Positive Indian Ocean Dipole Events Research Articles

Dynamics of the Barrier Layer Dipole in the Equatorial Indian Ocean

AbstractThe 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.

A mechanistic investigation into the unusual intensification of rainfall over Western India during the 2019 summer monsoon

This study presents a detailed analysis of the 2019 record-breaking seasonal (June–September) monsoon rainfall, since 1901, over large areas of Western India, which evolved in the backdrop of an intense positive Indian Ocean Dipole (pIOD) event. Analysis of ground-station-observed daily rainfall data revealed that the unusually excessive and flood-producing monsoonal rains of 2019 over Western India were driven by three intense rain episodes occurring between late June and September viz., 1) 28th June – 12th July 2) 24th July – 11th August 3) 2nd September – 13th September, characterized by large-scale bands of organized convection. We observed an abundance of stratiform precipitation from GPM Precipitation Radar swaths during the heavy-to-extreme rain events, indicating the presence of organized mesoscale convective systems with elevated heating over the region. Our findings suggest that the continual top-heavy stratiform heating forced a Rossby-wave pattern with regional stretching of mid-tropospheric potential vorticity across South and Southeast Asia, creating a favorable environment for sustaining widespread monsoon heavy-to-extreme rain events. Results reveal that the pIOD conditions that prevailed over tropical Indian Ocean during 2019 led to enhanced cross-equatorial moisture transport and were vital in fostering deep convection and heavy precipitation in this region. Interestingly, upon analyzing long-term data, we observed a 25% increase in the spatiotemporally aggregated rainfall over Western India contributed by seasonally abundant heavy rainfall events during pIOD years, as compared to non-pIOD years. Overall, this study underscores the potential hydrological consequences of intense pIOD manifestations on intense rain episodes and their impact on the monsoon-centric regions of South and Southeast Asia, particularly in a warming world.

The strengthened role of new predictors of Indian Ocean Dipole (IOD) during the recent decades of weakened ENSO-IOD relationship

Indian Ocean Dipole (IOD) is the major interannual ocean-atmosphere interaction phenomena in the Indian Ocean (IO) and is influenced by external forcing such as El Nino Southern Oscillation (ENSO). Meanwhile, its co-occurrence and stronger relationship with ENSO have decreased during recent decades. The IOD variability also reduced after 2000, accompanied by a shift of the western pole to central IO extending further southward. The study reports that the boreal fall (SON, September, October, November) season IOD has an intensified relationship with the previous winter Subtropical Indian Ocean dipole (SIOD) and spring season equatorial north tropical Atlantic (NTA) SST anomalies., while the ENSO relationship is reduced from its pre-2000 value. It is found that the persistent warming (cooling) in the western side of positive (negative) SIOD during the previous winter induces easterly (westerly) wind anomalies in the equatorial IO during the following summer and fall, leading to positive (negative) IOD events. These IOD events have the western pole shifted to south-central IO instead of the canonical northwestern warming. The spring season NTA SST anomalies induce stronger summer season circulation and SST gradient in the equatorial Pacific, like ENSO. During the SON season, this pattern is associated with cooling and easterly wind anomalies in the tropical eastern IO and IOD. These two patterns explain the major mode of IOD variability after 2000. While the IOD associated with NTA have co-occurring ENSO during summer and fall, the SIOD-induced IOD events are independent of ENSO in the Pacific. Thus, these two predictors provide long-lead predictability (2–3 seasons ahead) of IOD for both ENSO co-occurring and non-ENSO IOD events. However, the IOD predictability of seasonal prediction models mainly depends on the ENSO-IOD relationship, resulting in reduced IOD skills for many of them after 2000. The models such as COLA-CCSM4, which has improved skill have stronger than observed ENSO-IOD relationship than the pre-2000 period. A linear regression model including SIOD and NTA SST indices of the previous winter and spring season respectively as predictors simulates IOD with a skill of around 0.65 during the recent period, indicating the necessity of seasonal prediction models to capture the variability in the southern IO and NTA and their teleconnections for better prediction of IOD in the recent period of reduced ENSO skill.

Morphological Response of Coral Reef Islands to the 2019 Extreme Positive Indian Ocean Dipole Event

Morphological Response of Coral Reef Islands to the 2019 Extreme Positive Indian Ocean Dipole Event

Associations between Australian climate drivers and extreme weekly fire danger

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.

Exploring the Complex Dynamics of Tropical Cyclone Activity in the Southern Indian Ocean: A Multidecade Analysis

This study analyzes tropical cyclones (TCs) in the Southern Indian Ocean from 1970 to 2022, utilizing data from the International Best Track Archive for Climate Stewardship (IBTrACS) and Sea Surface Temperature (SST) data from ERA-5. Trends are assessed with Sen's slope method. Significant TC shifts are evident. Categories 1-2 TCs have substantially reduced in number, duration, and speed, indicating an overall decline in activity. Conversely, more powerful TCs (categories 3-5) have intensified, driven by rising SST. This intensification is associated with alarming increases in the Power Dissipation Index (PDI) and Accumulated Cyclone Energy (ACE), reflecting heightened destructive potential. TCs predominantly occur during DJFM, influenced by elevated SST compared to MJJASO. Climate drivers, including El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Madden-Julian Oscillation (MJO), significantly affect TC characteristics. Elevated TC activity coincides with La Niña, neutral ENSO phases, and positive IOD events. At the same time, MJO influences are nuanced, leading to a slight TC activity decline, potentially due to data limitations in specific MJO phases. This study offers vital climatological insights into evolving TC risks in the Southern Indian Ocean, aiding agencies and decision-makers in understanding TC-related risks in the region.

Dynamical Response of the Arabian Sea Oxygen Minimum Zone to the Extreme Indian Ocean Dipole Events in 2016 and 2019

AbstractThe 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.

Spatial Correlations of Regional Tropical Cyclone– and Nontropical Cyclone–Induced Severe Rainstorms during 2000–19

Abstract Severe rainstorms are one of the most devastating disasters in southeast China (SEC). A deep and comprehensive understanding of the spatial correlations of severe rainstorms is important for preventing rainstorm-induced hazards. In this study, tropical cyclone– and nontropical cyclone–induced severe rainstorms (TCSRs and NTCSRs, respectively) over SEC during 2000–19 are discussed. Co-occurrence probability and range values calculated using the semivariogram method are used to measure the spatial correlations of severe rainstorms. The extent to which potential factors [El Niño/La Niña, Indian Ocean dipole (IOD), latitudes, longitudes, temperature, elevation, and radius of maximum wind] affect the spatial structure of severe rainstorms is discussed. The spatial correlation distances for TCSRs (300–700 km) in typhoon season (July, August, and September) are longer than most of those for NTCSRs (150–300 km) in mei-yu season (June and July). The range values of TCSRs at each percentile (except for the minimum range values) tend to be omnidirectional. While NTCSRs tend to have a major direction of northeast–southwest. El Niño tends to increase the average spatial correlation distance of TCSRs in the northeast–southwest and NTCSRs in the north–northeast. La Niña tends to decrease the spatial correlation distance of TCSRs in the northeast–southwest. The occurrence of positive IOD (+IOD) and negative IOD (−IOD) events may increase the spatial correlation distance of TCSRs in the northwest–southeast, and −IOD events may decrease the distance in the northeast–southwest. IOD events, especially −IOD, may change the spatial correlation distance of NTCSRs in the east–northeast. Latitudes, longitudes, temperature, elevation, and radius of maximum wind significantly affect the spatial correlation distance of TCSRs in various directions. Significance Statement Spatial correlation distances of rainfall events, especially severe rainstorms induced by different weather systems such as tropical and nontropical cyclones (TCSR and NTCSR), can provide important information for rain-induced hazard risk mitigation. Moreover, factors that affect the variability of the spatial correlation distance of TCSRs and NTCSRs are also of interest. To this end, co-occurrence probability and semivariogram methods are used to obtain the spatial correlation distances. The spatial correlation distance of TCSRs varies between 300 and 700 km in typhoon season (July, August, and September) while it ranges between 150 and 300 km in mei-yu season (June and July) for NTCSRs over southeast China (SEC). El Niño/La Niña, Indian Ocean dipole (IOD), latitudes, longitudes, temperature, elevation, and radius of maximum wind have significant impacts on the spatial correlation distances of TCSRs. These findings can provide useful information for forecasting the spatial correlation distance of severe rainstorms over SEC.

Extreme Drought Assessment in Sumatra-Indonesia Using SPI and EDI

Even though Sumatra is very vulnerable to the effects of drought, research identifying drought on this island is very limited, especially extreme drought. This research was conducted to identify extreme drought in Sumatra using the Standardized Precipitation Index (SPI) and the Effective Drought Index (EDI). This study uses precipitation data from the Indonesian Meteorology, Climatology and Geophysics Agency (BMKG) and Global Precipitation Climatology Center (GPCC) reanalysis data. The composite indices were conducted to discover some phenomena that cause the drought based on El Niño and positive Indian Ocean Dipole (IOD) events. The results showed that the El Niño and positive IOD phenomena were more likely to influence extreme droughts. However, the droughts in 2014 and 2008 tended to be influenced by the negative Sea Surface Temperature anomaly (SSTA). The spatial analysis results show that the areas that experience extreme drought more often are the west coast of Sumatra (except Aceh province), especially based on SPI12 and EDI. The composite indices results show that the drought that occurred in Aceh province was more influenced by the El Niño phenomenon in the December-January-February (DJF) period and the positive IOD in the June-July-August (JJA) – September-October-November (SON) period. In addition, Aceh province is an area that is not affected by El Niño-positive IOD, especially during the SON-DJF period. These results can be used to mitigate drought, especially when El Niño-positive IOD phenomena cause it.

Predictability of Marine Heatwaves off Western Australia Using a Linear Inverse Model

Abstract Marine heatwaves (MHWs) off Western Australia (110°–116°E, 22°–32°S; herein, WA MHWs) can cause devastating ecological impacts, as was evidenced by the 2011 extreme event. Previous studies suggest that La Niña is the major large-scale driver of WA MHWs, while the Indian Ocean dipole (IOD) may also play a role. Here, we investigate historical WA MHWs and their connections to these large-scale climate modes in an ocean model (ACCESS-OM2) simulation driven by a prescribed atmosphere from JRA-55-do over 1959–2018. Rather than analyzing sea surface temperature, the WA MHWs and climate mode indices were characterized and investigated in vertically averaged temperature (VAT) to ∼300-m depth to afford the longer ocean dynamic time scales, including remote oceanic connections. We develop a cyclostationary linear inverse model (CS-LIM; from 35°S to 10°N, across the Indo-Pacific Ocean), to investigate the relative contributions of La Niña VAT and positive IOD VAT to the predictability of WA VAT MHWs. Using a large ensemble of CS-LIM simulations, we found that ∼50% of WA MHWs were preceded about 5 months by La Niña, and 30% of the MHWs by positive IOD about 20 months prior. While precursor La Niña or positive IOD, on their own, were found to correspond with increased WA MHW likelihood in the months following (∼2.7 times or ∼1.5 times more likely than by chance, respectively), in combination these climate mode phases were found to produce the largest enhancement in MHW likelihood (∼3.2 times more likely than by chance). Additionally, we found that stronger and longer La Niña and/or positive IOD tend to lead stronger and longer WA MHWs. Significance Statement This study examines seasonal-to-interannual time-scale predictability of marine heatwaves off Western Australia. We developed and applied a linear inverse model, informed by numerical model results, to generate a large number of 60-yr temperature simulations across the broader Indian–Pacific Ocean region to quantify this marine heatwave predictability. We found that La Niña typically increases the likelihood of marine heatwaves off Western Australia about 5 months (3–7 months) later, while positive Indian Ocean dipole events increase their likelihood about 20 months (18–22 months) later. Marine heatwaves can severely impact local marine ecosystems and the economy. Our findings are expected to be valuable for marine heatwave prediction system development on time scales that can be beneficial to marine ecosystem conservation and fishery management.

Influences of teleconnections on climate variables in northern and northeastern Thailand

Abstract Teleconnection events can influence normal regional weather patterns and affect weather forecast accuracy. To improve the forecast ability, the relationship between main teleconnections such as El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Madden–Julian Oscillation (MJO), and climate variables (rainfall, maximum and minimum surface temperature, vertical mixing ratio, and vertical maximum temperature) was established using lag correlation coefficient and t-test methods. The results reveal moderately significant correlations between El Niño, positive IOD and rainfall, and vertical mixing ratio, which can be associated with lower-than-usual rainfall. The coincidence between El Niño and positive IOD events can worsen drought. Even though the MJO and regional weather correlations were significant, the magnitude of correlation coefficients was negligible. In addition, the spatiotemporal distribution of ENSO shows that the strong El Niño has more influence on rainfall anomalies in the post-1980s. Since there are insufficient studies on the association between teleconnections and climate variables, especially vertical mixing ratio, our findings can benefit prediction development for teleconnection-induced regional climate anomalies for extreme events and water management preparations in northern and northeastern Thailand.

Significant Precipitation Anomalies over Indonesia in the Aftermath of iod Events

Indian Ocean Dipole (IOD) is one of the prominent climate phenomena affecting Indonesia. The well-documented impacts of IOD are dry and wet anomalies during positive and negative IOD events, respectively, which are clearly detected during the development and mature phases of IOD in the boreal summer to autumn (June-November). IOD impacts on other seasons were questionable because of unclear associated signals. Here, we find significant precipitation anomalies in the aftermath of IOD events, which reach a peak in the following spring (March-May). The delayed pattern is distinctive with the typical impact of IOD. During positive IOD, the quick recovery and warming of IOD-induced sea surface temperature (SST) in the eastern Indian Ocean due to increasing shortwave radiation is suggested to play a role. When the influence of El Niño-Southern Oscillation (ENSO) is linearly removed, the aftermath impacts of IOD in the western regions appear to be suppressed because of weaker positive (SST) anomalies. While the eastern regions show anomalous increase on precipitation due to anomalous increase of convergence. This finding reveals the complex impacts of IOD and ENSO and may benefit to developing a better seasonal forecast over Indonesia.

Wind Variability and its Impact on IOD and MJO Phenomena in Western Waters


 
 
 
 
 
 This study investigates the interplay between wind variability and the Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO) phenomena in the western waters of Sumatra during the period 1982-2021. Utilizing monthly average wind data from satellite images, the research employs a comprehensive approach involving GrADS, Microsoft Excel, Surfer, Matlab, and Panoply software for data processing. The analysis reveals distinct wind patterns and varying IOD and MJO phases, with notable instances such as a positive IOD in October 1982 and a negative IOD in March 1983, coupled with strong MJO activity in February 1982 and weak activity in October 1983. This research underscores the significant influence of wind conditions on the dynamics of IOD and MJO phenomena in the studied region, contributing to a deeper understanding of ocean-atmosphere interactions.
 Highlight:
 
 
 
 
 
 
 
 
 
 
 
 Temporal Analysis: This study examines wind variability's interplay with IOD and MJO phenomena over three decades, revealing distinct patterns and phases in the western waters of Sumatra.
 Comprehensive Approach: Utilizing advanced software tools, the research processes satellite-derived wind data, shedding light on the complex dynamics of ocean-atmosphere interactions.
 Phenomena Dynamics: Notable occurrences, including positive and negative IOD events and varying MJO strengths, emphasize the profound impact of wind conditions on shaping the behavior of these critical climate phenomena, contributing to enhanced understanding and prediction.
 
 
 
 
 
 
 
 
 Keyword: Wind Variability, Indian Ocean Dipole (IOD), Madden-Julian Oscillation (MJO), Ocean-Atmosphere Interactions, Western Sumatra
 
 
 
 
 
 
 
 

Role of the Australian High in Seasonal Phase Locking of the Indian Ocean Dipole

AbstractThis 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.

Characteristics of Marine Heatwaves in the Indonesian Waters during the PDO, ENSO, and IOD Phases and Their Relationships to Net Surface Heat Flux

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics.

Detecting the sensitivity of water content and deficiency rainfall during positive Indian Ocean Dipole events

This study aimed to detect the water content in peatland and cropland associated with rainfall deficiencies and positive Indian Ocean Dipole (IOD) events. The condition of water content is a biological indicator of peatland ecosystems that commonly contributes to hot spots in Sumatra. The water content and rainfall data were processed using the spatial average at two study areas, and the variabilities were analyzed with the Indian Ocean Dipole index (DMI). From 2015 until 2020, this study employed the Dipole Mode Index to evaluate five positive IOD events. The analysis of the water content conditions and deficiency rainfall correlated to positive IOD events. In the peatland, the water content was a sensitive parameter, and even though the value decreased by a small amount, it potentially inflamed. Even if the water content was low in cropland areas, it did not trigger forest fires. The effect of rainfall deficiency was related to land response to water content conditions. Different land types will have different sensitivities. Understanding the correlation between deficiency rainfall with water content can be helpful for an early warning system.

Impact of leading Indo‐Pacific climatic modes in modulating the wave power variability and their trends in the North Indian Ocean

AbstractThe present study analyses the trend and variability in wave parameters (wave height and wave period) and wave power (WP) in the North Indian Ocean (NIO). The impact of climatic modes such as El‐Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on the WP of NIO has been investigated using the European Centre for Medium‐Range Weather Forecasts reanalysis datasets (ERA5) for the period 1959–2021. It is observed that along the west coast of India (WCI), the coastal WP increased from south to north during summer whereas, in winter and other seasons the WP increased from north to south. Instead, along the east coast of India (ECI), WP is higher on the north than the south, always. The annual WP climatology along the Indian coasts reveals that WP potential is higher along/adjacent to WCI than the ECI. It was observed that the total significant wave height (SWH) WP anomalies during normal and positive IOD (pIOD) events are mainly dominated by swell WP, whereas during El Niño and combined ENSO‐IOD events, dominated by wind seas. During the peak of pure El‐Niño, the SWH WP shows a basin‐wide increase in WP anomalies. However, during summer, SWH WP showed increasing trends over Bay of Bengal. It is evident from the composite analysis that annual averaged WP anomalies are decreasing in normal, pure pIOD and combined events, but El‐Niño is found to increase the WP anomalies over the Arabian Sea in the region north of 20°N. The present study advocates that a detailed analysis of WP along the Indian coastal regions is beneficial to the renewable energy industries to utilize this as an important energy resource.

Air sea conditions facilitate a transformation of the positive Indian Ocean Dipole with distinct east pole characteristics into an extreme event

The Indian Ocean Dipole (IOD) is one of the dominant interannual variabilities in the Indian Ocean (IO), and an extreme IOD, in particular, has dramatic effects on the weather, agriculture, and ecosystem around it. Therefore, the formation of an extreme IOD has been a worldwide research focus. Among 13 positive IOD (PIOD) events, two type-east and two type-comparable PIODs developed into extreme events during the 1960–2020 period. This investigation focuses on the cause of the formation of the type-east extreme PIOD, as previous studies have discussed the origin of the type-comparable extreme PIOD. Composite analysis showed that, as an entity, the strong East Asian and Australian monsoon (EAAM) may result in an evident easterly wind anomaly around the Indonesian region of the Marine Continent during May to August of the years when type-east PIODs occurred. The easterly wind anomaly associated with the EAAM was stronger in the extreme IOD group, whereas it was relatively weak in the regular group. The difference in the easterly wind anomaly between the extreme and regular groups could result in a vertical motion anomaly by enhancing the anomalous westward current. The stronger vertical motion created an upwelling in the deep-layer cold water, resulting in a more distinct difference in the vertical temperature gradient. All these conditions promoted the transformation of the type-east PIODs that occurred in 1961 and 1994 into extreme events and are indicative of the importance of vertical advection terms in the formation of type-east extreme PIODs. This study reveals the cause of the formation of type-east extreme PIODs, which will be helpful in understanding IOD diversity.

Meridional‐Width Variability of Near‐Equatorial Zonal Currents Along 80.5°E on Seasonal to Interannual Timescales in the Indian Ocean

AbstractNear‐equatorial zonal currents along 80.5°E in the upper Indian Ocean are key to tropical inter‐basin hydrographic exchanges. However, their meridional‐width variability and underlying causes have remained elusive. In this study, we use satellite‐derived ocean current data to reexamine the latitudinal structure of Wyrtki jets (WJs), Monsoon Circulation (MC), and South Equatorial Countercurrent (SECC) along 80.5°E on seasonal to interannual timescales. The WJs are characterized by a transient semiannual feature within the latitudinal range of 2.5°S–3°N, whereas the MC and the SECC have a dominant annual period within the latitudinal range of 3°–5.5°N and 2.5°–6°S, respectively. Empirical orthogonal function (EOF) analysis separates the seasonal latitudinal boundaries of the WJs, the MC, and the SECC, and reveals their interannual variability in response to anomalous wind forcing. EOFs1–3 represent the WJ, MC, and SECC modes accounting for 54%, 19%, and 14% of the total variance, respectively. The near‐equatorial zonal currents are asymmetrically responsive to the Indian Ocean Dipole (IOD), with an abnormally strong westward current resulting from the latitudinal superposition of the WJ and SECC modes in the autumn of 1997, 2006, and 2015 during strong positive IOD events. However, the abnormally enhanced eastward WJs occurred only during weak IOD events in the autumn of 1995, 1999, and 2005, accompanied by latitudinal destructive interference of strong westward SECC. Model sensitivity experiments show that the asymmetric response of near‐equatorial zonal currents to climate modes is closely related to the spatial differences in large‐scale wind anomalies associated with wind‐driven and eastern‐boundary‐generated waves.

Role of Extreme Indian Ocean Dipole in Regulating Three‐Dimensional Freshwater Content in the Southeast Indian Ocean

AbstractThe tropical Southeast Indian Ocean (SEIO) is a key area linking the global freshwater and heat exchanges. The Indian Ocean Dipole (IOD) fundamentally modulates the Indian Ocean circulation and thus regulates the basin‐wide freshwater balance. However, our knowledge of this effect remains limited. Using observational‐based data sets, this study suggests that extreme positive IOD events have notable signatures on the three‐dimensional freshwater content of the SEIO, leading to the vertically opposite salinity anomalies in the surface and subsurface layers. The wind changes led to the northwestward extension of the South Equatorial Current and intensified Sumatra‐Java upwelling. The changing horizontal and vertical currents jointly result in the complicated salinity anomalies. The Equatorial Undercurrent serves as the conduit for water exchange between the equator and the SEIO. This work highlights a strong coupling between the equatorial circulation and the three‐dimensional freshwater inventory of the SEIO within the framework of the IOD.