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

The spatio‐temporal characteristics of the interannual variability and long‐term trend of the marine heatwaves (MHWs) and related dynamic mechanisms in the western equatorial Indian Ocean (WEIO) are investigated using satellite observations. A prominent MHW hot spot is found in a region of the WEIO (48°E−54°E, 2°S–2°N), with a mean MHWs' intensity, duration, and frequency of 1.54°C, 13.33 days, and 1.97 times, respectively. MHWs in the hot spot region have significant interannual variability after removing the long‐term trend, associated with Indo‐Pacific major climate modes. In 1982/1983, 1983/1984, 1987/1988, 1997/1998, 2006/2007, 2009/2010, 2011/2012, 2012/2013, 2014/2015, 2015/2016, and 2019/2020, the MHWs occurred with longer duration, higher frequency, and more total days. These years correspond to a positive Indian Ocean Dipole, or an El Niño event, or both. The occurrence of MHWs accompanied by anomalous positive sea surface height suggests that oceanic planetary wave processes modulate MHWs in the WEIO. Westward‐propagating downwelling equatorial Rossby waves triggered by anomalous equatorial easterly winds drive the convergence of warm upper‐ocean water and weaken the upwelling of cool subsurface water, which favor anomalously warm sea surface temperature (SST) and the occurrence of MHWs. In addition, the westward‐propagating off‐equatorial downwelling Rossby waves in the southern tropical Indian Ocean also affect MHWs in the WEIO through the propagation and reflection of waves. The annual MHW frequency, duration, and total days in the hot spot region increase up to 1.56 times, 4.95 days, and 31.72 days per decade, respectively, related to the significant increase in mean SST under global warming. Plain Language Summary: Marine heatwaves (MHWs) are anomalous warming events in the ocean that can be disastrous for the global marine ecosystem and lead to local socioeconomic losses. The western equatorial Indian Ocean (WEIO), as the western pole of the Indian Ocean Dipole (IOD) and the upwelling region where the Indian Ocean monsoon prevails, experiences strong MHWs. This study shows the characteristics and dynamic mechanisms of MHWs in the WEIO. The MHWs had a significant interannual variability during 1982–2020, associated with the positive IOD and El Niño events. It was found that oceanic planetary wave processes cause the occurrence of MHWs in the WEIO. Westward‐propagating downwelling equatorial Rossby waves and off‐equatorial downwelling Rossby waves in the southern tropical Indian Ocean increase sea surface temperature in the WEIO by suppressing upwelling and entrainment. Under global warming, increasing frequency, duration, and annual days of MHWs are found in the WEIO. Global warming is consistently one of the main causes of extreme warm events. Understanding the characteristics and mechanisms that drive the occurrence of MHWs in the WEIO is a step toward predicting these extreme events. Key Points: Marine heatwaves in the western equatorial Indian Ocean (WEIO) have significant interannual variability related to the Indo‐Pacific climateMarine heatwaves in the WEIO are mainly driven by westward‐propagating downwelling equatorial Rossby waves and off‐equatorial Rossby wavesThe frequency and duration of marine heatwaves exhibited significant increasing trends in the WEIO from 1982 to 2020 [ABSTRACT FROM AUTHOR]

This study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO. [ABSTRACT FROM AUTHOR]

The Southern Ocean (SO) is one of the key regions in absorbing and storing anthropogenic heat. An analysis of the CMIP6 models finds a distinct warming minimum/cooling and freshening in the subtropical ocean thermocline of the south Indian Ocean (SIO) under a medium-emission scenario (SSP245). The warming minimum/cooling has also been found in other warming scenarios in previous studies. However, the freshening here has received less attention. On account of increased precipitation in the models, the SO high latitudes get fresher in a warmer world. We show that this freshening anomaly is advected to the north of the deep mixed layer by the horizontal current and then subducts into the ocean interior in the SIO. As a result, the isopycnal surfaces become fresher, deeper, and cooler. This freshening and cooling signal then propagates to the north along isopycnals through the subtropical gyre and leads to freshening and cooling on the depth coordinates where the vertical movement of isopycnals (heaving) is insignificant. Lacking deep-enough mixed layers, the other two basins show smaller freshening and cooling signs in the models. Here the importance of freshening in temperature redistribution in the ocean interior in the SIO under extensive global warming is emphasized. The result helps interpret the future heat storage in the SO in a warmer world. Significance Statement: The Southern Ocean (SO) is a key region in absorbing and storing anthropogenic heat. The observed past and simulated future warming in the SO show maximum values between 40° and 45°S and minimum values downward and northward of the deep mixed layers. CMIP6 models show the same pattern, and it is found that this pattern is most distinguished in the south Indian Ocean. The freshening anomaly advected to the deep mixed layers subducts into the ocean interior, leading to freshening and cooling signals on isopycnals. These signals spread with the subtropical gyre and induce warming minimum/cooling there. The result would help interpret the warming pattern in the SO in a warmer world. [ABSTRACT FROM AUTHOR]

This study examines the climate response to a sea surface temperature (SST) warming imposed over the southwest tropical Indian Ocean (TIO) in a coupled ocean–atmosphere model. The results indicate that the southwest TIO SST warming can remotely modulate the atmospheric circulation over the western North Pacific Ocean (WNP) via interbasin air–sea interaction during early boreal summer. The southwest TIO SST warming induces a "C shaped" wind response with northeasterly and northwesterly anomalies over the north and south TIO, respectively. The northeasterly wind anomalies contribute to the north TIO SST warming via a positive wind–evaporation–SST (WES) feedback after the Asian summer monsoon onset. In June, the easterly wind response extends into the WNP, inducing an SST cooling by WES feedback on the background trade winds. Both the north TIO SST warming and the WNP SST cooling contribute to an anomalous anticyclonic circulation (AAC) over the WNP. The north TIO SST warming, WNP SST cooling, and AAC constitute an interbasin coupled mode called the Indo-western Pacific Ocean capacitor (IPOC), and the southwest TIO SST warming could be a trigger for IPOC. While the summertime southwest TIO SST warming is often associated with antecedent El Niño, the warming in 2020 seems to be related to extreme Indian Ocean dipole during the autumn of 2019. The strong southwest TIO SST warming seems to partly explain the strong summer AAC of 2020 over the WNP even without a strong antecedent El Niño. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long‐lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co‐occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward‐propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long‐lasting MHWs in the TIO. Plain Language Summary: Marine heatwaves (MHWs) are extreme sea surface warming events, which can have destructive impacts on marine ecosystems and socio‐economy. The tropical Indian Ocean (TIO) has experienced the most significant sea surface temperature warming among the global oceans in the satellite observing era since the 1980s. The increases in duration and frequency of MHWs were found statistically significant in the TIO, and further increases can be expected under global warming. Several long‐lasting MHWs occurred in recent years, associated with extreme climate modes. Internal ocean waves triggered by anomalous winds deepened the thermocline through the convergence of warm upper‐ocean water and reduced upwelling of subsurface cold water, resulting in a surface warm that facilitated the MHW formation. Thus, the consecutive propagating waves during the extreme climate events sustained the prolonged MHWs in the TIO. These results expand our understanding of the physical mechanisms responsible for MHWs in the TIO under the Indo‐Pacific climate variability and global warming. Key Points: Long‐lasting marine heatwaves (MHWs) occurred in the upwelling regions of the tropical Indian Ocean during 2015–2016 and 2019–2020The recent long‐lasting MHWs are related to the super El Niño in 2015–2016 and extreme Indian Ocean dipole in 2019Oceanic downwelling waves induced‐thermocline warming plays a crucial role in the long‐lasting MHWs [ABSTRACT FROM AUTHOR]

The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity. Plain Language Summary: The Indian Ocean Dipole (IOD) is an ocean‐atmosphere coupled climatic phenomenon which can cause severe social and economic losses in the surrounding regions such as drought in the Maritime Continent/Australia and flooding in East Africa. The IOD features a see‐saw structure accompanied by an anomalous sea surface temperature gradient, winds, and oceanic adjustments. The El Niño–Southern Oscillation in the Pacific is an important trigger to a strong IOD event. However, an extreme positive IOD event occurred in 2019 without a concurrent or ensuing El Niño. We show that the thermocline warming associated with anomalous ocean downwelling in the southwest tropical Indian Ocean triggered atmospheric convection, inducing anomalous easterly winds along the equator and hence, positive feedbacks associated with an IOD event. This study may help to understand the evolution of extreme IOD and improve IOD predictability. Key Points: The oceanic downwelling Rossby waves in the south tropical Indian Ocean is key to the 2019 extreme positive Indian Ocean Dipole (IOD)The Rossby waves induced thermocline warming, triggering wind‐evaporation‐SST feedback thus anomalous easterly winds along the equatorThe easterly wind anomalies further triggered the Bjerknes feedback and other positive feedbacks and established an extreme IOD [ABSTRACT FROM AUTHOR]

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]

The south tropical Indian Ocean (TIO) warms following a strong El Niño, affecting Indo-Pacific climate in early boreal summer. While much attention has been given to the southwest TIO where the mean thermocline is shallow, this study focuses on the subsequent warming in the southeast TIO, where the mean sea surface temperature (SST) is high and deep convection is strong in early summer. The southeast TIO warming induces an anomalous meridional circulation with descending (ascending) motion over the northeast (southeast) TIO. It further anchors a "C-shaped" surface wind anomaly pattern with easterlies (westerlies) in the northeast (southeast) TIO, causing a persistent northeast TIO warming via wind–evaporation–SST feedback. The southeast TIO warming lags the southwest TIO warming by about one season. Ocean wave dynamics play a key role in linking the southwest and southeast TIO warming. South of the equator, the El Niño–forced oceanic Rossby waves, which contribute to the southwest TIO warming, are reflected as eastward-propagating oceanic Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the southeast TIO warming. [ABSTRACT FROM AUTHOR]

An unusual positive Indian Ocean Dipole (IOD) event occurred in 2015 associated with the 2015/16 extreme El Niño. Unlike the canonical IOD, sea surface temperature (SST) warming in the west-central tropical Indian Ocean (TIO) dominated the strong zonal SST gradient as cooling off Sumatra-Java was weak. Over the southeastern TIO, deeper thermocline has suppressed the upwelling cooling since 2012. Such deepened thermocline related to a low-frequency adjustment and curtailed cool anomalies in the 2015 positive IOD but favored warm anomalies in the 2016 negative IOD. Based on statistical analyses, ocean assimilation data confirm that an IOD-like pattern exists in the TIO on decadal time-scale. During a negative decadal IOD-like phase, thermocline is deeper in the southeastern TIO; the thermocline-SST feedback is unfavorable for positive IOD occurrence and intensity, but conducive to negative IOD events. Thus, we propose that the 2015-2016 IOD events are modulated by the low-frequency variability of thermocline. [ABSTRACT FROM AUTHOR]