22.04.2013: FB1-Seminar

11:00 h, Hörsaal (West), Düsternbrooker Weg 20

S. Thomsen ⁽¹⁾, C. Eden ⁽²⁾ and L. Czeschel ⁽²⁾:  "Stability analysis of the Labrador Current" 

⁽¹⁾ University Kiel, Helmholtz Centre for Ocean Research Kiel, Kiel Germany sthomsen(at)geomar.de
⁽²⁾ University Hamburg, Centre for Marine and Climate Research, Hamburg Germany carsten.eden(at)zmaw.de

Abstract:

The water mass transformation process in the Labrador Sea during winter plays an important role for the Atlantic meridional overturning circulation and the global climate system. The Labrador Sea Water (LSW) is exported within the deep Labrador Current (LC) after the convection process. LSW takes up large amounts of atmospheric tracer gases as CO2 and oxygen, and is thus one of the major agent for ventilation of the abyssal ocean. It is shown that enhanced eddy kinetic energy (EKE) along the LC shows up in a 1/12° ocean model simulation during the transformation process. Moored in-situ measurements within the LC also show enhanced EKE levels during winter. This instability processes within the LC is important as it might alter the water mass properties of the (LSW) by frontal mixing processes during the water mass transformation and export within the LC.

The frontal instability process, which leads to enhanced EKE along the LC during winter is investigated using ageostrophic linear stability analysis. Dense and weakly stratified water masses produced during the wintertime transformation process lead to weaker stratification and a strengthening of the lateral density gradients within the LC. Weak stratification and enhanced vertical shear result in low Richardson numbers and the growth rate of baroclinic waves increases significantly within the shelf break LC during winter. Rapid frontogenesis along the whole LC sets in resulting in enhanced EKE. During the rest of the year strong stratification and weak vertical shear leads to larger Richardson numbers and smaller growth rates. Ageostrophic linear stability analysis shows that a geostrophic interior mode has similar wavelengths as the first wavelike disturbances in the model simulations. A shallow mode with lateral scales O(1 km) is also predicted, which can be associated with mixed layer instabilities and submesoscale variability but remains unresolved by the model simulation.