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Water vapour transport to the high latitudes : mechanisms and variability from reanalyses and radiosoundings

Abstract : The water vapour converging to the polar regions condenses into heat-trappingclouds and eventually precipitates, freshening the polar oceans and thickening the ice-sheets. Modulo circulation changes, the moisture transport is expected to increase in a warmer climate. While the extra precipitation could dampen the ice sheets’ contribution to sea level rise, the surplus of moisture could also feed back on the surface warming. However, the present variability of the polar moisture budgets must be known precisely before they can be projected with confidence into the future.This study examines the atmospheric water cycle of both the Arctic and the Antarctic in seven global reanalyses and in radiosonde observations covering the 1979-2013 period. The impacts of known model and assimilation flaws and of the various numerical approximations were evaluated and proven to be limited, at least for the moisture flux variable and the more recent reanalyses.In the Arctic, aside from a slight overestimation, the northward fluxes in reanalyses exhibit a remarkable agreement with the radiosoundings in terms of spatial and temporal patterns. In all reanalyses, transient eddies provide the bulk of the mid-latitude moisture imports – 89-94% at 70◦ N. In most datasets, evaporation, precipitation and precipitable water increase in line with what is expected from a warming signal. However fluxes do not scale with the Clausius-Clapeyron relation because the increasing humidity is not correlatedwith the meridional wind, particularly near the surface.The representations of the Antarctic atmospheric water cycle in reanalyses suffer from the scarcity of observations : the moisture convergence estimations vary from 117 to 156 mm per year. On the coast, the mean moisture flux results from the interplay between transient eddies and katabatic winds, which are particularly sensitive to the orography. The coastalradiosonde sites report significant increases of the southward moisture fluxes but otherwise there are practically no trends in reanalyses on a continental scale.Finally, the share of transient eddies in moisture advection is qualified using alternate methods. In particular, extratropical cyclones leave a characteristic imprint on the transport field, which can be detected and quantified.
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Ambroise Dufour. Water vapour transport to the high latitudes : mechanisms and variability from reanalyses and radiosoundings. Earth Sciences. Université Grenoble Alpes, 2016. English. ⟨NNT : 2016GREAU042⟩. ⟨tel-01562031⟩



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