GCM experiments on changes in atmospheric predictability associated with the PNA pattern and tropical SST anomalies

Abstract

Based on results from a simple three-level quasi-geostrophic model, Lin and Derome suggestedthat atmospheric predictability is influenced by the Pacific/North American (PNA) pattern. Inthe present study, predictability experiments are conducted with the Canadian Centre forClimate Modelling and Analysis general circulation model (CCCma GCM). A 47-yr integrationof theGCM with specified sea surface temperature (SST) for the years 1948—94 is first performed.Forecasts are initiated whenever the PNA pattern is in a strong positive or strong negativephase during this simulation. For each forecast, an ensemble of six initial conditions is generatedwith small random perturbations. Forecasts initiated when the PNA is in its positive phasehave smaller growth rates of ensemble standard deviation than forecasts initiated when thePNA is in its negative phase. Regional characteristics of the prediction spread are also examined.Similar experiments are conducted to determine the relationship between atmospheric predictabilityand SST anomalies in the tropical Pacific. Forecasts initiated when tropical SST anomaliesare positive have smaller growth rates of ensemble standard deviation than forecasts initiatedwhen tropical SST anomalies are negative. However, cases with positive tropical SST anomaliesbut without a strong PNA pattern show a similar prediction spread to cases with negative SSTanomalies. The results suggest that, in comparison to the PNA pattern, the influence of tropicalSST anomalies is only secondary. A set of three-layer diagnostic equations is used to analyzethe GCM results. It is speculated that the transient eddies have a stronger influence on thecirculation anomalies (and therefore reduce the atmospheric predictability more) in the negativePNA phase than in the positive PNA phase.