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Arctic: Where do wet air masses come from in winter?

​Thanks to the first full-length record of the isotopic composition of water vapor from Svalbard (Norway), climatologists at LSCE (CEA-CNRS-UVSQ) documented the origin of winter air masses during peak moisture events.

Published on 18 March 2021

In 2014, a team from LSCE put up a laser analyzer spectrometer in Ny-Ålesund, Svalbard, Norway.  This allowed them to continuously record, from May 2014 to September 2018, the isotopic composition of water vapor in this Arctic archipelago. This sequence is the longest ever observed in a polar region.

A second laser spectrometer installed 2 km from Ny-Ålesund, higher up (474 m above sea level), made it possible to check the consistency of the data provided by the two instruments for 20 days, eliminating a possible hyperlocal bias that could have affected the tracking instrument.

How do climate scientists use their measurements?  Oxygen-18 (δ18O) content signals temperature and humidity variations at the study site.  A more complex isotopic signal, called deuterium (2H) excess and denoted by the letter d (combining changes in deuterium (δ2H) and δ18O content) reflects changes in ocean temperature at the evaporation site.

Thus, an early February 2017 "event" is marked by dissonance between the two isotopic markers.  δ18O increases, indicating a warming of the air mass and an increase in humidity at the study site. The deuterium excess, d, decreases, indicating that the source of the evaporation warmed up.  This inference is coherent with the reconstruction of air mass trajectories by meteorological models, which indicate a rapid movement of evaporation sources from the Arctic to the warmer North Atlantic Ocean.

The situation is different for a December 2017 event, which shows no warming of evaporative sources. In this case, the backtrajectories show that the air masses carrying moisture remained in the Arctic area.

These results demonstrate the added value of long-term isotopic monitoring of water vapor to better understand the origin of Arctic moisture and atmospheric dynamics. They also open a new perspective for the analysis of snow cores drilled in Svalbard.

In addition, a study of snow in East Antarctica has established that some δ18O anomalies can be explained by large-scale atmospheric variations (lows or highs) in the southern hemisphere, which force relatively warm oceanic air masses into the interior of the Antarctic continent.

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