An Analysis of Equatorial Pacific Sea Surface Temperature Anomaly Experiments in General Circulation Models with and without Mountains

Abstract

Perpetual January experiments have been performed using versions of the NCAR Community Climate Model (CCM) with and without mountains. Features of the mean simulations of the ?no mountains? experiment are compared with those of the standard CCM, the ?mountains? experiment. The stationary waves in the ?no mountains? experiment have smaller amplitudes than those in the ?mountains? case, especially for zonal wavenumbers 2 and 3. The mean zonal wind in the ?no mountains? case also has weaker horizontal gradients than in the ?mountains? case. The response of these two versions of the CCM to equatorial Pacific sea surface temperature (SST) anomalies is investigated. Two anomalies are considered for each model configuration. Differences in the responses of the two models to the same anomalous forcing are discussed. The Northern Hemisphere midlatitude response of the ?no mountains? model has nearly the same spatial scale as that of the ?mountains? model, but details of the shape of the response pattern are different. The amplitude of the response in the Northern Hemisphere is weaker in the ?no mountains? case than in the ?mountains? case by about a factor of 2. On the other hand, the response in the Southern Hemisphere is stronger in the ?no mountains? case than in the ?mountains? case. It is shown that this is consistent with the interpretation that the Pacific/North American (PNA) teleconnection pattern extracts energy from the mean zonal flow by barotropic conversion. The importance of barotropic conversion in the Southern Hemisphere is also demonstrated. A linear barotropic vorticity equation model is used to compare the response to localized tropical forcing in each of the two basic states, for ?mountains? and ?no mountains,? produced by the CCM. When forced in the vicinity of the SST anomaly, the linear model shows a sensitivity to the state about which it is linearized that is similar to the sensitivity shown by the CCM to its climatic state. This sensitivity is shown to be influenced by barotropic conversion processes, which in turn are influenced by the basic state configuration. Furthermore, calculations indicate that forcing in virtually any region of the tropics tends to produce a stronger (weaker) Northern (Southern) Hemisphere response for the ?mountains? basic state than the ?no mountains? basic state. It is also shown that anomalous upper troposphere convergence around Indonesia may be contributing to the CCM response to the eastern Pacific SST anomalies being considered in this study. We conclude that the stationary waves in each CCM simulation affect the midlatitude response of that model to tropical forcing anomalies.