The GSDM describes a repeatable cycle of circulation variability that encompasses different time scales and physical processes. The linear framework is consistent with a superposition of atmospheric quasi-oscillations, which was advocated more than 80 years ago using regional and hemispheric weather data. The GSDM is based on daily values of the global relative atmospheric angular momentum (AAM) tendency whose extreme excursions are related with circulation transitions and extreme weather events. Circulation anomalies over the Asia-Pacific-North America sector normally make a large contribution to the global integral. Four distinct time scales ranging from baroclinic waves to the Madden-Julian Oscillation (MJO) are included explicitly in the model evolution, but their statistics are also sensitive to: a) the phase of ENSO, b) the seasonal cycle and c) the recent warming of tropical west Pacific sea surface temperatures.
Physical processes like the mountain torque, momentum transports and tropical convection conspire to produce a slow, poleward movement of zonal momentum anomalies during a cycle of the GSDM. The anomaly starts in the equatorial tropopause of the western hemisphere (WH), “propagates” poleward and eastward into the eastern hemisphere and then back into mid-latitudes of the WH where the anomaly reaches the surface. This behavior contributes to week 1-3 forecast skill, especially during a sequence of 2-3 well-defined MJOs. A systematic change in MJO statistics due to global change could impact such predictability. Real time monitoring of the last four years suggest a change in activity may have occurred.