Circulation and dynamics on the Northeastern Chukchi Sea Shelf

Abstract

The circulation on the northeastern Chukchi Sea shelf is controlled by the poleward pressure gradient between the Pacific and Arctic Oceans. Local winds modulate the upper ocean and can rapidly alter the flow field. Present understanding of the circulation is largely based on subsurface measurements, but the response of near-surface currents to the slowly-varying secular pressure gradient and rapidly-varying local winds has not been addressed. I analyzed surface current data, extending more ~150 km offshore in the northeastern Chukchi Sea, collected from shore-based high-frequency radar systems (HFR) during the open water season. I find three wind-induced circulation regimes. Two of these are related to strong northeasterly winds when wind speeds approach or exceed 6 m s⁻¹ and the third results from infrequent northwesterly winds at >~6 m s⁻¹ . I find two dynamically different regions separated along ~71.5°N associated with hydrographic changes. North of 71.5°N the water column is strongly stratified due to cold and dilute ice meltwaters, whereas the water column to the south is much less stratified. These differences are reflected in the current response to the winds. I also adapted and refined an HFR data processing technique and developed an economical way to assess HFR-derived data quality, which is beneficial when using HFR data collected from networks having suboptimal coverage. I investigated the poorly understood circulation around Hanna Shoal. North of the Shoal there is a zonal gradient in the thermohaline and flow fields. The eastern side of the Shoal is strongly stratified year-round and vertically sheared unlike the western side, where the flow is steadily northeastward over the water column. Dense bottom waters flow clockwise around Hanna Shoal, but zonal convergence is implied in the upper water column north of the Shoal. The circulation is influenced by the distribution of late summer sea ice and by clockwise-propagating topographic waves.