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The Indo-Pacific Region is a noteworthy area with the sophisticated ocean and atmosphere system. Here, we focus on the low-frequency signals related to the ocean and atmosphere variation and its impact on dynamic and thermodynamic processes in this region. Dynamically, the sea level rise signals in the tropical west Pacific (WTP) and the southeast Indian Ocean (IO) are closely linked to each other through the pathways of oceanic waveguide within the Indonesian Seas. The low-frequency westward-propagating wave originated in the tropical Pacific is the major cause for the southeast IO sea level change. At this linkage, the interannual wave signal is dominated by the wind variability along the equatorial Pacific and the interdecadal signal is driven mainly by the wind curl off the equatorial Pacific, respectively associated with the El Nino and the Pacific Decadal Oscillation. The sea level variability in the southwest IO is mainly related to the local wind. Thermodynamically, we find a contrasting trend pattern of sea surface salinity between the WTP and the southeastern tropical IO since mid-1990s, with significant salinity increase in the WTP and freshening in the southeastern tropical IO. These trends were associated a strengthening trend of the Walker Circulation over the tropical Indo-Pacific. Precipitation, surface level pressure and wind also underwent a decadal shift in mid-1990s. Understanding low-frequency processes in the tropical Indo-Pacific will better inform on how the tropical hydrological cycle will be affected by the natural variability and a warming climate. [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]

Remarkable warming of the Southern Indian Ocean during the recent two decades is assessedusing a heat budget analysis based on the Estimating the Circulation and Climate of theOcean version 4 release 3 model results. The annual mean temperature averaged in theupper-700 m Southern Indian Ocean during 1998-2015 has experienced significant warmingat a rate of 1.03×10−2∘C/year. A heat budget analysis indicates that the increase is mostlydriven by decreased cooling from net air-sea heat flux and increased warming from heatadvection. Increased Indonesian Throughflow advection is the largest contributor to warmingthe upper 700 m of the Southern Indian Ocean, while the reduction of surface turbulent heatflux is of secondary importance. These results expand our understanding of the decadalheat balance in the Indian Ocean and of Indo-Pacific decadal climate variability. [ABSTRACT FROM AUTHOR]