J. Climate

Annual Variations of the Thermocline in the Tropical Pacific Ocean

Bin WANG, Renguang WU, and Roger LUKAS
School of Ocean and Earth Science and Technology
University of Hawaii

Using the latest version (RA-6) of the NCEP/ODAS (Ocean Data Assimilation System) reanalysis data we identify two regimes of annual thermocline variation in the tropical Pacific Ocean, The Ekman regime is primarily located in the poleward side of the annual mean ITCZ and SPCZ, and in the region (2N-8N, 170W-120W). In this regime, the annual variation of thermocline is mainly driven by the local wind stress curl associated with the annual march of ITCZ and the western Pacific oceanic monsoon. The wave regime consists of the equatorial wave guide (3N-3S), where the annual maximum depth occurs progressively later eastward from November to January, and two off-equatorial Rossby waveguides along 5N and 6S west of about 140W, where prominent westward phase propagation with a longitudinally varying speed (0.5-1.0 m/s) dominates.

Numerical experiments with an intermediate tropical ocean model are preformed to explore the mechanisms governing the annual cycle. The pronounced and distinct annual cycle in the equatorial central Pacific (deepest in December and shallowest in May-June) is primarily due to remote forcing, especially from the western Pacific monsoon. The annual march of the eastern Pacific ITCZ may play an important role in phasing the maximum and minimum depths there. The December maximum and June minimum in the equatorial central Pacific then propagate westward in the off-equatorial waveguides all the way to the western boundary. The bimodal variation in the equatorial western Pacific results from combined effects of the monsoon westerly forcing during the transitional season and the westward propagation of equatorial Rossby waves. The bimodal variations in the equatorial far eastern Pacific are determined by the remote forcing through the eastward propagation of Kelvin waves.

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