Importance of the Saharan heat low in controlling the North Atlantic free tropospheric humidity budget deduced from IASI <i>δ</i>D observations

Abstract

Abstract. The isotopic composition of water vapour in the North Atlantic free troposphere is investigated with Infrared Atmospheric Sounding Interferometer (IASI) measurements of the D / H ratio (δD) above the ocean. We show that in the vicinity of West Africa, the seasonality of δD is particularly strong (130 ‰), which is related with the influence of the Saharan heat low (SHL) during summertime. The SHL indeed largely influences the dynamic in that region by producing deep turbulent mixing layers, yielding a specific water vapour isotopic footprint. The influence of the SHL on the isotopic budget is analysed on various time and space scales and is shown to be large, highlighting the importance of the SHL dynamics on the moistening and the HDO enrichment of the free troposphere over the North Atlantic. The potential influence of the SHL is also investigated on the inter-annual scale as we also report important variations in δD above the Canary archipelago region. We interpret the variability in the enrichment, using backward trajectory analyses, in terms of the ratio of air masses coming from the North Atlantic and air masses coming from the African continent. Finally, the interest of IASI high sampling capabilities is further illustrated by presenting spatial distributions of δD and humidity above the North Atlantic from which we show that the different sources and dehydration pathways controlling the humidity can be disentangled thanks to the added value of δD observations. More generally, our results demonstrate the utility of δD observations obtained from the IASI sounder to gain insight into the hydrological cycle processes in the West African region.

Contrasting stable water isotope signals from convective and large-scale precipitation phases of a heavy precipitation event in southern Italy during HyMeX IOP 13: a modelling perspective

“…More recent studies (e.g. Lee et al, 2016Lee et al, , 2017 pointed out a significant moisture contribution, one-quarter of the total integrated water vapour, from north Africa in the middle troposphere (3-5 km above sea level, a.s.l.) feeding the deep convective systems together with the local water vapour sources over the Mediterranean in the lower troposphere (below 2 km a.s.l.).…”

“…Thanks to a tremendous expansion in the number of datasets of water vapour isotopic composition and a substantially improved set of theories and models for interpreting them, the related studies have been expanded during the past several years (e.g. Pfahl et al, 2008;Bonne et al 2014;Aemisegger et al, 2015;Dütsch et al, 2018;Lacour et al, 2017;Christner et al, 2018).…”

“…Aemisegger and Papritz (2018) and Aemisegger and Sjolte (2018) showed that the important moisture uptake by cold and dry airstreams during events of strong large-scale ocean evaporation carries a distinct SWI signature in water vapour. Recent studies Lacour et al, 2017) analysed the influence of the Saharan heat low on the isotopic budget of offshore west Africa on various temporal and spatial scales, highlighting the importance of the Saharan heat low dynamics on the moistening and the SWI enrichment of air parcels in the free troposphere over the North Atlantic. In addition, Risi et al (2008) used stable isotopic signals to better understand convective precipitation processes.…”

A Framework to Study Mixing Processes in the Marine Boundary Layer Using Water Vapor Isotope Measurements

“…In a more realistic framework, the MBL water vapor can be considered as the result of mixing between the free troposphere and the evaporative flux. Mixing models between two air parcels can be elaborated from mass balance equations (Galewsky & Hurley, ; Noone et al, ) to deduce information on the moistening/dehydrating processes in the atmosphere (e.g., Conroy et al, ; Lacour et al, ; Worden et al, ). Usually, these mixing models use constant values for the two end‐members and the moist and enriched end‐member is often set as the water saturated at the ocean surface or as the subtropical water vapor.…”

Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes

“…The water vapor isotopic composition (e.g., δD = (R/R SMOW − 1) × 1000 expressed in per mill, where R is the D/H ratio and SMOW is the Standard Mean Ocean Water reference), has been shown to be sensitive to a wide range of atmospheric processes , such as continental recycling (Salati et al, 1979;Risi et al, 2013); unsaturated downdrafts (Risi et al, , 2010a; rain evaporation (Worden et al, 2007;Field et al, 2010); the degree of organization of convection (Lawrence et al, 2004;Tremoy et al, 2014); the convective depth (Lacour et al, 2017b); the proportion of precipitation that occurs as convective or large-scale precipitation (Lee et al, 2009;Kurita, 2013;Aggarwal et al, 2016); vertical mixing in the lower troposphere Galewsky, 2018a, b), mid-troposphere (Risi et al, 2012b) or upper-troposphere (Galewsky and Samuels-Crow, 2014); convective detrainment (Moyer et al, 1996;Webster and Heymsfield, 2003); and ice microphysics (Bolot et al, 2013). It is therefore very challenging to quantitatively understand what controls the isotopic composition of water vapor.…”