Hydrographic Structure and Modification of Pacific Winter Water on the Chukchi Sea Shelf in Late Spring

Pacific-origin winter water is a fundamental component of the Chukchi Sea ecosystem, as its elevated nutrient content spurs primary productivity. As such, it is of high importance to understand the pathways and residence time of winter water on the shelf and how it is modified through air-sea interaction. Here we present results from an extensive hydrographic survey of the northeast Chukchi Sea carried out in May-June 2014 as part of the SUBICE program. Winter water (near the freezing point) was prevalent throughout the survey region, and the water column could be characterized as a two-layer system with weakly stratified surface mixed layers atop well-mixed bottom layers. The height of the bottom boundary layers varied according to bottom slope and upwelling versus downwelling conditions, consistent with previous theory. The density jump separating the surface and bottom mixed-layers was generally very weak, which motivated an investigation of how quickly the water column would overturn due to re-freezing in a lead or small polynya, which were prevalent throughout the study region. Using a polynya model driven by realistic surface forcing, together with a one-dimensional mixed-layer model, we determined that the Chukchi Sea is poised for overturning at this time of year. Such convection would stir nutrients from the sediments into the water column, thus promoting phytoplankton growth throughout the shelf.