Land-air-sea interaction at the Antofagasta Region (Chile, 23 degrees S): The DICLIMA experiment

The extreme aridity of the desert along the west coast of South America fro m about 30 degrees S to the equator, has originated several hypotheses abou t physical mechanisms explaining the necessary additional subsidence to the large-scale one associated with the subtropical anticyclone. The DICLIMA e xperiment was aimed at the identification and quantification of the mechani sms responsible for the diurnal increase in the coastal subsidence toward w hich both the marine and the inland sloping boundary layers would contribut e. 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 19 98. The severity of the El Nino event that begun in March 1997 dominated th e global ocean and atmosphere conditions during the last two experiments. G lobal results show significant daily-cycle signals in the temperature, wate r vapor mixing ratio and wind at least until 4 000 m altitude, revealing th e conclusive influence of the insolation cycle at the western Andean slope over those signals and in the daytime divergence over the coastal strip. Da y to day Variations in the height of the base of the subsidence inversion s howed oscillations similar to those characterizing the passage of coastal l ows over higher latitudes in central Chile. Estimates based on surface ener gy budget measurements over land yield figures compatible with average offs hore 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-satu rated in CO2 were observed, with a strong dependence on physical factors de rived from the El Nino presence and to a localized biological pump action o nly in January 1997. In the frame of the global climate variability, the re gime shift (climatic jump) observed in the mid 70s was reflected in north a nd central Chile in the sea-surface and tropospheric temperatures, air mois ture and cloudiness. Those changes would be associated with a general weake ning of the subsidence inversion consistent with the fall in the average va lue of the Southern Oscillation index.