Why is the Equatorial Pacific Warm Water Volume (WWV) an El Nino predictor, and why has it not performed as well recently?

Abstract:

Simple formulae are developed to calculate equatorial Pacific WWV from wind stress. The WWV formulae are in agreement with WWV observations. They show that it is zonal equatorial convergence, rather than meridional convergence, that is mainly responsible for the WWV. Also, the theory shows that the WWV is closely related to the non-equilibrium variability that leads El Nino and can be used to predict it. In accordance with Bunge and Clarke (May 2014, J. Phys. Oceanogr.), vital to this predictability is that at interannual and lower frequencies equatorial Kelvin and Rossby wave modes combine to form long Rossby waves which propagate westward more slowly with increased latitude. Consequently, the signal delay at the western boundary increases with meridional distance from the equator and also with increasing zonal distance of the major equatorial wind forcing from the western boundary. Thus the further the equatorial wind forcing is from the western boundary, the bigger the signal delays at the western boundary and the more we can see into the future when the non-equilibrium variability reflects at the western boundary and contributes to the equatorial WWV. Since zonal ENSO wind-forcing has been, on average, closer to the western boundary since 1998, the size of the WWV and its lead over ENSO has on average decreased.