Recent observations of water columns and sea ice over the Chukchi Borderland

Abstract

Summertime Arctic Ocean expeditions have been intensively carried out since 2010 to examine how rapidly marine environment changes over the Pacific Arctic region. We present recent findings from water columns and sea ice over the Chukchi Borderland, with a focus on the variability of physical and biochemical properties. Yearlong mooring data show temporal variation of the Pacific summer water (PSW) over the Chukchi Plateau (CP). The PSW layer on the northern portion of CP remained over the winter of 2013. On the southern portion of CP, however, substantial heat within the PSW layer was released from October and mid-winter of 2014. It implies that the PSW heat influences on sea ice formation/melting along the PSW pathways. In two ice camp stations, physical and biochemical properties of melt ponds and under-sea ice water were investigated. In closed ponds, relatively high concentrations of nitrogen species were found whereas no SiO2 was detected. In comparison, in opened ponds, PO₄ and SiO₂ concentrations were similar to those in surface seawater and nitrogen species were depleted. These results suggest that nutrient can be used as an indicator to characterize the types of melt ponds. Under Arctic sea ice, we investigated vertical variability of Arctic copepod using an acoustic Doppler current profiler and its relations with water column structure (potential temperature, fluorescence, and dissolved oxygen). This study indicates that the vertical distribution of Arctic copepod under sea ice with 1-2 m thickness is effectively regulated by light intensity associated with the halocline depth at high dissolved oxygen. Furthermore, satellite observations indicate that surface chlorophyll concentration has a relatively significant negative relationship with sea ice retreat over the Chukchi Borderland, implying that chlorophyll-a concentration tends to increase as sea ice retreat occurs earlier.