Flow response to large-scale topography: the Greenland tip jet

Abstract

Numerical model simulations of idealized and observed flows are used to investigate thedynamics of low-level jet streams that form in stratified flow downstream of the vertex of largeelliptical barriers such as the southern tip of Greenland, hereafter referred to as ‘tip jets’. Thetip jet dynamics are governed by conservation of Bernouli function as parcels accelerate downthe pressure gradient during orographic descent. In some circumstances, the Greenland tip jetis influenced by baroclinic effects such as differential horizontal (cross-stream) thermal advectionand/or vertical shear. In contrast, in the barotropic situation upstream flow is diverted aroundand over the obstacle into laminar (Bernouli conservation) and turbulent (Bernouli deficit)regimes, respectively. In both situations, a downstream geostrophic balance is achieved, characterizedby baroclinicity and vertical shear associated with the surface-based tip-jet front. Thestrength of the tip-jet is most sensitive to changes in the basic state dimensionless mountainheight (Nh/U) and Rossby number, underscoring the importance of the orographic deflectionof airstreams and Lagrangian accelerations on the slope. Enhanced surface-based forcing of theocean circulation occurs in the region of the tip jet core through large air–sea energy exchange (upward surface-heat fluxes > 800W m^-2), and at the tip jet flank through localized surfacestress forcing.