Abstract/Summary

AbstractGlobal surface warming since 1850 consisted of a series of slowdowns (hiatus) followed by surges. Knowledge of a mechanism to explain how this occurs would aid development and testing of interannual to decadal climate forecasts. In this paper a global climate model is forced to adopt an ocean state corresponding to a hiatus (with negative Interdecadal Pacific Oscillation, IPO, and other surface features typical of a hiatus) by artificially increasing the background diffusivity for a decade before restoring it to its normal value and allowing the model to evolve freely. This causes the model to develop a decadal surge which overshoots equilibrium (resulting in a positive IPO state) leaving behind a modified, warmer climate for decades. Water mass transformation diagnostics indicate that the heat budget of the tropical Pacific is a balance between large opposite signed terms: surface heating/cooling due to air-sea heat flux is balanced by vertical mixing and ocean heat transport divergence. During the artificial hiatus, excess heat becomes trapped just above the thermocline and there is a weak vertical thermal gradient (due to the high artificial background mixing). When the hiatus is terminated, by returning the background diffusivity to normal, the thermal gradient strengthens to pre-hiatus values so that the mixing (diffusivity x thermal gradient) remains roughly constant. However, since the base layer just above the thermocline remains anomalously warm this implies a warming of the entire water column above the trapped heat which results in a surge followed by a prolonged period of elevated surface temperatures.