Frontogenesis in the North Pacific Oceanic Frontal Zones—A Numerical Simulation

Abstract

A primitive equation model [Geophysical Fluid Dynamics Laboratory?s (GFDL?s) MOM 2] with one degree horizontal resolution is used to simulate the seasonal cycle of frontogenesis in the subarctic frontal zone (SAFZ) and the subtropical frontal zone (STFZ) of the North Pacific Ocean. The SAFZ in the model contains deep (greater than 500 m in some places) regions with seasonally varying high gradients in temperature and salinity. The gradients generally weaken toward the east. The STFZ consists of a relatively shallow (less than 200 m in most places) region of high gradient in temperature that disappears in the summer/fall. The high gradient in salinity in the STFZ maintains its strength year round and extends across almost the entire basin. The model simulates the location and intensity of the frontal zones in good agreement with climatological observations: generally to within two degrees of latitude and usually at the same or slightly stronger intensity. The seasonal cycle of the frontal zones also matches observations well, although the subarctic front is stronger than observed in winter and spring. The model balances are examined to identify the dominant frontogenetic processes. The seasonal cycle of temperature frontogenesis in the surface level of the model is governed by both the convergence of the wind-driven Ekman transport and differential heating/cooling. In the STFZ, the surface Ekman convergence is frontogenetic throughout the year as opposed to surface heating, which is frontogenetic during winter and strongly frontolytic during late spring and summer. The subarctic front at 40°N in the central Pacific (not the maximum wintertime gradient in the model, but its location in summer and the location where variability is in best agreement with the observations) undergoes frontogenesis during spring and summer due to surface Ekman convergence and differential horizontal shear. The frontolysis during winter is due to the joint influence of differential heat flux and vertical convection in opposition to frontogenetic Ekman convergence. The seasonal cycle of salinity frontogenesis in the surface level is governed by Ekman convergence, differential surface freshwater flux, and differential vertical convection (mixing). For salinity, the differential convection is primarily forced by Ekman convergence and differential cooling, thereby linking the salinity and temperature frontogenesis/frontolysis. Below the surface level, the seasonal frontogenesis/frontolysis is only significant in the western and central SAFZ where it is due primarily to differential mixing (mostly in winter and early spring) with contributions from convergence and shearing advection during fall and winter. The shearing advection in the model western SAFZ is likely a result of the Kuroshio overshooting its observed separation latitude. The model?s vertical mixing through convective adjustment is found to be very important in controlling much of the frontogenesis/frontolysis. Thus, the seasonal cycle of the surface frontal variability depends strongly on the subsurface structure.