Modeling Ocean Circulation and Biogeochemical Dynamics in the Drake Passage
Jersild, Annika Lee
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The Southern Ocean is a critical component of global climate and carbon cycling. Although an overall regional carbon sink, observations indicate uptake behavior fluctuates on seasonal to decadal scales, and there is much uncertainty surrounding the drivers of that variability. Our focus is in the Drake Passage, the narrowest and southernmost constriction of the Antarctic Circumpolar Current and a well sampled sector of the Southern Ocean. We develop a new regional circulation and biogeochemical model based on the MITgcm in order to reproduce seasonal carbon flux patterns and understand the mechanisms that influence seasonal and interannual variability. With a 10km horizontal resolution, which explicitly resolves the small-scale transient movements in the region called mesoscale eddies, we are able to use the model to examine interplay between horizontal and vertical advection, ocean stratification, and biological carbon export influencing surface partial pressure of carbon dioxide. The model reveals that fronts and eddies play prominent roles in setting the spatial pattern, mean state, and variability of the carbon budget. We analyze the drivers of regional carbon flux and determine the carbon sources of gas exchange, vertical entrainment, and mean advection are balanced by the biological pump and mesoscale eddy transfer. We also use a sensitivity study to show that when the mesoscale eddies are suppressed or parameterized, the model is unable to reproduce those seasonal amplitudes and phases. Experiments with suppressed eddy activity demonstrate a significant change to the iron supply and phenology of phytoplankton blooms, and this mismatch in timing and intensity of the bloom causes significant biases in seasonal carbon cycle in the region. These results contribute to improved understanding of mesoscale eddies and biogeochemical cycling in the Southern Ocean. The outcome has implications for decisions on future modeling research requiring seasonal-level analyses in the region, and for the enigmatic biases in partial pressure of carbon dioxide in Earth System Models.