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

This 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 α k result from the large air–sea humidity gradients reflected in low relative humidity with respect to sea surface temperature ( h s = 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 h s 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 ( h s and SST) is analyzed during SLOE for different α k formulations and found to be coherent with d– h s and d–SST slopes determined from available observations. The d– h s 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 h s–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.