Seasonal variation of the upper ocean responding to surface heating in the North Pacific

Seasonal variations of the upper ocean, such as mixed layer depth (MLD) and sea surface temperature (SST), responding to the atmospheric forcing in the North Pacific (10 N-50 N), are investigated by analyzing the Argo and NCEP/NCAR reanalysis 1 data. The OAFlux data are also used for comparison. During the early heating period in the high-latitude ocean north of 30 N, where a seasonal thermocline is formed above the deep mixed layer under strong surface heating, the MLD h is found to be scaled as h (Lλ)1/2, where L is the Monin-Obukhov length scale and λ is the Ekman length scale. On the other hand, in the low-latitude ocean south of 30 N, where the preexisting MLD is shallow and surface heating is weak, h is found to be scaled by λ. It is found that a large amount of heat flux across the MLD occurs, especially in the high-latitude ocean during the late heating period, in which h is small. It suggests the contribution by turbulent mixing across the MLD in addition to radiation penetration, and the eddy diffusivity in the high-latitude ocean is estimated as Kv 10^-4-10^-3 m² s^-1. The heat budget of the mixed layer reveals that the contribution from the ocean heat transport is much smaller than the surface heat flux in the high-latitude ocean except in the Kuroshio region, but it is sometimes comparable in the low-latitude ocean.