Vortex Dynamics and interannual variability in the Labrador Sea


Deep convective ventilation in the Labrador Sea is partly controlled by mechanisms other than local surface forcing and local density gradients. There exists also an active eddy-driven restratification which is modulated by variations in boundary current dynamics. Large interannual variations in both eddy shedding and buoyancy transport from the boundary current have been observed but not explained (see figure below).
In this project we investigate the processes controlling eddy generation and associated buoyancy transport by combining realistic and idealized numerical modeling, data analysis, and theory. Ensembles of numerical experiments with a high-resolution regional model will explore the sensitivity of eddy generation and property transport to variations in local and external forcing parameters. Extended analysis of eddy and boundary current properties in data, centrally the now fifteen-year TOPEX/Poseidon and Jason altimeter records, will allow comparison of modeled and actual vortex characteristics over a wide range of oceanic conditions. Theory, supported by idealized experiments, will provide criteria to test candidate hypotheses as to the nature of the instability, and will suggest possibilities for its parameterization. The net result will be an understanding of the links between local and nonlocal forcing variability, and the eddy-driven buoyancy fluxes which limit deep convection. This process-oriented study should form an important step toward the larger goal of understanding and accurately modeling variability of the Atlantic Meridional Overturning Circulation in general.

modeled kinetic energymodel velocity and ssh

Eddy Kinetic Energy (EKE), cast as a speed VEKE ≡ √2EKE, from satellite altimetry (left panel) and in our regional Labrador Sea model (center panel). Modeled Sea Surface Height field and velocity vectors for a three-day average snapshot during the month of February are shown on the right panel. The vigorous eddy field originates predominatly from the West caost of Greenland. The eddy kinetic energy is constructed from despiked alongtrack data during the entirety of the TOPEX/Poseidon years 1992–2002. The regional Labrador Sea model has been run at 8km resolution for 60 years, under the climatological forcing.

This work has a direct benefit to the representation of the Labrador Sea branch of the Atlantic Meridional Overturning Circulation (AMOC) in large-scale climate models, in which details of the narrow boundary current instability are not possible to resolve. In the face of dramatically increasing freshwater discharge from the Arctic, it is critical to understand the transport of buoyancy from boundary current to the convection region, and in particular, to identify the factors underlying its variability.


A schematic of the Labrador Sea (left panel) together with isopycnal thickness (right panels) during the 1960s and 1990s showing the spreading of LSW across the subpolar North Atlantic at mid-depths, . Note how fresher is Labrador Sea Water layer during the 1990s. This fresh water was a major contributor to the marked freshening of the deep North Atlantic observed over the past decade. Figures courtesy of Igor Yashayaev.


Professor Annalisa Bracco
School of Earth and Atmospheric Sciences
Georgia Institute of Technology
abracco at gatech.edu
Earth and Space Research
Professor Joseph Pedlosky
Physical Oceanography Dept.
Woods Hole Oceanographic Insitution
Dr. Igor Yashayaev
Bedford Institue of Oceanography
Dept. of fisheries and oceans, Canada University




Background material (i.e. our most recent papers related to this topic):

A.Bracco and J. Pedlosky, Vortex generation by topography in locally unstable baroclinic flow, JPO vol. 33, pag. 207-219, 2003.

A. Bracco, J. Pedlosky and R. Pickart, Eddy formation near the west coast of Greenland, JPO., in revision, 2007 [pdf file]

J. Lilly, P. B. Rhines, F. Schott, K. Lavender, J. Lazier, U. Send and E. D’Asaro, Observations of the Labrador Sea eddy field, Prog. Oceanogr., 59, 75–176, 2003.

I. Yashayaev, Hydrographic changes in the Labrador Sea, 1960–2005, Prog. Oceanogr., 73, 242–276, 2007.