Convection in the Western North Atlantic Sub-Polar Gyre: Do Small-Scale Wind Events Matter?

Abstract

This chapter addresses aspects of open-ocean convection in the western North Atlantic, with particular emphasis on the area near southern Greenland and the impact of small-scale atmospheric patterns. Nearly a century ago it was hypothesized that deep convection occurred in the southwest Irminger Sea, and recently convection has been observed in the eastern Labrador Sea. It is argued that the interaction of wintertime low-pressure systems with the high topography of Greenland may lead to the deep mixed-layers in these two areas. Two small-scale atmospheric patterns, forward tip jets and reverse tip jets, result in intense winds on both sides of southern Greenland. The latter pattern is related to the barrier winds along southeast Greenland. In each case the winds blow over areas of closed oceanic circulation where the deep mixing appears to occur. The importance of the forward tip jet in the evolution of the mixed-layer of the southwest Irminger Sea has recently been established, and evidence suggests that during strong winters the overturning can be quite deep. Reverse tip jets may in turn have a similar impact on the eastern Labrador Sea, causing enhanced heat loss and convection. However, to demonstrate this conclusively, further work is necessary to sort out the interaction of the barrier winds and reverse tip jets with the oceanic circulation and pack-ice.

Extreme Variability in Irminger Sea Winter Heat Loss Revealed by Ocean Observatories Initiative Mooring and the ERA5 Reanalysis

“…The NAO (see, e.g., Hurrell et al, 2003, for an overview of this mode) has been suggested to influence tip jet occurrence with positive NAO conditions favoring northward displacement of the winter storm track, greater interaction with Greenland, and an increase in frequency of jet conditions (Moore, 2003;Moore et al, 2011;Pickart et al, 2008). The NAO index in December 2014 (1.63) is strongly positive, supporting this suggestion given the observed prevalence of jet conditions at this time.…”

“…This remained under debate (de Jong et al, 2012;Pickart et al, 2008) until recently. In the North Atlantic, formation of dense water has long been known to take place in the Labrador and Nordic Seas and was also suggested to occur in the Irminger Sea.…”

“…In the North Atlantic, formation of dense water has long been known to take place in the Labrador and Nordic Seas and was also suggested to occur in the Irminger Sea. This remained under debate (de Jong et al, 2012;Pickart et al, 2008) until recently. However, mooring and Argo profiling float observations now provide unequivocal evidence for Irminger Sea deep convection (de Jong & de Steur, 2016;de Jong et al, 2018;Fröb et al, 2016) and highlight the need for better understanding of ocean-atmosphere interaction in this basin.…”

Atmospheric conditions associated with oceanic convection in the south‐east Labrador Sea

“…An examination of buoyancy flux through the surface of the ocean led Martin and Moore [2007] to speculate that reverse tip‐jets could indeed be important in the deepening of the mixed‐layer. Pickart et al [2008] also speculate on the possibility that reverse tip‐jets have an impact on deep water formation, though they note that the air in the jets may be too modified to strongly affect the ocean. In this study we present an analysis of heat fluxes in the ECMWF ERA‐40 dataset over the convective site in the south‐east Labrador Sea.…”

Exploration of turbulent heat fluxes and wind stress curl in WRF and ERA‐Interim during wintertime mesoscale wind events around southeastern Greenland

“…As a result, large turbulent heat fluxes are possible during all types of wind events, and the large heat fluxes can cause surface water densification [ Renfrew et al , ; Harden et al , ; Oltmanns et al , ]. Westerly tip jets were the initial focus for forcing ocean convection in the Irminger Sea [ Pickart et al , ; Våge et al , , , ], but other types of mesoscale winds may also be important for both preconditioning and convection in the Irminger Sea and southeast Labrador Sea [ Lavender et al , ; Martin and Moore , ; Sproson et al , ; Pickart et al , ; Oltmanns et al , ]. Additional work can help uncover the potential effect of all types of wind events and to understand how short‐lived mesoscale wind events impact the ocean on climatic time scales.…”