The Interaction of Westerly Wind Bursts and the Spring Predictability Barrier in Determining the Size of the 1997/98, 2014/15, and Current El Niño Events

Abstract:

The ENSO growth rate has an annual cycle. The growth rate reaches a minimum in the spring (strongly damped) and increases to a maximum in the fall (weakly growing). This cycle of growth rate results in the spring predictability barrier. In this system, the stochastic forcing imposed before the growth rate minimum has a significantly reduced impact on the eventual El Niño magnitude, while stochastic forcing imposed after the growth rate minimum is more likely to trigger an El Niño event. In this framework, the Westerly Wind Burst (WWB) that occurred in February of 2014 occurred too early. The eastern edge of the warm pool that shifted in response to the WWB quickly returned to its climatological position, reducing the impact of additional noise forcing through the warm pool edge processes associated with state-dependent noise. The lack of additional WWBs doomed the 2014/15 El Niño event to weak status. This is in contrast to what was observed with the 1997/98 and the ongoing 2015 event where the initial WWBs helped condition the ocean for future WWBs. The sum total of the additional forcing provided by WWBs over the period from boreal spring to fall, while the growth rate is increasing, helped to produce the large El Niño events in 1997/98 and the rapidly developing event this year.