Equatorial Mountain Torques and Large-Scale Mountain Flow Dynamics

The evolution of the two components of the Equatorial Mountain Torque (EMT) is analyzed in the NCEP reanalysis. For periods between 2 and 30 days, we find a strong lagged correlation between its component along the Greenwich axis (EMT1) and its component along the 90°E axis (EMT2): a positive EMT1 is followed by a negative EMT2. This is in part due to the flow dynamics around the major mountain ranges (Antarctica, the Himalayas, Greenland, the Rockies and the Andes), in which case it is associated with regional features such as the cold surges: a positive EMT1 is associated to high pressure anomalies to the North of the massive, these anomaly subsequently moves south-eastward creating a negative EMT2.

We then argue that these torques represent the dynamical influence of the mountains on the atmosphere. We use for that a theoretical model for the linear atmospheric response to a mountain forcing on the sphere, that is adapted from the Eady model for baroclinic instability. With this model, we show that a low level westward flow with positive shear aloft, produces a positive EMT along the equatorial axis that is at 90° to the east of the longitude of the mountain. The lee anti-cyclone development that follows, produce a negative mountain torque along the Equatorial axis, that is at the longitude of the mountain. If we take realistic mountain profiles, the model EMTs compare well with those observed, and the surface patterns of pressure and Temperature are reminiscent of those occurring during cold-surges.