Heat and salt budgets of the mixed layer around the Subarctic Front of the North Pacific Ocean

Abstract

The Subarctic Front (SAF) of the North Pacific Ocean is important because it considerably affects the storm track in the troposphere, and mode water ocean mixed layer (OML) forms south of the front. However, processes controlling the sea surface temperature and salinity in the area of the SAF are not fully understood. Using Argo, satellite, and atmospheric reanalysis data, we investigate the OML heat budget seasonal cycle around the SAF in a domain defined by 150°–168°E and 40°–46°N. In summer, a large imbalance suggests a non-negligible cooling effect due to vertical mixing. In winter, we find that in the area of mode water OML, air–sea fluxes contribute to 60 % of heat dissipation. The remaining cooling is due to lateral advection and mixed layer base entrainment, while eddies warm the OML. North of the SAF, the geostrophic and Ekman flows have comparable cooling effects. Entrainment also cools the OML in fall, but vanishes in winter. In March, entrainment warming due to a temperature inversion below the OML is found to be negligible. We also estimate the salinity budget of the OML. In winter, entrainment contributes to a salinity increase north of the SAF, but it is negligible in the mode water OML area. The contribution of salinity to buoyancy loss during fall and winter OML deepening is relatively small, especially south of the front where it accounts for only 6 % of total buoyancy loss.