Mechanisms and drivers controlling spatio-temporal evolution of pCO2 and air-sea CO2 fluxes in the southern Java coastal upwelling system

Abstract

Upwelling systems are known for their complex dynamic processes spanning a wide variety of spatio-temporal scales, due to strong coupling between the ocean and atmosphere. One such upwelling system is found at the eastern boundary of the Indian Ocean along the southern coast of Java during the southeast monsoon (SM). This study examines the mechanisms and drivers involved in the spatial and temporal evolutions of surface pCO2 in this upwelling system over seasonal to inter-annual timescales using a coupled, high-resolution regional model. A decomposition analysis was used to quantify the changes in pCO2 in response to changes in surface-temperature (T), surface-salinity (S), dissolved inorganic carbon (DIC), and total alkalinity (ALK). The upwelling of deeper, cooler waters decreases the surface pCO2 by 50 ± 1.48 μatm from June to September. However, the presence of DIC-rich waters at significantly shallower depths increases the surface pCO2 by 27 ± 0.9 μatm. The surface pCO2 increases by 5 ± 0.50 μatm due to changes in salinity, which exerts less control than T-driven changes. Biological changes increase the surface pCO2 by 17.54 ± 2.74 μatm. The maximum seasonal amplitudes of the variations in pCO2 caused by seawater solubility and biology are in counterbalance during the SM. Therefore, the upwelling-driven combined effect of physical mechanisms dominates the effect of biological mechanisms in reducing surface pCO2 under these conditions. During the SM, the southern coast of Java acts as a seasonal CO2 sink (−1.36 ± 6.48 g C m−2 year−1), although consideration of the mean flux over 2006–2017 shows that the region acts as a source of CO2 (1.31 ± 5.03 g C m−2 year−1). Furthermore, following a negative Indian Ocean Dipole (IOD) year, a strong sink of atmospheric CO2 is observed during the SM.