Deep weakly-stratified layers in the Greenland Sea

Abstract

Deep weakly-stratified layers in the Greenland Sea constitute a mode of convection poorly understood. The Greenland Sea is where the densest portion of the overflow waters on either side of Iceland are formed through wintertime convection. Deep interior weakly-stratified layers are located in the core of sub-mesoscale coherent vortices (SCVs). SCVs have a cold and relatively fresh core that extends down to 2500 m. These structures are isolated from the surface in summer, so traditional routines used to determine the mixed-layer depth are not able to detect SCVs once they are isolated from the surface. Hence, open questions remain regarding the location, timing and relevance of SCVs in dense water formation. This study aims to determine the occurrence and the role that SCVs play in dense water formation. A novel routine to identify weakly-stratified layers is applied to a historical hydrographic data set to identify weakly-stratified layers present in the Greenland Sea. The findings suggest that SCVs could be formed as a result of deep surface mixed layers in the central Greenland Sea. They form in years of strong atmospheric forcing in winter and positive surface salinity anomalies in autumn. In this study we show that wintertime convection down to the bottom of the core stabilises the SCV avoiding its collapse. By using hydrographic surveys and Argo float data we confirm that the lifetime of SCVs can reach up to one year, although further investigation is needed to determine their typical endurance. SCVs formed in the central Greenland Sea are typically observed close to the same location south-west of the Greenland Fracture Zone. Reasons for this location are its low Eddy kinetic energy (EKE) and interaction of the SCVs with the topography. Once the SCVs reach the rim current they are advected around the Greenland Sea until they collapse. When the SCVs collapse their core water are suffeciently dense to contribute to the overflow waters on both side of Iceland indicating that they indeed contribute to the dense water formation in the Nordic Seas.