8.4.3 Pacific-North American Pattern

The Pacific-North American (PNA) pattern (see Chapter 3) is commonly associated with the response to anomalous boundary forcing. However, PNA-like patterns have been simulated in atmospheric GCM experiments subjected to constant boundary conditions. Hence, both external and internal processes may contribute to the formation of this pattern. Particular attention has been paid to the external influences due to SST anomalies related to ENSO episodes in the tropical Pacific, as well as those situated in the extratropical North Pacific. Internal mechanisms that might play a role in the formation of the PNA pattern include interactions between the slowly varying component of the circulation and high-frequency transient disturbances, and instability of the climatological flow pattern. Trenberth et al. (1998) reviewed the myriad observational and modelling studies on various processes contributing to the PNA pattern.

The ability of GCMs to replicate various aspects of the PNA pattern has been tested in coordinated experiments. Until several years ago, such experiments were conducted by prescribing observed SST anomalies as lower boundary conditions for atmospheric GCMs. Particularly noteworthy are the ensembles of model runs performed under the auspices of the European Prediction of Climate Variations on Seasonal to Interannual Time Scales (PROVOST) and the US Dynamical Seasonal Prediction (DSP) projects. The skill of seasonal hindcasts of the participating models’ atmospheric anomalies in different regions of the globe (including the PNA sector) was summarised in a series of articles edited by Palmer and Shukla (2000). These results demonstrate that the prescribed SST forcing exerts a notable impact on the model atmospheres. The hindcast skill for the winter extratropical NH is particularly high during the largest El Niño and La Niña episodes. However, these experiments indicate considerable variability of the responses in individual models, and among ensemble members of a given model. This large scatter of model responses suggests that atmospheric changes in the extratropics are only weakly constrained by tropical SST forcing.

The performance of the dynamical seasonal forecast system at the US NCEP in predicting the atmospheric anomalies given prescribed anomalous SST forcing (in the PNA sector) was assessed by Kanamitsu et al. (2002). During the large El Niño event of 1997 to 1998, the forecasts based on this system with one-month lead time are in good agreement with the observed changes in the PNA sector, with anomaly correlation scores of 0.8 to 0.9 (for 200 mb height), 0.6 to 0.8 (surface temperature) and 0.4 to 0.5 (precipitation). More recently, hindcast experiments have been launched using AOGCMs. The European effort was supported by the Development of a European Multimodel Ensemble System for Seasonal to Interannual Prediction (DEMETER) programme (Palmer et al., 2004). For the boreal winter season, and with hindcasts initiated in November, the model-generated PNA indices exhibit statistically significant temporal correlations with the corresponding observations. The fidelity of the PNA simulations is evident in both the multi-model ensemble means, as well as in the output from individual member models. However, the strength of the ensemble mean signal remains low when compared with the statistical spread due to sampling fluctuations among different models, and among different realisations of a given model. The model skill is notably lower for other seasons and longer lead times. Empirical Orthogonal Function analyses of the geopotential height data produced by individual member models confirm that the PNA pattern is a leading spatial mode of atmospheric variability in these models.

Multi-century integrations have also been conducted at various institutions using the current generation of AOGCMs. Unlike the hindcasting or forecasting experiments mentioned above, these climate simulations are not aimed at reproducing specific ENSO events in the observed system. Diagnosis of the output from one such AOGCM integration indicates that the modelled ENSO events are linked to a PNA-like pattern in the upper troposphere (Wittenberg et al., 2006). The centres of action of the simulated patterns are systematically displaced 20 to 30 degrees of longitude west of the observed positions. This discrepancy is evidently linked to a corresponding spatial shift in the ENSO-related SST and precipitation anomaly centres simulated in the tropical Pacific. This finding illustrates that the spatial configuration of the PNA pattern in AOGCMs is crucially dependent on the accuracy of ENSO simulations in the tropics.