Scientific Understanding of the Atlantic Basin Climate System – A Mid- CLIVAR Perspective

1. Introduction The US CLIVAR (Climate Variability and Prediction) Program, now in its eighth year, was designed to investigate the season-to-season, year-to-year, and decade-to-decade fluctuations in the climate. These climate variations influence nearly all spheres of human activity: our clothing, shelter, and energy needs; our ability to grow crops and raise livestock for food and fiber; and our safety from a wide range of natural hazards. Thus, while CLIVAR has a broad focus on understanding the mechanisms of climate variability, it also has a pragmatic focus on climate prediction, particularly of those climate fluctuations that affect humans and their livelihoods. Much, or even most, of climate variability on these inter-seasonal to interdecadal timescales involves interactions between the atmosphere and the oceans. The autonomous behavior of the atmosphere dominates shorter timescales, while interactions with ice sheets and glaciers, changes in the gross properties of the land surface, and astronomical forcing are likely important on much longer timescales. While land surface-atmosphere and cryosphere-atmosphere interactions are probably of some 3 importance on timescales of interest to CLIVAR, its primary focus has been on interactions between the atmosphere and the oceans. At its midpoint, CLIVAR is undergoing a transition from identifying phenomena and determining their underlying mechanisms, towards using this knowledge to make probabilistic climate predictions on seasonal time scales and longer, and towards accurately assessing climate-related risks and probabilities. During its first half, as CLIVAR sought to identify and understand the key relevant phenomena and processes of atmosphere-ocean interaction, the science was organized around regions of the globe, in particular, around specific ocean basins. For the Atlantic basin, a small number of key phenomena were identified: coupled atmosphere-ocean variability in the tropical Atlantic, the North Atlantic Oscillation, and the large-scale meridional overturning circulation. Thus, the business of the US CLIVAR program in the Atlantic (Atlantic CLIVAR, for short) can largely be summed up with three acronyms, TAV (Tropical Atlantic Variability), NAO (North Atlantic Oscillation), and MOC (Meridional Overturning Circulation). To these three acronyms we must add one more, ENSO (El Niño – Southern Oscillation), for while ENSO originates in the tropical Pacific Ocean, it strongly influences the Atlantic basin, especially in the tropics. It is, in fact, the complex interplay between externally forced variability, especially from ENSO, and that generated by internal processes that makes Atlantic-basin climate an especially challenging subject of study. 4 Building on what has been learned during its nominal …