Seasonal advection of Pacific Equatorial Water alters oxygen and pH in the Southern California Bight

Abstract:

Chemical properties of the California Undercurrent (CU) have been changing over the past several decades, yet the mechanisms responsible for the trend are still not fully understood. We present a survey of temperature, salinity, O₂, pH, and currents at intermediate depths (defined here as 50–500 m) in the summer (June 30 to July 10) and winter (December 8 to 15) of 2012 in the southern region of the Southern California Bight. Observations of temperature, salinity, and currents reveal that local bathymetry and small gyres play an important role in the flow path of the California Undercurrent (CU). Using spiciness (π) as a tracer, we observe a 10% increase of Pacific Equatorial Water (PEW) in the core of the CU during the summer versus the winter. This is associated with an increase in π of 0.2, and a decrease in O₂ and pH of 30 µmol kg^-1 and 0.022, respectively; the change in pH is driven by increased CO₂, while total alkalinity remains unchanged. The high-π, low-O₂, low-pH waters during the summer are not distributed uniformly in the study region. Moreover, mooring observations at the edge of the continental shelf reveal intermittent intrusions of PEW onto the shelf with concomitant decreases in O₂ and pH. We estimate that increased advection of PEW in the CU could account for approximately 50% of the observed decrease in O₂, and between 49 and 73% of the decrease in pH, over the past three decades.