Can polar lows lead to a warming of the ocean surface?

O Saetra,T Linders,J.B Debernard

doi:10.1111/j.1600-0870.2007.00279.x

Abstract

Possible surface warming by strong wind-forcing from polar lows in the North-Atlantic has been investigated using a numerical model for vertical entrainment of waters from a subsurface warm core, and microwave satellite images of sea-surface temperature during polar low events. The hypothesis is based on the frequently observed subsurface warm core in oceans influenced by the North-Atlantic current (NAC) or by outflowing surface water from the Arctic Ocean. CTD-soundings from the Nordic Seas reveal that the waters from the NAC are located under colder and less saline surface waters in winter. For sufficiently strong wind events, turbulent entrainment of this subsurface warm core may lead to a rapid surface warming. Our main findings is that the surface warming of more than 1 ◦C may take place within a few hours. The result is based on model runs with initial temperature and salinity profiles from CTD-observations. Observational evidence of surface temperatures that support the hypothesis are found in microwave satellite observations from a polar lowevent. In the case presented here, increased sea-surface temperatures between 1 and 2 ◦C were observed. We believe that rapid surfacewarming of this magnitude may be a potential positive feedback mechanism for the cyclone intensity.

Highlights

Polar lows are important for society because of the sometimes extreme and potentially destructive weather that accompanies them
This paper is partly motivated by the finding of Paper I, that a polar low can it self induce a warming of the ocean surface with around 1 ◦C and thereby increasing the heat fluxes from the ocean
Such an effect would enhance the forcing mechanism of Wind-Induced Surface Heat Exchange (WISHE), which is the state of the art theory for tropical cyclones

Summary

Introduction

Polar lows are important for society because of the sometimes extreme and potentially destructive weather that accompanies them. If the forcing process is latent heat release in deep convection it would mean that polar lows are similar to tropical cyclones. Tropical cyclones are even more intense than polar lows and globally a more frequent phenomenon with more imagined effects on climate. In the papers included in this thesis my coauthors and I investigate the interaction between polar lows and the ocean. Paper I deals with how polar lows may affect the upper ocean It starts with a look at the rather special vertical temperature profiles that occur in the Nordic Seas and continue with investigating how the strong winds associated with a polar low can change these profiles. Two different theories for polar lows, both assuming forcing by latent heat release in deep convection, are compared with the data set.

Summary of Paper I

Summary of paper II

Summary of paper III

Discussion