Does Recent Change in Tropical Pacific Forecast Skill Represent Variation in ENSO Predictability or Precursors?

Abstract:

Recently, it has been suggested that the predictability of ENSO has changed in the last few decades in correspondence with the observed increase of CP relative to EP ENSO events. We investigate this issue within the context of a statistically stationary stochastically forced multivariate linear dynamical system. Using a Linear Inverse Model (LIM) constructed from observed tropical Pacific oceanic and atmospheric anomalies, it is suggested that decadal changes in realized ENSO forecast skill may also reflect variations of different precursors to ENSO events and not necessarily a change in the underlying predictability of tropical Pacific coupled dynamics. These precursors can be excited by random weather forcing and subsequently result in SST anomaly amplification primarily through surface or thermocline feedbacks, respectively. That is, some ENSO events are inherently less predictable than others due to the relative importance of different physical processes, and periods when these events are more often randomly excited will be periods of reduced forecast skill.