Long-Term Precipitation Isotope Ratios (δ18O, δ2H, d-excess) in the Northeast US Reflect Atlantic Ocean Warming and Shifts in Moisture Sources

Abstract:

The global water cycle is exhibiting dramatic changes as global temperatures increase resulting in increases in: drought extremes, flooding, alterations in storm track patterns with protracted winter storms, and greater precipitation variability. The mechanisms driving these changes can be difficult to assess, but the spatial and temporal patterns of precipitation water isotopes (δ¹⁸O, δ²H, d-excess) provide a means to help understand these water cycle changes. However, extended temporal records of isotope ratios in precipitation are infrequent, especially in the US.

In our study we analyzed precipitation isotope ratio data from the Hubbard Brook Experimental Forest in New Hampshire that has the longest US precipitation isotope record, to determine: 1) the monthly composited averages and trends from 1967 to 2012 (45 years); ; 2) the relationships between abiotic properties such as local temperatures, precipitation type, storm tracks and isotope ratio changes; and 3) the influence of regional shifts in moisture sources and/or changes in N Atlantic Ocean water conditions on isotope values. The seasonal variability of Hubbard Brook precipitation isotope ratios is consistent with other studies, as average δ¹⁸O values are ~ -15‰ in January and ~ -5 ‰ in July. However, over the 45 year record there is a depletion trend in the δ ¹⁸O values (becoming isotopically lighter with a greater proportion of ¹⁶O), which coupled with less change in δ ²H leads to increases in d-excess values from ~ -10‰ around 1970 to greater than 10‰ in 2009. These changes occurred during a period of warming as opposed to cooling local temperatures indicating other processes besides temperature are controlling long-term water isotope traits in this region. We have evidence that these changes in precipitation isotope traits are controlled in large part by an increases in moisture being sourced from a warming N Atlantic Ocean that is providing evaporated, isotopically-depleted precipitation to the region. Thus, the warming of the N Atlantic Ocean appears to influence the climate and the precipitation isotopes of Northeastern coastal regions and could be a larger water source to watersheds in this North American region.