Annual variation in precipitation δ2H reflects vapor source region at Barrow, AK

Other literature type English OPEN
Putman, Annie L. ; Feng, Xiahong ; Sonder, Leslie J. ; Posmentier, Eric S. (2016)

Interpretation of variability in precipitation stable isotopic ratios often relies exclusively on empirical relationships to meteorological variables (e.g., temperature) at the precipitation site. Because of the difficulty of unambiguously determining the vapor source region(s), relatively fewer studies consider evaporation and transport conditions. Increasing accessibility of Lagrangian air parcel tracking programs now allows for an integrated look at the relationship between the precipitation isotope ratios and the evolution of moist air masses. In this study, 70 precipitation events occurring between January 2009 and March 2013 at Barrow, AK, USA, were analyzed for δ<sup>2</sup>H and deuterium excess. For each precipitation event, vapor source regions were identified with the Lagrangian air parcel tracking program, HYSPLIT, in back-cast mode. The results show that the vapor source region migrated annually with the most distal (proximal) and southerly (northerly) vapor source regions occurred during the winter (summer). This may be linked to equatorial expansion and poleward contraction of the Polar circulation cell and the extent of Arctic sea ice cover. Annual cycles of vapor source region latitude and δ<sup>2</sup>H in precipitation were in phase; depleted (enriched) δ<sup>2</sup>H values were associated with winter (summer) and distal (proximal) vapor source regions. Precipitation δ<sup>2</sup>H responded to variation in vapor source region as reflected by significant correlations between δ<sup>2</sup>H with the following three parameters: 1) total cooling between lifted condensation level and precipitating cloud at Barrow, &Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub>, 2) the meteorological conditions at the evaporation site quantified by 2&thinsp;m dew point, <span style="text-decoration: overline"><i>T</i></span><sub><i>d</i></sub>, and 3) whether the transport crossed the Brooks and/or Alaskan ranges, expressed as a Boolean variable, <i>mtn</i>. These three variables explained 52&thinsp;% of the variance (p&thinsp;<&thinsp;0.001) in precipitation δ<sup>2</sup>H with a sensitivity of &minus;3.25&thinsp;&plusmn;&thinsp;0.57&thinsp;‰&thinsp;°C<sup>&minus;1</sup> (p&thinsp;<&thinsp;0.001) to &Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub>, 3.80&thinsp;&plusmn;&thinsp;0.78&thinsp;‰&thinsp;°C<sup>&minus;1</sup> (p&thinsp;<&thinsp;0.001) to T<sub><i>d</i></sub>, and 34.29&thinsp;&plusmn;&thinsp;11.05&thinsp;‰ (p&thinsp;=&thinsp;0.0028) depletion when <i>mtn</i> is true. The magnitude of each effect on isotopic composition also varied with vapor source region proximity. For storms with proximal vapor source regions (where &Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub>&thinsp;<&thinsp;7&thinsp;°C), &Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub> explained 26&thinsp;% of the variance in δ<sup>2</sup>H, <span style="text-decoration: overline"><i>T</i></span><sub><i>d</i></sub> alone accounted for 48&thinsp;%, while <i>mtn</i> explained 5&thinsp;%. For storms with distal vapor sources (&Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub>&thinsp;>&thinsp;7&thinsp;°C), &Delta;<span style="text-decoration: overline"><i>T</i></span><sub><i>cool</i></sub> explained 13&thinsp;%, <span style="text-decoration: overline"><i>T</i></span><sub><i>d</i></sub> explained only 10&thinsp;%, and <i>mtn</i> explained 24&thinsp;%. The deuterium excess annual cycle lagged by 2&ndash;3 months the δ<sup>2</sup>H cycle, so the direct correlation between the two variables is weak. Neither vapor source sea surface temperature, nor vapor source relative humidity, nor a linear combination of the two, was a statistically significant predictor of precipitation deuterium excess. Vapor source region <span style="text-decoration: overline"><i>T</i></span><sub><i>d</i></sub> explained 24&thinsp;% of variance in deuterium excess, (&minus;0.53&thinsp;&plusmn;&thinsp;0.12&thinsp;‰&thinsp;°C<sup>&minus;1</sup>, p&thinsp;<&thinsp;0.001). The patterns in our data suggest that on an annual scale, isotopes of precipitation at Barrow may respond to changes in the southerly extent of the Polar circulation cell. We expect isotopes to respond similarly for longer-term climate-induced changes to the mean position of meridional circulation features, and expect that the most of the variation in isotopes measured in ice cores and other long term records are driven by changes in circulation, instead of fluctuations in local temperature.
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