Vertical velocity in the central tropical Pacific: a circulation model perspective for JGOFS

Abstract

Results from a variety of experiments with a primitive equation ocean general circulation model are examined to provide insight about variations 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 discussed: the seasonal average; the tropical instability waves (TIW); the interannual 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°W are presented. According 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, with significant structure in both the zonal and meridional directions. Remotely forced response can be very large on the day-to-day timescale, but diminishes typically to, at most, a few hundred cm day −1 up or down when averaged over 10 days. Interannual variation associated with 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 day −1 near the equator and somewhat more just north of the equatorial waveguide in the downwelling part of the seasonal circulation. The field program period was studied using the different available wind analyses to force the model over the 1991–1992 period. Because the wind analyses have substantial differences, they force substantially different W fields in the ocean model. Unfortunately, no wind analysis can be shown to be substantially superior to any other during this period. Thus, estimates of the net vertical transport of any quantity of JGOFS interest must take into account the wide range of possible W values.