The sensitivity of thermally driven circulations on horizontal resolution in a depth level coordinate ocean circulation model

Abstract

The structure of high latitude circulations from a depth level coordinate (GFDL/MOM) are investigated in a flat bottom spherical sector while increasing the horizontal resolution from 2 to 0.25deg. The model is forced using the Newtonian damping condition for temperature in the absence of surface wind stress and salinity. When the resolution is low, as in earlier studies watermass formation occurs near the northeastern corner of the basin.. Thermally driven circulations from a low resolution ocean circulation model a depth level coordinatebased on Park and Bryan (2001) show that in high latitude thermally driven circulations from a low resolution ocean circulation model based on depth level coordinate (z-model) and one based on isopycnal layer coordinate (layer model) are in opposite directions. In the layer model, there is a northward eastern boundary current along the eastern boundary and the surface flow is cyclonic as the observations or theory. They show that the difference is due to difference in the convection scheme and the vertical coordinate system used in each model. When the horizontal resolution of z-model is increased, z-model start to show an eastern boundary current that flows to the north. This eastern boundary current becomes northern boundary current to form cyclonic circulation as in the low resolution (2x2 degrees) layer model. When the horizontal resolution is increased to 0.25x0.25 degree, the velocity and thermal structures from the z-model becomes similar to those from the low resolution layer model. This suggests that to make low resolution z-model more realistic in we need a new convection scheme that produces a convectively mixed layer of non-zero stratification.