How ocean circulation and its atmospheric forcing are linked to global ocean carbon storage in warm climates simulated with CESM2

Abstract:

Ocean carbon storage is an important component in the climate system. Together with the atmospheric and terrestrial carbon reservoirs, the ocean circulates carbon on timescales that are relevant for decadal to millennial climate response. As the largest of the three carbon reservoirs, it is central to our understanding of the global climate system.

In a 12-member ensemble of climate states in the intermediate complexity model cGENIE, we found that the global ocean carbon storage decreases linearly with increased strength of the global ocean overturning circulation. The relationship is strongest between total carbon and circulation, while biological and physical components of the ocean carbon cycle respond to changes in circulation strength in a less predictable manner. Here we revisit those conclusions using the high-complexity CESM2 (Community Earth System Model 2) and a range of past and future warm climates from CMIP6 (lig127ka, midHolocene, historical, 1pctCO2, abrupt-4xCO2, ssp5-8.5). Specifically, we investigate the response in ocean carbon storage to changes in global overturning circulation, as the climate warms compared to pre-industrial. We find that the natural capacity for ocean carbon storage decreases as the climate gets warmer. In some simulations, we simultaneously see a weakening of the ocean overturning circulation, which is opposite to the response in cGENIE. We tentatively conclude that this difference can be at least partially attributed to some CESM2 experiments being transient simulations, while the cGENIE ensemble consists of equilibrium simulations. To further understand the transient response of the global ocean carbon storage to global changes in circulation, we investigate the correlation between circulation and individual carbon cycle components, as well as changes in atmospheric forcing (wind stress, precipitation), which influence air-sea gas exchange and ocean circulation.