Upwelling and Downwelling Driven by the North Equatorial Countercurrent and Internal Waves at Hatohobei Island and Helen Reef, Palau

Abstract

AbstractUpwelling and downwelling at two near‐equatorial islands in the western Pacific, Hatohobei Island and Helen Reef, Palau, are investigated using time series from a temperature array in the upper 57 m from 2014 to 2019 and a ship survey from 3 to 5 July 2013. During the survey, on the upstream side, upwelling/downwelling is found above/below an eastward subsurface current maximum due to an unusual mesoscale eddy. This eddy detached from a meander in the eastward North Equatorial Countercurrent in May 2013 and translated southwestward to the islands over 2 months bringing among the strongest currents seen at these islands in altimetry from 1993 to 2020. Following a streamline, the flow satisfied the Bernoulli equation outside of the wake. In both the cyclonic and anticyclonic wakes from the south and north points of Helen Reef, elevated estimated diffusivities from a shear‐based parameterization were similar at 10−5 m2 s−1 and a factor of 10 above background values. In the time series, downwelling in the upper 57 m is associated with a broader, surface‐intensified NECC converging at topography. Also, the shallower thermocline in 2014 and during the El Niños of 2015–2016 and 2018–2019 led to increased stratification at this depth and permitted greater internal wave activity. Peaked semidiurnal internal waves with peak‐to‐peak amplitudes of 4°C were equal to interannual changes. The net downwelling/warming of 0.5°C at the temperature array when smoothed over 2 months is consistent with both current‐driven downwelling and internal‐wave driven mixing of warmer waters downwards.