Intra-annual Principal Modes and Evolution Mechanism of the El Nino Southern Oscillation

The El Nino-Southern Oscillation (ENSO) is one of the most important phenomena in climate. By studying the fluctuations of surface air temperature within one year between 1979-01-01 and 2016-12-31 of the region (30S-30N, 0E-360E) with eigen-decomposition, we find that the temperature fluctuations are dominated by the two principal modes whose temporal evolutions respond significantly to the ENSO variability. According to introduce a micro-correlation, we find that the coupling between the first principal mode and the temperature fluctuations in the El Nino region could result in different ENSO phases. Without this coupling, the El Nino region is in a normal phase. With the strong coupling between the El Nino region and the Northern Hemisphere, an El Nino event will appear with a high probability. Then this coupling changes to be strong between the El Nino region and the Southern Hemisphere accounting for the fast decay of El Nino after boreal winter, even leading to a La Nina event. Moreover, the coupling between the El Nino region and the second principal mode is found to be strong in normal or La Nina phases in response to the normal or strong Walker Circulations. We conjecture that the temporal evolutions of these couplings for the first and second principal modes are controlled by the meridional and zonal ocean-atmospheric circulations respectively.