On the nature of winter cooling and the recent temperature shift on the northern gulf of Alaska shelf

Abstract

In spring 2006 and 2007, northern Gulf of Alaska (GOA) shelf waters were ∼-1.5°C below average throughout the ∼:250 m deep shelf and the salinity-dependent winter stratification was anomalously weak due to above (below) average surface (bottom) salinities. Spring 2007 and 2008 temperatures were also ∼-1.5°C below average, but the anomalies were confined to the upper 100 m due to moderate salt stratification. Shelf temperatures in these 2 years were among the lowest observed since the early 1970s, thus interrupting an approximately 30-year warming trend. We examined winter cooling processes using historical conductivity-temperature-depth (CTD) profiles and mooring data from hydrographic station GAK1. The 2006 and 2007 cooling was associated with anomalously strong atmospheric heat loss in November 2006 and March 2007 and belowaverage fall runoff, which weakened winter stratification and allowed the late cooling to penetrate throughout the water column. In 2007 and 2008, early winter cooling was weak, fall runoff large, and stratification moderate at 100 m so that spring temperature anomalies were trapped to the upper 100 m. Analysis of the 40 year GAK1 CTD record indicates that winter averaged air-sea heat flux and salinity stratification anomalies explain 81% of the variation in deep (100-250 m) GOA temperatures. Although the timing and magnitude of winter runoff influences the shelf temperature distribution, temperature anomalies are a consequence of three-dimensional circulation and mixing processes. These involve the complex, but poorly understood, interplay among the air-sea heat flux; the ocean heat flux convergences; the stabilizing influence of runoff; and the destabilizing effects of cooling, vertical mixing, and the wind-driven cross-shelf buoyancy flux. Copyright 2010 by the American Geophysical Union.