What controls deuterium excess in global precipitation?

Abstract

Abstract. The deuterium excess (d) of precipitation is widely used in the reconstruction of past climatic changes from ice cores. However, its most common interpretation as moisture source temperature cannot directly be inferred from present-day water isotope observations. Here, we use a new empirical relation between d and near-surface relative humidity (RH) together with reanalysis data to globally predict d of surface evaporation from the ocean. The very good quantitative agreement of the predicted hemispherically averaged seasonal cycle with observed d in precipitation indicates that moisture source relative humidity, and not sea surface temperature, is the main driver of d variability on seasonal timescales. Furthermore, we review arguments for an interpretation of long-term palaeoclimatic d changes in terms of moisture source temperature, and we conclude that there remains no sufficient evidence that would justify to neglect the influence of RH on such palaeoclimatic d variations. Hence, we suggest that either the interpretation of d variations in palaeorecords should be adapted to reflect climatic influences on RH during evaporation, in particular atmospheric circulation changes, or new arguments for an interpretation in terms of moisture source temperature will have to be provided based on future research.

Synoptic scale controls on the δ18O in precipitation across Beringia

“…These differences reflect the slower diffusivity of H 2 18 O relative to HDO during evaporation above the ocean surface [Dansgaard, 1964]. Specifically, high d-excess values arise when there is insufficient time for vapor to equilibrate between the saturated ocean surface layer and the subsaturated atmosphere [e.g., Pfahl and Sodemann, 2014]. Evaporated moisture entrained in the Arctic (as opposed to Pacific) is subject to such conditions, whereby large-humidity gradients between the ocean surface and the dry atmosphere above-particularly at the sea ice margin [e.g., Kurita, 2011]-will lead to strong nonequilibrium (kinetic) fractionation and an evaporate characterized by relatively high d-excess and low δ 18 O [e.g., Gat et al, 2003;Uemura et al, 2008;Pfahl and Sodemann, 2014].…”

“…Specifically, high d-excess values arise when there is insufficient time for vapor to equilibrate between the saturated ocean surface layer and the subsaturated atmosphere [e.g., Pfahl and Sodemann, 2014]. Evaporated moisture entrained in the Arctic (as opposed to Pacific) is subject to such conditions, whereby large-humidity gradients between the ocean surface and the dry atmosphere above-particularly at the sea ice margin [e.g., Kurita, 2011]-will lead to strong nonequilibrium (kinetic) fractionation and an evaporate characterized by relatively high d-excess and low δ 18 O [e.g., Gat et al, 2003;Uemura et al, 2008;Pfahl and Sodemann, 2014]. Furthermore, Adak receives increased snowfall during months with Arctic-derived moisture, and this will also be characterized by high d-excess due to nonequilibrium condensation during ice particle growth [Jouzel and Merlivat, 1984].…”

“…Deuterium excess (d-excess [Craig, 1961]) is determined by evaporative conditions of the source region (e.g., relative humidity and SST) [see Pfahl and Sodemann, 2014], as well as kinetic processes during vapor transport and condensation (e.g., ice-crystal formation and reevaporation) [Dansgaard, 1964;Jouzel and Merlivat, 1984;Gat, 1996]. Adak winter month d-excess and δ 18 O precip negatively correlate (r = À0.56, P < 0.01, n = 48) (Figure 3), and HYSPLIT output shows that d-excess values of Arctic-derived precipitation (+9.6‰) are significantly (P < 0.01) higher than Pacific derived (+4.6‰).…”

Factors controlling stable isotope composition of precipitation in arid conditions: an observation network in the Tianshan Mountains, central Asia

“…The isotopic data in the GNIP database, especially in the stations operated for several years, serve as an indispensable resource in many scientific disciplines on global and regional scales (e.g. Divine et al, 2011;Hughes and Crawford, 2012;Pfahl and Sodemann, 2014). Approximately one-third of the arid area of the world is located in central Asia (Fig.…”

Isotopic hydrograph separation in two small mountain catchments during multiple events

“…The value of d is primarily a function of the mean relative humidity of the atmosphere above the ocean water (Merlivat and Jouzel 1979). It can be used to determine the origin of air masses that brought precipitation, sub-cloud evaporation and continental moisture recycling (Froehlich et al, 2008), and is also used in paleoclimatic studies (Pfahl and Sodemann, 2014). We use deuterium excess to indicate water that underwent evaporation (d was significantly less than 10).…”