The proposed work will investigate climate variability and sensitivity mechanisms on intraseasonal to interannual time scales. The natural low-frequency variability generated within the atmosphere and its interaction with the oceanic mixed layer will be studied using observations as well as a hierarchy of models. The tropical intraseasonal oscillation and its interaction with midlatitude weather anomalies will also be investigated. Dynamic and thermodynamic mechanisms for setting the long time scale of the tropical intraseasonal oscillation will be tested using variations on the NCAR CCM3 GCM; these will be compared to the observational analysis. Interactions between clouds, radiation, SST and atmospheric circulation and the effect on tropical climate will also be investigated using a hierarchy of models. The work is important since an improved understanding of dynamics of the climate system in the middle latitudes and tropics has benefits both from a theoretical as well as practical point of view.