What caused the abrupt reduction of the South Indian Ocean heat & sea level in 2014–2016 and the ensuing quick recovery?

A decade-long increase of the basin-wide sea level and heat content in the subtropical southern Indian Ocean (SIO) during 2004–2013 ended abruptly, immediately following the onset of the strong 2014–2016 El Niño. Interestingly, this unprecedented drop of the SIO heat quickly recovered during the weak 2017–2018 La Niña. A study recently published in Science Advances used observations and model simulations to explore what caused the abrupt reduction and recovery of the SIO heat and sea level during 2014-2018.

The interannual-to-decadal variability of heat content and sea level in the SIO is strongly influenced by its connection with the Pacific and large-scale climatic forcing in the Indo-Pacific region primarily associated with El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM). The remote ENSO effect on the SIO heat content is twofold. First, ENSO drives changes in the upper-ocean heat content in the western equatorial Pacific and, therefore, modulates advection into the SIO from the Pacific via the Indonesian Throughflow (ITF), the “ocean tunnel” effect. Second, local changes in wind stress curl influencing the upper-ocean heat content in the SIO are also related to ENSO by the means of atmospheric teleconnections via the Walker Circulation, the “atmospheric bridge” effect. While the atmospheric bridge is a rather fast teleconnection, it takes years for the ocean tunnel signals to reach the western Indian Ocean. The IOD events are often triggered by ENSO. However, they are not always in-phase with ENSO because about one-third of IOD events occur independently of ENSO events.

According to the new study, the 2014–2016 El Niño did contribute to the observed cooling of the SIO through an unusual combination of both the reduced heat advection from the Pacific (dominant in the eastern SIO) and the basin-wide cyclonic wind anomaly that led to shoaling of isotherms (dominant in the western SIO). The ensuing recovery was mainly forced by an anticyclonic wind anomaly associated with stronger trade winds that caused deepening of isotherms and thus the suppression of 2014–2016 cooling signal propagating from the eastern boundary. The main results presented in this study highlight the complexity of the SIO heat content variability driven by the ocean tunnel and atmospheric bridge effects and their interactions.

Figure 1 from Volkov et al. (2020). Schematic showing the dynamic processes affecting the subtropical SIO heat content and sea level: Color shows the mean dynamic topography in the horizontal plane and temperature climatology in the vertical planes. The atmospheric circulation in the SIO is dominated by southeasterly trade winds. The general ocean circulation consists of the Indonesian Throughflow (ITF) that feeds the South Equatorial Current (SEC) and the Leeuwin Current (LC), and the South Indian Counter Current (SICC). ENSO affects the SIO heat content via the ocean route (ITF) and atmospheric route (atmospheric bridge due to Walker Circulation). The ITF volume and heat transports into the eastern SIO (ESIO) increase during La Niña conditions (stronger trade winds in the Pacific) and decrease during El Niño conditions (weaker trade winds in the Pacific). Besides advection, signals generated in the Pacific reach the coast of West Australia as coastally trapped waves. Then, these signals propagate toward the western SIO (WSIO) as eddies and Rossby waves. Local wind stress curl can modify the waves radiated from the eastern boundary and/or generate other waves.

Volkov, D., S.-K. Lee, A. L. Gordon, and M. Rudko, 2020: Unprecedented reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014–2018. Sci. Adv. 6(36), eabc1151. https://doi.org/10.1126/sciadv.abc1151.

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