Effect of the nodal declination tide on the thermohaline water structure, sea surface heights and geostrophic currents in the southwestern Bering Sea

Abstract

Satellite data on the sea level heights and data of Argo floats for the southwestern Bering Sea and adjacent area of the North Pacific are analyzed. Interannual variability of the thermohaline structure and water dynamics caused by the nodal (declination) tide with the period of 18.6 years is revealed. The tidal-induced 18.6-year variability is traced in variations of the sea level, water temperature and direction and velocity of the geostrophic currents. Extreme strengthening (in 2006) and weakening (in 1997 and 2015) of nodal tides led to lowering/rising of the sea level heights in the southwestern Bering Sea and in the areas southward from the Near Strait and Commander Islands. The sea level lowering was associated with increasing of salinity and water density in the layer of 50–400 m; as the result, the salinity and density at the depth of temperature minimum (~100 m) were higher in 2005–2006 than in 2015–2016 by 0.20 psu and 0.14 kg . m–3, respectively. This increasing can be explained by transformation of the Alaskan Stream waters due to intensified tidal mixing in the Aleutian Passes and in the Near Strait. No statistically significant correlation between the sea level and amplitude of the nodal tide was found in the areas remoted from the straits. Another consequence of the sea level lowering was an appearance of cyclonic circulation northward and southward from the Near Strait and weakening of the Alaskan Stream in spring of 2006, with cessation of the flow from the Near Strait to the mainland coast that caused SST decreasing in the southwestern Bering Sea. Intensification (weakening) of nodal tides was accompanied by decreasing (increasing) of sockeye salmon catches at East Kamchatka and Aleutian Islands.