Effects of wind and sea ice dirft on the seasonal variation of warm circumpolar deep water in the Amundsen Sea

Abstract

Spatial and temporal variation of the layer of warm and salty c irumpolar deep water (CDW) in the center of the Amundsen Shelf was measured during two oceanographic surveys and a two-year mooring array. A hydrographic transect from the deep ocean, across the shelf break, and into the Dotson Trough shows a local elevation of the warm deep water layer at the shelf break. On the shelf, CDW fows south-east along the trough. Along its pathway to the ice shelves, CDW gradually be comes colder and fresher, presumably because of mixing with surface water and/or glacial meltwater. The thickness of the warm layer displays a seasonal variation with maximum thickness in austral summer and minimum in austral winter. The amplitude of this seasonal variation is up to 60 m. The variation in layer thickness gives rise to a seasonal variation of the CDW's heat content. In order to investigate the e ffects of wind and sea ice drift on the heat content, ocean surface stress was calculated using the ERA interim reanalysis wind data and observed sea ice velocity and concentration from satellites. Fields of Ekman pumping velocity were obtained from the ocean surface stress. The Ekman pumping at the shelf break, where the warm layer is elevated, shows a strong seasonal variation coinciding with the mooring data. The average wind eld is eastward north of the shelf break and westward south of the shelf break for all seasons. The main effect of a layer of sea ice (between the wind and the water) is to reduce the surface stress and intensify the horizontal gradient of surface stress at the marginal ice zone. This is creates a divergence of the Ekman transport and a positive Ekman pumping at the marginal ice zone, if the wind direction is eastward. From February to April, the marginal ice zone is lose to the shelf break, and the wind eld is eastward, giving rise to a positive Ekman pumping that may explain the seasonal signal seen in the mooring data. During austral winter, the marginal ice zone is further north, so the Ekman pumping has been weakened due to the decreasing of horizontal gradient of surface stress at shelf break.