ENSO-related variations in the southern Pacific seasonal mean circulation: model versus observations

The objective of this paper is two-fold: 1) to study the influence of tropical Pacific sea surface temperature (SST) anomalies associated with El Nino on the southern Pacific atmospheric circulation using observations; and 2) to assess the ability of a fully comprehensive GCM to reproduce the observed El Nino-related variations. For this purpose the El Nino events were classified into (a) strong warming (SW) events; and (b) weak warming (WW) events. The observed circulation features for both types of events were derived from the National Center for Environmental Prediction (NCEP) reanalyses of observations. The GCM used here is the atmospheric component of the United Kingdom Meteorological Office (UKMO) unified model (UM). SST anomalies obtained from a version of GISST were used to drive the model at the sea surface for 35 years starting from 1 August 1960.

The stationary response during both the classes of events are studied using 300 hPa streamfunction anomalies and zonal-mean mass streamfunction. The tropical upper-level divergence and the subtropical upper-level convergence strengthen during SW events, resulting in a stronger stationary wave train. During WW events the stationary wave train is weaker and shifted eastwards. The influence of high frequency transients on the seasonal mean circulation is also investigated. It is found that the extratropical high frequency transients in the southern Pacific considerably influence the region of seasonal mean cyclonic forcing by extending eastwards across the dateline. This influence is relatively weaker during WW ENSO events. The intensity and eastward extension of storm tracks are consistent with the location and strength of the descending branch of the Hadley cell in the region. Model results are qualitatively similar to observations and will be presented in detail at the conference.