Abstract
Observations from cruises in the Arabian Sea and data from satellites are interpreted using different realizations of a multi-level primitive equation model and an eddy-permitting reduced-gravity shallow water model of the Indian Ocean. The focus is on the interannual circulation variability of the Arabian Sea, and especially of the meridional location of the Great Whirl (GW). The results suggest that the variability in the western Arabian Sea is not only due to the interannual variability in the wind field, but that a substantial part is caused by the chaotic nature of the ocean dynamics. Decreasing the friction coefficient from 1000 to 500m2s-1 in a 19o numerical reduced-gravity model, the variance of the GW location increases dramatically, and the mean position moves southward by one degree. In the eddy-permitting experiments analyzed, both mechanisms appear to determine the GW location at the onset of the GW dynamics in late summer.
Highlights
During the German Meteor and Sonne cruises in 1993, 1995 and 1996 and related moored observations in the western Arabian Sea, considerable year-to-year differences in the ocean circulation were found (e.g., Schott and McCreary, 2001)
When the dynamic layer becomes shallower than the mixed layer, deep water is entrained into the mixed layer and the thickness of the dynamic layer is set to the mixed-layer depth
For the surface heat flux, we have used the formulation by Barnier et al (1995), which can be interpreted as a relaxation of the model surface temperature towards an equivalent surface temperature determined from a climatology based on the data from ECMWF
Summary
During the German Meteor and Sonne cruises in 1993, 1995 and 1996 and related moored observations in the western Arabian Sea, considerable year-to-year differences in the ocean circulation were found (e.g., Schott and McCreary, 2001). Year-to-year changes in atmospheric forcing, which in the Arabian Sea mainly result from variability of the wind field, obviously lead to externally-forced variability in ocean dynamics. If the internal variability is not negligible, a direct model–data comparison even with a perfect model is meaningless except in the context of data assimilation, and one can only compare statistical parameters such as mean values, variances or correlations of quantities important to the ocean circulation. Internal variability has been argued to play no substantial role since the early work by Luther and O’Brien (1989) and usually is neglected altogether (e.g., Anderson and Carrington, 1993; Luther, 1999) This neglect is meaningful only when using models with relatively coarse resolution and/or rather high values of the friction parameters where the internal variability is small or nonexistent. Our purpose is to quantify and compare external and internal variability in the western Arabian Sea
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