Coastal climate dynamics of the Antofagasta region (Chile, 23S): The 1997–1998 DICLIMA experiment

The search for physical mechanisms explaining the extreme dryness of the coastal subtropical desert along western South America, that extends equatorward well beyond the area of influence of the subtropical anticyclone, has been the aim of mostly descriptive studies.

The DICLIMA field experiment was designed to test and quantify the hypothesis of an afternoon coastal subsidence enhancement that would prevent the marine air from penetrating inland beyond the coastal escarpment and to assess the relative contributions of the marine and the inland (sloping) boundary layers in producing that extra subsidence.

Three ten-day field campaigns were performed nearby Antofagasta (23 ° S) on January 20 - 30, 1997 (DICLIMA I); on July 14 - 24, 1997 (DICLIMA II) and on January 19-28, 1998 (DICLIMA III). The very strong ENSO episode that developed since March 1997 dominated the adjacent ocean and the regional atmospheric conditions during the last two field experiments.

Preliminary results show significant diurnal cycles in temperature, mixing ratio and wind above the coastal subsidence inversion and up to 600 hPa, revealing a deep influence of the solar heating cycle over the west slope of the Andes that reach altitudes in excess of 5000 m at that latitude. This resulted in a strong diurnal cycle of cross-shore boundary layer divergence/convergence in the afternoon/early morning encompassing a layer between 1200 and 2500 m altitude. Since the vertically integrated cross-shore mass transport cycle below the inversion is one order of magnitude less than above it, the diurnal mass divergence aloft seems to account for the required extra subsidence along the coastal strip.

Day to day variability in the height of the subsidence inversion revealed oscillations similar to those characterizing the passage of coastal-trapped atmospheric disturbances (e.g. coastal lows) further south. Such oscillations, while constituting relatively minor features during both austral summer campaigns, became significant in a particular event in the DICLIMA II winter field experiment.