Effects of Wind and Sea Ice Drift on the Seasonal Variation of Warm Circumpolar Deep Water in the Amundsen Sea

Abstract:

We examined the effect of wind and sea ice on seasonal variation in the thickness of circumpolar deep water, to better understand the processes causing mass loss in the West Antarctic ice sheet (WAIS). Spatial and temporal variation of the layer of warm and salty circumpolar deep water (CDW) at the center of the Amundsen Shelf was measured during two oceanographic surveys and a two-year mooring deployment. 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, the water flows south-east along the trough. The thickness of the warm layer displays seasonal variation with maximum thickness in austral summer and minimum thickness in austral winter. The variation in warm layer thickness gives rise to a seasonal variation of the modified CDW heat content. In order to investigate the effects 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. The Ekman pumping velocity was calculated from the ocean surface stress field. 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 field is eastward north of the shelf break and westward south of the shelf break during all seasons. The main effect of a layer of sea ice (between the wind and the water) is to reduce the surface stress which can intensify the horizontal gradient of surface stress at the marginal ice zone. This 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, a marginal ice zone close to the shelf break gives rise to a positive Ekman pumping that may explain the seasonal signal seen in the mooring data. At northern boundaries of coastal polynya, positive Ekman pumping appears under a westward wind field. During austral winter, the marginal ice zone is further north and there are almost no polynya (except for a few very small areas of open water surrounded by ice close to the coast), so Ekman pumping is reduced compared to the summer season and does not cause any flow of warm water onto the shelf.