The impact of ENSO on extratropical low-frequency noise in seasonal forecasts

This study examines the variability in forecasts of the January-February-Ma rch (JFM) mean extratropical circulation and how that variability is modula ted by the El Nino-Southern Oscillation. The analysis is based on ensembles of seasonal simulations made with an atmospheric general circulation model (AGCM) forced with sea surface temperatures observed during the 1983 El Ni no and 1989 La Nina events. The AGCM produces pronounced interannual differ ences in the magnitude of the extratropical seasonal mean noise (intraensem ble JFM variability). The North Pacific, in particular, shows extensive reg ions in which the 1989 seasonal mean noise kinetic energy (SKE), which is d ominated by a "Pacific-North American (PNA)-like'' spatial structure, is mo re than 2 times that of the 1983 forecasts. The larger SKE in 1989 is assoc iated with a larger-than-normal barotropic conversion of kinetic energy fro m the mean Pacific jet to the seasonal mean noise. The generation of SKE by submonthly transients also shows substantial interannual differences, thou gh these are much smaller than the differences in the mean flow conversions . An analysis of the generation of monthly mean noise kinetic energy and it s variability suggests that the seasonal mean noise is predominantly a stat istical residue of variability resulting from dynamical processes operating on monthly and shorter timescales. A stochastically forced barotropic model (linearized about the AGCM's 1983 and 1989 seasonal and ensemble mean states) is used to further assess the r ole of the basic state, submonthly transients, and tropical forcing in modu lating the uncertainties in the seasonal AGCM forecasts. When forced global ly with spatially white noise, the linear model generates much larger varia nce for the 1989 basic state, consistent with the AGCM results. The extratr opical variability for the 1989 basic state is dominated by a single eigenm ode and is strongly coupled with forcing over the tropical western Pacific and the Indian Ocean. Linear calculations that include forcing from the AGC M variance of the tropical forcing and submonthly transients show a small i mpact on the variability over the PNA region as compared with that of the b asic-state differences.