Comparison of bulk sea surface and mixed layer temperatures

Abstract

Mixed layer temperature (MLT) and bulk sea surface temperature (SST) are frequently used interchangeably or assumed to be proportional in climate studies. This study examines historical analyses of bulk SST and MLT from contemporaneous ocean profile observations during 1960–2007, looking for systematic differences between these variables. The results show that globally and time‐averaged MLT is cooler than SST by approximately 0.1°C. MLT is cooler than SST in upwelling zones where abundant net surface warming is compensated for by cooling across the base of the mixed layer. In the upwelling zone of the equatorial eastern Pacific this negative MLT‐SST difference varies out of phase with seasonal SST, reaching a negative extreme of −0.8°C in boreal spring when SST is warm, solar radiation is high, and winds are weak. In contrast, on interannual timescales MLT‐SST varies in phase with SST with small differences during El Niños as a result of low solar heating and enhanced rainfall and with large differences, approaching −0.8°C, during La Niñas. On shorter diurnal timescales, during El Niños, MLT‐SST differences associated with temperature inversions occur in response to nocturnal cooling in the presence of near‐surface freshening. Near‐surface freshening produces persistent shallow (a few meters depth) warm layers in the northwestern Pacific during boreal summer when solar heating is strong. In contrast, shallow cool layers occur in the Gulf Stream area of the northwest Atlantic in boreal winter when fresh surface layers caused by lateral exchange are cooled by abundant turbulent heat loss. The different impacts of shallow barrier layers on near‐surface temperature gradients in these different dynamical regimes are explored with a one‐dimensional mixed layer model.