Numerical simulation on the impact of global mountain uplift on the subtropical arid climate

Abstract

The global cooling, regional tectonic uplift, and land-sea redistribution have long been considered as the main drivers of the long-term evolution of dry climates during the Cenozoic. However, their relative importance is still a matter of hot debate. The effect of mountain uplift on forming the global mid-latitude dry climates has been widely investigated by lines of geologic evidences and numerical experiments based on atmospheric general circulation models. These previous simulation results are directly derived from the mountain climatic effects, without considering the superimposed effects due to sea surface temperature changes caused by mountain uplift. In order to address this issue, the impact of global topography on the arid environment is investigated by three numerical experiments of different mountain uplift scenarios performed with a global coupled atmosphere–ocean model CESM. The CESM consists of atmosphere, land, ocean, and sea ice components that exchange state information and fluxes through a coupler. The low-resolution version of CESM with less computationally expensive is used for this study. In addition, an aridity index is further taken to measure the global dryland area driven by mountain uplift. The aridity index is defined as the ratio of annual precipitation to potential evapotranspiration. The latter represents the evaporative demand of the atmosphere, which is composed of four meteorological parameters including surface air temperature, relative humidity, wind speed, and available energy. The relative contribution of these five factors to the aridity index is further evaluated to identify the underlying potential mechanism in response to the mountain uplift. By using the three simulations with modern topography, without global topography, and without the Tibetan Plateau, it is accessible to examine how the global mountain uplift affects the factors controlling the aridity index. In the global no-topography experiment, the subtropical arid zones in all continents exhibit an overall symmetrically zonal distribution. As the global mountain uplifts, on the one hand, southeastern and central Asia, southeastern North America, and central South America vary from arid to wet areas, while inland Eurasia and the coast of East Africa degenerate from humid to arid areas. The Tibetan Plateau uplift mainly affects the limited areas of Eurasia and eastern Africa. On the other hand, the topography of Eurasia and Africa relieves the aridification of coastal and northern North Africa, but to a certain extent, exacerbates the arid environment in central North Africa and western North and South America. The Tibetan Plateau uplift aggravates the drought climate on its western side, including the Middle East and most parts of North and East Africa. As a whole, the global mountain uplift leads to an overall decrease in the subtropical arid areas, with the expanded arid areas being less than the increased wet areas. The quantitative analysis of aridity index indicates that, the global topography uplift affects the terrestrial drought climate primarily by changing precipitation, and then by modifying the potential evapotranspiration. Their contributions to the aridity index are qualitatively consistent in the spatial pattern but different in magnitude. Among the impact factors of the potential evapotranspiration, the uplift of the terrain affects the aridity mainly through changes in relative humidity and near-surface air temperature, and then near-surface wind speed. To some extent, these simulation results agree qualitatively with the evidence of reconstructed geological records and previous atmosphere-only model results, indicating that the present study has scientific and palaeoclimate significance and thus helps to deepen the understanding of the climatic effect of global topography.