On the relationship between Indian Ocean sea surface temperature and the transition from El Nino to La Nina

The relationship between Indian Ocean sea surface temperature and the transition of El Nino events into either La Nina or El Nino-Southern Oscillation neutral conditions is examined in both observations and the retrospective ensemble hindcasts of the National Center for Environmental Prediction Climate Forecast System. The southern Indian Ocean is shown to demonstrate a particularly robust and consistent relationship with the evolution of these transitions. These associations are described, and a physical mechanism involving air-sea interaction in the Indian and western Pacific Oceans is proposed. Observations suggest that easterly surface wind anomalies in the western Pacific Ocean are associated with the emergence of La Nina during boreal summer and fall. Here it is shown that these winds are significantly correlated to southern Indian Ocean sea surface temperature in the preceding spring that is characterized by a large-scale zonal dipole of cool and warm anomalies in the southwestern and southeastern Indian Oceans, respectively. These associations are particularly pronounced for strong El Nino conditions, during the dissipation of which a pronounced wavetrain-like atmospheric pattern accompanies sea surface temperature anomalies in the southern Indian Ocean. Together, the circulation and sea surface temperature anomalies increase the meridional cross-equatorial temperature gradient in the western Indian Ocean and mute intraseasonal variability while strengthening surface equatorial easterly winds in the Indo-Pacific warm pool. Collectively, these anomalies favor subsequent La Nina development. On the basis of these observed associations, a predictive model that demonstrates skill in anticipating the nature of El Nino transitions, involving the southern Indian Ocean, Asian monsoon, and El Nino-Southern Oscillation, is proposed. In the National Center for Environmental Prediction Climate Forecast System, the relationships described above are simulated both consistently and realistically, despite model weaknesses, further bolstering a key role of southern Indian Ocean and predictive relationship. Comparison of fully coupled and sea surface temperature-forced simulations suggests a key role for air-sea interaction in the observed associations. Moreover, it is demonstrated that coupled simulations of El Nino-Southern Oscillation may benefit substantially from improved representation of Indian Ocean variability and Indo-Pacific interaction.