Intraseasonal Oscillations in the Mid-Latitudes: Observations, Theory, and GCM Results

We examine in this paper the role that oscillatory interactions between the jet stream and Northern Hemisphere (NH) mountain ranges play in the low-frequency variability (LFV) of the mid-latitude atmosphere. The basic observational evidence for intraseasonal oscillations in the NH extratropics is reviewed, along with the results of a hierarchy of models—from the simplest to full GCMs—that explain these observations as arising from an oscillatory topographic instability. We next report on recent reanalysis results that allow one to explicitly connect large-scale flow-pattern changes with oscillatory changes in atmospheric angular momentum, which appear to be driven by mountain-torque changes. An intermediate model with fairly realistic climatology and LFV is used to establish a relationship between these oscillatory circulation changes and the model's Markov chain of multiple flow regimes. We return to the reanalysis results to show that certain phases of the regional mountain-torque oscillations are fair predictors of flow-pattern changes up to 15 days in advance. Open questions are emphasized at the end.