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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 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]

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]

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months. [ABSTRACT FROM AUTHOR]