Ocean Forcing of San Francisco Bay: Intrusion of Upwelled Water

Ocean forcing is a critical factor in how west-coast estuaries will respond to climate change both in the US and globally. During periods where ocean forcing is dominant, variability in coastal upwelling on the shelf may control water quality and circulation variability within adjacent estuaries. Further, with climate change it is expected that waters upwelled onto the shelf will become more acidic and also more hypoxic, thus intrusion of newly upwelled waters can introduce stress to estuarine ecosystems. As a stratified bottom layer, deep upwelled waters can intrude prior to meaningful exposure to the atmosphere and near-surface light and thus deliver high-nutrient, hypoxic and/or acidic water far inside estuaries. The “reach of the ocean” has been used to refer to the spatial extent of ocean influence in estuaries, but the actual extent is not well known as it depends on the interaction between water flow patterns and biogeochemical reaction rates, which in turn depend on several other factors (e.g., stratification and light exposure). Our study focuses on San Francisco Bay, adjacent shelf waters, and comparable bays (e.g., Tomales Bay), where we aim to observe, quantify and understand the controls on intrusion and the influence of upwelled waters. Using a combination of time-series moorings and boat-based spatial surveys of water properties and flow structures of both shelf and San Francisco Bay waters, we were able to better understand the processes that account for the presence of deeper upwelled waters outside the mouth of San Francisco Bay, and those that may either allow or block the intrusion of these deep upwelled waters into the Bay.