Surface Waves as Major Controls on Particle Backscattering in Southern California Coastal Waters

Satellite observations of particle loads and optical backscattering coefficients (b_bp) in the Southern California Bight (SCB) have been thought to be driven by episodic inputs from storm runoff. Here we show however that surface waves have a larger role in controlling remotely sensed b_bp values than previously considered. More than 14 years of 2-km resolution SeaWiFS, MODIS and MERIS satellite imagery spectrally-merged with the Garver-Siegel-Maritorena bio-optical model were used to assess the relative importance of terrestrial runoff and surface wave forcings in determining changes in particle load in the SCB. The space-time distributions of particle backscattering at 443nm and chlorophyll concentration estimates from the model were analyzed using Empirical Orthogonal Function analysis, and patterns were compared with several environmental variables. While offshore values of b_bp are tightly related to chlorophyll concentrations, as expected for productive Case-1 waters, values of b_bp in a ~10km band near the coast are primarily modulated by surface waves. The relationship with waves holds throughout all seasons and is most apparent around the 40m isobath, but extends offshore until about 100m in depth. Riverine inputs are associated with elevated b_bp near the coast mostly during the larger El Nino events of 1997/1998 and 2005. These findings are consistent with bio-optical glider and field observations from the Santa Barbara Channel taken as part of the Santa Barbara Coastal Long-Term Ecological Research and Plumes and Blooms programs. The implication of surface waves determining b_bp variability beyond the surf zone has large consequences for the life cycle of many marine organisms, as well as for the interpretation of remote sensing signals near the coast.