An examination of hydrography and sea level variability in the Gulf of Alaska

Abstract

This thesis summarizes a detailed analysis of Seward sea level (SSL) in the Gulf of Alaska (GOA) and hydrography at oceanographic station GAK1 along the oceanographic monitoring line outside of Resurrection Bay, Alaska. SSL variability was examined with respect to forcing by tides, sea level pressure (SLP), wind, the steric contribution due to water column variations in temperature and salinity, and several climatic indices. In addition, multi-decadal trends in sea level and hydrography were also examined. The period of analysis spans 1970 -2010. Tidal motion (periods < 1day), account for ~97% of the total sea level (SL) variance while SLP variations, induced by the inverted barometer effect, account for ~2% of the total variance. After removing these influences, along-shore wind stress and sea level are highly coherent for the 2 - 60 day subtidal period range and account for ~40 - 50% of the SSL variance over these timescales. The steric contribution, based on the geopotential height referenced to 200 m (GH200), is also coherent with SSL, but it only accounts for ~10% of the SSL variance at these timescales. The along-shore wind stress-SSL coherence varies seasonally and is greatest in winter when winds are strongest and smaller in summer when wind variability is reduced. The annual cycle in SSL is coherent and in-phase with GH200. The latter is primarily controlled by the annual cycle in GOA coastal freshwater discharge and its effect on coastal salinities. SSL variations are also significantly correlated with the Pacific Decadal Oscillation (PDO) and the Southern Oscillation Index (SOI). Over the 40-year record examined here SSL shows a decreasing trend due to continental rebound. There are also statistically significant linear long-term trends in temperature and salinity as measured at GAK1. These trends indicate that the upper 100 m is warming at ~0.20 oC decade-1 and at 0.15 oC decade-1 between 100 - 200 m. Surface salinities are decreasing at 0.15 decade-1, while salinities between 100 and 200 m are increasing by 0.025 decade-1. The surface salinity trends are consistent with an increase in coastal freshwater discharge due to increased glacial ablation and increased precipitation. In aggregate, these trends indicate that the GOA shelf has become more stratified over the past 40 years.