Jim Moum (Oregon State University)
Sea surface temperature (SST) is a critical control on the atmosphere and numerical models of atmosphere-ocean circulation emphasize its accurate prediction. However, large, systematic biases in SST occur in climate models of the equatorial cold tongues of Atlantic and Pacific oceans, with the biases attributed to models’ inability to constrain turbulent mixing. Changes in the temperature of a water parcel at the sea surface are determined solely by the imbalance between atmospheric heating from above, and cooling by turbulent mixing from below. Away from the equator, the seasonal cycle in solar radiation dominates the seasonal cycle in SST. At the equator, the atmosphere heats the ocean throughout the year, yet equatorial oceans also exhibit a seasonal cycle in SST. While atmospheric inputs are well constrained on seasonal timescales, variations in turbulent mixing over such long time scales have never before been measured. Herein is shown the existence of a distinctive seasonal cycle of subsurface cooling via mixing in the equatorial Pacific cold tongue, using the first multi-year measurements of turbulence in the ocean. In boreal spring, SST rises by 2 K 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 K month-1) is equivalent to that required to counter the heating terms. These results provide the first quantitative assessment of how mixing varies on time scales longer than a few weeks, clearly showing its controlling influence on seasonal cooling of SST in a critical oceanic regime.