Paired Vapor-Precipitation Isotope Data from Manus, Papua New Guinea

Abstract:

We measured the stable isotopic composition of water vapor, precipitation and seawater over a period of 10 days at the highly instrumented Atmospheric Radiation Measurement (ARM) site located on Manus Island, Papua New Guinea. The island site, at 4 meters above sea level, sits close to the open ocean and is minimally impacted by terrestrial processes. Over the period of measurement, the mean δD value of near-surface water vapor (δD_v) was -109.7±39.7‰, overlapping average tropospheric emissions satellite (TES) δD_v values for the lower troposphere. We observed a 155‰ range in δD_v values, coinciding with a transition through a Madden-Julian Oscillation event into a quiescent period. Low δD_v values appear to reflect post-condensation exchange and the highest δD_v values measured were in equilibrium with seawater δD. Of all surface meteorological variables, δD_v was most strongly correlated with specific humidity, reflecting the importance of rainout and distillation in driving δD_v values. During precipitation events, δD_v dropped abruptly following maximum precipitation rates, likely due to mesoscale subsidence or rain evaporation.

Intra-event δD values of precipitation (δD_p) were most strongly correlated with δD_v, and only weakly correlated with surface meteorological variables, highlighting the important role of δD_v in controlling δD_p values. We did not observe an amount effect through all precipitation events, as the lowest δD_p values often lagged maximum precipitation rates. Precipitation deuterium excess values were significantly correlated with surface relative humidity and temperature, as well as cloud base height measurements, with higher deuterium excess values corresponding to higher cloud altitudes.