Turbulence variability at the equator in the central Pacific at the beginning of the 1991–1993 El Niño

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Abstract
A 38‐day, 5990‐cast microstructure study on the equator performed during the onset of the 1991–1993 El Niño shows the effect on small‐scale activity at 140°W of an equatorial Kelvin wave. By using two ships, data were taken continuously from November 4 to December 12, 1991, near the National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory mooring at O°N, 140°W. The ships occupied the station sequentially with a 3.5‐day overlap for intercalibration. Variability in currents was observed on tidal periods, and periods of 4 days (presumably equatorially trapped internal gravity waves), 8 days (cause unknown), 20 days (tropical instability waves), and longer (Kelvin waves). Variation in water structure occurred most prominently on the timescale of Kelvin waves. The diurnal cycle typical of that location was observed: nocturnal deepening of the surface mixed layer was accompanied by a “deep cycle,” bursts of turbulence penetrating into the stratified region below the nighttime mixed layer. During the observational period, one Kelvin wave trough and one crest passed through the site. Changes accompanying the phase change in the Kelvin wave included a reversal of the near‐surface current, a deepening of the thermocline, and a change of water mass. Changes in small‐scale activity included a tenfold decrease of the thermal dissipation rate and a fourfold decrease of the rate of heat transport downward from the mixed layer. The nighttime mixed layer deepened from 30 to 60 m. The thickness of the stratified region in which nocturnal turbulence bursts occurred, the deep cycle region, thinned from 40 to 20 m because it was confined between the bottom of the nighttime mixed layer and the low‐shear region near the core of the undercurrent. The decrease in downward heat flux observed at this passage of the downwelling Kelvin wave front could explain the rapid sea surface temperature (SST) increase seen at El Nino onsets. The magnitude of the change in vertical flux is similar to the magnitude of the change in horizontal advection. This process would produce a warmer SST much more quickly than could the advection of warm waters eastward.