Craig–Gordon model validation using stable isotope ratios in water vapor over the Southern Ocean

Shaakir Shabir Dar,Ankit Swaraj,Anil Kumar,Prosenjit Ghosh

doi:10.5194/acp-20-11435-2020

Abstract

Abstract. The stable oxygen and hydrogen isotopic composition of water vapor over a water body is governed by the isotopic composition of surface water and ambient vapor, exchange and mixing processes at the water–air interface, and the local meteorological conditions. These parameters form inputs to the Craig–Gordon models, used for predicting the isotopic composition of vapor produced from the surface water due to the evaporation process. In this study we present water vapor, surface water isotope ratios and meteorological parameters across latitudinal transects in the Southern Ocean (27.38–69.34 and 21.98–66.8∘ S) during two austral summers. The performance of Traditional Craig–Gordon (TCG) (Craig and Gordon, 1965) and the Unified Craig–Gordon (UCG) (Gonfiantini et al., 2018) models is evaluated to predict the isotopic composition of evaporated water vapor flux in the diverse oceanic settings. The models are run for the molecular diffusivity ratios suggested by Merlivat (1978), Cappa et al. (2003) and Pfahl and Wernli (2009), referred to as MJ, CD and PW, respectively, and different turbulent indices (x), i.e., fractional contribution of molecular vs. turbulent diffusion. It is found that the UCGx=0.8MJ, UCGx=0.6CD, TCGx=0.6MJ and TCGx=0.7CD models predicted the isotopic composition that best matches with the observations. The relative contribution from locally generated and advected moisture is calculated at the water vapor sampling points, along the latitudinal transects, assigning the representative end-member isotopic compositions, and by solving the two-component mixing model. The results suggest a varying contribution of the advected westerly component, with an increasing trend up to 65∘ S. Beyond 65∘ S, the proportion of Antarctic moisture was found to be prominent and increasing linearly towards the coast.

Highlights

The knowledge of factors governing the evaporation of water from the oceans is an essential part of our understanding of the hydrological cycle
The δ18O values of ocean surface water extracted from the Global Seawater Oxygen-18 Database (SWD) (Schmidt et al, 1999) are plotted
The isotopic composition of water vapor and surface water samples collected across a latitudinal transect from Mauritius to Prydz Bay in the Southern Ocean are described

Summary

Introduction

The knowledge of factors governing the evaporation of water from the oceans is an essential part of our understanding of the hydrological cycle. Evaporation of ocean water generates vapor and forms the initial reservoir for circulation in the hydrological cycle. Measurements of the isotope composition of water in the various reservoirs of the hydrological cycle operating over the oceans is useful to infer information about the origin of water masses and to understand the formation mechanisms, transport pathways and the precipitation processes (Craig, 1961; Dansgaard, 1964; Yoshimura, 2015; Gat, 1996; Araguás-Araguás et al, 2000; Noone and Sturm, 2010; Gat et al, 2003; Benetti et al, 2014; Galewsky et al, 2016).

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