Studying the inter-annual variability of the Canary Current Upwelling System with ROMS

Steven Herbette (1), Patrick Marchesiello(2), Leo Nykjaer(1)

(1) JRC/IES, Ispra, Italy
(2) IRD, Brest, France

An unusually strong upwelling event with very low sea surface temperatures was observed throughout the Southern California Bight in March 2002. About one week after the peak of the upwelling, very high concentrations of surface chlorophyll were measured by SeaWiFS. The temporal evolution of the upwelling was recorded by the Santa Monica Bay Observatory mooring in detail.

The upwelling was driven by persistently strong along-shore winds acting upon weakly stratified waters. At the mooring site, water from depths of over 80 m was upwelled to the surface, leading to a drop of about 3.5C within only two days. The injection of new nutrients into the euphotic zone led to the highest surface chlorophyll concentrations recorded by SeaWiFS since September 1997.

We have modeled this upwelling event with ROMS and an NPDZ-type ecosystem/biogeochemical model. The simulation was performed with a 4-level embedded model with horizontal resolutions of 20 km, 6.6 km, 2.2 km, and 700 m, and 40 vertical layers. The 20 km grid encompasses the entire US West Coast from Baja California to the Canadian border, and extends about 1200 km offshore. The finest grid covers Santa Monica Bay and San Pedro Basin. The model was forced with a blended wind product that combines MM5 and ETA model results at resolutions of up to 2 km.

The modeling results show the same patterns as the observations, both in its temporal and spatial evolution of surface temperatures and chlorophyll concentrations. We will use our model to estimate the contributions of advected and locally grown phytoplankton in Santa Monica Bay and further investigate the reasons why this upwelling event was so particularly strong.