High pCO2 conditions in San Francisco Bay originate from coastal upwelling from the California Undercurrent

Megan M Rutkowski, University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, PA, United States, James W Murray, University of Washington Seattle Campus, School of Oceanography, Seattle, WA, United States, Mary K Miller, Exploratorium, San Francisco, CA, United States, Adrienne J Sutton, University of Washington, Joint Institute for the Study of the Atmosphere and Ocean, Seattle, WA, United States and Christopher L Sabine, NOAA Seattle, Seattle, WA, United States
Abstract:
The eastern boundary upwelling system off the west coast of North America is more acidic than the open ocean. High NO3, high pCO2 conditions exist in the Salish Sea driven by upwelling of low O2, high NO3 and high pCO2 water from the California Undercurrent (CU). Similar conditions exist in San Francisco Bay. Scientists from NOAA and the USGS collected a time series of data for temperature, salinity, oxygen and pCO2 from March 2013 to June 2015 at the Exploratorium Wired Pier in San Francisco. pCO2 ranged from 550 to 1000 μatm and was always higher than atmospheric values. There was an inverse relationship between salinity, O2 and pCO2. pCO2 increased when O2 decreased and salinity increased. These properties are due to upwelling from the CU. The main process is natural aerobic respiration in the region where the CU originates, which is the oxygen minimum zone in the eastern tropical North Pacific. The properties in San Francisco Bay can be compared with properties in the CU south and north of San Francisco Bay, collected during Wecoma Cruise W0705A. These sections show that the core of the CU is located at the shelf break at depths of 175m to 250m. This water was last at the sea surface at approximately 50°S; 150°W in the west-central South Pacific and reached the US west coast via the Pacific Equatorial Water (PEW). As the CU flows poleward along North America, it mixes laterally with colder, fresher, younger Pacific Subarctic Upper Water (PSUW) which outcrops in the NW North Pacific (near 50°N, 170°E). The contribution of the low O2 PEW southern end-member decreases poleward and off the central California coast the CU has nominal contributions of 50% PEW and 50% PSUW. The anthropogenic “ocean acidification” contribution to DIC in the source waters of the California Undercurrent is presently ≤ 20% of the total increase in DIC, but it will increase in importance in the future. The remaining increase in DIC is due to natural aerobic respiration.
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