Interannual Variability in Stratification, Nutrients, and Water Mass Structure in the Chukchi Sea

Striking changes in the physical environment of the Chukchi Sea have been observed over the past decade. As a seasonally ice-covered sea, the advance and retreat of sea ice plays a dominant role. This seasonal evolution of ice is linked to variability in stratification and water mass structure, which have been shown to influence the ecosystem from phytoplankton to fishes and seabirds. Using mooring and shipboard data, we quantify interannual variability in stratification, water mass structure and associated nutrients. Stratification over the shelf increases over the summer until mid-September when increasing winds and surface cooling begin to mix the water column. Salinity stratification typically accounts for more than half of the total stratification during summer but the effects of temperature are not insignificant. Stratification and water mass structure are tightly linked. Timing of the transition between Winter Water and Summer Water is correlated with the timing of ice retreat in spring and ice arrival in fall. While transport of water masses from the south is likely important to water mass transitions, transport at the Icy Cape line is not significantly correlated with the timing of Summer Water arrival or the timing of ice retreat. However, the strongest average spring transport (March to May) in our record was observed in 2017, a year that both ice retreat and Summer Water arrival were more than a month earlier than average. The open water season in 2017 and the amount of time that Summer Water was present were both unprecedented in our record.