The ocean absorbs roughly 30% of the annual anthropogenic emissions of carbon dioxide (CO2). This uptake produces ocean acidification, which can have severe consequences for marine organisms and also reduces the ability of the ocean to uptake more CO2. The amount of CO2 absorbed by the ocean varies across the globe and is modulated by several interrelated processes. Sea-air exchange of CO2 depends primarily on the difference of partial pressures of CO2 between the air and the sea surface and on the wind speed. Biological processes also condition this exchange via photosynthesis and respiration. Oceanic circulation drives the redistribution of the surface ocean carbon and its vertical exchange with the deep ocean, thus also affecting the ocean-atmosphere carbon fluxes through changes in surface ocean content. Altogether, these processes imprint spatial, seasonal and interannual variability on the marine carbon system.
A recent study published in the journal Nature Scientific Reports used high-frequency multi-year data at three locations identified as climate change hotspots. Two of these IMOS moorings are located close to South Pacific boundary currents (the Leeuwin Current at the Kangaroo Island site, and the East Australian Current at Maria Island site) and the Southern Ocean Time Series mooring is located in the Subantarctic Zone (SAZ).
The authors identified and investigated the main drivers involved in the seasonal an interannual (2012–2016) variability of the carbon system at the three locations.
The seasonal variability at the boundary current sites is temporally different and highly controlled by sea surface temperature. Advection processes also play a significant role on the monthly changes of the carbon system at the western boundary current site (Maria Island).
Changes in volume transport, poleward extension and warming rates at oceanic boundary currents have been linked to shifts in climate regimes over large areas of the ocean. The interannual variability at the sites in the study most likely responds to long-term variability in oceanic circulation ultimately related to climatic indices such as the El Niño Southern Oscillation, the Pacific Decadal Oscillation and the Southern Annular Mode (SAM).
The warming associated with boundary currents, which is expected to increase in future scenarios, would consequently have a relevant effect on the carbon system of these and other similar regions. The authors state that the most probable change will be a reinforcement of the increase in the amplitude of the seasonal variability that has been observed for more than 30 years. However, the time series data needs to be extented, by maintaining long-term high-frequency oceanic observations at the study sites, to better understand the response of the marine carbon system to the atmospheric CO2 increase.
In the SAZ, advection and entrainment processes drive most of the seasonality, augmented by the action of biological processes in spring. Given the relevance of advection and entrainment processes at SAZ, the interannual variability is most probably modulated by changes in the regional winds linked to the variability of the SAM.
To read the full paper: https://www.nature.com/articles/s41598-019-44109-2