The role of the halted baroclinic mode at the central equatorial Pacific in El Nino event
Chu, Peter C.
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The role of halted “baroclinic modes” in the central equatorial Paciﬁc is analyzed. It is found that dominant anomaly signals corresponding to “baroclinic modes” occur in the upper layer of the equatorial Paciﬁc, in a two-and-a-half layer oceanic model, in assimilated results of a simple OGCM and in the ADCP observation of TAO. A second “baroclinic mode” is halted in the central equatorial Paciﬁc corresponding to a positive SST anomaly while the ﬁrst “baroclinic mode” propagates eastwards in the eastern equatorial Paciﬁc. The role of the halted second “baroclinic mode” in the central equatorial Paciﬁc is explained by a staged ocean-atmosphere interaction mechanism in the formation of El Nino: the westerly bursts in boreal winter over the western equatorial Paciﬁc generate the halted second “baroclinic mode” in the central equatorial Paciﬁc, leading to the increase of heat content and temperature in the upper layer of the central Paciﬁc which induces the shift of convection from over the western equatorial Paciﬁc to the central equatorial Paciﬁc; another wider, westerly anomaly burst is induced over the western region of convection above the central equatorial Paciﬁc and the westerly anomaly burst generates the ﬁrst “baroclinic mode” propagating to the eastern equatorial Paciﬁc, resulting in a warm event in the eastern equatorial Paciﬁc. The mechanism presented in this paper reveals that the central equatorial Paciﬁc is a key region in detecting the possibility of ENSO and, by analyzing TAO observation data of ocean currents and temperature in the central equatorial Paciﬁc, in predicting the coming of an El Nino several months ahead.
Advances in Atmospheric Sciences
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