Quasi‐Decadal Temperature Variability in the Intermediate Layer of Subtropical South Indian Ocean During the Argo Period

Abstract

It has been reported that the subtropical South Indian Ocean (SIO) has been rapidly warming over the past two decades and can therefore be characterized as one of the major heat accumulators among the oceanic basins. However, this strong warming is not uniformly distributed in the vertical direction. In comparison to the decade‐long warming in the upper layer (0–300 m) in 2004–2013, the intermediate layer (300–1,000 m) displays a shorter warming during 2004–2009 and an intense cooling during 2010–2016. By decomposing temperature variations into heaving and spice components, and performing a heat budget analysis, we show that temperature variations in the intermediate layer during these two periods are primarily contributed by isopycnal migrations driven by local wind forcing. Local wind change in the subtropical SIO can be explained by the Indian Ocean Dipole and El Niño–Southern Oscillation during 2004–2016, while Southern Annular Mode (SAM) favors anomalous wind change in mid‐latitudes and the formation of basin‐wide wind change in the SIO. Additionally, wind forcing in the Subantarctic Mode Water (SAMW) formation region, which is closely linked to the SAM, modulates the anomalous spreading of SAMW into the interior of the subtropical SIO. This, therefore, leads to the SAMW intrusion being of secondary importance to the quasi‐decadal temperature variability. Our findings demonstrate the independence of wind‐driven temperature changes on the quasi‐decadal scale in the intermediate layer of the subtropical SIO under the overall warming background of SIO waters.

Rapid Sea Level Rise in the Tropical Southwest Indian Ocean in the Recent Two Decades

“…We also examine the trends of multiple atmospheric forcing, including wind stress, Ekman pumping, as well as sea surface heat flux anomalies during 2002-2021 (e.g., Huang et al, 2022Huang et al, , 2023Li & Han, 2015). As shown in Figure 4a, atmosphere circulation is featured as an anticyclonic wind stress trend during 2002-2021, which induces downward Ekman pumping and can lead to increasing OHC (Figure 2).…”