Time-Spatial Features of Mix El Nino

The diversity of El Nino is a critical field of the climate research. The eastern Pacific (EP) and central Pacific (CP) types of El Nino have been identified in the previous studies. However, the extreme El Nino event that occurred in 2015-2016 is quite different from both the EP and CP El Nino events. The sea surface temperatures anomalies (SSTA) for this event widely spread in both the central and eastern Pacific and have a small zonal gradient in the central-eastern Pacific. Many researchers regarded this event as a mixed type of El Nino. Using the regression-EOF method, the Mix El Nino pattern is extracted from the tropical Pacific SSTA field during the period from 1900 to 2019. Here, we reveal that the Mix El Nino is a very usual rather than a new type of El Nino, it is just that the EP and CP El Nino events are more frequent since the 1980s, while the Mix El Nino events frequently appear before the 1980s. The time-spatial features of the Mix El Nino are further investigated. The results demonstrate a unique westward propagation of the maximum SSTA for the Mix El Nino from the far eastern Pacific to the central Pacific. In contrast, the SSTA center is locked in the far eastern Pacific region for the EP El Nino and the central Pacific region for the CP El Nino. The evolutions of subsurface ocean temperature anomalies and sea surface height anomalies are also examined to support this. The ocean-atmosphere interaction plays an important role in the evolution of the Mix El Nino. The anomalous atmospheric Walker circulation for the Mix El Nino is mainly in the western and central Pacific as well as very weak in the eastern Pacific. In contrast, there are significant westerlies/easterlies in the eastern Pacific for the EP/CP El Nino. The small gradient of SSTA in the central-eastern Pacific for the Mix El Nino leads to weak zonal wind anomalies, which further weaken the zonal gradient of SSTA. All this suggests that the Mix El Nino is not unusual and fundamentally different from the EP and CP El Nino with important implications for global climate effects