Extremes of Dissolved Oxygen in the California Current System

TL;DR

This study analyzes extreme dissolved oxygen levels in the California Current System using underwater glider data, revealing spatial and seasonal patterns of hyperoxic and hypoxic events linked to biological activity and upwelling.

Contribution

It introduces a novel analysis of dissolved oxygen extremes in the California Current using extensive glider data and identifies key environmental factors influencing these extremes.

Findings

Hyperoxic events occur mainly near-shore and during non-winter months.

Surface hyperoxia is associated with phytoplankton blooms and stratification.

Hypoxia is most frequent in upwelling regions with deep mixing.

Abstract

Dissolved oxygen (DO) is a non-conservative tracer of interactions at the air-sea interface, respiration and photosynthesis, and advection. In this manuscript, we study extremes in the degree of oxygen saturation (SO), the ratio of DO to the maximum concentration given the water's temperature, salinity, and depth with SO=1 critically saturated. We perform the analysis with the California Underwater Glider Network (CUGN), which operates gliders on four lines that extend from the California coast to several hundred kilometers offshore, profiling to 500m depth every 3km. Since ~2017, the gliders have been equipped with a Sea-Bird 63 optode sensor to measure the DO content. We find that parcels with SO>1.1, hyperoxic extrema, occur primarily near-shore in the upper 50m of the water column and during non-winter months. Along Line 90 which originates in San Diego, these hyperoxic events occur primarily in stratified waters with shallow mixed layers. We hypothesize that photosynthesis elevates DO in sub-surface water that can not rapidly ventilate with the surface. Along the three other lines, hyperoxic extrema occur almost exclusively at the surface and are correlated with elevated Chl-a fluorescence suggesting they are primarily driven by blooms of photosynthesis. We also examine hypoxic extrema, finding that parcels with SO<0.9 and z<50m occur most frequently along the northernmost line where upwelling has greatest impact.