Pantropical climate interactions
ENSO fluctuates between warm events (El Niño) and cold events (La Niña). These events force changes in the Atlantic and Indian Oceans than can feed back onto the Pacific. Indian Ocean variations. ENSO events also exhibit considerable diversity in their amplitude, spatial structure, and evolution, which matters for how they affect global climate. Sea surface temperature variations in the equatorial and north tropical Atlantic can significantly contribute to the diversity of these events. In addition, tropical interbasin linkages vary on decadal time scales. Warming during a positive phase of Atlantic Multidecadal Variability over the past two decades has strengthened the Atlantic forcing of the Indo-Pacific, leading to an unprecedented intensification of the Pacific trade winds, cooling of the tropical Pacific, and warming of the Indian Ocean. The Indo-Pacific temperature contrast further strengthened the Pacific trade winds, helping to prolong the cooling in the Pacific. These interactions forced from the tropical Atlantic were largely responsible for the recent hiatus in global surface warming. Changes in Pacific mean-state conditions during this hiatus also affected ENSO diversity considerably.
There is tremendous potential for improving seasonal to decadal climate predictions and for improving projections of future climate change in the tropics though advances in our understanding of the dynamics that govern interbasin linkages. The role of the tropical Atlantic, in particular, requires special attention because all climate models exhibit systemic errors in the mean state of the tropical Atlantic that compromise their reliability for use in studies of climate variability and change. Projections based on the current generation of climate models suggest that Pacific mean-state changes in the future will involve faster warming in the east equatorial basin than in the surrounding regions, leading to an increase in the frequency of extreme El Niños. Given the presumed strength of the Atlantic influence on the pantropics, projections of future climate change could be substantially different if systematic model errors in the Atlantic were corrected. Progress on these issues will depend critically on sustaining global climate observations; climate model improvements, especially with regard to model biases; and theoretical developments that help us to better understand the underlying dynamics of pantropical interactions and their climatic impacts.