08.06.2015: FB1-Seminar

11:00 h, Hörsaal, Düsternbrooker Weg 20

Abstract:

The Indian Ocean has sustained robust surface warming in recent decades, with warming rates exceeding those of other tropical ocean basins. However, it remains unclear what role multi-decadal variability in upper-ocean Indian Ocean thermal characteristics play in these trends. Using ocean model hindcasts, characteristics of Indian Ocean temperature changes are explored, focusing on the top 700m. Simulated temperature changes across the Indian Ocean in the hindcasts are consistent with those recorded in observational products and ocean reanalyses.
Assessment of Indian Ocean heat content since the 1950s suggests extensive subsurface cooling for much of the tropical Indian Ocean. Sensitivity experiments reveal that cooling trends in Indian Ocean heat content since the 1960s to the late 1990s are largely driven by wind-stress forcing, likely due to remote Pacific forcing associated with the Pacific Decadal Oscillation. As such, multi-decadal wind-forcing has masked increases in Indian Ocean heat content due to thermal forcing since the 1960s. However, wind and thermal forcing both contribute positively to Indian Ocean heat content since 1999 and thus drastic increases in Indian Ocean heat content in coming decades are likely, with implications for regional climate and vulnerable societies in Indian Ocean rim-countries.
For example, during the 2010/11 La Niña event that was characterized by exceptionally warm temperatures around the Maritime Continent, Australia experienced extreme rainfall conditions that resulted in devastating floods claiming 35 lives and billions of dollars in damages. Northeast Australian 2010/11 rainfall was amplified by 84% above average conditions, highly unusual even for a strong La Niña event. We demonstrate that the extreme Australian hydroclimatic conditions that led to destructive floods were significantly exacerbated by long-term Indo-Pacific surface ocean warming. Using atmospheric general circulation model experiments with 2010/11 ocean conditions including and excluding a long-term warming trend, we estimate the contribution of SST changes in generating this extreme event. The likelihood of exceptional rainfall in northeast Australia was significantly increased by warmer SST driving enhanced onshore moisture transport onto Australia and anomalous ascent over the eastern half of the continent. Our results highlight the role of low-frequency SST changes containing both natural and anthropogenic signals for intensifying rain-producing atmospheric circulation conditions, increasing the likelihood of extreme precipitation events.

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