paper, I will create a modern climate profile of the Southwestern United States.
The National Climate Assessment (NCA), National Oceanic and Atmospheric
Administration (NOAA), and various research institutions include different states
when defining the Southwestern United States climate region. This paper will
primarily focus on the climate of Arizona and New Mexico, these states being
constant in the interpretations of various climate research agencies.
climate data pertinent to this analysis includes temperature and drought trends,
as well as climate altering factors such as the El Niño Southern-Oscillation,
variations in the North American Monsoon System, and Sea Surface Temperature
Anomalies. This report is intended to give context to both past and future
climate analysis of the Southwest.
climate of the Southwestern United States is characterized as the hottest and
most arid region in the nation (Garfin, et al., 2014). The past century in
the Southwest can be roughly summarized as having long, sometimes decade long
droughts punctuated by fewer pluvial periods lasting shorter amounts of time (Carrillo, Castro, Chang, & Luong, 2017). The primary
mechanisms controlling yearly climate variations include the El Niño
Southern-Oscillation (ENSO) in the autumn and spring, mid-latitude storm
systems in the winter (Andrade & Sellers, 1988), and the North
American Monsoon System (NAMS) in the summer (Sheppard, et al., 1999).
The aim of
this paper is to provide a description of the modern climate setting of the
Southwestern United States that includes consistent climate patterns and the
factors that affect them. The following analysis will include a discussion on
multi-year droughts, seasonal climate trends, and a focus on outside mechanisms
that cause variations in these trends.
in the southwest follows a normal four season cycle with maximums in the summer
and minimums in the winter, decreasing with higher elevations. The mean annual
temperature of New Mexico, at 12°C, is lower than that of Arizona, at
17°C. Daily averages in the southwest vary from winter lows at
-7°C at higher altitudes to summer highs between 27°C to 35°C at
lower altitudes. (Sellers & Hill, 1974)
Droughts and the Effects of Global
Southwest owes much of its aridity to a quasi-permanent subtropical
high-pressure ridge over the region. Other contributing factors include high
temperatures, high levels of evapotranspiration, and rainshadow effects from
mountain ranges (Scott, 1991).
Droughts in the Southwest can be particularly intense, sometimes spanning
several decades. (Carrillo, Castro, Chang, & Luong, 2017) In analyzing the
past century of drought data using tree ring data complimented with the
Twentieth-Century Reanalysis product (20CR) and a long-term simulation that
dynamically downscaled this data, (Carrillo, Castro, Chang, & Luong, 2017) demonstrated that
most multi-year droughts can be linked to the El-Niño Southern-Oscillation,
with the exception of the 1892-1899 drought, which was related to inner
Figure 1. The JA MFC (July-August
Moisture Flux Convergence) anomaly time-series. The convergence and divergence
of the moisture flux are pointed out with vectors. The dotted bars show annual
changes and the solid line shows the running mean over 10-years. (Carrillo, Castro, Chang, & Luong, 2017)
by the National Climate Assessment in 2014, the Southwest is already
experiencing the effects of climate change. The period since the 1950’s has
seen the highest temperatures in the last 600 years (Garfin, et al., 2014). A higher frequency
in droughts, combined with human caused temperature increases, has led to
insect outbreaks, as well as increased forest fire frequency and tree
mortality. (Garfin, et al., 2014)
The El Niño Southern-Oscillation and
Spring/Autumn Climate Variations
The El Niño
Southern Oscillation (ENSO) is the largest cause for variation in the
Spring/Autumn dry seasons of the Southwestern climate. (Kiladis & Diaz, 1989) While it’s effects
are largely minimized in the winter by mid-latitude storm systems (Andrade &
large ENSO events can have an effect on cold season precipitation (Kiladis &
The effects of ENSO on summer precipitation are also minimized, primarily due
to the fact that the North American Monsoon System (NAMS) provides a fairly
constant influx of moisture (Sheppard, et al., 1999).
southwest region, the spring and autumn seasons tend to be affected greatly by
ENSO. Both seasons see a pronounced increase in precipitation due to unusually
warm water off the West Coast. This leads to the development of strong
west-coast troughs and weakens the tradewind inversion (Andrade & Sellers, 1988). In the autumn, this
causes stronger, more frequent development of tropical storms and hurricanes
off the West Coast. (Smith, 1986)
Cold Season Climatic Setting and
Southwest, being located south of the typical winter westerly storm tracks, is
generally not subject to the substantial precipitation brought to the northern
states (Sheppard, et al., 1999). Rather, these
westerly storms result in high winds and cloudy skies throughout the region (Sheppard, et al., 1999). When precipitation
does occur in the winter, it is often caused by large cyclones spanning a few
thousand kilometers that enter North America via California (Sellers & Hill, 1974). Heavy winter rains
in Arizona tend to coincide with unusually dry periods in northwestern states (Sellers & Hill, 1974).
variations in winter climate in the Southwest can be attributed to 4 major
causes: the Pacific/North American pattern, southwestern troughing, the El
Niño-Southern Oscillation, and the Pacific Decadal Oscillation (Sheppard, et al., 1999). The Pacific/North
American (PNA) pattern (Simmons, Wallace, & Branstator, 1983) can be linked to
above average Southwest precipitation depending on the east-west position of
the high pressure ridge, where a reverse PNA pattern results in below average
precipitation. Southwestern troughing, a phenomenon in which meridional flow is
displaced westward, the winter circumpolar vortex expands and displaces Pacific
storms to the south, causing them to absorb more moisture and contributing to
over sixty percent of January precipitation totals in southern Arizona (Sellers & Hill, 1974).
El-Niño events, as discussed in further detail in a different section of this
paper, result in the largest variation in annual climate in the troposphere (Kiladis & Diaz, 1989). Large warm ENSO
events (El Niño) result in cool and wet winters, with southwestern deserts
experiencing winter flooding (Kiladis & Diaz, 1989). Strong cold ENSO
events (La Niña), on the other hand, result in a reverse PNA pattern, resulting
in warmer and drier conditions in the southwest (Kiladis & Diaz, 1989).
Summer Climate Setting and
dominant feature of summers in the southwest is the North American Monsoon
System (NAMS) (Sheppard, et al., 1999). Monsoon systems
develop from the thermal contrast between continents and oceans in low-latitude
regions, and have a large effect on warm season precipitation patterns (Higgens, Yao, & Wang, 1997). The North American
Monsoon System affects much of the western United States and northwestern Mexico
(Higgens, Yao, & Wang, 1997). The onset of NAMS
typically occurs in Mexico in June, and later in higher latitudes such as in
New Mexico and Arizona (Higgens, Yao, & Wang, 1997).
in the North American Monsoon System are controlled by factors such as ENSO, soil
moisture, snow cover, and most importantly, Sea Surface Temperature Anomalies
(SSTA), (Higgins & Shi, 2000). (Higgins, Mo, & Yao, 1998) demonstrated that
wet summer monsoons in the southwest were usually preceded by dry winters in
the northwest and vice versa; the primary cause of this was attributed to
Pacific SSTA. In a study by (Higgins & Shi, 2000), it was concluded
that the mechanism relating SSTA to the summer monsoon was the impact of
changes in the Pacific jet on West Coast precipitation regimes from preceding
Figure 2. SSTA (8C) for NH winter
(January, February, March) for the 50’s (a) and 80’s (b). Computed as compared
to monthly means from the base period from 1961 to 1990. (Higgins & Shi, 2000)
climate of the southwest can classified as primarily arid, sometimes enduring
multi-year droughts (Carrillo, Castro, Chang, & Luong, 2017), and can be affected
by a number of outside factors throughout the year. The spring and autumn dry
seasons see a dramatic increase in precipitation during ENSO events (Kiladis & Diaz, 1989). The winter
precipitation is primarily varied by large cyclones from the West Coast (Sellers & Hill, 1974). The summers, being
characterized by NAMS, sees variations primarily due to Sea Surface Temperature
Anomalies (SSTA). Some of the questions I seek to answer in later assignments
include how we can use paleoclimate reconstruction techniques to understand how
changes in these weather systems have affected the climatic past of the Southwest.
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