The Walker circulation, as reflected in the sea level pressure gradient across the equatorial Pacific, has weakened over the last century. This change has been attributed to anthropogenic greenhouse gas forcing. However, over the last several decades, there has been a strengthening of the Walker circulation, at a time when the radiative forcing from greenhouse gases has been strongest. Is this internal or ‘natural' variability overprinting the longer-term anthropogenic changes? What are the mechanisms that can produce multi-decadal fluctuations in the Walker circulation? Can these mechanisms be distinguished from the externally forced changes?

In this seminar, I will present results from a hierarchy of climate model simulations in which we analyze the ocean and atmospheric mechanisms that produce variability in the tropical Pacific climate on interannual and decadal timescales, and in response to anthropogenic forcing. The main finding is that interactive ocean dynamics are not required to produce Walker circulation variability with realistic spatial structure in surface pressure, surface temperature, winds and precipitation. Interactive ocean dynamics increase the variability in the eastern equatorial Pacific on seasonal to interannual timescales as expected from the positive Bjerknes feedback. On decadal timescales, however, interactive ocean dynamics play little role because of the offsetting influences of zonal advection and thermocline fluctuations. In response to greenhouse gas forcing, interactive ocean dynamics appear to dampen the warming response of the eastern equatorial Pacific because of a sharpening and shoaling of the thermocline there.

The implications of these findings for detection, attribution and prediction of tropical Pacific climate changes are discussed.