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

Abstract

Understanding what controls the water vapor isotopic composition of the sub-cloud layer (SCL) over tropical oceans (δD 0 ) is a first step towards understanding the water vapor isotopic composition everywhere in the troposphere. We propose an analytical model to predict δD 0 motivated by the hypothesis that the altitude from which the free tropospheric air originates (z orig ) is an important factor: when the air mixing into the SCL is lower in altitude, it is generally moister, and thus it depletes the SCL more efficiently. We extend previous simple box models of the SCL by prescribing the shape of δD vertical profiles as a function of humidity profiles and by accounting for rain evaporation and horizontal advection effects. The model relies on the assumption that δD profiles are steeper than mixing lines, and that the SCL is at steady state, restricting its applications to timescales longer than daily. In the model, δD 0 is expressed as a function of z orig , humidity and temperature profiles, surface conditions, a parameter describing the steepness of the δD vertical gradient, and a few parameters describing rain evaporation and horizontal advection effects. We show that δD 0 does not depend on the intensity of entrainment, in contrast to several previous studies that had hoped that δD 0 measurements could help estimate this quantity.Based on an isotope-enabled general circulation model simulation, we show that δD 0 variations are mainly controlled by mid-tropospheric depletion and rain evaporation in ascending regions and by sea surface temperature and z orig in subsiding regions. In turn, could δD 0 measurements help estimate z orig and thus discriminate between different mixing processes? For such isotope-based estimates of z orig to be useful, we would need a precision of a few hundred meters in deep convective regions and smaller than 20 m in stratocumulus regions. To reach this target, we would need daily measurements of δD in the mid-troposphere and accurate measurements of δD 0 (accuracy down to 0.1 ‰ in the case of stratocumulus clouds, which is currently difficult to obtain). We would also need information on the horizontal distribution of δD to account for horizontal advection effects, and full δD profiles to quantify the uncertainty associated with the assumed shape for δD profiles. Finally, rain evaporation is an issue in all regimes, even in stratocumulus clouds. Innovative techniques would need to be developed to quantify this effect from observations.

Rain Evaporation, Snow Melt, and Entrainment at the Heart of Water Vapor Isotopic Variations in the Tropical Troposphere, According to Large‐Eddy Simulations and a Two‐Column Model

“…This suggests that the overestimated sensitivities of h, δD v , and δD p to domain-mean precipitation amount all share the same reason. A first possible reason is the neglect of horizontal isotopic gradients (Bony et al, 2008;Risi, Bony, Vimeux, & Jouzel, 2010;Risi et al, 2019). Horizontal advection acts to bring enriched water vapor from dry to deep-convective regions, damping daily δD v variations in regions of large-scale ascent by about 25% (Risi et al, 2019).…”

How Rossby wave breaking modulates the water cycle in the North Atlantic trade wind region

“…This anomalous behaviour is linked to a poleward shift in the midlatitude jet in January-February 2018 over the western North Atlantic and a positive North Atlantic Oscillation index (NAO; see Supplement S2). Several studies have shown that anticyclonic Rossby wave breaking (ARWB) over the North Atlantic occurs preferentially in the positive phase of the NAO (Benedict et al, 2004;Rivière and Orlanski, 2007). Furthermore, January-February 2018 was characterised by a relatively weak deep-convective activity in the northern part of South America as well as a southward shift in the ITCZ over the South Atlantic compared to climatology (see Supplement S2).…”

What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large‐Eddy Simulations

“…It is inspired by the SCL water budget presented in Risi et al. (2019), itself inspired by (Benetti et al., 2014, 2015). It is an extension of the closure equation by Merlivat and Jouzel (1979).…”

“…A first step is to understand the mechanisms controlling the isotopic composition of water vapor near the surface of tropical (30°S to 30°N) oceans, since it then supplies water to all other regions of the atmosphere (Risi et al, 2019). Indeed, this water vapor is an important moistening source to air masses traveling to land regions (Gimeno et al, 2010;Ent & Savenije, 2013) and toward higher latitudes (Ciais et al, 1995;Delaygue et al, 2000).…”