El Nino-related variations in the southern Pacific atmospheric circulation: model versus observations

Our objective is two-fold: (1) to study the influence of tropical Pacific sea surface temperature (SST) anomalies associated with the El Nino - Southern Oscillation (ENSO) on the southern Pacific atmospheric circulation using observations; and (2) to assess the ability of a fully comprehensive GCM to reproduce the observed ENSO-related variations. The observed circulation features were derived from the National Center for Environmental Prediction (NCEP) reanalyses of observations. The GCM used is the atmospheric component of the United Kingdom Meteorological Office (UKMO) unified model (UM). A detailed study of the impact of the ENSO on the seasonal mean circulation and on the intraseasonal variability on synoptic time scales has been carried out using both the model and NCEP data for austral summer season. We have also investigated the relationship between the high frequency (HF) transients and the quasi-stationary mean circulation during selected ENSO years. A regional Hadley cell intensity in the tropical Pacific has been used as an El Nino index to classify the warming and cooling years for our composite analysis. During El Nino years the regional Hadley cell acts as a medium through which the enhanced equatorial convection forces a train of Rossby waves to produce the quasi-stationary circulation anomalies in extratropics. The Hadley cell weakens during La Nina years, and the equatorial convection moves further westwards. The Rossby wave response to this situation is to reverse the phase of the quasi-stationary circulation anomalies by 180degrees, as expected. These features are fairly well simulated by the UM, though the extratropical circulation anomalies in response to the equatorial convection is found about 30degrees longitude further eastwards than observed during El Nino years. The model confirms the observational evidence that the storm track variations in the southern subtropical Pacific are largely driven by variations in the South Pacific Convergence Zone (SPCZ) associated with the El Nino and La Nina events. However the model struggles to reproduce the significant storm track activity, present in the NCEP data, related to the westerly jet stream during La Nina years. Finally, the relationship between the HF transients and the quasi-stationary anomalies found in both the model and observations supports the role of the HF transients in forcing the time-mean circulation, as reported by Hoerling and Ting 1994 for the Pacific - North American (PNA) region.