Roles of climate modes and inter-basin interaction in regulating upper-ocean heat redistribution and extreme sea level events in the western tropical Pacific and southeast Indian Ocean

Author:

Weiqing Han (University Of Colorado, Boulder)

Co-Investigator(s):

Suhas Dasenahalli Lingaraju (University Of Colorado, Boulder)

Project Summary:

The Southeast Indian Ocean (SEIO) and western tropical Pacific (WTP) host multitude islands and coasts where low-lying areas are vulnerable to sea level rise and extreme events. While the El Niño - Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO) affect the WTP and SEIO sea level through both atmospheric bridge and oceanic connection via the Indonesian Throughflow (ITF), their roles in regulating sea surface Height EXtreme (HEX) events and the Compounding Height-Heat EXtremes (CHHEX) have not been systematically investigated. In the SEIO, the Ningaloo Niño -- an intense marine heatwave off the west Australian coast - has been recently identified as a regional climate mode. Its effects on HEXs and CHHEXs in the SEIO and WTP, however, have not been explored. While recent studies show intensified inter-basin linkage between the SEIO and WTP sea surface temperature (SST) in recent decades, with forcing by volcanic eruptions playing a role, how the intensified inter-basin interaction affects HEXs and CHHEXs remains unknown. Whilst the Madden-Julian Oscillation (MJO) - the dominant mode of atmospheric intraseasonal oscillations (ISOs) - becomes stronger with more regular eastward propagation from the Indian Ocean to the WTP under global warming, how the changing behaviors of the MJOs affect HEX activities in the WTP and SEIO remains unclear.

The overall goal of the proposed research is to utilize the 30+ year (daily and monthly) multiple satellite altimeter data, satellite SST, tide gauge observations, other observed data and reanalysis products combined with model experiments to thoroughly investigate the roles of climate modes and inter-basin interaction in regulating HEX and CHHEX events in the SEIO and WTP region. Our period of interest is 1993-present, when satellite altimeter and SST data are both available. Specific objectives are to: 1) Characterize HEX and CHHEX activities (e.g., frequency, intensity, duration) and their changing behaviors in recent decades; examine the relative importance of surface wind stress versus buoyancy flux forcing in causing these extremes, and explore the relevant processes (e.g., upper-ocean heat and mass redistribution including the ITF transport); 2) Qualify and quantify the effects of ENSO, Ningaloo Niño and SEIO-WTP inter-basin interaction at interannual timescale on HEX and CHHEX events; 3) Investigate the roles of the IPO, North Pacific Gyre Oscillation (NPGO), and inter-basin impacts from the Indian Ocean SST - which are linked to volcanic eruptions and greenhouse gas warming - in causing decadal-to-multidecadal sea level variability in the WTP and SEIO in recent decades, and their subsequent modulations on the HEX and CHHEX events; 4) Assess the impacts of atmospheric ISOs- especially the MJOs - on the HEXs and CHHEXs in the SEIO and WTP, and explore the processes involved.

The proposed research is imperative, because the HEXs and compounding extremes can have large ecological, economic, and social consequences. The project is highly relevant to NASA OSTST program, because satellite altimeter data are vital for detecting HEX events and serve as a reliable proxy for large-scale ocean heat content, heat and mass redistribution and connection with coastal ocean in the study region, where in situ measurements are limited. Although the altimeter data have land contamination issue, our results show remarkable agreements between tide gauge and altimeter data in coastal and island regions of the SEIO and WTP. The proposed research directly contributes to priority research Theme 1 by utilizing altimeter data to improve our understanding of large-scale physical processes, regional sea level variations and heat and mass transport.