Frontogenesis over the Gulf Stream

Sensible heat fluxes associated with sea surface temperature (SST) fronts affect the genesis and evolution of atmospheric fronts. But how strong is this effect? A highly simplified analytic model, based on Keyser et al. (1988), is developed and used to explore this question. The model describes a two-dimensional vortex, representative of an extratropical cyclone, and its associated fronts, interacting with an SST front. The model predictions are compared with climatologies of atmospheric fronts over the North Atlantic Ocean identified in ERA5 reanalyses.

The climatologies are divided into times when fronts are detected at a point and times when they are not, and compared with model results with and without fronts in their initial conditions. In the front case, there is a period of relatively weak frontogenesis as the model front crosses the SST front, but because of the upstream and downstream frontolysis, the SST front is unimportant in determining the long-term strength of the atmospheric front. Importantly, this result changes little in the model as the width of the SST gradient changes. In line with the climatology, the long-term strength of the potential temperature gradient characterizing the atmospheric front is largely determined by the deformation. If the deformation is sufficiently strong, the adiabatic frontogenesis overwhelms the frontolytic effects of diabatic frontogenesis on either side of the SST front. In the no front cases, there is sustained frontogenesis in the model only when the deformation is sufficiently strong or when the translation speed is low, as advection otherwise weakens the potential temperature gradient. As the cold airmass crosses the SST front, the model produces strong diabatic frontogenesis followed by frontolysis. This result agrees with the climatologies for the North Atlantic, where the dominant contribution to the climatological diabatic frontogenesis along the SST comes from the no front cases.

The simple models explain why more fronts are detected along the SST front than either side of it. Strong diabatic frontolysis decreases equivalent potential temperature gradients either side of the SST front, while the combination of diabatic frontogenesis (or weaker diabatic frontolysis) and adiabatic frontogenesis increases equivalent potential temperature gradients along the SST front. Consequently, an equivalent potential temperature gradient is more likely to exceed the detection threshold for a font over the SST front than either side of it.

There are many omissions from the model, but the main ones were the lack of a boundary layer and the associated turbulent dissipation, a simplified treatment of the surface sensible heat fluxes, and the lack of moist processes. Despite these omissions, the model proved to has a great deal of explanatory power, suggesting the main dynamical and diabatic processes has been retained.

Keyser, D., M.J. Reeder and R.J. Reed. 1988. A generalization of Petterssen's frontogenesis function and its relation to vertical motion. Mon. Wea. Rev., 116, 762-780.