Atmospheric Response to a Midlatitude SST Front: Along-front Winds

Satellite observations and modeling studies show that midlatitude SST fronts influence the marine atmospheric boundary layer (MABL) and atmospheric circulation. Here we use the Weather Research and Forecasting (WRF) mesoscale model to explore the atmospheric response to a midlatitude SST front in an idealized, dry, two-dimensional configuration, with a background wind V oriented in the along-front direction.

The SST front excites an along-front wind anomaly in the free atmosphere, with peak intensity just above the MABL. This response is nearly quasigeostrophic, in contrast to the inertia-gravity wave response seen for cross-front background winds. The free atmosphere response increases with the background wind V, in contrast to previously proposed SST frontal MABL models.

The MABL winds are nearly in Ekman balance. However, a cross-front wind develops in the MABL due to friction and rotation, such that the MABL cross-front Rossby number ε ≈ 0.2. The MABL vorticity balance and scaling arguments indicate that advection plays an important role in the MABL dynamics. Surface wind convergence shows poor agreement with MABL depth-integrated convergence, indicating that the MABL mixed-layer assumption may not be appropriate for SST frontal zones with moderate to strong surface winds.