Understanding the resilient carbon cycle response to the 2014–2015 Blob event in the Gulf of Alaska using a regional ocean biogeochemical model

Abstract

Marine heatwaves (MHWs), characterized by anomalously high sea surface temperatures, are increasing in frequency and intensity and strongly impact ocean circulation, biogeochemistry, and marine ecosystems. During the 2014–2015 MHW (commonly called the Blob) in the NE subarctic Pacific, moored observations at Ocean Station Papa (OSP; 145° W, 50° N) showed a moderate decrease in oceanic pCO2, contrary to the increase expected from warming-induced solubility reduction alone. Using a regional model that reproduces the observed pCO2 variability and trend at OSP, we show that this decline resulted from a decrease in dissolved inorganic carbon (DIC) supply that outweighed the warming-driven increase in pCO2. The DIC reduction was primarily caused by weakened vertical transport associated with enhanced upper-ocean stratification and reduced Ekman pumping prior to the onset of the Blob, which suppressed the upwelling of DIC-rich subsurface waters. Horizontal transport also contributed locally, particularly at OSP. These results demonstrate that anomalous physical circulation, rather than biological processes, was the primary driver of the enhanced CO2 uptake during the Blob and highlight the importance of resolving physical transport mechanisms when assessing carbon cycle responses to extreme warming events.