Recent Trends Of The Tropospheric Vapor Deuterium To Hydrogen Ratio And Their Climatological Significance

Abstract:

Preliminary analysis of satellite-borne Tropospheric Emission Spectrometer (TES) data reveals recent trends in both the 900 hPa deuterium to hydrogen (D/H) ratio of water vapor, and the 500-800 hPa water vapor mixing ratio, r_E. In the Arctic, vapor dD decreased at 1.9‰/yr (p=0.041) from 2004 to at least 2009, and r_E increased at 0.041 g kg^–1/yr (p=0.065). The ratio of the two trends is -46‰/ g kg^–1, comparable with the dD vs. r_E regression slope of –29‰/ g kg^–1 (p<0.0001). In low latitudes (15-40ºN) the dD trend is much smaller and opposite in sign, but statistically insignificant (+0.42‰/yr, p=0.64), while r_E also has a positive but statistically insignificant trend of +0.037 g kg^–1/yr (p=0.58). During this period, Arctic sea surface temperature (SST) increased slightly, so its positive effect on equilibrium fractionation is the wrong sign to explain the observed decrease of Arctic dD.

Making the preliminary assumption that these temporal variations in vapor are robust relative to TES data calibration, we investigate them using a marine boundary layer (MBL) model with isotopes. The model includes meteorological processes such as subsidence and convergence of air from the mid and upper troposphere, and height-dependent turbulent mixing in the MBL. The convergence component allows us to investigate the effect of the mixing ratio of air aloft (r_E) on the isotopic composition of the MBL.

Model results show that vapor δD in the MBL decreases as r_E increases. This is because the admixture of isotopically depleted vapor from the upper atmosphere, which increases with r_E, results in isotopic depletion near the surface. However, the sensitivity of dD to r_E is strongly nonlinear; it is -30.33 ‰/g kg^-1 in the Arctic, but only -5.25 ‰/g kg^-1 in the subtropics. This explains both the negative trend of dD in the Arctic and lack of trend in the subtropics. In addition, the simulated Arctic sensitivity compares well with the observed values of -46 and –29‰/ g kg^–1. The model also shows that the effects of other variables, such as SST and turbulent mixing rate, have minor influence during this period.

The recent gain in moisture of the Arctic upper troposphere, which has been both directly observed and inferred from the decrease of MBL δD, is most likely due to global warming or decreasing Arctic sea ice.