Global Synoptic Dynamic Model (GSDM)
This quasi-phase space diagram has global relative atmospheric angular momentum (AAM) anomalies on the x-axis plotted against global relative AAM time tendency anomalies on the y-axis. The labels represent standardized anomalies where a 1968-96 mean climatology and a 1968-2006 standard deviation have been used. The standard deviation for global relative AAM is 1.2x1025 kg m2 s-1 and for the global tendency is 1.8x1019 kg m2 s-2. The daily global AAM tendency is estimated from the global AAM time series using a 5-point numerical scheme. The chart provides a gross assessment of the stage of the Global Synoptic Dynamic Model (GSDM) discussed in Weickmann and Berry (2007, pdf here).
The clockwise rotation of the trajectory depicts an atmospheric "mode" that is termed the global wind oscillation (GWO). The GWO varies in time and space but remains quite simple as one goes beyond the variations in global mean AAM. For zonal AAM averages, GWO zonal wind anomalies build up in the upper troposphere near the equator and shift poleward and downward as they expand or move into mid-latitudes. There is also a movement of anomalies from the Western to the Eastern Hemisphere. Thus the zonal mean or wavenumber zero component is closely linked to a wavenumber 1 disturbance. The temporal behavior of the GWO is a mixture of slow oscillations and non-oscillatory, red noise-type variations. On average, the GWO evolves coherently between 30N-30S. The global AAM anomaly is a useful but simple monitoring index of this behavior.
The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively. Stages 2 and 4 are transitional.
Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field. Troughs are probable across the western USA with a ridge over the southeast. High impact weather is favored across the Plains.
Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4. A cold regime is probable across the central USA.
Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed favoring high impact weather events along the USA west coast.
Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.
When the Madden-Julian Oscillation (MJO) is active (MJO info) , it contributes to well-defined, broad "trajectories" in the GWO phase-space. Another slow, subseasonal process is a friction-mountain torque index cycle that also contributes to forcing the trajectories. It appears these two "slow modes" can interact and possibly excite one another. The friction-mountain torque index cycle includes teleconnection patterns like the PNA and the NAO. These equivalent barotropic patterns contribute to exchanging large amounts of angular momentum between the ocean-earth and the atmosphere through their surface wind anomalies.
Despite the emphasis on the mountain and frictional torques for the global AAM variations, the subseasonal zonal AAM variations are dominated by zonal mean momentum fluxes. On average there is a difference between the latitude band where a torque generates an AAM anomaly, the band where the anomaly appears in the atmosphere and the latitude band where it is removed. This can be attributed to the role of momentum fluxes. The flux convergence of AAM is the dominant forcing for the poleward movement of zonal AAM anomalies. The torques appear as a response to the propagation of anomalies. The systematic poleward moving source or sink of AAM is produced by the phasing of fluxes due to tropical zonal mean mass circulations and those due to mid-latitude synoptic and other eddies.