The Maluku Sea features strong Chlorophyll-a (Chl-a) bloom and sea surface temperature (SST) cooling during the Southeast Monsoon season. Using high-resolution Chl-a and SST satellite observations over a long time window, this study aims to investigate the effect of El Niño Southern Oscillation (ENSO) on the seasonal variability of Chl-a and SST in the Maluku Sea. During the El Niño and La Niña, the maximum Chl-a concentration (greater than 0.5 mg/m3) and minimum SST (less than 27.5 °C) are observed in August. This finding is in line with prior investigation. Moreover, we reveal that the ENSO does not alter the seasonal variability of Chl-a and SST in the region, but the magnitudes of summertime Chl-a bloom and SST cooling are enhanced (reduced) by about 0.1 mg/m3 and 0.5 °C during the El Niño (La Niña). This is caused by an increase (decrease) in the sea level pressure gradients, which possibly enhanced (reduced) wind-driven mixing in the Maluku Sea.

It has been widely known that the coastal upwelling along the southern coast of Java is generated by southeasterly wind which induces offshore Ekman Mass Transport (EMT) during southeast monsoon. However, the variability of EMT has not been fully described in previous studies. The present study investigated the variability of Ekman dynamics which consist of EMT and Ekman Pumping Velocity (EPV) along the southern coast of Java, based on remotely sensed data. We demonstrated the incongruity between the distribution of southeasterly wind speed and sea surface temperature (SST) during southeast monsoon which is related to the distribution of Ekman dynamics. Offshore EMT at the western region of the southern coast of Java is stronger than offshore EMT at the eastern region. However, stronger offshore EMT at the western part is inhibited by downwelling EPV while weaker offshore EMT at the eastern part is accelerated by upwelling EPV. Consequently, SSTs at the eastern parts are lower than those at the western parts. Thus, the changes of offshore EMT intensity from eastern to western parts are balanced by their EPV distributions which explain the incongruity between the distribution of wind speed and SST during southeast monsoon. On an interannual timescale, the combination of La Niña and negative Indian Ocean Dipole (IOD) events tends to weaken offshore EMT and EPV which reduce the intensity of Chl-a bloom and SST cooling during southeast monsoon season. Furthermore, ENSO has a less significant impact on the Ekman dynamics variability than IOD.

The advection of relatively fresh Java Sea water through the Sunda Strait is presently responsible for the low‐salinity “tongue” in the eastern tropical Indian Ocean with salinities as low as 32‰. The evolution of the hydrologic conditions in the eastern tropical Indian Ocean since the last glacial period, when the Sunda shelf was exposed and any advection via the Sunda Strait was cutoff, and the degree to which these conditions were affected by the Sunda Strait opening are not known. Here we have analyzed two sediment cores (GeoB 10042–1 and GeoB 10043–3) collected from the eastern tropical Indian Ocean off the Sunda Strait that cover the past ~40,000 years. We investigate the magnitude of terrigenous supply, sea surface temperature (SST), and seawater δ¹⁸O (δ¹⁸O_sw) changes related to the sea level‐driven opening of the Sunda Strait. Our new spliced records off the Sunda Strait show that during the last glacial, average SST was cooler and δ¹⁸O_sw was higher than elsewhere in the eastern tropical Indian Ocean. Seawater δ¹⁸O decreased ~0.5‰ after the opening of the Sunda Strait at ~10 kyr B.P. accompanied by an SST increase of 1.7°C. We suggest that fresher sea surface conditions have persisted ever since due to a continuous transport of low‐salinity Java Sea water into the eastern tropical Indian Ocean via the Sunda Strait that additionally increased marine productivity through the concomitant increase in terrigenous supply.

The evolution of an extreme positive Indian Ocean Dipole (pIOD) that took place in the tropical Indian Ocean during the late boreal summer to early winter of 2019 is examined in terms of coupled ocean–atmosphere dynamics. The patterns of anomalous sea-surface temperature (SST) revealed a typical pIOD characteristic: cooling (warming) in the southeastern (western) tropical Indian Ocean. Based on the Dipole Mode Index (DMI), the evolution of the event started in mid-July and gradually strengthened with an abrupt weakening in early September before coming to its peak in late October/early November. It quickly weakened in November, and then it terminated in mid-December. During the peak phase of the event, the SST anomaly in the southeastern (western) tropical Indian Ocean reached about −2 °C (+1 °C). The pattern of anomalous SST was followed by an anomalous pattern in precipitation, in which deficit precipitation was observed over the eastern Indian Ocean, particularly over the Indonesia region. Earlier study has shown that dry conditions associated with the pIOD event created a favorable condition for a forest-peat fire in southern Sumatra. The number of fire hotspots has increased significantly during the peak phase of the 2019 pIOD event. In addition, anomalously strong upwelling forced by strong southeasterly wind anomalies along the southern coast of Java and Sumatra had induced a surface chlorophyll-a (Chl-a) bloom in this region. High surface Chl-a concentration was collocated with the negative SST anomalies observed off the southwest Sumatra coast and south Java.

Seasonal variation of chlorophyll-a (Chl-a) concentration in the Gulf of Tomini (GT) is investigated using SeaWiFS-derived sea surface Chl-a, QuikSCAT-derived ocean surface wind vector and Pathfinder sea surface temperature (SST) measurements. Results show that Chl-a concentration in the GT attained maximum in August (0.59 mg/m ³ ) and is primarily controlled by monsoon winds. During summer, strong winds pass through the Maluku archipelago and blast out toward the Maluku Sea, and impinge the coastal mountain on the southeastern GT (GT tip). As a result, a strong wind jet observed at the open-ocean of GT and is suspected to be the main driving force for the Chl-a bloom in the region. The center of the bloom is located consistently with the wind stress maximum (0.28 Nm ^-2 ) and low SST (27.3 ^° C ). The present study describes the seasonal and spatial distributions of the Chl-a bloom in GT and its associated oceanographic features.

Long-term satellite data are used to investigate the variability of ocean surface chlorophyll-a (chl-a) concentration in the Halmahera Sea (HS) under influence of the Australian-Indonesian Monsoon (AIM), the El Niño-Southern Oscillation (ENSO), and the Indian Ocean Dipole (IOD). In this study, we first analysed the seasonal variability of chl-a, and then examine the relationship between surface chl-a, sea surface temperature (SST), and sea surface wind stress in the area. Our results suggest that prevailing southeasterly winds play a fundamental role in generating chl-a blooms in the HS. Particularly on a seasonal timescale, through the mechanism of Ekman mass transport, strengthening of southeasterly wind stress during the Southeast Monsoon season (June – August) produces enhanced chl-a concentrations associated with ocean surface cooling in the area of study. On the other hand, the chl-a bloom completely diminishes during the Northwest Monsoon season (December – February) due to weakening of wind stress and Ekman transport. On an interannual timescale, sea level pressure and wind stress are coherent with ENSO and IOD phases. During El Niño and positive IOD events (La Niña and negative IOD events), both sea level pressure and wind stress greatly increase (decrease) over the HS. These conditions cause an anomaly in southerly (northerly) wind stress, which is favourable to an enhancement (reduction) of the chl-a concentration in the region. This study demonstrates that sea level pressure and wind stress are the critical factors in determining the magnitude of chl-a bloom in the HS.