Polar lows as arctic hurricanes

Abstract

Satellite observations of the polar oceans have revealed the presence of small, intense vortices that often resemble hurricanes, having clear central eyes surrounded by deep convective clouds. Recent aircraft and dropsonde data also show that these storms, like hurricanes, occur within deep moist adiabatic atmospheres and possess warm cores. We propose that at least some polar lows are indeed arctic hurricanes. Using a recently developed theoretical model of the mature hurricane, we show that the observed difference between the moist entropy of the troposphere and that representing saturation at sea surface temperature can sustain moderately intense hurricanes. Unlike the environments of tropical hurricanes, much of this moist entropy difference results from an air-sea temperature difference. Numerical experiments using an axisymmetric nonhydrostatic model confirm that intense hurricanes can develop in environments typical of those in which polar lows are observed to develop. Due to the relatively large values of the Coriolis parameter, these storms have smaller diameters than do hurricanes. The experiments also show that the deep, cold cut-off lows which create favorable thermodynamic environments for polar lows also inhibit their development because of the large intertial stability of their circulations. Finally, we show that, like hurricanes, surface flux-driven polar lows cannot arise spontaneously, but require an independent and presumably non-axisymmetric dynamical mechanism for their initiation.