Modelling of atmospheric climate variations at NIWA
A version of the United Kingdom Meteorological Office (UKMO) unified model (UM) has been installed at the National Institute of Water and Atmospheric Research (NIWA) for modelling studies of climate variability. One of the main objectives of our modelling effort is to relate circulation features in the Southern Pacific to idealised El Nino - Southern Oscillation (ENSO) events. The UM was first tested for its correct set up and consistent performance at NIWA using an ensemble of integrations prior to its application. It is proposed to examine the influence of ENSO events on the South Pacific seasonal means by: (1) carrying out a "control" simulation forced by observed sea surface temperatures (SSTs) over 1960-1995; and (2) setting up idealised SST anomaly simulations. In this paper we concentrate on the success of the control run in simulating the observed climatology and interannual variations. The basic climatological circulation features over New Zealand - Australia region represented in the control simulation are validated against the National Center for Environmental Prediction (NCEP) re-analyses of observations. It is found that the seasonal variations in sea level pressure, precipitation and latitude-height cross section of zonal wind simulated by the model are in good agreement with observations for the most part, although the model westerly winds tend to be too strong over the Southern Ocean. The UM has also been found to respond reasonably well to the tropical Pacific sea surface forcing anomalies by realistically simulating seasonal mean patterns over New Zealand region during ENSO events. This gives us confidence in applying this general circulation model in subsequent experiments where we will try to understand the mechanisms of interannual variability that affect New Zealand climate.