Impact of downward heat penetration below the shallow seasonal thermocline on the sea surface temperature

Observational data are used to investigate summer heat penetration into the subsurface ocean in order to quantify the heat capacity of the upper ocean with respect to surface heat exchange. Sea surface temperature is strongly modulated by the change in heat capacity, which could influence the overlying atmosphere and hence trigger climate variations, even during the warming season, when the ocean has been regarded as being rather passive. Few studies have focused on the heat exchange process in surface and subsurface layers because of the existence of a strong seasonal thermocline at the bottom of thin summer mixed layers (ML). By introducing the concept of the heat penetration depth (HPD), defined as the depth to which the downward net heat flux (Q_net) distinctly penetrates, we here characterize the heat capacity in terms of the heat content above the HPD using a simple, one-dimensional vertical model during the warming season. Seasonal changes in the HPD indicate that the thermal effects of Q_net gradually penetrate below the shallow seasonal thermocline due to vertical eddy diffusivity. Downward heat penetration into the layer below the shallow seasonal thermocline occurs widely throughout the North Pacific, and two-thirds of Q_net penetrates below the ML. In a hypothetical analysis of the case where the observed Q_net accumulates only within the ML, the change in SST is unrealistically larger than that of the observed SST. These results indicate that heat penetration plays a crucial role in climate variations during the warming season.