Surface thermal centroid anomaly of the eastern equatorial Pacific as a unified Nio index

By analyzing the variability of global SST (sea surface temperature) anomalies, we propose a unified Nio index using the surface thermal centroid anomaly of the region along the Pacific equator embraced by the 0.7A degrees C contour line of the standard deviation of the SST anomalies and try to unify the traditional Nio regions into a single entity. The unified Nio region covers almost all of the traditional Nio regions. The anomaly time series of the averaged SST over this region are closely correlated to historical Nio indices. The anomaly time series of the zonal and meridional thermal centroid have close correlation with historical TNI (Trans-Nio index) indices, showing differences among El Nio (La Nia) events. The meridional centroid anomaly suggests that areas of maximum temperature anomaly are moving meridionally (although slightly) with synchronous zonal movement. The zonal centroid anomalies of the unified Nio region are found helpful in the classification of the Eastern Pacific (EP)/Central Pacific (CP) types of El Nio events. More importantly, the zonal centroid anomaly shows that warm areas might move during a single warming/cooling phase. All the current Nio indices can be well represented by a simple linear combination of unified Nio indices, which suggests that the thermal anomaly (SSTA) and thermal centroid location anomaly of the unified Nio region would yield a more complete image of each El Nio/La Nia event.