Effects of mesoscale SST fronts on the marine boundary layer

A numerical modeling study is presented using a large-eddy simulation with open boundaries to examine the effects of sea-surface temperature fronts on the marine boundary layer. Two cases are considered, cold to warm and warm to cold, with identical upstream, neutral conditions. Simulations show that the spatial order of warming and cooling has a direct impact on both the boundary-layer structure and surface fluxes. Downstream boundary layer depth is larger when warm water is upstream from cold water because of enhanced convective forcing and mixed layer deepening. When cold water is upstream from warm water, a cool internal boundary layer forms over the warm water, which must be eroded from above before mixing can extend to the original boundary layer top. Analysis of the momentum budget indicates that turbulent momentum flux dominates the velocity field, with pressure forcing accounting for about 20% of the changes in momentum. Results suggest that SST variability on scales of 5-20 km should be directly simulated in mesoscale models, since they would be very difficult to parameterize as subgrid processes. Results indicate that smaller scale fronts (< 5 km) might be represented in mesoscale models by relating the effective heat flux to the strength of SST variance.