Seasonal and Interannual Variability in the California Current System over the Washington Continental Slope observed with Seagliders

Nearly continuous Seaglider repeat sections over the Washington continental slope since August 2003 reveal seasonal and interannual patterns of circulation, oxygen and phytoplankton distribution. Structure in the upper 1 km of the water column has been resolved to ~5 km along sections completed roughly fortnightly using Seaglider long-range autonomous underwater vehicles. More than 11,000 profiles of temperature, salinity, dissolved oxygen, chlorophyll fluorescence and optical backscatter on over 25,000 km of survey track have been collected in 5 years along a pair of sections normal to the coast. Both seasonal and interannual signals are prominent in the observations. Isopycnal tilts delineating the equatorward California Current offshore and the poleward inshore countercurrent reverse seasonally. Alongshore current fluctuations as well as property surfaces tend to propagate offshore near the speed of a gravest mode long Rossby wave. Annual signals are nearly oppositely phased from the continental shelf edge to the end of the survey track, 220-240 km offshore. Interannual differences in water type and depth averaged transport in the upper 1 km are apparent. Alongshore transport variations are strongly interannual, amounting to a few Sverdrups poleward or equatorward in different years. A particularly large equatorward transport pulse was observed over a several month episode centered on January 2007, where average transports exceeded 10 Sverdrups with a few fortnightly estimates exceeding 20 Sverdrups.