Gradients in a Tropical Mountain Ecosystem — a Synthesis

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The climate of the research area is mainly influenced by the easterlies of the trade wind regime (see Chapter 8 in this volume) and thus by the Atlantic circulation patterns. The Pacific circulation patterns play a minor role but associated westerlies modify the regional weather in the study area, especially in the austral summer. Due to the dominance of the Atlantic circulation, the Pacific ENSO phenomenon does not cause extraordinary anomalies, whereas La Nina situations induce somewhat stronger effects (see Chapter 20). Weather extremes in the study area are initiated by aperiodic but regularly occurring circulation patterns. Cold air surges (friajes) from the south can cause severe plunges in air temperature, which in turn can affect plant life. One unexpected finding is the marked impact of nocturnal mesoscale circulation processes in the near Amazon on cloudiness and rainfall in the Rio San Francisco valley. Also remarkable is the strong spatial gradient of humidity between Loja and the RBSF, over a beeline distance of less than 20 km where the crest of the eastern Cordillera is a clear weather divide. While rainfall at RBSF peaks in austral winter, the basins west of the main Cordillera are characterized by two rainy seasons peaking in March and October, respectively. The rainfields that generate the perhumid climate of the RBSF cannot reach the basin of Loja because they are released from relatively shallow orographic cap clouds in the upper part of the RBSF area, which are not able to pass the main ridge of the eastern Cordillera. The summer rains in the austral summer, especially in the basins of Loja and Vilcabamba, are related to a westerly stream flow initiated by the Pacific circulation. Ionic concentrations of rain and fog precipitation water in the RBSF area are generally low. But global and to a lesser extent regional circulation processes are also responsible for substantial atmospheric matter intake , mostly via wet deposition along the atmospheric pathway (see Chapter 22). Biomass burning in the Amazon is the major source of atmospheric matter intake via the easterlies. This remote source especially provides significantly enhanced