Marine boundary layers above heterogeneous SST: Alongfront winds

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds Vg = 10 ms-1 and heterogeneous sea surface temperature (SST) is examined using fine-mesh large-eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with jumps Δθ = (2, -1.5) K varying over a horizontal x distance of 1 km characteristic of an upper-ocean mesoscale or submesoscale front. The geostrophic winds are oriented parallel to the SST isotherms (i.e., the winds are alongfront). Previously, Sullivan et al. examined a similar flow configuration but with geostrophic winds oriented perpendicular to the imposed SST isotherms (i.e., the winds were acrossfront). Results with alongfront and across-front winds differ in important ways. With alongfront winds, the ageostrophic surface wind is weak, about 5 times smaller than the geostrophic wind, and horizontal pressure gradients couple the SST front and the atmosphere in the momentum budget. With across-front winds, horizontal pressure gradients are weak and mean horizontal advection primarily balances vertical flux divergence. Alongfront winds generate persistent secondary circulations (SC) that modify the surface fluxes as well as turbulent fluxes in the MABL interior depending on the sign of Du. Warm and cold filaments develop opposing pairs of SC with a central upwelling or downwelling region between the cells. Cold filaments reduce the entrainment near the boundary layer top that can potentially impact cloud initiation. The surfacewind-SST-isotherm orientation is an important component of atmosphere-ocean coupling. The results also show frontogenetic tendencies in the MABL.