Evolution of South American high pressure systems during late summer 1997

A high pressure system between 20degreesS and 35degreesS, moving from east to west, passed over the Andes mountains on 27 and 28 February 1997. Some days earlier, on 20 February, another centre of anticyclonic vorticity bad moved westward without crossing the Andes. These events developed during a South American Seesaw System (SASS) event that bad been displaced southward from its climatological position. This southward displacement of the local circulation pattern was accompanied by intense convective activity during the positive phase of the event, which favoured the development of strong storms in north and central Argentina. The unusual configuration of the motion field gave rise to atypically high temperatures in the mid-latitude Argentinian region. Both anticyclonic systems reached the mountain barrier: one from the north and the other from the south. The analysis of the factors involved in this phenomenon reveals that the static stability in each system behaved differently near the mountain barrier. Advection of potential vorticity was responsible for the westward motion of the system, while the radiative processes influenced the production of cyclonic and anticyclonic potential vorticity which balanced out during each day.