On the Physical Origin of the Semiannual Component of Surface Air Temperature over Oceans

With the understanding that seasonal cycle of the temperature is forced principally by the annually evolving solar irradiance, many previous studies have defined seasonal cycle of surface air temperature (SAT) as the sum of yearly-period sinusoidal component and its harmonics, especially semiannual component. This perception of directly forced response is confirmed for tropical SAT in this study. However, in mid-latitude and subpolar oceans, the ratio between the semiannual and annual components of solar irradiance is negligibly small but that of the SAT over oceans is not, which remains to be understood. To solve this puzzle, a simple energy budget model including main energy sources and sinks of oceanic mixed layer is designed. It is revealed that, when the oceanic mixed layer is prescribed as a layer of constant depth, the phase and amplitude of the modeled SAT are not consistent with these of observations. However, when the annually changing heat capacity of oceanic mixed layer is included, both the amplitude and phase of the modeled SAT share these of the observed SAT, proving that the semiannual component of SAT over mid-latitude and subpolar oceans is a result of the heat capacity-varying oceanic mixed layer in response to annually evolving solar irradiance.