Deepening of Winter Mixed Layer in the Canada Basin in Response to Pacific Summer Water Pathway Change

Abstract

The surface mixed layer plays a critical role in heat, carbon, and nutrient exchange within the atmosphere–ice–ocean system. Using the latest observations from the ice-tethered profilers in the Canada Basin, we find the mixed layer became 9.6 m deeper and 0.7 psu saltier from 2006–2013 to 2014–2022. The mixed layer deepening coincides with the warming and freshening in the subsurface layer, thereby diminishing stratification beneath the mixed layer. The physical mechanisms of the mixed layer changes were investigated from two perspectives: the mixed layer local freshwater budget and remote control of Pacific inflow. Results indicate the saltier mixed layer maybe attributed to reduced freshwater inflow from the boundary driven by the southeastward contraction of the Beaufort Gyre. Lagrangian particle tracking experiments with GLORYS12 reanalysis reveal that subsurface layer salinity decreases, which can be explained by intensified entrainment of the fresh Pacific Summer Water (PSW) into the basin. It is driven by the local and remote effects: (a) Weakened northeasterly winds over Barrow Canyon facilitate PSW further approaching the southern edge of the basin, moving westward at deeper isobaths along the Chukchi slope and finally into the basin. (b) The southeastward contraction of the Beaufort Gyre drives more PSW from the Chukchi slope northeastward into the basin with the clockwise circulation. If the deepening of the mixed layer and the weakening of subsurface stratification persist, increased upward entrainment of heat and nutrients from the halocline may significantly accelerate ice melt and impact the local ecosystem.