THE ROLE OF THE NORTHERN ANNULAR MODE IN FOCUSING CLIMATE CHANGE IN THE WESTERN UNITED STATES
Stephanie McAfee & Joellen Russell
DEPARTMENT OF GEOSCIENCES
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ABSTRACT
Recent studies suggest that much of the recent hydrological change
in the western United States is due to anthropogenic global
warming. However, the pattern of climate change in the western
United States appears to be distinct from the global pattern of
warming. The western United States is warming more than the
global average, much of that trend is concentrated in the spring, and
there appears to be an abrupt shift to warmer conditions in the
mid-1980s. This earlier spring onset is reflected in earlier snowmelt
and increased wildfire activity later in the summer. We suggest that
increases in the Northern Annular Mode (NAM) index drive earlier
spring onset. The NAM experienced a step change to more positive
values in the mid-1980s. High index values are associated with 1) a
reduction in storm activity, 2) increased 500 hPa heights, 3) warmer
spring temperatures, 4) changing precipitation patterns, and 5) early
browning (reduced NDVI) over much of the western United States,
particularly during the spring and when the mid-winter NAM index
reaches values greater than +1 or +1.5, which has happened more
frequently since the mid-1980’s. Conversely, the winter NAM index
has not fallen below -1 since the mid-1980’s, despite doing so more
than 10 times between 1900 and 1984. Models and climate theory
suggest that surface warming and stratospheric cooling related to
ozone destruction lead to higher NAM index values. This implies
that while the pattern of climate change seen in the West is not
unique – in fact it seems typical of high NAM index conditions – the
increased frequency of those events may be linked to human
alteration of the climate system and their impacts exacerbated by
the general warming trend.
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THE UNIVERSITY OF ARIZONA
TUCSON, ARIZONA
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THE NORTHERN ANNULAR MODE HAS GREATEST CLIMATE AND ECOSYSTEM IMPACTS IN THE SPRING
Below are map correlations between the JFM NAM index and stormtrack, 500-hPa anomalies, temperature and precipitation (1978-1998), and GIMMS NDVI (1982-2002). Correlations significant at 90% by a 10,000 realization
Monte Carlo simulation are indicated by the heavy black dashed line. The climatological stormtrack is shown in gray. The fine black line follows the 2000-m elevation contour. Major rivers shown in blue.
NOVEMBER
DECEMBER
JANUARY
FEBRUARY
MARCH
APRIL
MAY
JUNE
STORMTRACK
In addition, there are significantly fewer low (<-1) JFM values after
1985 than would be expected considering the entirety of the dataset
(1900-2007).
Observed
Expected
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1900-1984 1985-2007 1900-1984 1985-2007
17
0
13.4
3.6
0.03
TEMPERATURE
The spring (AMJ) NAM index displays a trend (AMJ = 0.004*t -0.22,
p<0.05 for both coefficients), rather than a step-change in value
over time, as shown by the plot of the index and its 11-year moving
average shown below.
PRECIPITATION
NDVI
Although the NAM is strongest during the mid-winter (January to
March, JFM), it is active all year. In addition, the average midwinter NAM appears related to its states in the preceding fall and
subsequent spring (April - June, AMJ), as shown in this merged index
time series from Colorado State (D. Thompson) and the Climate
Prediction Center.
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Stormtrack is calculated as the monthly variance of the first-difference of daily meridional wind at 300-hPa. V-wind and geopotential height are from the NCEP/NCAR Reanlaysis (Kalnay et al., 1996). Temperature is from the
University of Delaware and is gridded at 0.5°x 0.5° (Wilmot and Matsuura, 1995). Precipitation is from the CPC Unified dataset and is at 0.25° x 0.25° resolution (Higgins et al., 2000). GIMMS NDVI at 0.25° x 0.25° resolution
was used (Pinzon et al., 2004; Pinzon, 2002, Tucker et al., 2005).
The NAM has the greatest impact on the western United States during the late winter and early spring, and the hallmarks of high-index winters include:
• A northward shift, followed by fragmentation of the stormtrack over the Pacific Ocean.
• Higher 500-hPa heights, particularly over the Southwest
• Cooler temperatures over much of the winter, by strong warming during early spring.
• Decreased precipitation in the Southwest and increased precipitation over the northern Great Plains,
particularly during spring
• Earlier than normal decreases in NDVI over much of the western U.S., indicative of drying and suggesting the
potential for early onset of the fire season.
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ACKNOWLEDGEMENTS
CPC US Unified Precipitation data, University of Delaware Air
Temperature data, and NCEP reanalysis data provided by the NOAA/
OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://
www.cdc.noaa.gov/. Naturalized Colorado Basin flow provided by the
Bureau of Reclamation from their website http://www.usbr.gov/lc/region/
g4000/NaturalFlow/index.html. CPC and Colorado State University AO
indices available online at http://www.jisao.washington.edu
A moving t-test comparing adjacent 10-year windows finds
significant increases in the JFM NAM in the early- and mid-1980’s.
Below is the difference between 10-year average values of the JFM
NAM. Black, blue and green diamonds tan dots indicate differences
significant at 90%, 95%, and 99%.
500 hPa
THE NORTHERN ANNULAR MODE
DESCRIBES HEMISPHERE-WIDE PRESSURE
AND WIND PATTERNS
The Northern Annular Mode (NAM) is the first empirical orthogonal
function of sea-level pressure (SLP) north of 20°N. Positive values of
the index are associated with higher pressure in the mid-latitudes,
lower pressure over the pole and stronger, more zonal westerly
winds, as shown below. On the left is the regression of the NAM on
JFM SLP. 1950-2005. On the right is the regression against zonal
wind over the same period.
NAM, SPRING AND ABRUPT CHANGE
The NAM also shows a significant step change toward higher values
during the mid-1980’s.
IMPACTS ON WATER RESOURCES IN THE COLORADO BASIN
Despite the correlations between the JFM NAM and temperature and precipitation, there were few significant
correlations between the annular mode and stream flow. Rather, there are spatially consistent, significant negative
correlations between annual naturalized flow and spring (April - June, AMJ) NAM indices. The AMJ NAM index is
positively correlated with the JFM index (r = 0.13), but is lower in amplitude.
Naturalized water year flow data are courtesy of the United States Bureau of Reclamation.
Correlations between detrended naturalized flow at 29
stations and the detrended AMJ NAM, 1906-2005. All
correlations are negative.
• Red indicates p < 0.01
• Orange indicates p < 0.05
• Y ellow indicates p < 0.10
Unfilled dots indicate insignificant correlations
This work was funded by the National Oceanic and Atmospheric Administration's Climate Program Office through grant # NA16GP2578 with the Climate Assessment for the Southwest program at the University of Arizona
CONCLUSIONS
The West is warming more rapidly than the rest of the world, with
subsequent ecosystem and water resources impacts, and the
impacts seem to be particularly notable in the spring. The
Northern Annular mode is a good candidate for producing this
focused climate change.
• The NAM has shifted to higher mid-winter and spring values,
particularly during the mid-1980’s.
• Higher phases of the NAM appear to trigger an earlier shift from
cool- to warm-season circulation patterns.
• The biggest impacts of the NAM on geopotential height,
temperature and precipitation occur in the spring.
• The NAM has impacts on NDVI (early drying during high-index
winters) that are consistent with the mid-1980’s increase in wildfire
frequency at that time (Westerling et al., 2006).
• The NAM is negatively correlated with streamflow in the Colorado
River basin, consistent with the mid-1980’s shift toward earlier
snowmelt (McCabe and Clark, 2005).
REFERENCES
McCabe, GJ & MP Clark (2005). Trends and variability in snowmelt
runoff in the western United States. J. Climate, 6: 476-482.
Westerling, AL et al. (2006). Warming and earlier spring increase
western U.S. forest wildfire activity. Science, 313: 940-943.
Tucker, CJ et al. (2005). An Extended AVHRR 8-km NDVI data set
compatible with MODIS and SPOT vegetation NDVI data. Int. J.
Remote Sensing, 26:4485-4498.