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

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