Vecchi

The response of the tropics to increasing CO2 is a central topic in climate change research, as tropical climate conditions can have far-reaching effects and they set the background for changes in the character variations, such as those of El Niño. Theoretical and modeling understanding, and observational evidence for long-term changes to the tropical climate system will be highlighted. Changes in the oceanic circulation and thermal structure will be discussed, as will changes in atmospheric circulation and precipitation patterns - including changes in the Hadley and Walker Circulations. The problem of defining interannual variations in a changing mean climate will also be discussed.

The change to the tropical Pacific mean state is complex, controled by both oceanic and atmospheric processes. In models with a simplified representation of atmospheric physics, feedbacks originating in the ocean drive the system to a “La Niña-like” state. In models with atmospheric general circulation components, thermodynamic constraints result in a reduction of the strength of the atmospheric overturning circulation – manifest primarily in the zonally-asymmetric (Walker) rather than zonal-mean (Hadley) component. In these models changes over the tropical Pacific Ocean resemble “El Niño-like” conditions, although the mechanisms are fundamentally different from those of El Niño, as are many of the impacts. Even though modeling studies can help reconcile aspects of the diverging theoretical understanding, a true reconciliation requires observational evidence. Observations of sea level pressure indicate that over the 20th Century the Pacific Walker circulation has weakened; however, differing reconstructions of historical SST are inadequate to distinguish between an increase or decrease in East-West SST gradient across the Pacific. We outline a way forward, with paleo-proxy observations, to reconcile these diverging views. Implications of the modeled changes are also discussed.