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| Monsoon Inter-annual
variability? PDF
version |
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Monsoon variability
from one summer to the next is substantial, and exceeds the normal monsoon
seasonal precipitation at most locations. For example, the normal monsoon
precipitation at Tucson, AZ is 6.06 inches. The driest monsoon season
measured 1.59 inches, and the wettest measured 13.84 inches. Therefore
a variation between seasons of 12.25 inches exists, which is over twice
the normal monsoon precipitation at Tucson. Understanding the causes for
this huge variation is the first step in developing an ability to forecast
an upcoming monsoon season.
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| Research within the
past decade or so has investigated the possible causes behind North American
Monsoon variability. Specific factors examined include: |
- Sea surface temperature
anomalies.
- Large-scale circulation
patterns.
- Land surface conditions.
- Tropical convergence
zones.
- Moisture transport
mechanisms.
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| All of these research
factors uncovered important details affecting the monsoon in the Southwest,
but none of them provided a perfectly clear picture of all conditions affecting
its variability. These factors are related to each other and are not
independent. For example, sea surface temperatures affect all the other
factors to some extent. In additional to inter-annual monsoon variability, multi-decadal
variability has been observed. In other words, data from 1963 through 1996
may show results in one aspect, while larger data sets from 1900 through
1963 uncover results not seen in the later time periods. |
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| The
role of sea surface temperature anomalies: |
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From a climatic perspective,
an important factor to examine in any precipitation regime is the surrounding
sea surface temperatures (SSTs). Atmosphere-ocean coupling is a strong
driver in many seasonal patterns observed around the globe. The El Nino/Southern
Oscillation is perhaps the most well known. Sea surface temperature anomalies
(SSTAs) are the primary data set used in these studies. SSTAs can be statistically
correlated to many observed conditions such as precipitation, maximum
temperature or snow cover, and SSTs vary slowly when compared to changing
weather patterns. Monsoon variability research shows that Pacific SSTs
are an important factor in determining summertime drought or rainy conditions
in both the Southwest and the Great Plains. Additionally these Pacific
SSTs help modulate the previous winter season precipitation amounts. These
analyses show winters preceding early-onset monsoons are characterized
by cold SSTA in the mid-latitudes of the North Pacific and warm SSTA in
the subtropics of the North Pacific; the opposite is true for late onset
monsoons. The SSTA analysis shown in Figure 1 depicts a typical early
onset monsoon pattern. Blue colors represent colder than normal temperatures;
yellows and reds represent warmer than normal. The actual temperature
scale in degrees Celsius is given at the bottom.
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| Figure
1: Sea surface temperature anomalies with cooler
than normal temperatures (blue) off the western U.S. Cooler water in this
location is typical of an early onset monsoon pattern. |
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| A significant
correlation exists in the observed data such that wet winters are generally
followed by dry monsoons, and dry winters are generally followed by wet
monsoons. Dry and wet conditions are classified as the bottom or top 25%
of the years examined. Therefore the Southwest summer rainfall is modulated
by the Pacific SSTs. Unfortunately SSTs are not the only factor involved,
since 16 consecutive wet (dry) winters and wet (dry) monsoons have been
observed in the past 100 years. Likewise near normal winter or monsoon precipitation,
the middle 50% or half of the years examined were unrelated to each other. |
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| The
role of the large scale circulation |
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| The North
American monsoon is closely linked to the large scale (continental) weather
patterns over the United States. The monsoon onset is connected with a shift
in the Bermuda high toward the west, which in turn provides light easterly
flow at mid and upper levels into Mexico and the Southwest. The exact location
of the summertime ridge over the United States is a big factor in monsoon
variability from one year to the next. Additionally the location of this
ridge is affected by SSTs in the months of May of June. These two figures
illustrate an idealized relationship between the ridge location and the
character of monsoon observed over the Southwest. |
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| Two idealized configurations
of the ridge exist. The first configuration (Figure 2) has a strong
ridge located in the Great Plains. Upper level easterlies carry moisture
and disturbances from the Gulf of Mexico south of the ridge. In this configuration
the monsoon arrives early in the Southwest and is wet. The second configuration
(Figure 3) centers the ridge in northwest Mexico because a trough
occurs in the western U.S. This situation directs moisture into the Great
Plains. The Southwest is dry and monsoon onset is late. |
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| Figure
2: Strong upper level ridge located in the Great Plains is positioned
for carrying moisture from the Gulf of Mexico into the Southwest. |
Figure
3: Upper level ridge centered in northwest Mexico blocks the moisture
in the Gulf of Mexico from entering the Southwest. |
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| Another
important concept in examining the large scale circulation is the thermal
contrasts between the land and sea. This thermal contrast is what establishes
a large scale thermally direct circulation of hot, rising air over the continent
and cooler, sinking air over the ocean. The monsoon thermally driven circulation
is physically similar to a sea breeze observed near an ocean or lake, except
on a much larger scale. |
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| The North
Pacific sea surface temperature patterns affect these large scale circulations
by influencing the summer precipitation in the Great Plains. Warmer water
in the eastern North Pacific favors a ridge in that area with a trough over
the West. Conversely cold water in the eastern North Pacific supports a
strong ridge in the Great Plains. Many studies have found a strong relationship
between the Southwest and the Great Plains. During the summertime, when
the Great Plains is persistently wet, the Southwest will tend to be dry.
If the Great Plains is experiencing a summer drought, the Southwest will
tend to be wet. This reversal of phase observation is directly related to
the large scale circulations present, and is a factor in monsoon variability
from one year to the next. |
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| The
role of land surface conditions |
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| A significant
correlation exists in the observed data such that wet winters in the Southwest
are generally followed by dry monsoons, and dry winters are generally followed
by wet monsoons. One of the possible physical explanations for this observation
is found in examining the spring snow pack in the southern Rocky Mountains.
A heavy snow pack in southern Utah and Colorado may delay the development
of the summer ridge over the U.S. because more solar energy goes toward
melting snow instead of heating the atmosphere necessary to build the monsoon
ridge. A statistical analysis of this feedback indicates it influences western
New Mexico the most and to a lesser extent Arizona and the Plains. |
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| Figure
4 illustrates the relationship between spring snowfall and summer rainfall
in New Mexico. Solid dots above the horizontal center line occur when the
water equivalent of the spring snow pack is above normal. Below the center
line indicates less than normal snow pack. The vertical center line differentiates
above (right of line) or below (left of line) normal monsoon rainfall. As
you can see, most of the solid dots occur in the upper left quadrant (above
normal snow, below normal rainfall) and lower right quadrant (below normal
snow, above normal rainfall). These data are from a 30 year period 1961
to 1990. |
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| Figure
4: Snow water equivalent (SWE) on April 1 plotted with the amount of
New Mexico rainfall observed that year in July and August (from Gutzler,
2000). |
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| The evolution of the
monsoon as it develops over North America is characterized by the rains
starting in southwest Mexico in May and moving northward into the U.S. by
July. As the rains move northward, a rapid green up in vegetation is observed
and is associated with this increase in precipitation. The abundant vegetation
then improves the ability for moisture to remain in the lower levels of
the atmosphere, due to transpiration by plants, and assists the moisture
moving northward. Drier lower levels in the atmosphere can impede moisture
movement, since much of the rainfall evaporates before it reaches the ground.
Hence the lower levels never reach saturation. These monsoon moisture conditions
remain until a large-scale circulation pattern change occurs and dries out
the atmosphere. The effect of vegetation on atmospheric moisture is most
important during the latter part of the monsoon in August and September.
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| Given this evolution
of the monsoon, a question arises: does an early start in Mexico imply an
early start in the Southwest? This question is important because an early
start to the monsoon in the Southwest invariably leads to a wet monsoon.
Therefore, if we know what is happening in Mexico we may be able to anticipate
what happens in Arizona and New Mexico A late start often has a "sputtering"
monsoon with below normal precipitation. Figure 5 depicts how the
start date affects the amount of precipitation received and was created
using data from 1963 to 1988. |
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| Figure
5: Precipitation amounts received in Arizona and New Mexico throughout
a year. Dash-dot line represents dry monsoons; dashed line wet monsoons
and the solid line is a composite of all monsoons (from Higgins, 1999). |
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| In Figure
5 the dashed line represents wet monsoons, the dash-dot line represents
dry monsoons and the solid line is a composite of all monsoons. Additionally
Figure 5 illustrates the relationship between a dry winter/wet monsoon and
wet winter/dry monsoon. |
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| Coming
back to the question about how an onset date in Mexico relates to Arizona
and New Mexico; the answer is no relationship exists between the southwest
Mexico onset date and seasonal rainfall anomalies. Thus an early monsoon
start in Mexico is not related to the monsoon precipitation characteristics
elsewhere. |
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| The role of the
inter-tropical convergence zones |
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| The inter-tropical
convergence zone (ITCZ) is an area of convection located between 5o and
10oN as shown in Figure 6. On a satellite picture, the ITCZ appears as an
area of thunderstorms north of the equator. The activity in this zone off
the coast of Mexico during the spring directly impacts the type of monsoon
observed. For example, a wet monsoon in the Southwest is associated with
suppressed precipitation in the eastern Pacific ITCZ and enhanced precipitation
north and south of this zone. Likewise an active ITCZ area during the spring
preceding monsoon onset is indicative of a dry monsoon. |
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| The shifts
in the ITCZ can be affected by sea temperatures. Warmer temperatures favor a
stronger and more southward displaced ITCZ off the Mexican coast. Cooler
temperatures tend to weaken the convergence zone and displace it further north.
Thus cooler sea surface temperatures can both increase the land-sea temperature
gradient and inhibit the ITCZ; warmer SSTs have the opposite effect. |
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| Figure
6: Visible satellite picture illustrating an inter-tropical convergence
zone. |
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| The physical
reason behind the ITCZ/monsoon interaction is not entirely clear. A tropical
convergence zone is maintained by lower level westerly winds from the equator
colliding with lower level easterly winds from the northern latitudes. The
stronger the intensity of these winds, the stronger the storms in this convergence
zone. When ITCZ activity off the southern coast of Mexico is weak, a plausible
explanation related to the monsoon is that the land-sea interactions associated
with the monsoon can more easily develop. Moisture and abundant rainfall
then enters southern Mexico under these conditions. A strong ITCZ inhibits
this critical circulation pattern from developing over Mexico. |
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| The role of moisture
transport |
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| Moisture
entering the Southwest during the North American Monsoon generally comes
from two locations, the Gulf or Mexico or the Gulf of California. Analysis
data shows mid and upper level moisture primarily originates from the Gulf
of Mexico, while lower level moisture advection originates from the Gulf
of California. Of course, once thunderstorms start in Arizona or New Mexico,
all levels of the atmosphere become moist. However the variability of where
this moisture originates in the first place helps explain the variability
from one monsoon to the next. |
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| Local
sea surface temperatures (SSTs) are an important aspect of this moisture
transport. Research has shown that roughly 80% of the rainfall in Arizona
and New Mexico occurs after SSTs in the northern Gulf of California exceed
28.5oC. Likewise positive correlations with SST anomalies over the Gulf
of Mexico have been found too. Hence the warm water in these areas increases
the availability of moisture to be transported elsewhere. |
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| One common
method for transporting moisture from one place to another is by increasing
the low level winds. Regions of lower level wind speed maximum are referred
to as lower level jets. Two lower level jets exist affecting the Southwest;
one in the Gulf of California and another over the Great Plains. A lower
level jet coming up the Gulf of California directly transports moisture
into Arizona and is called a gulf surge. One factor affecting the variability
of moisture transport in the Gulf of California is the SSTs along the California
and northern Baja coasts. An increased thermal gradient in this region promotes
the formation of lower level jet episodes as shown in Figure 7. |
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| Figure
7: Cooler sea surface temperatures off the Baja coast is a favorable
condition for moisture transport from the Gulf of California into Arizona
and California. |
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| The lower
level jet in the Great Plains primarily determines the type of summer precipitation
over Texas, Oklahoma and Kansas. The eastern portion of New Mexico may directly
experience affects from the Great Plains lower level jet moisture, but most
of the Southwest does not. The importance of this jet is that it weakens
when the large-scale circulation pattern reduces precipitation in the Plains.
Instead of moving north, this moisture gets shunted into Mexico and eventually
into the Southwest as the source of mid and upper level moisture. |
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| Figure
8: Warm sea surface temperatures in the Gulf of Mexico which can promote
mid and upper level moisture transport into the Southwest. |
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| Climate
Change and the Monsoon |
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| A question
of concern is how the North American Monsoon will be altered in the future
as a result of climate change. Global warming projections are given by numerical
computer models, such as those documented by the Intergovernmental Panel
on Climate Change. Unfortunately the IPCC models poorly represent the North
American Monsoon in the Southwest. Hence this question does not have an
accurate answer at this time. |
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| Summary |
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| Many factors
influence the variability from one monsoon season to the next. All of these
factors are inter-related to each other, and likewise none of them totally
account for monsoon variability. The scientific understanding of this variability
has increased substantially over the past 15 years and remains an active
research area. |
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| Further
Reading and References: |
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| Carleton,
et.al., 1990: Mechanisms of Interannual Variability of the Southwest United
States Summer Rainfall Maximum. J. Climate, 3, 999-1015.
Castro, C.L., McKee,
T. B. and Pielke, R.A., 2001: The Relationship of the North American Monsoon
to Tropical and North Pacific Sea Surface Temperatures as Revealed by
Observational Analyses. J. Climate, 14, 4449-4473.
Castro, C.L., Pielke,
R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the
Contiguous United States and Mexico Using the Regional Atmospheric Modeling
System (RAMS). Part I: Model Climatology (1950-2002). J. Climate, 20,
3844-3865.
Castro, C.L., Pielke,
R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the
Contiguous United States and Mexico Using the Regional Atmospheric Modeling
System (RAMS). Part II: Model Climate Variability. J. Climate, 20, 3866-3887.
Gutzler, David and
Preston, Jessica, 1997: Evidence for a relationship between spring snow
cover in North America and summer rainfall in New Mexico. Geophysical
Research Letters, 24, 2207-2210.
Gutzler, David S.,
2000: Covariability of Spring Snowpack and Summer Rainfall across the
Southwest United States. J. Climate, 13, 4018-4027.
Higgins, R.W., Mo,
K.C. and Yao, Y., 1998: Interannual Variability of the U.S. Summer Regime
with Emphasis on the Southwestern Monsoon. J. Climate, 11, 2583-2606.
Higgins, R.W., Chen,
Y. and Douglas, A.V., 1999: Interannual Variability of the North American
Warm Season Precipitation Regime. J. Climate, 12, 653-679.
Higgins, R.W. and
Shi, W., 2000: Dominant Factors Responsible for Interannual Variability
of the Summer Monsoon in the Southwestern United States. J. Climate, 13,
759-776.
Higgins, R.W. and
Shi, W., 2001: Intercomparison of the Principal Modes of Interannual and
Intraseasonal Variability of the North American Monsoon System. J. Climate,
14, 403-417.
Mo, Kingtse C. and
Paegle, Julia Nogues, 2000: Influence of Sea Surface Temperature Anomalies
on the Precipitation Regimes over the Southwest United States. J. Climate,
13, 3588-3598.
Vera, C. et. al.,
2006: Toward a Unified View of the American Monsoon Systems. J. Climate,
19, 4977-5000.
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| What
is a monsoon? | North American Monsoon
| Gulf Surges | Monsoon
progression | Monsoon Inter-annual
variability | Severe Thunderstorm and
Flash Flooding patterns | Upper
Level Lows and the Monsoon | Monsoon Safety
| For more reading |
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