New publication in "ScienceAdvances": Unprecedented reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014–2018
The subtropical southern Indian Ocean (SIO) is one of the major heat accumulators among the oceanic basins. A decade-long basin-wide warming and associated sea level rise in the 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. In a study published in Science Advances, we used observations and idealized model simulations to explore what caused the abrupt reduction and recovery of the SIO heat and sea level during 2014-2018.
Denis L. Volkov1,2, Sang-Ki Lee2, Arnold L. Gordon3, Michael Rudko1,2
1 Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, FL, USA
2 National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteorological Laboratory, Miami, FL, USA
3 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
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) (see schematic in Figure 1). 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 the advection of heat 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.
We showed that 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.
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