Seasonal to interannual variabilities of sea–air CO 2 exchange across Tropical Maritime Continent indicated by eddy–permitting coupled OGCM experiment

The lack of long–term observational data has limited research on sea–air CO 2 exchange variabilities in the 10 Tropical Maritime Continent (TMC). To address the issue, we utilized a three–dimensional high–resolution physical– biogeochemical ocean numerical model and applied it to simulate sea–air CO 2 exchange in the region over the last decade (2010–2019). Some key features like atmospheric CO 2 source signature and high sea surface pCO 2 environment inside the TMC were captured by the model. Within the TMC, model results indicated strong CO 2 degassing along the south of Java associated with the seasonal cycle of the upwelling system. Abundant supply of inorganic carbon during upwelling season 15 and strong wind speed results in CO 2 degassing that could reach as high as 30 gC m –2 year –1 around the area. In addition to the region acting as a full–year atmospheric CO 2 source, the TMC also exhibited interannual modulation in both sea–air CO 2 flux and sea surface pCO 2 which can be related to the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Large–scale anomalous strong CO 2 degassing and high sea surface pCO 2 from 2015 to 2016 in response to the 2015/2016 El Niño evolution was observed and dominated by modulation within the TMC. It is further found that 20 modulation of CO 2 degassing related to IOD were confined along the west of south of Java with a higher magnitude derived from empirical model (Iida et al. 2021) and machine learning (Landschützer et al., 2016; not shown) with 1° × 1° horizontal resolution did not exhibit such contrasting differences. Excess alkalinity in the sea surface, calculated as difference between total alkalinity and DIC, from simulation indicates that there is a considerable gradient between the TMC and surrounding open ocean which corresponds to higher sea surface pCO 2 inside the TMC compared 195 with open ocean. Modeled excess alkalinity in the open ocean were still lower compared with reconstruction in Iida et al (2021) which may explain the higher–than–observed sea surface pCO 2 in the open ocean. Indo–Pacific climatic forcing over the 2010–2019 period along with its possible mechanism from sea–air interaction perspective. This has never been done before considering the aforementioned limitations. Generally, the CO 2 degassing anomalies showed in–phase relationship with both IOD and ENSO (i.e., positive sea–air CO 2 flux anomalies during positive phase of IOD or ENSO, and 395 vice versa ). The ENSO tends to induce larger scale of sea–air CO 2 flux modulation while the IOD showed confined influence around South of Java but with higher magnitude of modulation. It was further suggested that the latest extreme climate event such as the 2015/2016 El Niño and 2019 pIOD were responsible for slowing the secular trend in the CO 2 degassing to atmosphere from the region. While results presented may provide insight about sea-air CO 2 exchange variabilities in TMC, it can be utilized also to invite interdisciplinary research collaborations regarding establishment of a 400 continuous sea–air CO 2 exchange monitoring system across the region and enrich our understanding of its dynamics under changing environments.