A numerical simulation has been conducted using a general circulation using a general circulation model of the tropical Atlantic Ocean forced with observed monthly surface wind stress for 1964-87 and parameterized surface heat flux. The simulated sea surface temperature (SST) and upper ocean heat content (HC) are used to examine the low frequency variability in the ocean. A comparison with the SST observations shows that the model realistically simulates the interannual SST variations with time scales of two to three years and the major features of the decadal variability at the sea surface, such as the fluctuation of the SST dipole pattern. (or the meridional gradient).
The simulated HC anomalies are ysed ti examine the variations of the thermocline depth and the effects of ocean dynamics. A principal iscillation pattern (POP) analysis is performed to distinguish the spatial structures of decadal and interannual variations. It is found that the interannual variations are associated with tropical oceanic waves, stimulated by the flunctuations of the equatorial easterlies, which propagate eastward along the equator and westward to the north and south, resulting in an essentially symmetric structure about the equator at these scales. The periods of these modes are determined by the meridional width of the equatorial wind anomaly. The decadal mode, however, is associated with the ocean's adjustment in tesponse to a basinwide out-of-phase fluctuation between the northeast and southeast trade winds. For instance, forced by a weakening of the northeast winds and a simultaneous strengthening of the southwest winds, the thermocline deepens in a belt extending from 5° N in the west to the North African coast. At the same time, the thermocline shoals from the southeast coast to the equatorial ocean. The associated SST pattern exhibits a strong dipole structure with positive anomalies in the north and negative anomalies in the south. When the wind anomalies weaken, the warm water accumulated in the northern tropical oceanir released and redistributed within the basin. At this stage, the SST dipole dissppears. In the framework of this separation of the variability into two dominant time scales, the extraordinay large warm SST anomalies int he southeast ocean in the boreal summer of 1984 are a result of in-phase interference of the decadal and interannual modes.
Complete copies of this report are available from:Center for Ocean-Land-Atmosphere Studies
last update: 1 July 1996
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