Similarities between the tropical Atlantic seasonal cycle and ENSO: An energetics perspective

The tropical Pacific and Atlantic Oceans have similar mean states - easterly winds and a zonally sloping thermocline which shoals in the east - but strikingly different sea surface temperature variability. Seasonal and interannual variations have comparable amplitudes, and correspond to different modes of oscillation in the Pacific. In the Atlantic, the seasonal cycle is dominant and has properties of both those modes. The Bjerknes feedback between the wind and SST, and its associated delayed, negative feedback constitute a free (interannual) mode of the Pacific, but in the Atlantic influence the seasonal cycle. This difference between the two oceans is attributable to the smaller dimensions of the Atlantic. An energetic analysis shows a circular relationship between available potential energy and the work done by the wind from April until September in the Atlantic. This energetics perspective suggests that a seasonally excited thermocline mode of coupled variability plays an active role in the seasonal cycle during these months, after which point seasonal forcing regains control.