Compound marine cold spells and hypoxic events in a nearshore upwelling system

Prolonged periods of extreme warm and cold seawater temperature anomalies, known as marine heatwaves (MHWs) and marine cold spells (MCSs), respectively, can have significant impacts on coastal marine ecosystems. Prior research has examined the co-occurrence of multiple extremes [low dissolved oxygen (DO) and/or low pH] during MHWs, with the impact of these compound events potentially exceeding that of a single stressor due to synergistic effects. However, we are not aware of any studies that have examined compound extreme events during MCSs, despite the ecosystem implications. Along Eastern Boundary Current Upwelling Systems (EBUS), strong-wind driven upwelling has led to reduced warming trends and persistence of MCSs, as well as an increased seasonal risk for coastal hypoxia via the cross-shelf advection of cold, low DO subthermocline waters. In this short communication, we investigated for the first time compound MCS-hypoxic events at a nearshore site in central California using several decades of nearshore water temperature data, short-term nearshore DO data, and several decades of DO data from the California Cooperative Oceanic Fisheries Investigations program. From 1988 to 2020, we identified 55 MCS events, with 50 events (-90.9%) initiated during anomalously strong upwelling. Using long-term temperature -DO relationships, we identified 20 of the 55 MCS events (-34.6%) as potential compound MCS-hypoxic events, with an average duration of 11.7 days. These compound events occurred almost exclusively during the major upwelling season (March to June) when there is a high propensity for the advection of cold, low DO subthermocline waters from the adjacent shelf into the nearshore. This work provides the first known investigation into the co-occurrence of MCS and hypoxic events in a nearshore upwelling system and provides a baseline for assessing future changes to these compound events in a changing climate, with important implications for ecosystem health in EBUS globally.