An analysis of equatorial Pacific sea surface temperature anomaly experiments in general circulation models with and without mountains

• Published in: Journal of the atmospheric sciences 1987-07, Vol.44 (14), p.1828-1844
• Main Authors: BLACKMON, M. L, BRANSTATOR, G. W, BATES, G. T, GEISLER, J. E
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Physics of the oceans; Sea-air exchange processes
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/1520-0469(1987)044<1828:AAOEPS>2.0.CO;2

Perpetual January experiments were performed by 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. Conversely, 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 which 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. It is concluded