Detecting moisture transport pathways to the subtropical North Atlantic free troposphere using paired H<sub>2</sub>O-<i>δ</i>D in situ measurements

Abstract

Abstract. We present two years of in situ 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 in the subtropical North Atlantic. We show that the data -if measured during night-time -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 air mass transport from high altitudes and high latitudes shows two different scenarios. The first scenario brings dry air masses to the stations, as the result of condensation events occurring at low temperatures. The second scenario brings humid air masses to the stations, due to cross-isentropic mixing with lower-level and more humid air during transport since last condensation (LC). The third pathway is transportation 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 the different pathways leave distinct fingerprints on the measured H 2 O-δD pairs.

The isotopic composition of near‐surface water vapor at the Maïdo observatory (Reunion Island, southwestern Indian Ocean) documents the controls of the humidity of the subtropical troposphere

“…The strong common finding between our study and previous ones at Mauna Loa observatory in Hawaii [Noone et al, 2011] and on Tenerife Island [González et al, 2016] is the capacity of pairing δ v measurements with q v to capture mixing processes at the diurnal scale between the ABL and the free troposphere although upslope flow dynamic can vary from the windward to the leeward side of island [Bailey et al, 2015].…”

“…We examine whether atmospheric large-scale subsidence can explain such isotopic depletions and how important extratropical transport is in setting the free tropospheric source in mixing processes (section 4.2). At last, we compare the dynamical processes identified in Reunion Island to influence the tropospheric humidity with processes identified in other similar subtropical locations in Hawaii [Noone et al, 2011;Bailey et al, 2013Bailey et al, , 2015 and Tenerife [González et al, 2016] Journal of Geophysical Research: Atmospheres 10.1002/2017JD026791 marine air at the MO during the day because of the upslope and sea breeze flow regimes. Then, the day value of δ 18 O V0 (À11.3‰ between 11 A.M. and 5 P.M.) has to be compared to our observations during daytime (À13.4‰).…”

“…Among these studies, isotopic measurements on stations located in a subtropical marine environment have provided new insights about the controls of the humidity of the atmospheric boundary layer (ABL) and the free troposphere. Noone et al [2011], Hurley et al [2012], and Bailey et al [2013] on Mauna Loa, Hawaii (subtropical North Pacific), and González et al [2016] in Tenerife, Canary Islands (subtropical North Atlantic), showed the relevance of water vapor exchange between the boundary layer and the free troposphere due to shallow convection at local scale. The later can transport boundary layer moisture across the trade wind inversion while entraining dry free tropospheric vapor into the boundary layer.…”

“…Subtropical humidity is also strongly affected by large-scale moisture transport and subsidence. Pairing measurements of humidity with isotopic observations have been used to detect extratropical moisture transport into the subtropics and to examine how important this transport is to influence moisture budget of the subtropical free troposphere [Hurley et al, 2012;Bailey et al, 2015;González et al, 2016].…”

Tropospheric water vapour isotopologue data (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, and HD<sup>16</sup>O) as obtained from NDACC/FTIR solar absorption spectra

“…For identifying and analysing different tropospheric moisture pathways, the distribution of {H 2 O, δD} pairs (e.g. Galewsky et al, 2005;Noone, 2012;González et al, 2016) or deuterium excess (d = δD − 8δ 18 ; e.g. Pfahl and Sodemann, 2014;Steen-Larsen et al, 2014;Aemisegger et al, 2014) are particularly promising.…”