The Ocean's Influence on North American Drought
We have already seen that the ocean influences the atmosphere:
- The ocean is the primary heat source driving the atmospheric circulation.
- The ocean is the primary source of moisture that falls on land.
Modes of Atmospheric Variability
The interaction of the ocean and atmosphere involves variability of
all periods from much less than a day to centuries to millions of years.
The interactions change the transport of water to the continents, leading
to flooding at one extreme to drought on the other. Some of the interactions
are particularly important and relatively well known.
This is perhaps the most important and well
known example of an interannual interaction. This is described in the El
the northern hemisphere annular mode, this is the year-to-year variability
pf weather patterns at high latitudes in the northern hemisphere. The
variability was first noticed in the north Atlantic, and the interaction
was called the North Atlantic Oscillation. Winds in the north Atlantic
varied as the Icelandic Low and the Bermuda High pressure areas strengthened
and weakened. Further studies showed the variations in weather
patterns extended throughout the northern hemisphere, and the oscillation
The two phases of the Arctic Oscillation: Left– Warm Phase; Right–
Cold Phase. Notice that storms come ashore in Europe at different latitudes
in the two phases. This strongly influences European weather, especially
From The National Snow and Ice Data Center's
web page on the Arctic
For other figures, see the Annular
Modes Web site: Figures.
- The oscillation is characterized by sea-level pressure anomalies
of one sign in the Arctic and anomalies of opposite sign centered
- The high index or warm phase of the Arctic Oscillation is defined
as periods of below normal Arctic sea-level pressure, enhanced surface
westerlies in the north Atlantic, warmer and wetter than normal
conditions in northern Europe, and stronger trade winds at lower
latitudes. This phase is shown on the left in the figure above. It
is associated with the positive values of the North Atlantic Oscillation
index (shown below).
- The low index or cold phase is characterized by higher sea-level
pressure in the Arctic, weaker westerlies, colder winters in North
America and Europe, and weaker trade winds. This phase is shown on
the right in the figure above. It is associated with negative values
of the North Atlantic Oscillation index (shown below).
- The cause of the AO and the NAO is still unknown.
The historical variability of the Arctic Oscillation.
From Joint Institute for the Study of the Atmosphere and Ocean, Arctic
Oscillation Time Series Web Page.
The NOAA Climate Prediction Center gives recent
values and predictions of future values a season in
Pacific Decadal Oscillation
similar to El Niño,
but operating over periods of 20-30 years.
Left: Warm (positive)
Cool (negative) phase of the Pacific Decadal
From Joint Institute for the
Study of the Atmosphere and the Ocean, University of Washington, web page on
the Pacific Decadal Oscillation.
Pacific Decadal Oscillation Index.
From the Climate Impacts
group of the
Center for Science in the
Earth System web page on the Pacific
The Pacific Decadal Oscillation is related to weather over north America.
Influence of the Pacific Decadal Oscillation on weather
over North America.
Top: October to march air
temperature anomalies during the warm phase, contours in 0.2°C.
Contour map of correlation coefficients between gridded North American December-February
(DJF) precipitation and the SST-based Pacific Decadal Oscillation index, based
upon data for the period 1900-93.
From Mantua: The
Pacific Decadal Oscillation and Climate Forecasting for North America.
Positive PDO values
are usually associated with wetter conditions in the
Southwestern United States, while negative PDO values
are suggestive of persistent drought in the Southwest.
From Robert H. Webb, Richard Hereford, and Gregory J. McCabe. (2000)
Atlantic Multidecadal Oscillation
This is the pattern of changes in surface temperature over
large areas of the tropical Atlantic over periods of several decades.
Recent research suggests that the AMO
is related to the past occurrence of major droughts in the Midwest
and the Southwest. When the AMO is in its warm phase, these droughts
tend to be more frequent and/or severe (prolonged?). Vice-versa
for negative AMO. Two of the most severe droughts of the 20th
century occurred during the positive values of the AMO between
1925 and 1965: The Dust bowl of the 1930s and the 1950s drought.
Florida and the Pacific Northwest tend to be the opposite - warm
AMO, more rainfall.
Multidecadal Oscillation web page of the Atlantic Oceanographic
and Meteorological Laboratory.
The Atlantic Multidecadal Oscillation index
of North Atlantic temperature anomalies.
Climate Pacemaker for Millennia Past, Decades Hence? by
Richard Kerr (2005).
Red and blue colored dots represent positive
and negative correlations of Northern Hemisphere summer rainfall
with the AMO index. When the value of the AMO is positive (warm Atlantic)
there is less rainfall over most of the United States and northeastern
South America, and more rainfall in southern Alaska, northern Europe,
west Africa and Florida.
Multidecadal Oscillation web pages at the NOAA Atlantic
Oceanographic and Meteorological Laboratory.
The Atlantic Multidecadal Oscillation also influences the number
of hurricanes in the Atlantic.
From United Nations Environmental Programme Grid
Arendal web page on Intense
hurricanes in the Atlantic Basin.
Paths of Atlantic hurricane during the two phases of the Atlantic
Top: Cold Phase. Bottom: Warm Phase.
From Atlantic Multidecadal
Oscillation web pages at the NOAA Atlantic Oceanographic and Meteorological
North American Drought
Drought over north America has been correlated to the Atlantic Multidecadal
Oscillation and the Pacific Decadal Oscillation.
The correlations account for the drought in the Great Plains during the 1930s
dust bowl and more recent droughts.
The relationship between drought in the continental US and the phases
of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal
Oscillation (AMO). The most severe droughts occur when the PDO is in
a negative phase, and the AMO is in a positive phase.
From McCabe (2004).
More than half (52%) of the space and time variance
in multidecadal drought frequency over the conterminous United States
is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic
Multidecadal Oscillation (AMO). An additional 22% of the variance in
drought frequency is related to a complex spatial pattern of positive
and negative trends in drought occurrence possibly related to increasing
Northern Hemisphere temperatures or some other unidirectional climate
trend. Recent droughts with broad impacts over the conterminous U.S.
(1996, 1999-2002) were associated with North Atlantic warming (positive
AMO) and northeastern and tropical Pacific cooling (negative PDO).
Much of the long-term predictability of drought frequency may reside
in the multidecadal behavior of the North Atlantic Ocean. Should the
current positive AMO (warm North Atlantic) conditions persist into
the upcoming decade, we suggest two possible drought scenarios that
resemble the continental-scale patterns of the 1930s (positive PDO)
and 1950s (negative PDO) drought.
The Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation
From McCabe (2004).
Fraction of Colorado in drought averaged over a running 48-month period.
From Nolan (2004) The
Climate of Colorado: an update and status report.
For more information, read Climatic
Fluctuations, Drought, and Flow in the Colorado River Basin by
the US Geological Survey (2004).
Fire in the American West
Forest fires in the western United States are significantly correlated
with ENSO. The correlation accounts for about half of the variance of areas
burned by fires from 1905 to 1985. Fires are common during La Niño
conditions, and much less common during El Niño (Swetnam 1990).
Time series of annual area burned in Arizona and
New Mexico and the Southern Oscillation Index during December, January, and
February (DJF). Low values of the index lead to rains in the
spring growing season and less fire the following summer.
From Swetnam (1990).
Kerr, R. A. (2005). CLIMATE CHANGE: Atlantic Climate Pacemaker for
Millennia Past, Decades Hence? Science 309
McCabe, G. J., M. A. Palecki, et al. (2004). Pacific and Atlantic
Ocean influences on multidecadal drought frequency in the United States.
Proceedings of the National Academy of Sciences,
101 (12): 4136-4141.
Swetnam, T. W. and J. L. Betancourt (1990). Fire-Southern Oscillation
Relations in the Southwestern United States. Science 249
Fire scar and tree growth chronologies (1700 to 1905) and fire statistics
(since 1905) from Arizona and New Mexico show that small areas burn
after wet springs associated with the low phase of the Southern Oscillation
(SO), whereas large areas burn after dry springs associated with
the high phase of the SO. Through its synergistic influence on spring
weather and fuel conditions, climatic variability in the tropical
Pacific significantly influences vegetation dynamics in the southwestern
United States. Synchrony of fire-free and severe fire years across
diverse southwestern forests implies that climate forces fire regimes
on a subcontinental scale; it also underscores the importance of
exogenous factors in ecosystem dynamics.
5 August, 2009