Research Article
Received: 7 March 2012
Revised: 27 May 2012
Accepted: 28 May 2012
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2012, 26, 1893–1898
(wileyonlinelibrary.com) DOI: 10.1002/rcm.6298
ENSO effects on d18O, d2H and d-excess values in precipitation
across the U.S. using a high-density, long-term network (USNIP)
Jeffrey M. Welker*
Environment and Natural Resources Institute Stable Isotope Laboratory and Department of Biological Sciences, University of
Alaska Anchorage, 3211 Providence Drive, EBL 117, Anchorage, AK 99508, USA
RATIONALE: Isotope (d18O and d2H values) ratios in precipitation have been central to understanding changes in Earth’s
climate as recorded in ice, coral, speleothems, lake varves and long-lived plants. Understanding how climate phases (i.e.
ENSO) affect the spatial and temporal patterns of d18O and d2H values in precipitation has, however, been uncertain
across the USA.
METHODS: A spatial precipitation isotope network (USNIP) has been established that aims to: (1) characterize the d18O,
d2H and d-excess values in precipitation across the USA with the highest spatially dense network of measurements yet
undertaken; (2) quantify the annual and seasonal patterns of precipitation d18O and d2H values that may be affected
by ENSO climate phases; and (3) provide a new isotope database for scientific studies that can be incorporated into
NEON, BASIN, GNIP, and IsoMAP.
RESULTS: On average, precipitation d18O and d2H values are very low in the northern Rocky Mountain region (~ 15%
d18O, and ~ 120% d2H), and precipitation d18O and d2H values are relatively higher along the Gulf Coast (~ 5% d18O
and 10% d2H) and in the Southeast. During El Niño periods the precipitation d18O and d2H values are lowest in northwest Montana, with precipitation that is depleted in 18O and 2H extending into northern Colorado, while moisture that is
enriched in 18O and 2H continues to dominate the Gulf Coast. The annual average differences between the climate phases generally show especially depleted 18O and 2H in precipitation across the Rocky Mountain region during El Niño, compared
with Neutral periods.
CONCLUSIONS: Detailed spatial and seasonal patterns of d18O, d2H and d-excess values provide fine-scale resolution not
previously recognized. Climate phases of ENSO have major effects on the spatial patterns of d18O, d2H and d-excess
values, being especially important on a seasonal basis in the Desert Southwest. Copyright © 2012 John Wiley & Sons, Ltd.
Isotope (d18O and d2H values) ratios in precipitation have
been central to understanding changes in the Earth’s climate
as recorded in ice[1] as well as in long-lived plants.[2] Typically,
isotope ratios have been interpreted with a focus on changing
temperatures although typically temperature can only
account for 50%–60% of the variance, indicating that other
facets may be influencing the isotope (d18O and d2H) ratios
in precipitation. Understanding how climate phases (i.e. the
El Niño Southern Oscillation – ENSO) affect the spatial and
temporal patterns of d18O and d2H values in precipitation
has, however, been uncertain across the USA, limiting our
climate proxy interpretations and continental-scale ecohydrology insight. In addition, understanding the processes
governing the spatial and temporal patterns of d18O and d2H
values in precipitation has taken on new importance because
of climate proxy reanalysis programs, and precipitation isotope
ratios are central to emerging monitoring programs, research
networks, isotope forensics and isoscapes.[3–8]
Rapid Commun. Mass Spectrom. 2012, 26, 1893–1898
Copyright © 2012 John Wiley & Sons, Ltd.
1893
* Correspondence to: J. M. Welker, Environment and Natural
Resources Institute Stable Isotope Laboratory and Department
of Biological Sciences, University of Alaska Anchorage, 3211
Providence Drive, EBL 117, Anchorage, AK 99508, USA.
E-mail: afjmw1@uaa.alaska.edu
Today, understanding the isotopes of precipitation is
central to delineating the attributes and changes in the US
hydrologic cycle within the National Ecological Observatory
Network (NEON).[9,10] Precipitation isotope data also forms
a basis from which the North American Carbon Program
(NACP) is documenting linkages between the carbon and
water cycles.[11] In addition, the watershed focus of the
Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI)[12]and of the Critical Zone Observatories (CZO)[13] will require precipitation isotope data to
enable quantification of residence times of ground water,[14]
and the Biogeosphere-Atmosphere Stable Isotope Network
(BASIN) is using precipitation isotope data as a cornerstone
of its estimates of water and carbon cycling at the ecosystem,
regional and global scales.[15] A high-resolution, spatially
dense network across the USA that quantifies isotope/climate
and isotope/geographic relationships has tremendous application and utility across earth and biological sciences today.
The U.S. Network for Isotopes in Precipitation (USNIP) is
fundamental to these needs in the USA.[16]
Divergent phases of the El Niño/Southern Oscillation
Index (ENSO) – El Niño, La Niña or Neutral (Normal) – are
major components of weather across the USA and globally.[17]
These climate phases are defined as periods when the normalized difference in surface pressure between Tahiti, French
J. M. Welker
Polynesia and Darwin, Australia, is a measure of the strength
of the trade winds, which have a component of flow from
regions of high to low pressure. High ENSO (large pressure
difference) is associated with stronger than normal (Neutral
Phase) trade winds and La Niña conditions, and low ENSO
(smaller pressure difference) is associated with weaker than
normal trade winds and El Niño conditions.
Prior studies have indicated that ENSO phases may affect
precipitation patterns and amounts across the USA and in the
tropics.[18,19] For instance, strong El Niño periods in continental
USA are associated with extreme weather patterns and result in
significant regional deviations in the amounts and seasonality
of precipitation and surface temperatures.[18] In addition,
oscillations in ENSO have been recorded in the isotopic values
of climate proxies such as corals.[20,21] However, there are
important facets of the fundamental patterns and processes of
the water isotopes in precipitation (d18O and d2H ) during
different ENSO phases in the USA that are unresolved because
there has not been a systematic analysis of modern d18O and
d2H values in precipitation during divergent ENSO phases.
Understanding these isotopic patterns is essential for the
accurate interpretation of paleoclimate records, the quantifying
of spatial patterns of isotopes in precipitation at continental and
global scales,[4] the refining of General Circulation Models
(GCMs),[22] and the understanding of the water cycle of
today as it moves into new states with changing climates and
land use.
The aims of this project were thus to: (1) characterize the
annual averages of the d18O, d2H and d-excess values in
precipitation across the USA with the highest spatially dense
network of measurements yet undertaken; (2) quantify the
seasonal patterns of precipitation d18O and d2H values between
1989 and 1995, a period that included two Neutral (Normal)
(1989 and 1995) and five El Niño (1990–1994) phases of
ENSO; (3) provide a new isotope database for scientific
studies, to be incorporated into NEON, BASIN, the Global
Network for Isotopes in Precipitation (GNIP),[23] and
IsoMAP;[24] and (4) provide a database for collaborative
research ventures spanning the earth, biological and ecological sciences.
EXPERIMENTAL
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Precipitation samples were collected at 73 sites across the
USA, involving over 12 000 isotopic measurements taken
from 1989 through 1995, a period with two Neutral (Normal)
ENSO phases and five El Niño phases.[25,26] The samples
were collected by the National Atmospheric Deposition
Program (NADP), archived in cold storage, and then analyzed
for d18O and d2H values using isotope ratio mass spectrometry
(IRMS) with IAEA VSMOW (Vienna Standard Mean Ocean
Water), GISP (Greenland Ice Sheet Precipitation) and
VSLAP (Vienna Standard Light Antarctic Precipitation) as
the standards.[3,27,28]
The isotope values were amount-weighted by month,
averages were calculated for the two different ENSO phases,
and a 12-point spherical kriging interpolation scheme was used
to create the isocapes on an annual and seasonal basis.[5] The
data is presented by season – fall (OND), winter (JFM), spring
(AMJ), and summer (JAS) – as these monthly groupings conform to those used to describe the intensity of the ENSO.
wileyonlinelibrary.com/journal/rcm
RESULTS AND DISCUSSION
On average, the d18O and d2H values of precipitation are
very low in the northern Rocky Mountain region (~ 15%
d18O, and ~ 120% d2H) and relatively high along the Gulf
Coast (~ 5% d18O and 10% d2H ) and in the Southeast
(Figs. 1(a) and 2(a)). These findings are in agreement with
surface water isotope surveys,[10,29] but provide details not
articulated by the coarse scales of the IAEA analysis, which
was based on seven sites collecting samples sporadically in
the 1960s.[30] During the Neutral Phase, the plume of
precipitation most depleted in 18O and 2H weakens, while
moisture enriched in 18O and 2H occurs from the Gulf of
Mexico into Kentucky (Figs. 1(b) and 2(b)). During El Niño
periods the isotopic values of precipitation are most
depleted in 18O and 2H in northwest Montana, with precipitation depleted in 18O and 2H extending into northern
Colorado, while moisture enriched in 18O and 2H continues
to dominate the Gulf Coast (Figs. 1(c) and 2(c)). The annual
average differences between the climate phases are presented
in Figs. 1(d) and 2(d), depicting precipitation that is generally
depleted in 18O and 2H values in the Desert Southwest, with
only slight differences across the eastern and western USA.
This spatial heterogeneity of precipitation d18O values
during El Niño across the USA corresponds to the global
estimates reported be Welp et al.,[31] who found depleted
18
O values across most of the USA.
d-excess values for precipitation as annual averages, for
the two climate phases and for the differences, are found
in Figs. 3(a) through 3(d). Typically, the d-excess values are
lower in the northern Rocky Mountain region and higher
along the Gulf Coast and along the western and eastern
seaboards, with slight deviations during the El Niño and
Neutral phases. The difference d-excess isoscape, however,
indicates that the Desert Southwest is a region in which
d-excess values are higher during El Niño than in Neutral
phases, by >2.5%. These d-excess patterns correspond to
those presented by Liu et al.[32] and reflect a Pacific Ocean
moisture source in the northwest, a moisture source with
precipitation that is enriched in 18O and 2H along the
Gulf Coast, and possibly greater recycling in convective
thunderstorms in the Desert Southwest during El Niño than
in Neutral phases.[8]
Because ENSO is known to affect seasonal patterns and
magnitudes of precipitation, the effects of ENSO phases on
d18O and d2H and d-excess values on a seasonal basis are
presented, including a set of difference isoscapes (Supplementary Figs. S1–S6, see Supporting Information). In the fall
of Neutral phases, precipitation depleted in 18O and 2H
occurs throughout the western USA, especially along the
Montana-Canada border. At this time, precipitation enriched
in 18O and 2H is confined to the Gulf Coast states (Supplementary Figs. S1 and S3, see Supporting Information). In the
winter of Neutral phases, precipitation depleted in 18O and
2
H occurs in the northern Great Basin and along the upper
Great Lakes region (Supplementary Figs. S1 and S3, see
Supporting Information). In spring, precipitation enriched
in 18O and 2H extends into the central USA and into the
southern portions of the Southwest, while precipitation most
depleted in 18O and 2H is found in the northern Great Plains
(Supplementary Figs. S1 and S3, see Supporting Information).
In summer, precipitation enriched in 18O and 2H extends
Copyright © 2012 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2012, 26, 1893–1898
Precipitation isotopes across the USA
Figure 1. Patterns of the spatial characteristics of amount-weighted averages of the d18O values of
precipitation across continental USA. (a) Weighted annual average d18O values of precipitation during
1989–1995, (b) weighted annual average d18O values of precipitation during Neutral Southern
Oscillation climate phase, (c) weighted average d18O values of precipitation during El Niño Southern
Oscillation climate phase, and (d) differences in the d18O values of precipitation between El Niño and
Neutral climate phases.
Rapid Commun. Mass Spectrom. 2012, 26, 1893–1898
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm
1895
Figure 2. Patterns of the spatial characteristics of amount-weighted averages of the d2H values of precipitation across continental USA. (a) Weighted annual average of d2H precipitation during 1989–1995,
(b) weighted annual average d2H values of precipitation during Neutral Southern Oscillation climate
phase, (c) weighted average d2H values of precipitation during El Niño Southern Oscillation climate phase,
and (d) differences in the d2H values of precipitation between El Niño and Neutral climate phases.
J. M. Welker
Figure 3. Patterns of the spatial characteristics of amount-weighted averages of the d-excess values of
precipitation across continental USA. (a) Weighted annual average of d-excess precipitation during
1989–1995, (b) weighted annual average d-excess of precipitation during Neutral Southern Oscillation
climate phase, (c) weighted average d-excess of precipitation during El Niño Southern Oscillation
climate phase, and (d) differences in the d-excess of precipitation between El Niño and Neutral
climate phases.
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throughout the eastern USA, with the monsoon precipitation
regime being apparent in the Southwest (Supplementary
Figs. S1 and S3, see Supporting Information).
During the fall of El Niño phases, precipitation isotope data
across the USA exhibits a large region depleted in 18O and 2H
throughout the northern Rocky Mountains and into the
northwestern area of the Great Plains (Supplementary Figs. S1
and S3, see Supporting Information). In winter, this very large
pool of precipitation depleted in 18O and 2H extends
into the northern Great Lakes region and into portions of
the Great Basin and northwest Colorado (Supplementary
Figs. S1 and S3, see Supporting Information). In spring,
moisture that is enriched in 18O and 2H extends into the
central USA and, in summer, precipitation that is enriched
in 18O and 2H is found throughout the eastern USA, especially
along the southern regions of the states along the Gulf of
Mexico (Supplementary Figs. S4 and S6, see Supporting
Information).
Seasonal differences between El Niño and Neutral phases
are depicted in Supplementary Figs. 2 and 4 (see Supporting
Information), which show differences in d18O values as large
as 4%, while d2H values exhibit differences of up to 30% for
all seasons. During the fall, winter, and spring there is an
extensive and large isotopic difference seen between the El
Niño and Neutral phases, especially in the western USA.
There is also a core of precipitation that is depleted in 18O
in the Desert Southwest (from western Texas to Utah to
Arizona) in fall and with less intense depletion of 18O and
2
H in winter. These differences are probably attributable to
differences in moisture sources, due to storm tracks delivering more tropical moisture to the region.[7] In spring, there
are higher d18O and d2H values in the Desert Southwest
wileyonlinelibrary.com/journal/rcm
during El Niño than during Neutral phases, which result
from internal recycling of moisture and/or intense rain
storms with large amounts of rain-out effects.[31]
Interpreted in terms of temperature, the differences in d18O
values between the two ENSO phases would require a 3
to 4.5 C difference using the full, theoretical isotope/
temperature slope of 0.7%/ C. Smaller slopes, often
observed in sub-annual plots of d18O values versus temperature, would yield even larger expected temperature changes.
Such large swings in temperature between El Niño and Neutral phases are not observed in these regions (Supplementary
Fig. S2, see Supporting Information).
d-excess patterns are spatially complex between seasons
(Supplementary Figs. S5 and S6, see Supporting Information).
Higher d-excess values are present in the Northeast during fall
for both phases, but in El Niño phases higher values are
present in the Desert Southwest (Supplementary Fig. S5(e),
see Supporting Information). During the spring and summer
d-excess values become higher over the northern Great Plains,
especially during Neutral phases (Supplementary Figs. S8(c)
and S8(d), see Supporting Information). Differences in the
d-excess values between phases are especially noticeable in
the western USA in JFM (Supplementary Fig. S6(b), see
Supporting Information), during which time higher values
occur along the West Coast and lower values in the Gulf
Coast region. These patterns correspond to shifting moisture
sources across the USA seasonally and with shifts in
moisture sources during different climate phases.[32]
This new network and associated d18O and d2H values, and
d-excess isoscapes and data provide a major improvement in
the understanding of isotope geochemistry of precipitation
across the USA. This database has application to
Copyright © 2012 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2012, 26, 1893–1898
Precipitation isotopes across the USA
understanding hydrologic, atmospheric, climatological and
biological processes of today, in the future, and in the past.
For example, temporal shifts in the d18O values of stalagmite
and pedogenic carbonate can now be interpreted in terms of
El Niño and Neutral climate phases which may account for a
portion of the 1%–3% shifts observed over ~150 000 years in
the central USA.[33] In addition, our high-resolution findings
will facilitate spatial modeling of isotopes in precipitation. For
instance, Bowen and colleagues have estimated that the d18O
values of precipitation in the northern Rocky Mountain region
average ~ 12%,[4] while, according to our measurements, the
actual values are closer to ~ 17% , with similar model/data
discrepancies for the Great Basin and the Northeast. This
database will allow us to refine the relationships between these
and other models to predict isotope values of precipitation
throughout the USA. In addition, this network database will
be used in the development of animation maps of the USA on
monthly time-steps,[34] and the findings reported here may also
contribute to the use of water isotopes in GCM simulations and
regional isotope models, as they provide ’ground-truthing’ for
calibrations of past, modern and future freshwater cycles
across the USA.[22] Finally, this database provides a basis for
continental-scale forensic climatology, especially with regard
to animal migration processes (e.g. identifying wintering and
breeding locations of migratory birds) and the connections
between temperate, subarctic and arctic habitat use.[35,36]
SUPPORTING INFORMATION
Additional supporting information may be found in the
online version of this article.
Acknowledgements
This research was supported by the National Science Foundation’s Earth System History Program (ESH-0012324, 0012367).
The author would like to thank the NADP network and the
NADP Coordination Office – especially Chris Lehmann,
Brenda Kelly, Van Bowersox, and Karen Harlin – for its support
and cooperation. Thanks are also extended to all of the NADP
site operators who contribute to the weekly data collections,
including the US Geological Survey, the US Forest Service, the
Bureau of Land Management, the National Park Service and
others. The isotopic analysis has been made possible by the
National Science Foundation’s Instrumentation and Facilities
Program, including awards to Colorado State University,
the University of Alaska Anchorage, and the University of
Colorado, Boulder. Kara Bastin, Dan Reuss, Bruce Vaughn,
Mark Simpson, Brian Cohn and Matt Rogers were instrumental in assisting with isotopic analysis, mass spectrometer
maintenance, data synthesis and sample preparation. This
paper is dedicated to Dave Bigelow, who was the NADP
QA/QC manager for 10 years, and a friend.
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Supplemental Figures:
Supplemental Figure 1. Seasonal (Fall, Winter, Spring and Summer) patterns of δ18O in
precipitation during Neutral (a),(b),(c),(d) and El Niño (e),(f),(g),(h) climate phases.
Supplemental Figure 2. Seasonal (Fall, Winter, Spring and Summer) differences in the
δ18O in precipitation (a),(b),(c),(d) between El Niño and Neutral climate phases and
corresponding temperature differences (e),(f),(g),(h).
Supplemental Figure 3. Seasonal (Fall, Winter, Spring and Summer) patterns of δD in
precipitation during Neutral (a),(b),(c),(d) and El Niño (e),(f),(g),(h) climate phases.
Supplemental Figure 4. Seasonal (Fall, Winter, Spring and Summer) differences in the
δD in precipitation (a),(b),(c),(d) between El Niño and Neutral climate phases and
corresponding temperature differences (e),(f),(g),(h).
Supplemental Figure 5. Seasonal (Fall, Winter, Spring and Summer) patterns of d-excess
in precipitation during Neutral (a),(b),(c),(d) and El Niño (e),(f),(g),(h) climate phases.
Supplemental Figure 6. Seasonal (Fall, Winter, Spring and Summer) differences in the dexcess in precipitation (a),(b),(c),(d) between El Niño and Neutral climate phases and
corresponding temperature differences (e),(f),(g),(h).