Mesoscale structure and its seasonal evolution in the northern California Current System

Abstract

Abstract During spring and summer 2000, two mesoscale mapping cruises were carried out in the northern California Current System between 41 . 9 ∘ N and 44 . 6 ∘ N and between the shallow, inner continental shelf and up to 150 km offshore. Measurements were made using a towed undulating vehicle equipped with a conductivity-temperature-depth instrument and a chlorophyll fluorometer. A shipboard acoustic Doppler current profiler (ADCP) measured water velocities, and surface drifter trajectories and satellite sea-surface temperature imagery provide context for the mesoscale maps. Nearly identical upwelling favorable wind stresses of up to 0.2 N m - 2 existed during both the spring and summer surveys. Early in the season (late May, early June) the upwelling front and jet followed the continental shelf bottom topography. There was cold water inshore of the shelfbreak all along the coast with pockets of elevated phytoplankton biomass (chlorophyll (chl) up to 4 mg m - 3 ) near the coast. Mesoscale activity was minimal. During late-summer (August), the upwelling front and jet were much more convoluted, including significant meanders offshore associated with a major submarine bank (Heceta Bank, 44.0– 44 . 6 ∘ N ) and a large coastal promontory (Cape Blanco, 42 . 8 ∘ N ). High levels of phytoplankton biomass were found over Heceta Bank (chl ∼ 20 mg m - 3 ) and near the coast south of Cape Blanco (chl ∼ 10 mg m - 3 ). Low velocities inshore of Heceta Bank, measured by both shipboard ADCP and surface drifters, indicate the potential for retention of water over the Bank, leading to favorable conditions for phytoplankton biomass accumulation. The large offshore meander near Cape Blanco carried cold, nutrient-rich, high phytoplankton biomass (chl of 2– 5 mg m - 3 ) water over 100 km offshore. This flow–topography interaction feature was generated in mid-June and remained an important part of the regional circulation for about 2.5 months.