Upper ocean stratification: regional differences between high and low resolution climate models

Abstract:

Notable improvements are seen in upper ocean stratification in both the Labrador Sea and Subantarctic Zone at high-resolution (HR, ~10-25 km grid spacing) compared to low-resolution (LR, ~100 km) in long climate model simulations performed by the International Laboratory for High-Resolution Earth System Prediction (iHESP). The simulations were made with Community Earth System Model (CESM) v1.3. We explore reasons for the improvements, focusing on ocean mesoscale eddy effects in the Labrador Sea and the quality of the ocean mean geostrophic flow for the Subantarctic Zone. In the Labrador Sea the vertical eddy tracer fluxes are very different between the eddy-parameterized model (LR) and eddy-resolving (HR), which will affect the stratification. Here, HR shows the effect of eddies that propagate from the west Greenland shelf/slope to the basin interior and act to restratify: this is difficult to represent in eddy-parameterized models. Meanwhile the key difference in the Subantarctic zone is resolution of the mean flow, such as the Agulhas Return Current and Antarctic Circumpolar Current, which in turn modulate SST gradients and surface heat loss. A non-local (downstream) response of stratification to enhanced surface heat loss is important in deepening the mixed layer depth in HR relative to LR. These examples are then contrasted with other regions where resolution makes less difference to the stratification, like the eastern sub-polar gyre. Finally, we examine how the resolution impacts the change of stratification under greenhouse gas forcing.