Sack Lunch Seminar (SLS)

Sea surface temperature in the central-eastern equatorial Atlantic has a seasonal cycle far bigger than that of the Pacific, but inter-annual anomalies smaller than those of the Pacific. Given the amplitude of seasonal SST variability, one wonders whether the seasonal cycle in the Atlantic is so dominant that it is able to strongly influence the evolution of its inter-annual variability. In this study, inter-annual upper-ocean variability within the tropical Atlantic is viewed from an energetics perspective, and the role of ocean dynamics, in particular the role of ocean memory, within Atlantic zonal mode events (also referred to as Altantic Nino events) is investigated. Unlike in the Pacific where seasonal and inter-annual variability involve distinctly different processes, the results suggest that the latter is a modulation of the former in the Atlantic, whose seasonal cycle has similarities with El Nino and La Nina in the Pacific. The ocean memory mechanism associated with the zonal mode appears to operate on much shorter time scales than that associated with the El Nino–Southern Oscillation, largely being associated with inter-annual modulations of a seasonally active delayed negative feedback response. Differences between the El Nino–Southern Oscillation and the zonal mode can then be accounted for in terms of these distinctions. Anomalous wind power over the tropical Atlantic is shown to be a potential predictor for zonal mode events. However, because zonal mode events are due to a modulation of seasonally active coupled processes, and not independent processes operating on inter-annual time scales as seen in the Pacific, the lead time of this potential predictability is limited.