Decadal Cooling Events in the South Indian Ocean During the Argo Era

Abstract

Despite the rapid warming in the south Indian Ocean (SIO) during the recent two decades, the upper 300 m of the tropical southwest Indian Ocean (SWIO) experienced significant cooling during 2005–2011. The decreasing rate of the observed temperature reached −0.50 ± 0.1°C/decade. Heat budget analysis reveals that the cooling trend over SWIO results from the competing effect between local air‐sea heat flux and horizontal heat advection. Moreover, all the three observational datasets indicate that in the subtropical SIO, a prolonged cooling event occurred beneath the thermocline during 2010–2016. Although there exist some discrepancies between the observations and estimating the circulation and climate of the ocean (ECCO) reanalysis before 2010, the observations and ECCO reanalysis agree in terms of the post–2010 temperature variability in the intermediate layer of the subtropical region, probably because of the better constraint of the ECCO assimilation product by the increased sampling of Argo floats in the deeper ocean. The heat budget analysis indicates that during the period 2010–2016, the decadal cooling in the 400–1,000 m layer is primarily modulated by vertical ocean advection, and the horizontal ocean advection makes a secondary contribution. This study expands our knowledge of the decadal temperature variability and heat balance in the SIO during the Argo era.

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

“…The Argo program began in the early 2000s and has significantly increased the frequency and spatial coverage of observations in the upper 2,000 m of the regional and global ocean (Riser et al, 2016), providing an opportunity to better illustrate the water mass variability beneath the upper ocean. A recent work by L. Huang et al (2022) noted a cooling trend in the intermediate layer of the subtropical SIO during 2010-2016, which is inconsistent with the contemporaneous temperature variability in the upper SIO. However, the relevant dynamic processes and climate forcings have not been well evaluated.…”

“…A recent work by L. Huang et al. (2022) noted a cooling trend in the intermediate layer of the subtropical SIO during 2010–2016, which is inconsistent with the contemporaneous temperature variability in the upper SIO. However, the relevant dynamic processes and climate forcings have not been well evaluated.…”

“…The temperature variability below the thermocline in the subtropical ocean can be influenced by the isopycnal processes that transmit anomalous warming/cooling signals from other regions (Häkkinen et al., 2016). Thus, compared to previous studies that conducted heat budget analysis over fixed depths in the SIO (e.g., L. Huang et al., 2022; Y. Li & Han, 2015; Zhang et al., 2018), we diagnose the heat balance between the changing isopycnal surface to investigate the mechanisms controlling quasi‐decadal temperature variability in the intermediate layer of the subtropical SIO (15°–30°S, 55°–110°E) based on ECCOv4r4. Due to the little impact of surface heat flux below the upper ocean, the heat budget for the intermediate layer is evaluated as follows, $$$\frac{\partial \theta }{\partial t}=\mathrm{A}\mathrm{d}\mathrm{v}+R\mathrm{e}\mathrm{s}$$$ where θ is the potential temperature.…”

“…The temperature variability below the thermocline in the subtropical ocean can be influenced by the isopycnal processes that transmit anomalous warming/cooling signals from other regions (Häkkinen et al, 2016). Thus, compared to previous studies that conducted heat budget analysis over fixed depths in the SIO (e.g., L. Huang et al, 2022;Y. Li & Han, 2015;Zhang et al, 2018), we diagnose the heat balance between the changing isopycnal surface to investigate the mechanisms controlling quasi-decadal temperature variability in the intermediate layer of the subtropical SIO (15°-30°S, 55°-110°E) based on ECCOv4r4.…”

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).…”