VERTICAL VELOCITY IN THE CENTRAL TROPICAL PACIFIC - A CIRCULATION MODEL PERSPECTIVE FOR JGOFS

Results from a variety of experiments with a primitive equation ocean general circulation model are examined to provide insight about variat ions in vertical velocity (W) that may have occurred during the JGOFS Equatorial Pacific field program. Measurements of W during this period are not available, so a model offers the only plausible, dynamically consistent way to obtain this information. Variability associated with four different aspects of the tropical Pacific circulation is discuss ed: the seasonal average: the tropical instability waves (TIW); the in terannual variability in strong and modest ENSO events; and the Kelvin response associated with remote forcing from westerly wind events in the western Pacific. Results from 140 degrees W are presented. Accordi ng to the model, TIW provide the greatest source of W variability Near and somewhat north of the equator W can range from about 1500 cm day( -1) upward to 1000 cm day(-1) downward over a typical 30-day period, w ith significant structure in both the zonal and meridional directions. Remotely forced response can be very large on the day-to-day timescal e, but diminishes typically to, at most, a few hundred cm day(-1) up o r down when averaged over 10 days. Interannual variation associated wi th a major ENSO event can involve complete disruption of the seasonal cycle, the disappearance of TIWs and great diminution or disappearance of the normal equatorial upwelling. The seasonal cycle variations are of the smallest amplitude; the maximum Variation is perhaps 100 cm da y(-1) near the equator and somewhat more just north of the equatorial waveguide in the downwelling part of the seasonal circulation. The fie ld program period was studied using the different available wind analy ses to force the model over the 1991-1992 period. Because the wind ana lyses have substantial differences, they force substantially different W fields in the ocean model. Unfortunately, no wind analysis can be s hown to be substantially superior to any other during this period. Thu s, estimates of the net vertical transport of any quantity of JGOFS in terest must take into account the wide range of possible W values. Pub lished by Elsevier Science Ltd.