Sea ice and water column structure on the eastern Bering Sea shelf

Abstract Seasonal sea ice is a defining characteristic of the eastern Bering Sea shelf, and plays a critical role in determining the vertical structure of temperature and salinity over this shelf. Ice movement relative to local winds, ice composition, and the impact of both arrival and retreat of ice on the water column at four mooring sites over the middle shelf are examined. Ice forms primarily in coastal regions and is advected over the southern and outer shelves. Ice drift from satellite data for two representative years, 2003 and 2007, was ~2% of local NCEP wind speed and oriented 44° to the right of the winds ( r 2 =0.25). Measurements from 30 ice cores collected in 2006–2009 gave an average salinity of 5.62±0.88, and an average nitrate concentration of 0.99±0.83 µM. Time series data collected at the biophysical moorings in the Bering Sea (1995–2012) were used to explore the evolution of the water column under ice. At the northern mooring, M8, the water column had mixed and cooled to ~−1 °C prior to the arrival of ice. Little melt occurred after ice arrival. At the other three moorings, the ocean temperature was 2–4 °C when ice arrived, resulting in extensive melt. Melting ice freshened and cooled the upper water column, resulting in stratification, which persisted for 10–25 days. Wind-induced water-column mixing occurred more slowly under the ice than in ice-free waters. An estimated 1.4 m of ice melted with the first arrival of ice at the three southern moorings where the latent heat of fusion accounted for approximately half the observed cooling. During ice retreat, there appeared to be little ice melt around the southern two moorings, but an estimated 0.8 m at M5 and M8. The extent of ice melt sets up the water column for the following summer.