Séminaire
The climate results from the interaction of the atmospheric (and, sometimes, oceanic) large-scale circulation with the regional geography (mountains ranges, coastline geometry, presence of lakes, etc). Accordingly, a significant fraction of climate variability and climate change is caused by variations in the large-scale circulation, although the functional relationship can be quite complex.
In this work we investigate the large-scale factors that control the climate variability of Patagonia in southern South America. The region is embedded in the circumpolar belt of westerly flow and the storm track of the Southern Hemisphere, and divided by the Andes cordillera leading to a marked precipitation gradient between the hyper-humid conditions in the Pacific side and dry conditions to the east of the mountains. Previous studies have shown that year-to-year changes in zonal flow (U) are strongly and positively (negatively) correlated with annual-mean precipitation (R) changes to the west (east) of the Andes. We now extend this analysis to seasonal averages of precipitation and surface air temperature (SAT), and augmented the poor observational records by using results from a regional (25 km horizontal resolution) climate model (PRECIS) integrated between 1960 and 1990. The recalculated U-R relationship is quite stable year round and mostly similar to the previous (observational) result. In contrast, the new U-SAT relationship exhibits significant seasonal and geographical dependence. For instance, years with stronger than normal westerly flow are associated with milder than normal winters in western Patagonia but colder than normal summers in eastern Patagonia. The variations in SAT are interpreted in terms of the wind-mediated changes in temperature advection and surface energy balance. As a relevant application, we use the U-SAT and U-P relationships to obtain the changes in temperature and precipitation over Patagonia that are congruent with changes in the zonal wind during the second half of the XX century.