The interaction of alongshore coastal currents with large headlands has been shown to increase the retention of planktonic organisms through the formation of headland eddies or upwelling shadows in their lee. This study investigates the circulation within Bodega Bay (in the lee of a small headland), in an upwelling region, and the potential for retention of plankton. During the upwelling season of 2004, time series of temperature and velocity were recorded throughout Bodega Bay, conductivity-temperature-depth (CTD) surveys were conducted, and surface drifters were released during upwelling, downwelling, and relaxation conditions. Postlarval settlement was monitored daily over two periods coinciding with CTD surveys. Under strong upwelling favorable conditions, wind-driven surface currents were equatorward both offshore and throughout the bay. However, there was significant current shear along the eastern shore of the bay where cold bottom waters move poleward, counter to the direction of the wind-driven surface flow. During downwelling and relaxation conditions, flow was poleward throughout the water column along the eastern shore of the bay. Postlarvae settled during all wind conditions, but greatest settlement was observed at the onset of upwelling favorable conditions. While no “typical” upwelling shadow is evident in the lee of the Bodega headland, subsurface recirculation driven by the alongshore flow past Bodega Head may facilitate the retention of plankton in the bay. Previous studies have generally focused on large headlands; however, it is likely that other small embayments in the lee of small headlands may also provide retention opportunities for planktonic organisms in upwelling regions.

The paradox of upwelling is the relationship between strong wind forcing, nutrient enrichment, and shelf productivity. Here we investigate how across-shelf structure in velocity and hydrography plays a role in the retention (inshore) and export (offshore) of particles such as nutrients, plankton and larvae. We examine the spatial structure of the coastal currents during wind-driven upwelling and relaxation on the northern Californian Shelf. The field work was conducted as part of the Wind Events and Shelf Transport (WEST) project, a 5-year NSF/CoOP-funded study of the role of wind-driven transport in shelf productivity off Bodega Bay (northern California) from 2000 to 2003. We combine shipboard velocity profiles (ADCP) and water properties from hydrographic surveys during the upwelling season to examine the mean acrossshelf structure of the hydrography and velocity fields during three contrasting upwelling seasons, and throughout the upwelling-relaxation cycle. We also present results from two winter seasons that serve as contrast to the upwelling seasons. During all three upwelling seasons clear spatial structure is evident in velocity and hydrography across the shelf, exemplified by current reversals inshore and the presence of a persistent upwelling jet at the shelf break. This jet feature changes in structure and distance from the coast under different wind forcing regimes. The jet also changes from the north of our region, where it is a single narrow jet, adjacent to the coast, and to the south of our region, where it broadens and at times two jets become evident. We present observations of the California Under Current, which was observed at the outer edge of our domain during all three upwelling seasons. The observed across-shelf structure could aid both in the retention of plankton inshore during periods of upwelling followed by relaxation and in the export of plankton offshore in the upwelling jet.

Historical records of near-surface water temperatures in the Southern Californian Bight often show a preferential cooling in the lee of headlands such as Point Dume, Palos Verdes, and Point Loma. At times, this cooler water is associated with an increase in chlorophyll-a as is evident in satellite images of ocean color from the region. Here we combine hydrographic data from a 1 day cruise aboard the RV Roger Revelle (a precursor to the 0304 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise) with high-frequency (HF) radar (CODAR) measurements, satellite images, and long-term thermistor records of near-surface temperature to identify a small-scale, isolated upwelling in the lee of Point Loma (32.5°N). Associated with the more saline water downstream of the headland are higher nutrient concentrations, an increase in chlorophyll-a concentration, and a bloom of chain-forming diatoms, indicative of a mature upwelling system. It is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the Point Loma headland. Time series of surface vorticity calculated from HF radar measurements of sea surface velocity show that as the flow separates from the headland, relative vorticity increases offshore of the cape. Inshore, the time series of divergence/convergence shows a tendency toward divergence at the surface, indicating a preferential upwelling which appears to raise the thermocline, thus resulting in a flux of cold nutrient-rich water to the surface. In the presence of high nutrients and light, photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents.

[ 1] Observations from an intensive oceanographic field program which took place in 1998 - 1999 about the separation point of the East Australian Current (EAC) show significant spatial and temporal variability of the EAC. Upstream of the separation point, southward flowing currents are strong, with subinertial velocities of up to 130 cm s(-1) in the near-surface waters, whereas downstream currents are highly variable in both strength ( 1 - 70 cm s(-1)) and direction. Upwelling is observed to occur through both wind-driven and current-driven processes, with wind effects playing a lesser role. By contrast, the encroachment of the EAC upon the coast has a profound effect on the coastal waters, accelerating the southward (alongshore) currents and decreasing the temperature in the bottom boundary layer (BBL) by up to 5 degreesC. As the axis of the jet moves onshore, negative vorticity increases in association with an increase in nonlinear acceleration. During this time, bottom friction is increased, the Burger number is reduced, and the BBL shut-down time lengthens. The observed upwelling is attributed to enhanced onshore Ekman pumping through the BBL resulting from increased bottom stress as the southerly flow accelerates when the EAC encroaches across the continental shelf.

The "Wind Events and Shelf Transport" (WEST) program was an interdisciplinary study of coastal upwelling off northern California in 2000-03. WEST was comprised of modeling and field observations. The primary goal of WEST was to better describe and understand the competing influences of wind forcing on planktonic productivity in coastal waters. While increased upwelling-favorable winds lead to increased nutrient supply, they also result in reduced light exposure due to deeper surface mixed layers and increased advective loss of plankton from coastal waters. The key to understanding high levels of productivity, amidst these competing responses to wind forcing, is the temporal and spatial structure of upwelling. Temporal fluctuations and spatial patterns allow strong upwelling that favors nutrient delivery to be juxtaposed with less energetic conditions that favor stratification and plankton blooms. Observations of winds, ocean circulation, nutrients, phytoplankton and zooplankton off Bodega Bay and Point Reyes (38 degrees N) were combined with model studies of winds, circulation and productivity. This overview of the WEST program provides an introduction to the WEST special issue of Deep-Sea Research, including the motivation for WEST, a summary of study components, an integrative synthesis of major research results to-date, and background on conditions during field studies in May-June 2001 (the upwelling period on which this special issue is focused). (c) 2006 Elsevier Ltd. All. rights reserved.