Equivalent forcing depth in tropical oceans |
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| Authors: | Takeshi Doi Tomoki Tozuka Toshio Yamagata |
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| Institution: | a Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan |
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| Abstract: | The forcing efficiency for the first and the second baroclinic modes by the wind stress in tropical oceans has been discussed by calculating equivalent forcing depth from annual mean, seasonal, and pentadal density profiles of the observational data. In the annual mean field, the first mode is forced preferentially in the western Pacific and the Indian Ocean, whereas the second mode is more strongly excited in the Atlantic and the eastern Pacific. This difference is mostly due to the pycnocline depth; the second mode is more dominantly forced where the pycnocline depth is shallower. We also revealed large seasonal variations of the second mode's equivalent forcing depth in the western Indian Ocean. The first mode is more dominantly forced during boreal spring and fall in the western Indian Ocean, while the second mode becomes more dominantly forced during boreal summer and winter. Those are due to seasonal variations of both the zonal wind and the pycnocline depth. Moreover, we show that the excitation of the second mode in the western Pacific increases after the late 1970s, which is associated with the decreasing trend of the zonal pycnocline gradient. Revealing the variation of the equivalent forcing depth will be useful for understanding the oceanic response to winds in tropical oceans and the improvement in the predictability of air-sea coupled climate variability in the tropics. |
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| Keywords: | Equivalent forcing depth Tropical oceans Baroclinic mode |
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