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[Pisky]

Il ruolo del AAM (Atmospheric Angular Momentum) e il Mountain Torque:

"Atmospheric angular momentum (AAM) provides a convenient framework to study the role of mountains, surface wind stresses and various transport mechanisms in variability ranging from intraseasonal to interdecadal and beyond. Quantitative studies are feasible with current global assimilated datasets which show a good budget balance for global integrals, intraseasonal variations and during northern winter/spring. The budgets get much worse when gravity wave drag is included, if zonal integrals are considered or during summer/fall seasons. AAM is useful as an index of the large scale zonal flow since it is highly correlated with independent length-of-day measurements and with phenomena such as the QBO, ENSO, the MJO and possibly global warming".

e poi:

"A composite study of the MJO AAM signal was performed using 30-70 day filtered data and global AAM tendency as a linear regression "predictor". Large-scale eddies forced by tropical convection were found to play a dominant role in forcing the AAM response during northern winter (November-March). As these eddies move eastward within the asymmetric basic state, they produce a poleward movement of momentum transports. When this upper tropospheric momentum source reaches the subtropics, it induces a vertically deep mass circulation and a subtropical friction torque anomaly, which causes AAM to change. For the mountain torque, a combination of equatorial Kelvin waves, forced by convection over Indonesia and then impinging on the Andes and mass exchanges along the east slopes of the Himalayas and Rockies were found. The former is more directly linked with the MJO while the latter is a residual of large amplitude, ubiquitous torques induced by mid-latitude synoptic wavetrains. In fact, a case study of an individual MJO, where the mountain torque was 3x larger than the frictional torque, prompted a linear model investigation of the torques and their role in forcing intraseasonal AAM anomalie".

"A simple Markov model of the three-component system comprised of total AAM, friction and mountain torque was developed. Statistical analysis of the observed data suggested AAM anomalies are damped on a 30 day time scale by the friction torque and forced stochastically by a "white" mountain torque and a "red" friction torque. The resulting Markov model has variances and lagged covariances that compare favorably to the observations, although systematic differences representing approximately 20-30% of the variance also emerge (see Fig. 4.9). The most prominent differences are the greater coherence between the mountain and friction torque for 10-90 day periods in the observations (see Fig. 4.9e) and the excessive AAM variance at ~2*pi*30 days (the AAM decay time scale) in the modeled spectrum (not shown)".
Immagine

Fig. 4.9 Coherence squared (left column) and phase (right column) between the variables labeled at the top of each panel. Three curves are shown in each panel: the thick solid lines are for the observations, the smooth curves are the theoretical cross spectra and the dashed lines are based on output from a red noise model. The coherence squared and phase (in radians) are plotted as a function of Fourier frequency (cycles/366 days) with selected labels along the abscissa showing the corresponding period in days. A positive phase means the first variable labeled above each panel lags the second variable.The synoptic features associated with global mountain and frictional torques were also studied. The primary synoptic structure producing the mountain torque during northern winter is seen in Fig. 4.10. Growth of quasi-geostrophic disturbances upstream of the mountains is followed by downstream dispersion of energy across the mountains. At the surface, mass anomalies propagate southward, likely reflecting topographic Rossby wave activity forced by low level flow impinging on the mountain slopes. Zonal mean momentum anomalies produced by the torque are transported out of the latitude band of the mountain. This is accomplished partially by the quasi-geostrophic wave activity. The momentum is dissipated in adjacent latitude bands by the friction torque which responds to the upper level momentum source produced by the transports. The mass anomalies are consistent with quasi-geostrophic balance of the surface flow. The decay time scale of the global mountain torque is ~1.5 days, i.e., it is nearly white for daily data.
Immagine

Fig. 4.10 The 250 mb streamfunction (contours) and sea level pressure (shading) anomalies that accompany a mountain torque over Asia (left column) and North America (right column) at days -3, 0, +3. The fourth panel in each column shows the mountain torque (Hadleys) as a function of latitude for each day. The curves labeled A, B and C correspond to days -3, 0 and 3 respectively.




gltaum.90day.gif

Mat, non ci ho capito granchè!

Sostanzialmente l'attrito con le Rocky Mountanins inizierà ad ondulare il getto?