A Climatology of Strong Large-Scale Ocean Evaporation Events. Part II: Relevance for the Deuterium Excess Signature of the Evaporation Flux

Abstract

AbstractThis paper discusses the relevance of transient events of strong large-scale ocean evaporation (SLOE) for the deuterium excess of marine boundary layer vapor d using a theoretical framework that invokes the closure assumption. We argue that during SLOE events, d is essentially determined by the evaporation flux signature. Distinct high d during SLOE with global-mean values in the range of 12??23? depending on the nonequilibrium fractionation factor αk result from the large air?sea humidity gradients reflected in low relative humidity with respect to sea surface temperature (hs = 53% ± 9%) that characterize these events. Extratropical cyclones are highlighted as an important driver for the variability of d. On the one hand, they are themselves associated with high hs and low d, especially in areas of cloud formation and precipitation in the warm sector. On the other hand, cyclones are the main driver inducing SLOE events with high d in regions of cold-air advection upstream of their path. The sensitivity of d to its direct climate controls (hs and SST) is analyzed during SLOE for different αk formulations and found to be coherent with d?hs and d?SST slopes determined from available observations. The d?hs relationship exhibits a robust negative correlation as opposed to the d?SST relationship, which shows regional and time-scale-dependent variations in strength and sign that are induced by indirect hs?SST cross-correlation effects. The dynamical features involved in SLOE generation appear to exert a key control on the moisture source properties relevant for d in the extratropics.