Isotopes in Arctic Precipitation Yield Details about Water Vapor Sources
ARM data yield insights into water vapor transport and sources of moisture for precipitation at Barrow, AK; results provide clues for interpreting long-term climate records such as ice cores.
Potential changes to spatial patterns of water vapor transport and precipitation may have impacts on fresh water resources, severe weather, and the Earth’s radiation balance. Over the past few years, the Arctic has exhibited a particularly strong hydrologic response to warming temperatures, including a notable increase in Arctic precipitation. Like changes in the timing or amount of precipitation, changes in the relative abundance of heavy-isotope water molecules in precipitation may reflect the effects of a changing hydrological cycle. The objective of this study is to understand how water vapor source and meteorology trajectories contributes to event-scale variations in the precipitation isotopic ratios and how such contributions vary seasonally. The authors investigate the isotopic ratios of precipitation from event-scale sampling combined with radar observations from the Atmospheric Radiation Measurement (ARM) site at Barrow, AK. Radar observations are used to identify the altitude and rate of condensation in the precipitating clouds in order to initialize Lagrangian air parcel tracking.
The study found that the vapor source regions for storms at Barrow, AK, USA, exhibited interannual, annual, and substantial inter-event variability. On average, vapor came from the North Pacific and Gulf of Alaska, the most southerly vapor source areas, in cold months when the polar circulation cell extended southward. Vapor came from the Bering, Chukchi, and Beaufort seas, the most northerly sources, in warm months when the polar cell contracted northward. The cycle of winter depletion and summer enrichment of deuterium exhibited by the precipitation followed the annual changes in the latitude of the vapor source region, as a result of source region controls on evaporation, transport, and condensation conditions. However, substantial intra-season variability occurred in both source and changes in the isotopic ratios indicating scatter in the seasonal relationship.
The study highlights how variations in stable isotopes of precipitation measured on an event-by-event basis can be interpreted in the context of the vapor source. The mechanisms identified, most notably the north-south migration of the vapor source region in phase with expansion and contraction of the polar circulation cell, may also operate on timescales longer than that of this study and may be a source of variation in isotopes measured in ice cores, pedogenic carbonates, and speleothems. Thus, the results from this study may aid in the interpretation of long-term climate records such as ice cores.
In this study, precipitation isotopic variations at Barrow, AK, USA, are linked to conditions at the moisture source region, along the transport path, and at the precipitation site. Seventy precipitation events between January 2009 and March 2013 were analyzed for changes in hydrogen isotope ratios (d2H) and deuterium excess. The sampling equipment was installed on a skydeck within the North Slope of Alaska facility of the Atmospheric Radiation Measurement Facility. If the precipitation was rain, a rain funnel was used to collect the sample. If the precipitation was snow, the fresh snow was scooped into a plastic bag from a designated surface on the skydeck. Samples were gathered less than 24 h after the event ended and often as soon as snow ended.
For each precipitation event, vapor source regions were identified with the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) air parcel tracking program 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 occurring during the winter (summer). This may be related 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 d2H in precipitation were in phase; depleted (enriched) d2H values were associated with winter (summer) and distal (proximal) vapor source regions. Precipitation d2H responded to variation in vapor source region as reflected by significant correlations between d2H with the following three parameters: (1) total cooling between lifted condensation level (LCL) and precipitating cloud at Barrow, (2) meteorological conditions at the evaporation site quantified by 2m dew point, and (3) whether the vapor transport path crossed the Brooks and/or Alaskan ranges. These three variables explained 54% of the variance in precipitation d2H. The magnitude of each effect on isotopic composition also varied with vapor source region proximity. Vapor source region relative humidity with respect to the sea surface temperature explained 34% of variance in deuterium excess. The patterns in the data suggest that on an annual scale, isotopic ratios of precipitation at Barrow may respond to changes in the southerly extent of the polar circulation cell, a relationship that may be applicable to interpretation of long-term climate change records like ice cores.
Annie L. Putman
This project was supported by the National Science Foundation grant 1022032, the Intensive Operational Period (IOP) Program of the Atmosphere Radiation Measurement, and Dartmouth College.
Putman, A.L., X. Feng, L.J. Sonder, and E.S. Posmentier. 2017. Annual variation in event-scale precipitation d2H at Barrow, AK, reflects vapor source region. Atmos. Chem. Phys., 17, 4627-4639. doi:10.5194/acp-17-4627-2017.