It has been speculated that the collapse of the Maya civilization in the Yucatan region of Mexico around 900 AD was caused by drought. A 10,000-year simulation with the CSIRO Mark 2 coupled global climatic model has been used to investigate such a possibility. The model replicates sporadic, severe drought over the Yucatan consistent with the above speculation. It was found that these droughts were specifically constrained to the Central American area, with no obvious linkages to other regions. An investigation of the mechanisms associated with rainfall fluctuations over the Yucatan indicates that these were not caused by sea surface temperature variations. Fluctuations in the intensity of the topographically constrained meridional wind systems located on both the western and eastern coasts of the Americas were found to be the dominant influence. The sensitivity of the Yucatan to drought episodes arises from its location at the convergence zone of these wind systems. It is concluded that the severe drought episodes in this region are a consequence of stochastic fluctuations of these wind systems and that external influences are not necessary.
Climatic simulations suggest that only slight changes in climatic variability are likely to be induced owing to greenhouse warming, and that these changes will project onto existing modes of climatic variability. A lesser studied aspect of the interaction of climatic variability with climatic change is how such a mutual interaction manifests itself as interannual climatic fluctuations. A number of examples are examined in this paper using simulations made with the Commonwealth Scientific and Industrial Research Organization (CSIRO) Mark 2 coupled climatic model. The simulations included an ensemble based on four Special Report on Emission Scenarios (SRES) cases, as well as individual ensembles for two selected SRES cases. In general, the intra-ensemble variability for a given SRES case was quite similar to the inter-ensemble variability of the four individual SRES cases. Time series of selected climatic variability and probability density function displays are used to illustrate the character of climatic variability for simulations out to 2100. Other examples include variations in droughts and pluvial events, and associated runoff, as the greenhouse effect progresses. The differing responses in the frequency of dry events among four SRES cases are also illustrated. The outbreak of cold events around 2050 is used to highlight the impact of climatic variability. Finally, case studies involving two large-scale climatic phenomena are used to show the ongoing dominance of climatic variability. RÉSUMÉ [traduit par la rédaction] Selon diverses simulations climatiques, le réchauffement dû à l'effet de serre ne devrait engendrer que de petits changements dans la variabilité climatique et ces changements s'exprimeront à l'intérieur des modes de variabilité climatique existants. Un aspect moins étudié de l'interaction entre la variabilité climatique et le changement climatique est la façon dont une telle interaction se manifeste comme des fluctuations climatiques interannuelles. Dans cet article, on examine un certain nombre d'exemples en se servant de simulations réalisées au moyen du modèle climatique couplé Mark 2 du Commonwealth Scientific and Industrial Research Organization (CSIRO). Les simulations comprenaient un ensemble basé sur quatre cas présentés dans le Special Report on Emission Scenarios (SRES) ainsi que des ensembles distincts pour deux cas SRES sélectionnés. En général, la variabilité intra-ensemble pour un cas SRES donné était assez semblable à la variabilité inter-ensemble des quatre cas SRES. On emploie des séries chronologiques de variabilité climatique sélectionnée et des tableaux de fonction de densité pour illustrer le caractère de la variabilité climatique pour des simulations s'étendant jusqu'en 2100. D'autres exemples portent sur les variations dans les sécheresses et les événements de pluie, de même que le ruissellement correspondant, à mesure que l'effet de serre progresse. On fait aussi ressortir les différences de réponses dans la fréquence des événements secs parmi les quatre cas SRES. On se ...
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