Seasonal sea surface cooling in the equatorial Pacific cold tongue controlled by ocean mixing

In the tropics, a strong seasonal cycle in sea surface temperature exists despite comparatively constant radiation inputs; turbulent mixing from below is now shown to control the cooling phase of the seasonal cycle in the equatorial Pacific ‘cold tongue’ at 140° W. Seasonal cycles in the equatorial Pacific In much of the non-tropical ocean, the seasonal cycle is dominated by seasonal variations in solar radiation. Yet in the tropics, there is a strong seasonal cycle in sea surface temperature despite comparatively constant radiation inputs. James Moum and colleagues present multi-year observations that show that turbulent mixing from below accounts for much of the magnitude of the seasonal cycle of sea surface temperature in the equatorial Pacific cold tongue at 140° W. These findings should contribute to improved understanding of the El Niño/Southern Oscillation cycle and to more accuracy in many coupled ocean–atmosphere climate models. Sea surface temperature (SST) is a critical control on the atmosphere 1 , and numerical models of atmosphere–ocean circulation emphasize its accurate prediction. Yet many models demonstrate large, systematic biases in simulated SST in the equatorial ‘cold tongues’ (expansive regions of net heat uptake from the atmosphere) of the Atlantic 2 and Pacific 3 oceans, particularly with regard to a central but little-understood feature of tropical oceans: a strong seasonal cycle. The biases may be related to the inability of models to constrain turbulent mixing realistically 4 , given that turbulent mixing, combined with seasonal variations in atmospheric heating, determines SST. In temperate oceans, the seasonal SST cycle is clearly related to varying solar heating 5 ; in the tropics, however, SSTs vary seasonally in the absence of similar variations in solar inputs 6 . Turbulent mixing has long been a likely explanation, but firm, long-term observational evidence has been absent. Here we show the existence of a distinctive seasonal cycle of subsurface cooling via mixing in the equatorial Pacific cold tongue, using multi-year measurements of turbulence in the ocean. In boreal spring, SST rises by 2 kelvin when heating of the upper ocean by the atmosphere exceeds cooling by mixing from below. In boreal summer, SST decreases because cooling from below exceeds heating from above. When the effects of lateral advection are considered, the magnitude of summer cooling via mixing (4 kelvin per month) is equivalent to that required to cou