TURBULENCE VARIABILITY AT THE EQUATOR IN THE CENTRAL PACIFIC AT THE BEGINNING OF THE 1991-1993 EL-NINO

A 38-day, 5990-cast microstructure study on the equator performed duri ng the onset of the 1991-1993 El Nino shows the effect on small-scale activity at 140 degrees W of an equatorial Kelvin wave, By using two s hips, data were taken continuously from November 4 to December 12, 199 1, near the National Oceanic and Atmospheric Administration Pacific Ma rine Environmental Laboratory mooring at 0 degrees N, 140 degrees W. T he ships occupied the station sequentially with a 3.5-day overlap for intercalibration. Variability in currents was observed on tidal period s, and periods of 4 days (presumably equatorially trapped internal gra vity waves), 8 days (cause unknown), 20 days (tropical instability wav es), and longer (Kelvin waves). Variation in water structure occurred most prominently ox the timescale of Kelvin waves. The diurnal cycle t ypical of that location was observed: nocturnal deepening of the surfa ce mixed layer was accompanied by a ''deep cycle,'' bursts of turbulen ce penetrating into the stratified region below the nighttime mixed la yer. During the observational period, one Kelvin wave trough and one c rest passed through the site. Changes accompanying the phase change in the Kelvin wave included a reversal of the near-surface current, a de epening of the thermocline, and a change of water mass. Changes in sma ll-scale activity included a tenfold decrease of the thermal dissipati on rate and a fourfold decrease of the rate of heat transport downward from the mixed layer. The nighttime mixed layer deepened from 30 to 6 0 m. The thickness of the stratified region in which nocturnal turbule nce bursts occurred, the deep cycle region, thinned from 40 to 20 m be cause it was confined between the bottom of the nighttime mixed layer and the low-shear region near the core of the undercurrent. The decrea se in downward heat flux observed at this passage of the downwelling K elvin wave front could explain the rapid sea surface temperature (SST) increase seen at El Nino onsets. The magnitude of the change in verti cal flux is similar to the magnitude of the change in horizontal advec tion. This process would produce a warmer SST much more quickly than c ould the advection of warm waters eastward.