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European Geosciences Union, Atmospheric Chemistry and Physics Discussions, 19(15), p. 27219-27251

DOI: 10.5194/acpd-15-27219-2015

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Detecting moisture transport pathways to the subtropical North Atlantic free troposphere using paired H2O-δ D in situ measurements

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

We present two years of measurements of water vapour (H 2 O) and its isotopologue ratio (δD, the standardized ratio between H 2 16 O and HD 16 O) made at two remote mountain sites on Tenerife Island in the subtropical North Atlantic. We show that the data – if measured during nighttime – are well representative for the lower/middle free troposphere. We use the measured H 2 O-δD pairs, together with dust measurements and back-trajectory modelling for analysing the moisture pathways to this region. We can identify four principally different transport pathways. The first two pathways are linked to transport from high altitudes and high latitudes, whereby the respective air can be dry, due to last condensation occurring at low temperatures, as well as humid, due to cross isentropic mixing with lower level and more humid air during transport since last condensation. The third pathway is transport from lower latitudes and lower altitudes, whereby we can identify rain re-evaporation as an occasional source of moisture. The fourth pathway is linked to the African continent, where during summer dry convection processes over the Sahara very effectively inject humidity from the boundary layer to higher altitudes. This so-called Saharan Air Layer (SAL) is then advected westward over the Atlantic and contributes to moisten the free troposphere. We demonstrate that different pathways leave distinct fingerprints on the measured H 2 O-δD pairs.