Weickmann, K., 2003: Mountains, the global frictional torque, and the circulation over the Pacific-North American Region. Mon. Wea. Rev., 131, 2608-2622.


ABSTRACT

The global mountain (M) and frictional (F) torques are lag correlated within the intraseasonal band, with F leading M. The correlation accounts for 20%-45% of their variance. Two basic feedbacks contribute to the relationship. First, the mountain torque forces global atmospheric angular momentum (AAM) anomalies and the frictional torque damps them; thus, dF/dt -M. Second, frictional torque anomalies are associated with high-latitude sea level pressure (SLP) anomalies, which contribute to subsequent mountain torque anomalies; thus, dM/dt F. These feedbacks help determine the growth and decay of global AAM anomalies on intraseasonal timescales.

The low-frequency intraseasonal aspect of the relationship is studied for northern winter through lag regressions on F. The linear Madden-Julian oscillation signal is first removed from F to focus the analysis on midlatitude dynamical processes. The decorrelation timescale of F is similar to that of teleconnection patterns and zonal index cycles, and these familiar circulation features play a prominent role in the regressed circulation anomalies.

The results show that an episode of interaction between the torques is initiated by an amplified transport of zonal mean-zonal momentum across 35N. This drives a dipole pattern of zonal mean-zonal wind anomalies near 25 and 50N, and associated SLP anomalies. The SLP anomalies at higher latitudes play an important role in the subsequent evolution. Regionally, the momentum transport is linked with large-scale eddies over the east Pacific and Atlantic Oceans that have an equivalent barotropic vertical structure. As these eddies persist/amplify, baroclinic wave trains disperse downstream over North American and east Asian topography. The wave trains interact with the preexisting, high-latitude SLP anomalies and drive them southward, east of the mountains. This initiates a large monopole mountain torque anomaly in the 20-50N latitude band. The wave trains associated with the mountain torque produce additional momentum flux convergence anomalies that 1) maintain the zonal wind anomalies forced by the original momentum transport anomalies and 2) help drive a global frictional torque anomaly that counteracts the mountain torque. Global AAM anomalies grow and decay over a 2-week period, on average.

Over the Pacific-North American region, the wave trains evolve into the Pacific-North American (PNA) pattern whose surface wind anomalies produce a large portion of the compensating frictional torque anomaly. Case studies from two recent northern winters illustrate the interaction.