A Strong Atlantic Meridional Mode Event in 2009: The Role of Mixed Layer Dynamics

In the first half of 2009, anomalous cooling of sea surface temperatures (SSTs) in the equatorial North Atlantic (ENA; 2 degrees-12 degrees N) triggered a strong Atlantic meridional mode event. During its peak in April-May, SSTs in the ENA were 1 degrees C colder than normal and SSTs in the equatorial South Atlantic (5 degrees S-0 degrees) were 0.5 degrees C warmer than normal. Associated with the SST gradient were anomalous northerly winds, an anomalous southward shift of the intertropical convergence zone, and severe flooding in Northeast Brazil. This study uses in situ and satellite observations to examine the mechanisms responsible for the anomalous cooling in the ENA during boreal winter and spring of 2009. It is found that the cooling was initiated by stronger than normal trade winds during January and February 2009 associated with an anomalous strengthening of the subtropical North Atlantic high pressure system. Between 6 degrees and 12 degrees N, unusually strong trade winds cooled the ocean through wind-induced evaporation and deepened the mixed layer anomalously by 5-20 m. Closer to the equator, surface equatorial winds responded to the anomalous interhemispheric SST gradient, becoming northwesterly between the equator and 6 degrees N. The anomalous winds drove upwelling of 0.5-1 m day(-1) during March-April, a period when there is normally weak downwelling. The associated vertical turbulent heat flux at the base of the mixed layer led to unusually cool SSTs in the central basin, further strengthening the anomalous interhemispheric SST gradient. These results emphasize the importance of mixed layer dynamics in the evolution of the meridional mode event of 2009 and the potential for positive coupled feedbacks between wind-induced upwelling and SST in the ENA.