RAPID: Microstructure Observations of Rapid Surface Freshening in the Labrador Sea

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A longstanding mystery in deep convection regions concerns the rapid surface capping after deep convection. In the Labrador Sea, this capping takes the form of a thin freshwater layer, regularly seen across the entire 600 km diameter basin in April or May shortly after the deepest convection in March. This freshwater appears to have been imported from the boundary currents at the basin's periphery, but Ekman transport distributed over the 30 m mixed layer would be far too slow. Since models of overturning circulation variability frequently begin with a freshwater perturbation over the deep water formation region, the mechanism for transporting boundary freshwater to the convected interior is an important link in the climate system.The origin of the rapid freshwater capping will be investigated during the spring 2010 hydrographic cruise across the Labrador Sea by the Bedford Institute of Oceanography (BIO). A novel measurement platform, the Air-Sea Interaction Profiler (ASIP) developed by Brian Ward of the University of Galway, will be used. This upward-profiling autonomous instrument records microstructure temperature and conductivity as well as turbulent shear, and in particular resolves the uppermost centimeters of the ocean including the centimeter scale salinity skin layer. Repeated measurements will give an indication of the spatial and temporal variability with far greater resolution than the roughly 50 km CTD station spacing. Microstructure shear measurements would help quantify the turbulent mixing within the halocline and therefore shed light on its formation. Different lateral flux processes could be distinguished by their degree of vertical and horizontal homogeneity and by the associated levels of mixing activity. This untethered instrument will sample the upper ten meters which are normally contaminated by the ship wake in the CTD stations.The intellectual merit of this project is an improved understanding of freshwater transport and halocline formation in the deep convecting region of the Labrador Sea, one of the key regions in the Atlantic Meridional Overturning Circulation. The primary broader impact is the relevance this work has for our understanding of climate variability, in particular, the sensitivity of the overturning circulation to freshwater perturbations. A post-doc will received training in making at sea measurements in a multi-national project.