A Greenland Sea Perspective on the Dynamics of Postconvective Eddies*

Abstract

Open ocean deep postconvection contributes to the formation of the dense waters that fill the global deep ocean. The dynamics of postconvective vortices are key to understanding the role of convection in ocean circulation. Submesoscale coherent vortices (SCVs) observed in convective regions are likely to be the anticyclonic components of hetons. Hetons are dipoles, consisting of a surface cyclone and a weakly stratified subsurface anticyclone, that can be formed by convection. Here, key postconvective processes are investigated using numerical experiments of increasing sophistication with two primary goals: 1) to understand how the ambient hydrography and topography influence the propagation of hetons and 2) to provide a theoretical context for recent observations of SCVs in the Greenland Sea.It is found that the alignment of hetons is controlled by ambient horizontal density gradients and that hetons self-propagate into lighter waters as a result. This provides a mechanism for transporting convected water out of a cyclonic gyre, but the propagation is arrested if the heton meets large-amplitude topography. Upon interaction with topography, hetons usually separate, and the surface cyclone returns toward denser water. The anticyclone usually remains close to topography and may become trapped for several hundred days. These findings may explain the observed accumulation and longevity of SCVs at the Greenland Fracture Zone, on the rim of the Greenland Sea gyre. The separation and sorting of cyclones from anticyclones have likely implications for the density and vorticity budgets of convective regions.

Shift towards the dominance of boreal species in the Arctic: inter-annual and spatial zooplankton variability in the West Spitsbergen Current

“…This could explain the positive relation between C. glacialis abundance and sea ice concentrations in our analyses. The stations in the north-western part of the study area, where the water masses of the WSC mix with fresher and colder waters originating from the Greenland Sea (Boreal Basin Gyre) or the Fram Strait (Loeng & Drinkwater 2007, Oliver et al 2008, had increasing water depth and, consequently, characteristic species at these stations, Calanus hyperboreus and Themisto libellula, are typically associated with deep waters of the Greenland Sea (Hirche 1997), supporting the relationships found in our analyses.…”

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

“…The low stratification of the eddy core cannot be explained by pure adiabatic vortex stretching alone as this mechanism will result in cyclonic vorticity, assuming that f dominates the relative vorticity. Accordingly, the low stratification in the eddy core must be the result of some kind of preconditioning induced by for example upwelling, deep convection (Oliver et al, 2008) or diapycnal mixing near the surface or close to boundaries (D'Asaro, 1988) before eddy generation takes place (McWilliams, 1985). D'Asaro (1988), Molemaker et al (2015) and Thomsen et al (2015) highlight the importance of flow separation associated with headlands and sharp topographical variations for the generation of ACMEs.…”

Baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling

“…[55] Although some model studies recently discussed the formation and evolution processes of the SCVs [e.g., Rubino et al, 2007;Oliver et al, 2008], their scales were prescribed, and what determines their scales (or what are their origins) was unknown. Many similarities mentioned above, including the scale, imply that the anticyclonic eddies are a possible candidate for the origin of the SCVs in weakly stratified polar (subarctic) oceans.…”