Evidence for a time-varying pattern of greenhouse warming in the Pacific Ocean

The observed warming trend in recent decades has had an El Nino-like character, but the cause of this pattern has been a contentious issue. Here we report results from the CSIRO climate model forced by historical and projected future increases in greenhouse-gas concentrations. These show a warming pattern that initially has the largest warming in the extratropics with a weak La Nina-like pattern in the tropics, but is eventually (after the 1960s) El Nino-like. This behaviour is present in each of an ensemble of three simulations. The switch in warming pattern is driven by the extratropical warmed water that subducts and penetrates through the subtropics and into the tropics, where it upwells and initiates the change. This result strengthens the argument, in terms of a physical mechanism, that the recent observed El Nino-like warming trend may be at least in part greenhouse-induced.