Factors controlling decadal variations in ENSO-teleconnections and seasonal climate predictions

Analysis of the instrumental record reveals considerable decadal variation of the link between North American seasonal anomalies and ENSO. Only a weak association existed between the two during the period 1950-75 , whereas strong ENSO related anomalies have been the norm during the most recent quarter-century. Given the well-known relationship between the vigour of ENSO teleconnections and the skill of seasonal climate forecasts, it becomes important to identify the origin for such low frequency variations in tropical-extratropical interactions.

Results are presented that examine the efficacy of several factors that may control such multi-decadal variations. These include (i) decadal variations of ENSO SST forcing itself, (ii) decadal variations of the North Pacific SSTs as described by the so-called Pacific-Decadal Oscillation (PDO), and (iii) intrinsic red-noise atmospheric processes which themselves can produce decadal variations independent of changes in boundary forcings. Analysis is based on a suite of atmospheric GCM simulations of the last half-century. Four different GCMs are forced with the monthly evolving global SSTs, and multiple re-enactments spanning that period are created with each model.

Our main finding is that the observed multi-decadal variation of the ENSO teleconnections appears unrelated to variations in global SSTs. A trend toward stronger ENSO variations since 1976 is shown to induce a trend toward stronger seasonal teleconnections, but this effect falls far short of explaining the observed decadal teleconnection changes. No evidence is found of an atmospheric response to the North Pacific SSTs, and thus the notion that the PDO may induce a decadal modulation of ENSO teleconnections appears to be without merit. Intrinsic atmospheric variations, however, are shown to be a viable factor that may explain much of the observed decadal variability.