Coastal ocean variability in the US Pacific Northwest region: seasonal patterns, winter circulation, and the influence of the 2009–2010 El Niño

Abstract

A 2-km horizontal resolution ocean circulation model is developed for a large coastal region along the US Pacific Northwest (34–50N) to study how continental shelf, slope, and interior ocean variability influence each other. The model has been run for the time period September 2008–May 2011, driven by realistic surface momentum and heat fluxes obtained from an atmospheric model and lateral boundary conditions obtained from nesting in a global ocean model. The solution compares favorably to satellite measurements of sea surface temperature and sea surface height, observations of surface currents by high-frequency radars, mooring temperature time series, and glider temperature and salinity sections. The analysis is focused on the seasonal response of the coastal ocean with particular emphasis on the winter circulation patterns which have previously garnered relatively little attention. Interannual variability is examined through a comparison of the 2009–2010 winter influenced by El Nino and the winters in the preceding and following years. Strong northward winds combined with reduced surface cooling along the coast north of Cape Mendocino (40.4N) in winter 2009–2010, resulting in a vigorous downwelling season, characterized by relatively energetic northward currents and warmer ocean temperatures over the continental shelf and upper slope. An analysis of the time variability of the volume-averaged temperature and salinity in a coastal control volume (CV), that extends from 41 to 47N and offshore from the coast to the 200-m isobath, clearly shows relevant integrated characteristics of the annual cycle and the transitions between winter shelf circulation forced by northward winds and the summer circulation driven primarily by southward, upwelling-favorable winds. The analysis also reveals interesting interannual differences in these characteristics. In particular, the CV volume-average temperature remains notably warmer during January–March 2010 of the El Nino winter.