Climate Response of the Equatorial Pacific to Global Warming

by Di Nezio, Pedro N.

The climate response of the equatorial Pacific to increased greenhouse gases is investigated using numerical experiments from five climate models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. Changes in the heat budget of the surface layer in response to CO2 doubling (2xCO2) are analyzed in experiments with full-coupled ocean dynamics; and compared to experiments with uncoupled ocean dynamics. In full-coupled experiments, weaker ocean zonal currents driven by a slowing down of the Walker circulation reduce the ocean heat flux divergence throughout the equatorial Pacific. The resulting ocean dynamical heating enhances the surface warming due to increased clear-sky surface radiation in response to 2xCO2. The total radiative plus ocean dynamical heating are stabilized by evaporation and cloud feedbacks over the warm pool and by increased ocean vertical heat transport over the cold tongue. Increased near-surface thermal stratification enhances vertical heat transport in the cold tongue despite a reduction in vertical velocity. This ocean dynamical cooling is the dominant negative term in the heat budget changes over the eastern Pacific; and represents a strengthening of the processes leading to the annual cycle of the cold tongue, which increases by 0.4 K as a result. The stratification response is found to be a permanent feature of the equilibrium climate potentially linked to both thermodynamical and dynamical changes within the equatorial Pacific. To conclude, the relationship between the heat budget changes and the SST response is discussed along with implications for detecting these signals in the modern observational record.