Numerical Simulations of winter cyclone modifications by the Andes

The dynamical influence of mountains chains on the atmospheric flow is a topic that has been the focus of interest of many scientist on the world. This influence has a great dependence on the height, wide and orientation of the mountain chains. So the results obtained over the Alps can not be applied directly over the Rockies or the Andes. Furthermore, the two greatest mountain chains of America have very different height and wide, even though they have the same orientation, so they produce different effects over the atmospheric flow.

The main focus of the present study is to investigate how the Andes features, like its height and wide, modify the structure and propagation of baroclinic waves crossing it. The model used is an hydrostatic, dry, primitive equation model based on the anelastic Boussinesq approximation. The initial wind field comprises a zonal jet that resembles the Southern Hemisphere winter mean subtropical jet. Numerical simulations were performed considering as initial conditions the observed subtropical jet structure at 180ºW (maximum intensity). A localized barotropic vorticity perturbation of small amplitude is superimposed to the initial wind field and it is allowed to grow over flat terrain into a mature baroclinic wave. Then, the real Andes were introduced and the interaction between the baroclinic perturbation and the orography is simulated. This model run was taken as the control run (CR). Two others experiments were performed: one increasing the Andes wide to the double (E1) and the other one reducing the Andes height to the half (E2).

The CR case simulates many of the features of the interaction between baroclinic waves and the Andes in agreement with diagnostic studies, including the north-eastward propagation as perturbation cross the Andes. Upper level cyclonic perturbations cross the Andes splitting in two centers, one located around 40ºS and a second center is observed further north. Both centers intensify and maintain its identity as they propagate eastward. The perturbation acquires a NW-SE orientation as the subtropical center propagates slower. Both centers are also present in the E1 run, being the southern one weaker than the CR case. In the E2 case, the subtropical center is not present although the perturbation maintains a weaker north-eastward propagation. At lower levels, perturbations tend to conform to the shape of the Andes and they suffer an abrupt migration to the equator. This is particularly strong in the CR case. The cyclone on the lee tends to form 1000 km far of the mountain ridge and in the E1 case it forms lightly north than the CR case. Anticyclonic perturbations were also studied. These perturbations cross the Andes further south compared with the cyclonic perturbations and it shows a more latitudinal displacement.