of monsoon retreatThe climatological condition and interannual variability of BOBSM retreat are studiedThe Indian Ocean Dipole plays an important role in the interannual variability of the BOBSM retreat [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) are anomalously warm events that profoundly affect climate change and local ecosystem. During the summer of 2012 (June-September), intense MHWs occurred in the tropical Indian Ocean (TIO) concurrently with an unseasonable positive Indian Ocean Dipole (pIOD) event. The MHW metrics (duration, frequency, cumulative intensity and maximum intensity) were characterized by northwestward-slanted patterns from west Australia to the Somalia coast. The analysis confirmed that these MHWs were closely associated with the unseasonable pIOD 2012. The weakening of Western North Pacific Subtropical High and strengthening of Australian High in spring induced an interhemispheric pressure gradient that drove two anticyclonic circulation patterns over the eastern TIO. The first anticyclonic circulation featured cross-equatorial wind anomalies from south of Java to the South China Sea/Philippine Sea, which led to strong upwelling off Sumatra-Java during the subsequent summer. The second anticyclonic circulation excited downwelling Rossby waves that propagated from the southeastern TIO to the western TIO. Thus, downwelling in the western pole and upwelling in the eastern pole led to a strong pIOD event peaking in summer, namely, the unseasonable pIOD 2012. These downwelling Rossby waves deepened the thermocline by more than 60 m and caused anomalous surface warming, thereby contributing to the occurrences of MHWs. With the development and peak of the unseasonable pIOD 2012, anomalous atmospheric circulation transported moisture from the TIO to the subtropical Western North Pacific (WNP), favoring a strong cyclonic anomaly that profoundly affected the summer monsoon rainfall over the subtropical WNP. This study provides some perspectives on the role of pIOD events in summer climate over the Indo-Northwest Pacific region. [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 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 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 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]

As the upstream region of the Asian summer monsoon, the Bay of Bengal summer monsoon (BOBSM) system has impacts on rainfall patterns in East Asia. In this study, we investigate the impact of the interannual variability of the BOBSM retreat on China precipitation in early summer (June) of the following year. When the BOBSM retreat occurs earlier in the previous year, we find enhanced rainfall in both the northeastern and eastern parts of China. Conversely, when the retreat of the BOBSM is delayed in the previous year, there is a tendency for decreased rainfall in most of northeastern and eastern China, while rainfall in the northern part of the Taiwan island region tends to increase. Statistical analysis demonstrates the co-variability between China's June precipitation anomalies and preceding wind anomalies in the eastern Bay of Bengal. The results indicate a strong relationship between the preceding BOBSM retreat and China precipitation anomalies in the following June. Furthermore, the analysis suggests that the BOBSM retreat is more of an independent signal rather than modulated by an Indian Ocean Dipole event. [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]

Study area: Tana River Basin in Kenya. Study Focus: Flood-related impacts and losses have been rising. Therefore, understanding flood characteristics, drivers, and predictability is critical for informed decisions in the ongoing flood early warning (FldEWS) projects. This study presents an in-depth analysis of hydro-meteorological, Sentinel Mission (SM), and ensemble hydrological model datasets. We examine flood characteristics using observed hydro-meteorological and SM datasets, followed by statistical analysis of climate drivers of flood events at inter-annual and sub-seasonal (S2S) time scales. Finally, reforecasts from Global Flood Awareness System (GloFAS) are assessed against observed river flows. New hydrological insights for the study region: There is a high inter-annual variability of flood events with flood peaks occurring in May and December. SM satellites have the ability to map flooded areas in near-real time. At inter-annual timescales, positive Indian Ocean Dipole (IOD) and warm El Niño Southern Oscillation (ENSO) drives short rains (October to December). At Sub-Seasonal (S2S) timescales, Madden Julian Oscillation (MJO; phases 2-4) is a notable driver of flood related extreme rainfall. GloFAS offers reliable forecasts depending on the flood magnitude, trigger probability, and 'anticipation window' and it meets the tolerable skill requirements for flood preparedness actions (FAR < 50% and POD > 50%) with up to a 20-day lead time for 1 and 2-year return periods. We subsequently discuss how our research findings can inform the development of FldEWS in Kenya, with an emphasis on improved co-production of flood forecast information with relevant stakeholders.

The Indian Ocean Dipole (IOD) is a critical coupled ocean-atmosphere oscillation system associated with significant weather anomalies in the global climate, particularly in the Indian Ocean rim countries. The paper presents a novel deep learning (DL) model, which we call the “spatio-temporal dilated ConvLSTM (STDNet) model”, for forecasting the Dipole Mode Index (DMI) using global sea surface temperature (SST) and heat content (HC) data. The model combines the techniques of dilation and fine-tuning to learn efficiently from the training data. CMIP6 historical simulation data from 5 modeling centres for 1861–2014 is used to train the model. Furthermore, the model is fine-tuned on reanalysis data from 1871–1973. During the testing period (1982–2019), the dipole correlation coefficient (DCC) was the highest when compared with state-of-the-art dynamical North American Multi-Model Ensemble (NMME) models, a convolutional neural network (CNN) and a dilated CNN. On a lead of 12 months, the DCC is 0.40 for the CNN, 0.44 for the dilated CNN, and 0.51 for the STDNet, and all the NMME models have negative correlations. The results show that the STDNet efficiently forecasts the DMI at leads of up to 12 months. The STDNet shows results to overcome the winter predictability barrier.

Abstract Seasonal prediction of the Indian Ocean dipole (IOD) is important, considering its impact on the climate of surrounding regions. Here we compare the prediction of the IOD in two generations of prediction system developed by the China Meteorology Administration (CMA), i.e., the second-generation climate model prediction system (CPSv2) and CPSv3. The results show that CPSv3 has better ability to predict the variability and spatial pattern of the IOD than CPSv2, especially when the lead time is long. CPSv3 maintains a certain level of credibility when predicting IOD events with 6-month lead time. The improved data assimilation in CPSv3 has reduced the predictability error of eastern Indian Ocean sea surface temperature (SST) and contributed to improvements in IOD prediction. Enhanced simulation of the El Niño-Southern Oscillation (ENSO)–IOD relationship promotes better prediction skill of ENSO-related IOD events in CPSv3. Our results suggest that upgrading data assimilation and the simulation of the ENSO–IOD relationship are critical for improving the prediction of the IOD in coupled climate models.

Recent occurrences of marine heatwaves (MHWs) in coastal China seas have caused serious impacts on marine ecosystem services and socio-economics. Nevertheless, the underlying physical process, including local drivers and remote associations, remains poorly understood, thereby hindering accurate predictability. In this study, we reported an extreme MHW event in the East China Seas (ECSs, including the Bohai, Yellow, and East China Sea), lasting for 75 d with a maximum intensity of 1.96 °C relative to 1982–2011 during the summer 2022. This ECSs MHW event was triggered by a combination of anomalous atmospheric and oceanic conditions, including enhanced insolation, weakened surface wind speed, suppressed latent heat loss from ocean, a shallower mixed layer, and upper ocean current anomaly. Mixed-layer temperature budget diagnosis suggested that changes in the ECSs temperature were dominated by the surface net heat flux, largely due to strong shortwave radiation flux, during the development and decay of the MHW event. Oceanic advection also created favorable conditions for the maintenance of the MHW. These physical drivers were further regulated by the westward expanded and intensified western Pacific subtropical high (WPSH), potentially linked to the negative phase of Indian Ocean Dipole (IOD). Despite the three years (2020–2022) consecutive La Niña events, the ECSs summer MHWs appeared to be more closely linked to negative IOD events, with a lagging period of 1–3 mon. The seasonal precursor signals of the negative IOD have the potential to affect local physical drivers of ECSs MHWs through regulating the strength and position of WPSH, thus serving as a promising predictor for the ECSs MHWs. The future likelihood and intensity of the ECSs MHWs are projected to increase substantially in the coming decades, largely due to broad-scale warming attributed to anthropogenic climate change. Consequently, there is an urgent need to develop MHW forecasting and early warning systems, and robust approaches to address climate change.

The bimodal wave climate of the semi-protected shallow Gulf St Vincent in South Australia has been analyzed through a forty-year (1980-2020) wave hindcast and an investigation into the climatic drivers of wave climate anomalies is presented. The sea and swell partitions of the wave climate were modelled independently as well as using an integrated model with both partitions represented. The wave hindcast was validated against two wave buoys located off the coast of Adelaide’s metropolitan beaches and key wave parameter anomalies were calculated across the gulf. Teleconnections were investigated, and the Southern Annular Mode is found to have the strongest correlations to wave parameter anomalies while the Southern Oscillation Index and the Dipole Mode Index fluctuations are found to correlate seasonally with wave parameter anomalies. Projected future trends of these climate drivers from literature have been related to the teleconnections found in this study to inform future trends of bimodal wave conditions in the gulf. The Southern Annular Mode is projected to trend positive which will reduce wave height and the westerly component of waves in the gulf, while the Southern Oscillation Index is projected to become more variable in the future which will lead to more extreme winter and spring wave conditions. An understanding of these trends allows coastal managers to pre-emptively manage the impacts of waves on the coastline at a seasonal to annual basis and provides insight into future wave conditions beyond these time periods.

As the upstream region of the Asian summer monsoon, the Bay of Bengal summer monsoon (BOBSM) system has impacts on rainfall patterns in East Asia. In this study, we investigate the impact of the interannual variability of the BOBSM retreat on China precipitation in early summer (June) of the following year. When the BOBSM retreat occurs earlier in the previous year, we find enhanced rainfall in both the northeastern and eastern parts of China. Conversely, when the retreat of the BOBSM is delayed in the previous year, there is a tendency for decreased rainfall in most of northeastern and eastern China, while rainfall in the northern part of the Taiwan island region tends to increase. Statistical analysis demonstrates the co-variability between China’s June precipitation anomalies and preceding wind anomalies in the eastern Bay of Bengal. The results indicate a strong relationship between the preceding BOBSM retreat and China precipitation anomalies in the following June. Furthermore, the analysis suggests that the BOBSM retreat is more of an independent signal rather than modulated by an Indian Ocean Dipole event.

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]

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 barrier layer (BL) significantly impacts the upper ocean circulation and thermodynamic structure by inhibiting the heat and momentum exchange between the mixed layer (ML) and the subsurface layer. There exist sea surface temperature and salinity dipole modes in the tropical Indian Ocean, however, a BL dipole mode has not yet been identified. Using the latest observations and ocean reanalysis, here we show a robust BL dipole mode in the central and eastern equatorial Indian Ocean, which is highly correlated with the Indian Ocean Dipole (IOD) events. Composite analysis shows that the BL thickness anomalies peak in autumn and are much larger during positive IOD events than during negative IOD events. We show that a positive BL dipole phase is characterized by positive BL thickness anomalies in the central equatorial Indian Ocean and negative BL thickness anomalies in the eastern equatorial Indian Ocean, and vice versa for a negative BL dipole phase. During positive IOD events, negative surface salinity anomalies slightly affect the ML depth along the equatorial Indian Ocean. Positive subsurface temperature anomalies deepen the isothermal layer (IL) in the central equatorial Indian Ocean and strong negative subsurface temperature anomalies significantly lift the IL in the eastern equatorial Indian Ocean, controlling the BL thickness anomalies and forming a positive BL dipole pattern. This operates in an opposite direction during negative IOD events. Our study shows a close relationship between the BL dipole and the IOD and has far‐reaching implications for better understanding and predicting the IOD events. Plain Language Summary: The ocean barrier layer (BL) is an intermediate layer of water between the base of the ML and the base of the isothermal layer (IL). The BL acts as a "barrier" to entrainment cooling and vertical mixing between the surface ML and subsurface layer. In the tropical Indian Ocean, whether there is a BL dipole mode co‐existing with the Indian Ocean Dipole (IOD) is still unknown. This study identified a BL dipole mode in the central and eastern equatorial Indian Ocean. During the IOD events, the anomalous zonal currents cause surface salinity anomalies and further impact the ML depth along the equatorial Indian Ocean. Subsurface temperature anomalies associated with equatorial waves and anomalous currents significantly change the IL depth in the central and eastern equatorial Indian Ocean. We show that temperature and salinity anomalies can induce a BL dipole, which is highly correlated and co‐varies with the IOD. This BL can potentially enhance the positive feedback of the IOD by changing the ocean circulation and thermodynamic structure in the upper ocean. This study will be beneficial to better understand the evolution of the IOD. Key Points: A barrier layer (BL) dipole is identified in the central equatorial Indian Ocean and eastern equatorial Indian OceanThe BL dipole is highly correlated and co‐occurs with the Indian Ocean Dipole event, which peaks in autumnThe isothermal layer depth dominates the evolution of the BL thickness along the equatorial Indian Ocean [ABSTRACT FROM AUTHOR]

The variability of the South Java Current (SJC) was observed by using reanalysis data spanning the years 1993 to 2021. This was done in order to determine whether or not the SJC was more influenced by the Indian Ocean Dipole (IOD), the El Niño-Southern Oscillation (ENSO), or a combination of the two. Employing empirical orthogonal function (EOF) analyses, we were able to determine that the time series of the principal component in the first mode (PC1) had an association with one of these occurrences. During the northwest monsoon in December, January, and February (DJF), it would appear that the IOD has a greater impact on the SJC than ENSO does, with a correlation of more than 0.8. During the first transition, which occurs in March, April, and May (MAM), the time series PC1 demonstrates that the SJC has a greater association with the ENSO (coefficient correlation more than 0.7). The study demonstrates that the PC1 has a negative association with both the IOD and the ENSO during the months of JJA, with a coefficient value less than 0.4. The JJA's SJC, however, is positively influenced by the coastal Kelvin wave in the vicinity of western Sumatra and southern Java. Moreover, the magnitude of the SJC, which was observed in DJF months, is affected by the Rossby wave that is moving in a westward direction south of 9˚S. [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 Indian Ocean Dipole (IOD) has been extensively studied for its significant impact on salinity distribution in tropical regions. However, its off‐equatorial influence in the northeastern Indian Ocean (IO) has received limited documentation thus far. In October 2019, a buoy in the central Andaman Sea (AS) observed an extreme freshening event that resulted in intensified upper‐ocean stratification and increased internal wave activities. The salt budget evaluation revealed the dominant role of horizontal advection. Interannual variability in autumn and winter sea surface salinity (SSS) showed a significant correlation with the IOD. Extreme freshening was observed exclusively during strong positive IOD (pIOD) years. This freshening primarily resulted from the outflow of low‐salinity water from the northeastern coast of the AS, driven by an anomalous anti‐cyclonic coastal‐trapped circulation exclusive to strong pIODs in the northeastern IO. This circulation tends to hinder southward freshwater transport in the western region while enhancing it in the eastern region. This circulation pattern is primarily influenced by Kelvin wave forcing, which is triggered by robust equatorial easterly anomalies that are typically more pronounced during strong pIOD events and weaker during weak pIOD events. The anticipated increase in the frequency of extreme freshening events due to the greenhouse warming has the potential to significantly modify the salinity distribution and freshwater transport in the future. Plain Language Summary: Salinity variability in the northeastern Indian Ocean (IO) is widely concerned owing to the substantial freshwater influx and its significant impact on oceanic and climatic dynamics. While the impact of the Indian Ocean Dipole (IOD) on salinity distribution in the tropical IO has been well‐studied, we have limited information about its influence outside of the equatorial region in the northeastern IO. A buoy deployed in the central AS in 2019 recorded a sharp freshening event. This event caused stronger salinity differences between different depths of the ocean and increased stability in the upper layers. Consequently, it resulted in increased internal wave activities near the ocean surface. By studying the mechanism behind the salinity variations, researchers found that this event was caused by a unique clockwise pattern of coastal currents during strong pIOD years. These currents bring freshwater from the northeastern AS toward the open ocean. This circulation pattern, likely driven by long waves originating in the equatorial region, has the potential to alter freshwater transport in the northeastern IO. It is anticipated that such extreme freshening events will occur more frequently under future projections of increased strong pIOD occurrences, warranting further investigation into the associated climatic, chemical, and ecological impacts. Key Points: The first in‐situ observation captured the extreme freshening event in the central Andaman SeaThe observation unveiled unique coastal‐trapped, anti‐cyclonic circulation anomalies in the northeastern IO exclusive to strong pIODThe contrasting impacts of strong and weak pIODs on the circulation and salinity distribution in the northeastern IO were firstly revealed [ABSTRACT FROM AUTHOR]

An effort is made to understand the role of El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events on the malaria transmission intensity over India during the period 1951–2020 (70 years) with the help of a realistically simulated dynamical malaria model. The results suggest that the La Nina years pose a greater threat of malaria disease, especially in the densely populated Indian states. During El Nino years, the malaria transmission intensity and distribution over India greatly reduce, except in the regions such as Orissa, Chhattisgarh, Jharkhand, Western Ghats, parts of Madhya Pradesh, and Andhra Pradesh. It is found that in the positive IOD years, the malaria transmission intensity increases (decreases) over the entire central Indian region and along coastal regions of Tamil Nadu and Kerala (southern peninsular states of India and northeast India). An almost opposite behavior is seen during the negative IOD years. The malaria transmission variability over India is becoming increasingly heterogeneous in recent decades during the El Nino and La Nina years as a result of global warming. The period of 1986–2020 witnessed a substantial decrease (increase) in the malaria transmission intensity during the positive (negative) IOD years, except for a few regions of India. The implications of the results presented in the paper linking the ENSO and IOD signals with the intensity and distribution of malaria over India in a warming world are enormous, especially for the densely populated Indian states. [ABSTRACT FROM AUTHOR]

Aims: We investigate the associations between major Australian climate drivers and extreme weekly fire danger throughout the year. Methods: We use a composite-based approach, relating the probability of top-decile observed potential fire intensity to the positive and negative modes of the El Niño Southern Oscillation, Indian Ocean Dipole, Madden–Julian Oscillation, Southern Annular Mode, split-flow blocking and Subtropical Ridge Tasman Highs, both concurrently and at a variety of lag times. Key results: The chance of extreme fire danger increases over broad regions of the continent in response to El Niño and positive Indian Ocean Dipole events, the negative mode of the Southern Annular Mode, split-flow Blocking Index and Subtropical Ridge Tasman High, and Madden–Julian Oscillation phases 5, 6, 2 and 8 in Austral summer, autumn, winter and spring respectively. These relationships exist not only concurrently, but also when a climate event occurs up to 6 months ahead of the season of interest. Conclusions: These findings highlight the importance of considering the influence of diverse climate drivers, at a range of temporal lag periods, in understanding and predicting extreme fire danger. Implications: The results of this study may aid in the development of effective fire management strategies and decision-making processes to mitigate the impacts of fire events in Australia. This paper explores the relationships between the major forces influencing Australian weather and climate, and the chance of severe fire seasons. The findings could be valuable in decision making and preparation for upcoming fire seasons to avoid more seasons with devastating outcomes such as the 2019–2020 Black Summer. This article belongs to the Collection Fire and Climate. [ABSTRACT FROM AUTHOR]