25.07.2016: FB1-Seminar

11:00 h, Hörsaal, Düsternbrooker Weg 20

Abstract:

The talk includes two parts. Part I discusses the structures and dynamics of the Equatorial Undercurrent (EUC) of the Indian Ocean. Different from the equatorial Pacific and Atlantic Oceans where EUC is quasi-permanent in nature, the Indian Ocean EUC is a transient  feature. Observations and ocean model experiments using an OGCM and a linear model show that the EUC regularly occurs in each boreal winter-to-spring, particularly during February and April, and reappears during summer–fall of most years. In the western basin, the EUC results primarily from equatorial Kelvin and Rossby waves directly forced by equatorial easterly winds. In the central and eastern basin, however, reflected Rossby waves from the eastern boundary play a crucial role. While the first two baroclinic modes make the largest contribution, intermediate modes 3–8 are also important. The summer–fall EUC exhibits apparent interannual variability in the eastern basin, and it is particularly strong during the positive Indian Ocean dipole (IOD) years, when upwelling occurs in this region normally occupied by “warm pool”. It results largely from Rossby waves reflected from the eastern boundary, with directly forced Kelvin and Rossby waves also having significant contributions. Part II explores the role played by the “transient EUC” is sustaining the summer-fall equatorial eastern basin upwelling. On seasonal time scales, the EIO upwelling is mainly supplied by the salty water from the western basin through a buffering process: The winter-spring EUC carries the salty water from the western basin eastward, induces downwelling in the EIO, and pushes portion of the salty water below the central thermocline, which subsequently upwells to the central thermocline during summer-fall and sustains the EIO upwelling. On interannual time scales, enhanced upwelling occurs during positive +IOD years. The strong summer-fall EUC supplies water for the intensified upwelling.

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