Influences of ‘La Niña’ on the increased frequency of snowfalls over the low lands of Cuyo during the 2007 winter

ARANEO, DIEGO C.

Santiago de Chile

Conferencia; Fourth EGU Alexander von Humboldt International Conference. The Andes: Challenge for Geosciences; 2008

The frequency of the snowy events in the low lands of the Argentinean Cuyo region during the winter of 2007 was among the highest in the last years. At the same time, the Sea Surface Temperature (SST) anomalies over the Pacific and Indian oceans evolved toward a "La Niña" pattern, which reached maturity during January and February of 2008.This work analyses the main dynamic mechanisms and hemispheric patterns of the atmospheric circulation that induced the recurrent snowfalls on the Cuyo plain, in connection with the atmospheric forcings that were originated in the anomalous SST on the tropical Pacific and Indian oceans.The results showed an anomalous divergence on the Indian Ocean at high levels in the troposphere, associated to positive SST anomalies on the region. This divergence gave origin to a quasi-stationary Rossby waves source at major latitudes which zonally propagated eastward among ~45° and 70° S, getting to affect the south of South America from the west and reaching the South Atlantic. The generated wave train showed a hemispheric three-wave pattern, while the mean zonal wind component at 200-300 hPa levels helped maintaining its quasi-stationary condition. The negative SST anomalies over the equatorial Pacific favored the straightness of the Pacific semi-permanent Anticyclone, with positive geopotential height (GH) anomalies at ~45°S-110°W, observed through all of the troposphere levels. This centre of positive GH anomalies was part of the mentioned wave train, which helped to its almost constant persistence from early June to late September. On the South Atlantic in front of the Argentinean coasts, a center of negative GH anomalies was observed extending a trough axis that crossed the continent through the high Andes of central Argentina and Chile. The resultant anomalous circulation between both centers left the South of the continent dominated by a southern flow which advected polar air over the Cuyo region.Under these conditions, the snowy events over the Cuyo plain took place when the blocking exerted by the high pressures (over the position of the positive GH anomalies), drove the low-pressure systems from the south toward the continent. This then inclined a trough axis in NW-SE direction at high levels crossing the central part of Argentina and Chile and leaving a cut-off trough over the Pacific in front of the central Chile coasts. At low levels, the entrance of an anticyclonic system took place around the southern extreme of the continent from the south in SW-NE direction. The system deepened on the oriental flank of the ridge at high levels, thus advecting polar air over the Cuyo region, while a system of low pressure was located in front of the Argentinean coasts. The conjunction of the polar air advected by the anticyclone on surface, added to the permanency of the cut-off trough on the central Chile coasts in height, gave origin to the snowfalls on the Cuyo plain. The high frequency of these events along the analyzed period had its origin in the persistence of the hemispheric pattern of the described quasi-stationary Rossby waves. The pattern could be simultaneously associated to the dominant configurations of the SST anomalies over the tropical Pacific and Indian oceans observed during those months of 2007.