Annual Cycle of Pacific–North American Seasonal Predictability Associated with Different Phases of ENSO

Abstract

The potential for seasonal mean predictability over the Pacific and North American regions is evaluated as a function of the amplitude of equatorial Pacific sea surface temperature forcing, the phase of that forcing, and the phase of the annual cycle. The potential predictability is measured as the ratio of the seasonal mean SST-forced signal and the internally generated seasonal mean noise. The authors? assessments are derived from the output of ensemble atmospheric general circulation model experiments forced with observed monthly SSTs for 1950?94. Using a perfect prognostic approach, results are presented on the predictability of upper-tropospheric circulation, North American land temperature, and precipitation. Seasonal predictability is shown to depend on the amplitude of the SST-forced signal, whereas the background noise is largely independent of SSTs. To zero order, that signal grows linearly with the amplitude of anomalous SSTs. An important departure from this is with respect to the phase of tropical Pacific SST anomalies, and the simulated atmospheric signals were stronger for ENSO?s extreme warm phases compared to ENSO?s extreme cold phases. This asymmetry can be traced throughout the teleconnection chain that links the ENSO forcing region with North American climate. With regard to the annual cycle?s role, the North American climate is shown to be most predictable during the late winter and early spring season of warm events. This stems from the fact that the SST-forced signal during warm events at that time of year is only slightly weaker than in midwinter, whereas the background noise is substantially reduced. Predictability during spring is significantly greater than that occurring in fall, due to a much weaker fall signal. Observational analyses are presented that corroborate these key model results, in particular enhanced skill during ENSO?s warm phase and a springtime predictability peak. Finally, a comparison is made between the classic ratio of variance measure of predictability that commingles all warm, cold, and non-ENSO years to yield a single estimate, against such a ratio calculated for individual events. North American seasonal predictability for specific events can greatly exceed this single gross measure, and it is shown that the latter is a poor yardstick of the prospects for skillful predictions during extreme ENSO states.