Adjustment of the coupled ocean–atmosphere system to a sudden change in the Thermohaline Circulation

The adjustment of the coupled ocean–atmosphere system to a fresh water anomaly in the North Atlantic Ocean is investigated using a coupled GCM. In response to the anomaly the Atlantic Thermohaline Circulation (THC) collapses, and the associated reduction in northward heat transport causes sea surface temperatures (SST) to fall over the North Atlantic, and to increase over the tropical South Atlantic. Atmospheric feedbacks are excited as soon as the SST anomalies reach sufficient magnitude. A key stage in the adjustment process occurs 4–6 years after the perturbation is introduced when a significant SST dipole develops in the tropical Atlantic. This dipole causes a southward shift of the ITCZ and leads, in year 7, to the triggering of an El Niño event. It is concluded that atmospheric feedbacks could spread globally the influence of a sudden change in the THC much more quickly and efficiently than could ocean processes alone.