Low-frequency variability of the zonal mean circulation induced by sea surface temperature variation and eddy feedback

This study is an investigation of the dynamics of the variabilities of the zonal mean atmospheric circulation induced by the variation of the sea surface temperature (SST) and by the nonlinear interaction between unstable baroclinic waves and the zonal mean flow.; The influence of SST is investigated with a zonally-symmetric model. A simple parameterization is used to describe the tropical condensational heating as a function of SST and the large-scale circulation. When forced by a time-dependent SST with an idealized annual cycle, the time-dependent model circulation shows a large difference in response time between the Hadley cell and the subtropical jet. While the Hadley cell quickly adjusts to the change of SST, the response of the subtropical jet lags the Hadley cell by nearly two months. This lag is a combined effect of the acceleration by the poleward angular momentum transport of the Hadley cell and the deceleration by the sub-grid scale diffusion and the downward momentum transport by the Hadley cell. The interannual variability of the Hadley circulation is simulated with the observed zonal mean SST from 1949 to 1992. Composite analysis of the model circulation shows pronounced intensification of the Hadley cell and the subtropical jet during El Nino (warm) years and vice versa in La Nina (cold) years. The anomaly of the subtropical jet extends over a much longer time period than the Hadley cell, and resembles the observed zonal wind anomaly during the 1982/83 El Nino. It suggests that the subtropical zonal wind anomaly is largely induced by the tropical SST anomaly.; The dynamics of wave-mean flow interaction is investigated with a three-dimensional, global primitive-equation model forced by a zonally-symmetric heating, in a sequence of experiments with progressively more complex versions of the model. The first version allows only a single zonal wave interacting with the zonal mean flow. The modification to the zonal mean flow is found to be largely determined by the linear normal mode structure of the wave. A quasi-periodic oscillation with period of 80 days in the interaction with wavenumber 3 resembles the "zonal index cycle". In a more complex version where a wave triad (wavenumbers 3, 6 and 9) is allowed to interact with the zonal mean flow, the variability of the zonal mean flow is irregular, and is characterized by a low-frequency oscillation with period of about two months. It is revealed that the wave-mean flow interaction is the primary mechanism in generating the low-frequency oscillation in the zonal mean flow. The wave-wave interaction is secondary. This is further verified in an experiment with a full wave spectrum. The low-frequency oscillation in this experiment is qualitatively similar to that in the wave triad experiment. EOF analyses reveal an oscillating pattern in the zonal mean zonal wind similar to the observed "zonal index cycle", and also suggest that the "zonal index cycle" is primarily driven by the eddy forcing.