The AMO as a Driver of Multi-decadal ENSO Variability

Multi-decadal shifts in ENSO variability have been observed, but it is unclear if this variability is just a random variation in the ENSO cycle or whether it is forced from outside the tropical Pacific basin.  El Niño events represent a collapse of the typical tropical Pacific annual cycle.  Using the ENSO spring persistence barrier strength to represent the tropical Pacific annual cycle, we find an inverse relationship between ENSO variance and the amplitude of the annual cycle can be found on decadal timescales. Using a combination of observations, reconstructions, reanalysis, proxy data, and coupled ocean-atmosphere model experiments, we find that the cause of this inverse relationship is the extent of the northward excursion of ITCZ during the boreal summer and its effect on ENSO stability seasonally and annually.  Furthermore, we find that a major control of the northward extent of the boreal summer ITCZ on these timescales is strength of the Pacific trade winds, which in turn are significantly influenced by tropical Atlantic SSTs.  On these multidecadal timescales, the long term pattern of tropical Atlantic SSTs are controlled by the Atlantic Multidecadal Oscillation.