Spatiotemporal variability of air-sea CO fluxes in response to El Niño-related marine heatwaves in the tropical Pacific Ocean.

The tropical Pacific is the largest oceanic source of carbon dioxide (CO) emissions, where persistent marine heatwaves (MHWs) frequently occur. During persistent MHW events which are associated with strong El Niño events, CO outgassing is notably reduced, however, its detailed spatiotemporal response to MHWs has not been fully characterized. In this study, we showed a high degree of consistency between CO source regions in the central and eastern tropical Pacific Ocean and the occurrence regions with average annual MHW days exceeding 45 days (co-occurring area covers 80% of the area where MHWs occur). The spatiotemporal variability of the air-sea CO flux on interannual and longer timescales can be reconstructed from annual MHW days and occurrence frequency, respectively, in the central and eastern Pacific Ocean of the co-occurring region. In this region, El Niño-related MHWs reduce the air-sea CO flux density up to 0.4-0.8 molC/m/yr per 100 MHW days, corresponding to a reduction of CO emissions by approximately 0.1 PgC per 100 MHW days. This is a 10%-40% reduction in CO emissions during MHW periods, with the strongest impact (30%-40% CO emission reduction) in the equatorial Pacific (5°S-5°N) of the central and eastern Pacific Ocean. In contrast, air-sea CO flux variations in coastal eastern upwelling region of the co-occurring region are mainly subjected to seasonal mixed layer variations, and thus not notably affected by El Niño-related MHWs on interannual timescales. By establishing the reproducibility between MHWs and air-sea CO flux variations, our results pave a way for detailed future spatiotemporal evolutions of MHW-induced changes in air-sea CO flux in the tropical Pacific Ocean.