The Beaufort Sea: Dynamics of the Mackenzie River plume on the inner Beaufort Shelf during summer

The oceanographic conditions of the Mackenzie River plume in the Arctic Ocean were examined during a 12-day period in August 2007. Field observations in the river channel and the delta region (2~6 m depth), ship-based observations on the shelf and satellite observations of sea surface temperatures indicate that movements of plume density fronts cause changes in water temperatures of over 10 C over a few days. We used a 1D model to compare the strength of stratification versus surface wind stress, and a 3D numerical model to simulate the plume motions under forcing from the river flows, local wind and water level variations from tides and wind-driven surge. The results indicate that the coastal region is stratified with a ~2 m thick surface plume even in water depths of 3e4 m, resulting in strong vertical variation of horizontal currents. Moderate easterly winds of 5e10 m/s are sufficient to induce offshore transport of the surface plume and onshore transport of the deeper shelf water, leading to large fluctuations in temperature and salinity in the coastal region. This study examined a period of offshore transport and mean water level set-down, and indicates the rapid response of the plume to wind over the shallow delta.

a)-b) Sea surface temperatures from MODIS satellite observations in August 2007, showing the extent of the Mackenzie River plume in the delta region. Easterly winds cause upwelling, stronger mixing and lower temperature of the plume in a); light winds with minimal mixing and entrainment of underlying shelf water allow the higher temperature plume to spread as a thin surface layer in b). c)-d) Model results for sea surface temperature at the same times as satellite data, indicating movement of the plume though the instrument array denoted by crosses.