Stochastic Forcing Structures Conducive to Eastern and Central Pacific ENSO Characteristics

The predictability of El Nino and the Southern Oscillation (ENSO) events depends upon the characteristics of the initial conditions as well as the role of stochastic forcing. Stochastic forcing is capable of improving the predictability of the ENSO event if it excites optimal initial conditions that maximize ENSO growth. However, it can also be detrimental to the predictability of the event, if after the forecast is initialized, it excites enhanced, unpredictable growth or introduces interference. This study uses Linear Inverse Modeling to identify spatial and temporal characteristics of stochastic forcing that contribute the development of Central Pacific (CP) or Eastern Pacific (EP) ENSO characteristics. First, the optimal initial conditions that maximize CP and EP ENSO growth are identified. The optimal initial condition for CP events resembles the Pacific Meridional Mode (PMM) while the optimal for the EP events shows less structural characteristics. Second, the stochastic forcing structures associated with generating the optimal initial conditions are examined. We find seasonally dependent, symmetric atmospheric structures associated with CP growth. EP growth appears to be related to springtime equatorial zonal wind anomalies. Implications of the results for development and predictability of CP or EP ENSO events will be discussed.