PAOC Colloquium: Caroline Ummenhofer (WHOI)
Title: Multi-decadal variability in Indo-Pacific heat content and implications for hydroclimatic extremes
Speaker: Caroline Ummenhofer, Department of Physical Oceanography, Woods Hole Oceanographic Institution
Abstract: The Indian Ocean has sustained robust surface warming in recent decades, with warming rates exceeding those of other tropical ocean basins, though the role of multi-decadal variability remains unclear. Strong warming trends since the 1950s are limited to the surface and south of 30°S, while extensive subsurface cooling occurred over much of the tropical Indian Ocean. Using high-resolution ocean model hindcasts and sensitivity experiments, the temporal evolution of Indian Ocean subsurface heat content reveals distinct multi-decadal variations associated with the Interdecadal Pacific Oscillation. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean Dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline in the 1970s and 1980s was associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin's leading mode of variability.
This has important implications for regional hydroclimatic extremes, such as droughts and floods in surrounding countries (Australia, Southeast Asia, and East Africa): for example, we showed that iconic multi-year droughts in Australia over the last century are linked more robustly to Indian Ocean variability, rather than Pacific Ocean conditions, as traditionally assumed. We also explored how the long-term Indo-Pacific ocean warming contributed to the extreme hydroclimatic conditions in Australia during the strong La Niña in 2010/11, which were associated with an observable drop in global sea level, as large amounts of water were temporarily shifted from the ocean onto Australia and led to a rare filling of Lake Eyre. Using atmospheric general circulation model experiments with 2010/11 ocean conditions, we evaluate the mechanisms and changes in the likelihood of record rainfall in northeast Australia due to long-term ocean warming.
Speaker's website: https://ummenhofer.whoi.edu
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