Variability of Heat Content and Eddy Kinetic Energy in the Southeast Indian Ocean: Roles of the Indonesian Throughflow and Local Wind Forcing

Abstract

Prominent interannual-to-decadal variations were observed in both heat content and mesoscale eddy activity in the southeast Indian Ocean (SEIO) during 1993-2020. The 2000-2001 and 2008-2014 periods stand out with increased 0-700 m ocean heat content (OHC) by ∼4.0 × 1021 J and enhanced surface eddy kinetic energy (EKE) by 12.5% over 85°–115°E, 35°−12°S. This study provides insights into the key dynamical processes conducive to these variations by analyzing observational datasets and high-resolution regional ocean model simulations. The strengthening of the Indonesian throughflow (ITF) and anomalous cyclonic winds over the SEIO region during the two periods are demonstrated to be the most influential. While the ITF caused prevailing warming of the upper SEIO, the cyclonic winds cooled the South Equatorial Current and attenuated the warming in the subtropical SEIO by evoking upwelling Rossby waves. The EKE increase exerts significant influence on OHC only in the Leeuwin Current system. Dynamical instabilities of the Leeuwin Current give rise to high EKEs and westward eddy heat transport in climatology. As the Leeuwin Current was enhanced by both the ITF and local winds, the elevated EKEs drove anomalous heat convergence on its offshore flank. This process considerably contributes to the OHC increase in the subtropical SEIO and erases the wind-driven cooling during the two warm periods. This work highlights the vital role of eddies in regional heat redistribution, with implications for understanding time-varying ocean heat storage in a changing climate.

Decadal variability of sea surface salinity in the Southeastern Indian Ocean: Roles of local rainfall and the Indonesian throughflow

“…As such, Exp-B retains only the forcing effect from the lateral boundaries, as indicated by the "B" (for boundary) in its naming. As has been validated by previous studies (Guo et al, 2021;Li et al, 2022), variability in Exp-B mainly originates from the ITF. In other experiments, atmospheric forcing is exerted in addition to the ITF effect.…”

“…ROMS is a hydrostatic, primitive equation model that implements a free surface, horizontal curvilinear coordinate, and a terrain-following (sigma-type) vertical coordinate system (e.g., Haidvogel et al, 2000). The model domain covers a sector-shaped region between 80°-122°E, 42°S-5°N, bordered by the Western Australian coast to the east and Sumatra-Java islands to the north (Guo et al, 2021;Li et al, 2022). The horizontal resolution is ~3 km near the Western Australian coast and degrades to ~12 km near the western boundary at 80°.…”

“…However, the center of precipitation changes locates to the north of SSS changes. This can be explained by the transport of ocean surface circulation; the rainfall-driven salinity changes are advected southward by the prevailing southward surface Ekman currents (induced by the southeast trade winds in climatology) in the region (e.g., Wang et al, 2020;Li et al, 2022) and spread to the entire SEIO region. response to SST variability in the SEIO (e.g., Tozuka et al, 2013;Doi et al, 2015;Li et al, 2019).…”

“…Weakening of ITF transport leads to SSH falling over the SEIO (Feng et al, 2010;Feng et al, 2015;Li et al, 2017), especially at the salinity front between 15 o -30 o S (Figure 14D). The SEIO is governed by the southeasterly trade winds in climatology, and the southward Ekman transport carries low-latitude low-salt water to the south, forming a convergence in the subtropical sector (Wang et al, 2020;Li et al, 2022). This process acts to reduce the SEIO SSS, while the excessive evaporation in the region acts to raise SSS.…”

“…On interannual timescales, El Niño (La Niña) events tend to cause SSS increases (decreases) in the SEIO, respectively, through the teleconnection signatures on rainfall and ocean circulation of the SEIO (e.g., Zhang et al, 2016a;Hu et al, 2019;Guo et al, 2021). The SEIO is also a region with pronounced decadal variability (e.g., Feng et al, 2010;Du et al, 2015;Feng et al, 2015;Li et al, 2017;Hu et al, 2019;Li et al, 2019;Li et al, 2022). The SEIO region showed a persistent SSS decrease during (e.g., Du et al, 2015Nie et al, 2020).…”