Baffin Bay sea ice extent and synoptic moisture transport drive water vapor isotope ( δ 18 O, δ D, d - excess ) variability in coastal northwest Greenland

Abstract. At Thule Air Base on the coast of Baffin Bay (76.51° N 68.74° W), we continuously measured water vapor isotopes (δ18O, δD) at high frequency (1 s−1) from August 2017 through August 2019. Our resulting record, including derived deuterium-excess (dxs) values, allows for analysis of isotopic–meteorological relationships at an unprecedented level of detail and duration for High Arctic Greenland. We examine isotopic variability across multiple temporal scales from daily to annual, revealing that isotopic values at Thule are determined by five interacting factors: (a) local air temperature, (b) local marine moisture availability, (c) the NAO, (d) surface wind regime, and (e) land-based evaporation/sublimation. Each factor's relative importance changes in response to seasonal shifts in Thule's environment, largely driven by the sea ice extent in northern Baffin Bay. Winter sea ice coverage forces distant sourcing of vapor that is isotopically light from fractionation during transport and prevents isotopic exchange with local waters. Late spring sea ice breakup triggers a rapid isotopic change at Thule as the newly open ocean supplies warmth and moisture that is 10 ‰ and 70 ‰ higher in δ18O and δD, respectively, and 13 ‰ lower in dxs. Sea ice retreat also leads to other environmental changes, such as sea breeze development, that dramatically alter the nature of relationships between isotopes and many meteorological variables in summer. On shorter temporal scales, enhanced southerly flow promoted by negative NAO conditions produce higher δ18O and δD values and lower dxs values. Diel isotopic cycles are generally very small as a result of a moderated coastal climate and counteracting isotopic effects of the sea breeze and local evaporation. Future losses in Baffin Bay sea ice extent will likely shift mean annual isotopic compositions toward more summer-like values, and past reductions should be similarly preserved in local glacial ice. These findings highlight the strong influence local environment has on isotope dynamics and the need for dedicated, multi-season monitoring to fully understand the controls on water vapor isotope variability.