Modeling the impact of polar mesocyclones on ocean circulation

High resolution, eddy resolving numerical ocean models are often forced with atmospheric reanalysis datasets (e.g. ERA40, NCEP). However, these products are gridded at a spatial resolution that inadequately resolves atmospheric subsynoptic (<1000 km diameter) weather systems. Focusing on a suite of high latitude, mesoscale cyclones (polar mesocyclones), that are both ubiquitous across the North Atlantic, and capable of having localized wind speeds of hurricane force and heat (sensible + latent) fluxes >500 W m-2, we show that approx. 75% and 100% of these vortices are not resolved in the ERA40 and NCEP reanalysis products, respectively.

This limitation results in the under-forcing of the ocean in numerical models by failing to accurately represent the transfer of momentum and heat from the atmosphere to the ocean, and points to an urgent requirement to parameterize the affects of these vortices. Here we report on the development of a novel parameterization to include mesoscale atmospheric activity in the wind field of ocean models by representing individual vortices determined to be missing, or underrepresented, in the existing reanalysis, as Rankine vortices.

The parameterization significantly improved the quality of the existing reanalysis data, bringing localized wind speeds into closer agreement with observations in all case-studies examined. Forcing an ocean sea-ice numerical model for two years with reanalysis data populated with parameterized polar mesocyclones, we observed both enhanced surface latent and sensible heat fluxes across the northeast Atlantic, and a dramatic increase in the cyclonic rotation of the Nordic Seas gyre. In response to these changes, Greenland Sea Deep Water (GSDW) formation generally increased, indicating more active open ocean convection, while an increase in the volume transport of intermediate and deep water overflowing the Denmark Strait suggests an important coupling between short-lived, intense atmospheric activity and deep ocean circulation.

As ocean models are integrated at increasingly finer spatial resolutions, it is imperative that we capture the energy spectrum of mesoscale weather over the ocean. We conclude by reporting on preliminary results from a longer (20 year), higher (1/6 degree) resolution ocean sea-ice model integration.