Tropical cells, tropical instability waves, and a secondary circulation near the northern front of the equatorial Pacific cold tongue
Perez, R.C., M.F. Cronin, and W.S. Kessler
J. Phys. Oceanogr., 40(9), doi: 10.1175/2010JPO4366.1, 2091-2106
Shipboard measurements and a model are used to describe the mean structure of meridional-vertical tropical cells (TCs) in the central equatorial Pacific, and a secondary circulation associated with the northern front of the cold tongue. The shape of the front is convoluted by the passage of tropical instability waves (TIWs). When velocities are averaged in a coordinate system centered on the instantaneous position of the northern front, the measurements show a near-surface minimum in northward flow north of the surface front (convergent flow near the front). This convergence and inferred downwelling extend below the surface mixed layer, tilt poleward with depth, and are meridionally bounded by regions of divergence and upwelling. Similarly, the model shows that on average, surface cold tongue water moves northward towards the frontal region and dives below tilted front, while subsurface water north of the front moves southward towards the front, upwells, and then moves northward in the surface mixed layer. The model is used to demonstrate that this mean quasi-adiabatic secondary circulation is not a frozen field that migrates with the front, but is instead highly dependent on the phase of the TIWs: southward-upwelling flow on the warm side of the front tends to occur when the front is displaced southward, while northward-downwelling flow on the cold side of the front occurs when the front is displaced northward. Consequently, when averaged in geographic coordinates, the observed and simulated TCs appear to be equatorially-asymmetric, and show little trace of a secondary circulation near the mean front.
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA