The Unprecedented Reduction and Quick Recovery of the South Indian Ocean Heat Content and Sea Level in 2014-2018

In a recent study published in Science Advances, a team of scientists at AOML led by Denis Volkov used observations and idealized model simulations to explore what caused the abrupt reduction and ensuing recovery of the South Indian Ocean heat and sea level in 2014-2018.

The study showed that the 2014-2016 El Niño, the warm phase of the El Niño-Southern Oscillation (ENSO), contributed to the observed cooling of the southern Indian Ocean. This occurred through an unusual combination of both the reduced heat advection from the Pacific and the basinwide cyclonic wind anomaly that led to an upwelling of cold deep water. The study also found that the southern Indian Ocean heat content was nearly restored during the 2017-2018 La Nina, which is the cold phase of ENSO. The recovery of the heat content was mainly forced by an anticyclonic wind anomaly associated with stronger trade winds that led to the upper-ocean warming in the interior of the southern Indian Ocean and the suppression of the 2014-2016 cooling signal.

The ocean absorbs about 93% of the excess heat accumulated in the Earth’s climate system, with the subtropical southern Indian Ocean being one of the major heat accumulators among the oceanic basins. Observations showed that a decade-long basinwide warming and associated sea level rise in the southern Indian Ocean ended abruptly with an exceptionally strong cooling anomaly during the 2014–2016 El Niño.

The interannual-to-decadal variability of heat content and sea level in the southern Indian Ocean is strongly influenced by its connection with the Pacific and large-scale climatic forcing in the Indo-Pacific region primarily associated with ENSO and Indian Ocean Dipole.

Figure 1 from Volkov et al. (2020). Schematic showing the processes affecting the subtropical southern Indian Ocean (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 southern Indian Ocean 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 Countercurrent (SICC). ENSO affects the southern Indian Ocean 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 southern Indian Ocean (ESIO) increase during La Niña conditions (stronger trade winds in the Pacific) and decrease during El Niño conditions (weaker trade winds in the Pacific). Signals generated in the Pacific reach the coast of West Australia as coastally trapped waves. These signals then propagate toward the western southern Indian Ocean (WSIO) as eddies and Rossby waves. Local wind stress curl can modify the waves radiated from the eastern boundary and/or generate other waves.

The ENSO effect on the southern Indian Ocean heat content is twofold. First, ENSO drives changes in the upper-ocean heat content in the western equatorial Pacific and modulates the advection of heat into the southern Indian Ocean via the Indonesian Throughflow (ITF) – the “ocean tunnel” effect. The Indonesian Throughflow is an ocean current that provides a pathway for a constant supply of warm water to move from the Pacific to the Indian Ocean. Second, changes in local wind forcing influence the upper-ocean heat content in the southern Indian Ocean, and they are also related to the ENSO by the means of the Walker Circulation – the “atmospheric bridge” effect. The Walker Circulation is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere) and is shown in figure 1.

The results of the study showed that the 2014-2016 El Niño contributed to the cooling of the southern Indian Ocean through a combination of the reduced heat advection from the Pacific and the basinwide cyclonic wind anomaly that led to an upwelling of cold deep water. The results presented highlight the complexity of the southern Indian Ocean heat content variability driven by the ocean tunnel and atmospheric bridge effects and their interactions.

For future work, the research team aims to explore how the 2014-2016 cooling in the southern Indian Ocean affected the amount of heat transported from the Indian Ocean to the Atlantic Ocean.