A high pressure system crossing westward the Andes Mountains

From February 25 to March 2 1997, a high pressure system moved exceptionally from the ENE to WSW, crossing the Andes Mountain (February 27-28) between 20° S and 35° S, this system was extended in almost all the troposphere. The purpose of this paper is to study the trajectory of this system and to analyze the mechanisms that allowed it to pass through the Andes. Both, synoptic and dynamic understanding of this case is complemented and deepened by the analysis of isentropic potential vorticity. Data from observations and analysis provided by the Servicio Meteorológico Nacional Argentino were used for the synoptic study. Furthermore, the grid point data available from the National Center for Atmospheric Research was used to build the isentropic dataset.

The Southern Hemisphere circulation during February was dominated by above normal heights throughout the middle latitudes and below normal heights at high latitudes. This pattern was associated with a continued poleward shift of the mid-latitude westerlies. The circulation during March presented similar characteristics and, in this month, well above-normal temperatures were observed across southern South America (Climate Diagnostics Bulletin, February and March, 1997). From February 20 to March 2, the dominant wave was the number 3. The field was cuasi - stationary in middle latitudes from February 25 and it became retrograde from February 28 to March 1 nearby 100° W.

In February the anomaly field of 500 hPa. was positive but weak in the subtropical region. In March, the subtropical belt at 500 hPa. was composed by two anticyclonic centers, one in the Bolivian Plateau and the other on the southern of Brazil and the adjacent Atlantic Ocean.

The potential vorticity combines both the wind and the mass fields into one quantity, for this reason it was used to understand and visualize the evolution of the system in isentropic coordinates. The selected level was the 315 K surface. An anticyclonic potential vorticity center located at 25° S and 55° W approximately, began to move westward (February 25). On February 27, this center arrived to the Andes Mountains a little more southern than its origin. Next day, an appendix of the anticyclonic potential vorticity center appeared at West Side of the mountains. Between March 1 and 2, it was on the Pacific Ocean and then it began to move northward. Considering the thermodynamic and mechanic effects, in different points of the trajectory, showed that both of them acted in joint form. The centers of anticyclonic relative vorticity agree with those of potential vorticity, whereas low values of static stability accompany the system. It is interesting to notice that the thermodynamic effects seemed to contribute to the system decay (near 25° S on March 2), when it lacked the heat source, represented by the continental area.