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 occurring with increasing frequency and intensity, profoundly impacting ocean circulation, biogeochemistry, and marine ecosystems. The MHW known as the Blob, which persisted in the subarctic NE Pacific from 2014 to 2015, significantly affected surrounding ecosystems. Warming-induced solubility reduction is expected to raise the partial pressure of carbon dioxide (pCO₂) in the surface water, causing outgassing of CO₂ to the atmosphere. Outgassing of CO₂ is another source of atmospheric CO₂ in addition to anthropogenic fossil fuel burning. However, moored observations at Ocean Station Papa (OSP; 145° W, 50° N) shows a moderate decrease in oceanic pCO₂ during the Blob, resisting the warming-induced outgassing of CO₂. This response is opposite of what is expected from warming alone, and instead has been attributed to reductions in dissolved inorganic carbon (DIC), although the mechanisms driving this reduction have remained unclear. We employed a regional model that accurately reproduces the temporal variability of oceanic pCO₂ at OSP to investigate the cause of decrease pCO₂ during the Blob. The analysis of model outputs indicates that the observed oceanic pCO₂ decline resulted from the offset between warming-induced solubility reduction (increasing pCO₂) and weakened physical transport of DIC (decreasing pCO₂), with the latter dominating. Both horizontal and vertical transports played important roles. The near-surface carbon budget over the broad region was primarily driven by changes in the vertical transport. The decrease in DIC during the Blob resulted from the suppression of upwelling of DIC-rich subsurface waters in the winter of 2013. In this period, the horizontal transport also contributed substantially to DIC reduction. In particular, at OSP, the effect of the horizontal transport was comparable to that of the vertical transport, reflecting the northward advection of low-DIC water masses. These findings indicate that changes in physical circulation were the primary driver of the moderately enhanced CO₂ uptake observed during the Blob. This study provides a critical insight into the complexity of biogeochemical response to extreme warming events and underscores the importance of resolving physical transport processes in assessing oceanic carbon uptake during MHWs.