干旱区生态环境知识资源中心

The extreme aridity of the desert along the west coast of South America from about 30 degrees S to the equator, has originated several hypotheses about physical mechanisms explaining the necessary additional subsidence to the large-scale one associated with the subtropical anticyclone. The DICLIMA experiment was aimed at the identification and quantification of the mechanisms responsible for the diurnal increase in the coastal subsidence toward which both the marine and the inland sloping boundary layers would contribute. Ten-day field experiments were performed at 23 degrees S in January 1997, at the end of the 1995-1996 La Nina event; in July 1997 and in January 1998. The severity of the El Nino event that begun in March 1997 dominated the global ocean and atmosphere conditions during the last two experiments. Global results show significant daily-cycle signals in the temperature, water vapor mixing ratio and wind at least until 4 000 m altitude, revealing the conclusive influence of the insolation cycle at the western Andean slope over those signals and in the daytime divergence over the coastal strip. Day to day Variations in the height of the base of the subsidence inversion showed oscillations similar to those characterizing the passage of coastal lows over higher latitudes in central Chile. Estimates based on surface energy budget measurements over land yield figures compatible with average offshore net heat export by the mean zonal currents measured simultaneously in January 1997. In the estimates of the CO2 fugacity over the water column (2 to 5 m) through PH and total alkalinity measurements, wide areas over-saturated in CO2 were observed, with a strong dependence on physical factors derived from the El Nino presence and to a localized biological pump action only in January 1997. In the frame of the global climate variability, the regime shift (climatic jump) observed in the mid 70s was reflected in north and central Chile in the sea-surface and tropospheric temperatures, air moisture and cloudiness. Those changes would be associated with a general weakening of the subsidence inversion consistent with the fall in the average value of the Southern Oscillation index.