The seasonal cycle of mixed layer temperatures in a global ocean general circulation model

We examine mixed layer temperatures in a global ocean general circulation model subjected to seasonally varying climatological forcing. Harmonic analysis of monthly mixed layer temperatures and climatological sea surface temperatures (SSTs) shows that, on the average, the annual harmonic accounts for 90% of the total seasonal variance in both fields, while the semiannual harmonic accounts for about 8%. The semiannual signal is mostly confined to equatorial and high-latitude regions. The model mixed layer temperatures underestimate the mean amplitude of the annual harmonic in middle latitudes (65 degrees]\theta\]10 degrees) by about 26%, while lagging climatological SSTs by 22 days, on average. In several parameter sensitivity experiments, these differences could be reduced to as little as 12% and 12.5 days, respectively, though most of this gain occurred when the mixed layer was unrealistically shallow (mean depth less than 65 m). At least part of the differences in amplitudes and phases of the annual harmonic is linked to the uncoupled formulation of the surface heat flux, which is computed using specified and seasonally varying climatological air temperatures. In ice-free areas, seasonal amplitudes and phases of air temperatures are almost identical to those of climatological SSTs. Thus, differences between model mixed layer temperatures and climatological SSTs give rise to Newtonian relaxation to SSTs, which then leads to amplitude damping and time lags in mixed layer temperatures relative to the SSTs.