Geostrophic Adjustment: A Mechanism for Frontogenesis

Abstract

A simple two-dimensional model is used to determine the geostrophically adjusted state of an initially motionless fluid with lateral density gradient. It is an extension of the much studied two-layer models to the case when the initial stratification is continuous, and allows a more detailed examination of the density and current fields. In addition, the model has demonstrated that geostrophic adjustment with its accompanying transverse circulation can produce regions of convergence and thus steepen the initial density gradient. In cases when the initial density transition is sharp enough, fronts, or density discontinuities, are formed at these convergence regions, along which the surface fluid particles drawn in from both sides of the fronts move into the interior. The density jump across the fronts is the greatest at the top and bottom surface and diminishes toward the interior where differential vertical motion during the adjustment can significantly level the isopycnals. One observed frontal structure south of Nantucket Shoals exhibits some qualitative feature predicted by the model.