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Marine heatwaves (MHWs) are extremely warm ocean temperature events that significantly affect marine environments, but their effects on the coastal carbonate system are still uncertain. In this study, we systematically quantify MHWs' impacts on air‐sea carbon dioxide (CO2) flux anomalies (FCO2′) in the Mid‐Atlantic Bight (MAB) and South Atlantic Bight (SAB) from 1992 to 2020. During the longest MHW in both regions, oceanic CO2 uptake capabilities substantially decreased, primarily due to significant increases in the seawater partial pressure of CO2 (pCO2sea). For all cases, MHWs played a more significant role in driving pCO2sea changes in the MAB than the SAB, where non‐thermal drivers dominated pCO2sea variability. In the MAB, weakened wind speeds related to wintertime atmospheric perturbations increase ocean temperatures and pCO2sea, further reducing CO2 uptake during winter MHWs. This work is the first to connect extreme temperatures to coastal air‐sea CO2 fluxes. The reduction in CO2 absorption noted during MHWs in this study has important implications for coastal regions to act as continued sinks for excess CO2 emissions in the atmosphere. Plain Language Summary: The transfer of carbon dioxide (CO2) between the atmosphere and ocean is sensitive to sea surface temperature (SST) changes because warmer SSTs increase the sea surface partial pressure of CO2 and reduce the ocean's ability to absorb CO2 from the atmosphere. It is, therefore, conceivable that marine heatwaves (MHWs), which are extremely warm ocean temperature events, could modify how carbon moves between the ocean and the atmosphere. This study provides the first attempt to evaluate the impacts of MHWs on the air‐sea CO2 flux (FCO2) anomalies along the US East Coast, encompassing the Mid‐Atlantic Bight (MAB) and South Atlantic Bight (SAB) during 1992–2020. Both regions experienced reduced CO2 absorption in response to the longest MHWs in each region. These extreme temperatures had a larger impact on CO2 absorption in the MAB compared to the SAB, where non‐temperature factors were more influential. The coastal ocean plays an important role in helping to mitigate human‐induced climate change by absorbing excess CO2 from the atmosphere. As such, the demonstrated reduced absorption of the ocean associated with MHWs in this study, which might also apply to other coastal locations, has vital implications for the efficiency of the ocean in offsetting global warming impacts. Key Points: Marine heatwaves (MHWs) primarily generated positive sea surface pCO2 (pCO2sea) anomalies in the Mid‐Atlantic Bight (MAB) and South Atlantic Bight (SAB) but had a larger impact on air‐sea CO2 flux anomalies in the MABReduced wind speeds amplified MHW contributions during CO2 sink months and counteracted them during CO2 source monthsIn the MAB, wintertime atmospheric perturbations related to zonal shifts in the jet stream produce slower wind speeds which aid in generating air‐sea heat flux type MHW events that ultimately reduce oceanic CO2 uptake [ABSTRACT FROM AUTHOR]

A proxy named 'upwelling age', defined as the ratio of wind time scale to 'advection time', was developed to quantify the local tendency for coastal upwelling. The formulation of the 'advection time' was derived from Ekman theory. A 3D numerical model was used to validate and refine the theoretical formula by simulating a total of 30 cases of different bottom topographies and wind stresses. The results agree reasonably well with the theoretical formulation although some modifications are necessary. The final formulation of the 'advection time' was parameterized as a function of pycnocline depth, bottom slope, and wind stress. [ABSTRACT FROM AUTHOR]