Space and time variability of surface currents in Central San Francisco Bay: observations from long-term HF radar measurements

Cassia Pianca, Piero Mazzini

San Francisco Bay (SFB) is the largest estuary along the West Coast of the United States, through which the majority of California runoff drains to the ocean, and is surrounded by a major metropolitan area comprised of the cities of San Francisco, Oakland, San Jose and more.

The outer estuary is a 3-way junction between the northern bay-delta system, the low-inflow south bay, and the ocean. This is known as Central Bay. Circulation and stratification here is a major factor in the exchange between SFB and the ocean, influencing not only intrusions of seawater but also the outflow of low-salinity water over the shelf that impacts ecosystems in the Greater Farallones National Marine Sanctuary.

Monitoring of near-surface currents in SFB began in 2006, funded by a State program – the Coastal Ocean Currents Monitoring Program (COCMP) – and has been continued through support from NOAA through the national Integrated Ocean Observing System (IOOS) and the Central and Northern California Ocean Observing System (CeNCOOS). Several high-frequency (HF) radar stations have been set up in SFB, with long-term data from 5 stations around Central Bay.

These stations operate at 42 MHz providing currents every 30 minutes with a spatial resolution of 500 m that fully resolves the spatio-temporal pattern of the tidal inflow jet and its interaction with the complex topography of Central Bay. These HF-radar data cover a region of approximately 10 km by 12 km. These decade-long, high-resolution measurements in this region allow us an unprecedented opportunity to investigate near-surface circulation over time scales ranging from tidal to interannual, including the effects of changes in runoff, ocean conditions, and local wind forcing.

Animation of (left) surface currents and (right) time series of sea level, temperature and salinity obtained at the EOS pier. (For full resolution use Firefox or Safari browsers).