Characteristics of the tropical intraseasonal oscillation in the NCAR community climate model

The structure of the intraseasonal oscillation in a perpetual January integration of the NCAR Community Climate Model (CCM) is analysed and compared with observations. The simulated oscillation is identified as an eastward‐moving perturbation in the equatorial velocity‐potential field, with predominantly zonal wave‐number‐1 structure. Analysis of its vertical and horizontal structure, based on composites, shows good agreement with the observed oscillation from station data, and from numerical weather prediction model analyses. The simulated intraseasonal oscillation is evident in the wind fields at all longitudes but its appearance in the precipitation and outgoing long‐wave radiation is confined to the eastern hemisphere. The perturbation to the zonal wind component is characterized by an out‐of‐phase behaviour between the lower and upper troposphere which is most coherent in the convective regions of the Indian Ocean and West Pacific. The stream‐function anomalies in these regions are suggestive of a coupled Rossby‐Kelvin wave, whilst in the western hemisphere the oscillation is more characteristic of a Kelvin wave. The role of cloud‐radiation interaction was studied using a parallel integration in which the cloud long‐wave forcing in the atmosphere was removed. This was found to influence the strength of the oscillation but had no significant impact on the period. In common with other model simulations of the intraseasonal oscillation, the period was found to be too short, near 25 days. Reasons for this are discussed in the context of a similar analysis of a perpetual July integration in which a much slower (period near 54 days) and less coherent oscillation was identified.