Increased occurrences of consecutive La Nina events under global warming

Most El Nino events occur sporadically and peak in a single winter(1-3), whereas La Nina tends to develop after an El Nino and last for two years or longer(4-7). Relative to single-year La Nina, consecutive La Nina features meridionally broader easterly winds and hence a slower heat recharge of the equatorial Pacific(6,7), enabling the cold anomalies to persist, exerting prolonged impacts on global climate, ecosystems and agriculture(8-13). Future changes to multi-year-long La Nina events remain unknown. Here, using climate models under future greenhouse-gas forcings(14), we find an increased frequency of consecutive La Nina ranging from 19 +/- 11% in a low-emission scenario to 33 +/- 13% in a high-emission scenario, supported by an inter-model consensus stronger in higher-emission scenarios. Under greenhouse warming, a mean-state warming maximum in the subtropical northeastern Pacific enhances the regional thermodynamic response to perturbations, generating anomalous easterlies that are further northward than in the twentieth century in response to El Nino warm anomalies. The sensitivity of the northward-broadened anomaly pattern is further increased by a warming maximum in the equatorial eastern Pacific. The slower heat recharge associated with the northward-broadened easterly anomalies facilitates the cold anomalies of the first-year La Nina to persist into a second-year La Nina. Thus, climate extremes as seen during historical consecutive La Nina episodes probably occur more frequently in the twenty-first century.