Abstract
The plume of Mississippi and Atchafalaya River system (MARS) is normally confined to the continental shelf and extends westward. During March 2010, while weather in North America was dominated by unusually low temperatures and winter storms in conjunction with large low-pressure systems, we observed an unusually broad extension of the MARS plume. Winds favorable for offshore transport along with high freshwater discharge contributed to a widespread MARS plume, which was statistically different from those normally occurring in spring. A strong relationship was observed between the location of this widespread plume and a reduced partial pressure of carbon dioxide (P-CO2) and elevated satellite-derived chlorophyll a. Furthermore, P-CO2 in the plume was reduced by 6.3 Pa (62 ppm) in March 2010 in contrast to a reduction of only 2.4 Pa in April 2009, thus exhibiting a greater CO2 uptake capacity in March 2010. This wide plume also covered at least twice the area and was a stronger CO2 sink by a factor of five when compared to more typical plume conditions in April 2009. Extending across the Louisiana shelf to the pelagic ocean, this widespread MARS plume encompassed regions that would normally exhibit nearly net zero air-sea CO2 exchange and resulted instead in a strong net sink for atmospheric CO2. Our findings demonstrate that regional weather-associated variations in wind forcing and river discharge may be important in influencing air-sea CO2 fluxes in marginal sea environments.