Southern Hemisphere Extratropics Research Articles

A study of wintertime circulation anomalies during past El Niño events using a high resolution general circulation model. I: Influence of model climatology

AbstractIn this paper the response of an 11‐layer general circulation model (GCM) to an enhanced composite El Niño sea surface temperature (s.s.t.) anomaly is studied. The integrations were run for 540 days in a perpetual January mode, and had either mean or envelope orography.A basic objective of this paper is to study the influence of model climatology on the response to a tropical s.s.t. anomaly. This was done both by comparing our results with those from a lower resolution GCM (the NCAR Community Climate Model) with an identical s.s.t. anomaly, and by performing control and anomaly integrations with both mean and envelope orography. The specification of orography strongly influences the strength of the mean zonal flow, and stationary and transient wave activity in our model. Whereas the runs with envelope orography produced an anomalous Pacific/North American (PNA) teleconnection pattern in the northern hemisphere, the runs with mean orography did not. In the extratropical southern hemisphere. our model produced a statistically significant response which had a substantial zonally symmetric component. By contrast, the NCAR model, with weaker zonal flow, produced a weaker and mainly statistically insignificant response in the southern hemisphere. Finally, in the tropics, over the Central and Eastern Pacific, the anomalous rainfall and vorticity fields correlated strongly with the corresponding fields in the control integration. Overall, therefore, we conclude that a GCM's response is strongly sensitive to the basic model climatology.In trying to understand dynamically the GCM response it was concluded that the PNA pattern may not simply be the result of Rossby wave dispersion from a tropical source. The PNA pattern may also be associated with a combination of two separate influences, the interaction with the anomalous mean flow of enhanced storm track activity over the Pacific, and with orography on the North American continent.The implications of the model's tropical response for simple linear theory were also discussed. In these composite El Niño anomaly experiments enhanced rainfall and upper divergence occurred well to the west of the upper tropical anticyclone pairs, implying that a Sverdrup type balance in the vorticity equation cannot hold. It is suggested that the strength of the anomalous vortex stretching term could be a small residual compared with the horizontal advection terms, and that a simple linear relationship between anomalous latent heating and the position and strength of the anomalous anticyclone pairs is not always valid. An alternative qualitative explanation for the relative position of these anticyclone pairs is given in terms of a reduction in the zonally asymmetric component of upper tropospheric vorticity across the tropical Pacific, whilst the position of the rainfall maximum is located close to the s.s.t. maximum.