Marine heatwaves and concurrence of sea level surges in the South Indian Ocean: characterization, causes and effects of upwelling and heat redistribution

Author:

Weiqing Han - (University Of Colorado, Boulder)

Co-Investigator(s):

Lei Zhang (University Of Colorado, Boulder)

Abstract:

Marine heatwaves (MHWs) have serious consequences on marine ecosystems and regional climate. Intense MHWs have been observed across the South Indian Ocean (SIO) in the past few decades; their frequency of occurrence, intensity and duration have increasing trends but exhibit large spatial and temporal variations. During the recent global surface warming “hiatus” decade, the upper Indian Ocean (IO) stored ~2/3 of the excess heat that entered earth’s climate system, by enhanced interocean heat transport from the tropical Pacific into the SIO via the Indonesian Throughflow (ITF), and the Southeast IO stored a significant portion of the excess heat. The increased upper-ocean heat content (OHC) in the Southeast IO has been suggested to increase the frequency of MHWs off the Australian west coast. In the rest of the SIO regions, however, causes for MHWs remain largely unexplored, and how oceanic upwelling and heat redistribution in the upper ocean regulate MHWs remains unknown. When sea surface height surges concur with MHWs, the extreme heat-height-surge (HHS) compound events lead to higher societal impacts. Yet, the HHS compounds in the SIO have not yet been investigated.

The overall goal of the proposed research is to utilize the ~28yr multiple satellite altimeter data, other satellite data, in situ observations, ocean-atmosphere reanalysis products combined with model experiments to provide a thorough investigation of MHWs and HHS compounds in the SIO. We will characterize MHW & HHS activities, explore the physical processes that give rise to these events, examine the roles of basin-scale upwelling and heat & mass redistribution in regulating these events, and assess the effects of major climate modes on MHWs & HHSs. Our period of interest is from 1993-present, when satellite altimeter and sea surface temperature (SST) data are both available. Specific objectives are to: [1] Characterize MHW activities (intensity, duration, spatial extent & frequency) and their spatiotemporal variability in tropical and subtropical SIO; identify HHS compound events, and explore the causes for MHWs and HHSs; [2] Examine the roles of large-scale oceanic upwelling and heat and mass redistribution in regulating MHW and HHS behaviors at interannual and decadal timescales; develop a detailed understanding of the associated processes; assess the impacts of atmospheric intraseasonal oscillations on MHWs & HHSs; [3] Qualify and quantify the effects of ENSO impacts on MHWs and HHSs in SIO via both atmospheric bridge and ITF; [4] Assess the impacts of major Indian Ocean climate modes – the Indian Ocean Dipole (IOD) and Subtropical IO Dipole (SIOD) on MHWs & HHSs. In addition, effects of monsoons will be assessed. We focus on climate-induced MHWs & HHSs that last for 10days to months. Therefore, daily (and monthly) altimeter data are suitable for the proposed study.

Scientifically, this study will, for the first time, systematically investigate MHWs and HHSs in the SIO, and provide insights into how large-scale upwelling and heat & mass redistribution – which are strongly influenced by climate modes - regulate MHWs and HHSs. This study is important, because MHWs have devastating impacts on marine ecosystems and serious consequences on regional climate, and HHS compound events have larger impacts than MHW alone. The proposed study is highly relevant to NASA OSTST program, because satellite altimeter data are vital for detecting HHS events and serve as a reliable indicator for large-scale upwelling and proxy for heat and mass redistribution in the SIO, which is poorly observed by in situ measurements. The proposed research directly contributes to priority research Theme 1 of the OSTST by utilizing altimeter data to improve our understanding of regional sea level variability, large-scale ocean circulation and variability, heat and mass redistribution in the upper ocean, and their impacts on extreme MHW & HHS events.

Supported by NASA