East–west differences in water mass, nutrient, and chlorophyll a distributions in the sea ice reduction region of the western Arctic Ocean

Abstract

The R/V Mirai conducted hydrographic surveys in the western Arctic Ocean during summer 2004 across a front between cold Arctic water and warm water from the Pacific Ocean where sea ice cover has been largely reduced in recent summers. The hydrographic data indicate a new type of vertical temperature minimum water west of the front along isohaline surfaces with approximate salinity (S) of 32, which is fresher than the typical temperature minimum (S ≈ 33) caused by spreading of Pacific winter water (PWW) mainly to the east of the front. Both of the temperature minimum waters are characterized by low potential vorticity with near‐freezing temperature, suggesting that they are formed by winter convection with sea ice formation. A difference between the waters results from a large contribution of sea ice meltwater to the fresh temperature minimum (frTmin) water of S ≈ 32. The distributions of the sea ice meltwater contribution and nitrogen deficit suggest that summer shelf water, largely influenced by the sea ice melt in the Chukchi Sea, is modified by winter convection on its way to the Chukchi Abyssal Plain to form the frTmin water. This water supplies nutrients through the water distribution to the west of the front at depths shallower than the nutrient maximum layer caused by the PWW spreading. The shallower nutrient supply by the frTmin water combined with light penetration without sea ice cover could produce a prominent chlorophyll a maximum layer west of the front.