Abstract analysis will include a discussion on
Abstract In thispaper, I will create a modern climate profile of the Southwestern United States.The National Climate Assessment (NCA), National Oceanic and AtmosphericAdministration (NOAA), and various research institutions include different stateswhen defining the Southwestern United States climate region. This paper willprimarily focus on the climate of Arizona and New Mexico, these states beingconstant in the interpretations of various climate research agencies. Majorclimate 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 TemperatureAnomalies. This report is intended to give context to both past and futureclimate analysis of the Southwest. Introduction The modernclimate of the Southwestern United States is characterized as the hottest andmost arid region in the nation (Garfin, et al., 2014).
The past century inthe Southwest can be roughly summarized as having long, sometimes decade longdroughts punctuated by fewer pluvial periods lasting shorter amounts of time (Carrillo, Castro, Chang, & Luong, 2017). The primarymechanisms controlling yearly climate variations include the El NiñoSouthern-Oscillation (ENSO) in the autumn and spring, mid-latitude stormsystems in the winter (Andrade & Sellers, 1988), and the NorthAmerican Monsoon System (NAMS) in the summer (Sheppard, et al., 1999). The aim ofthis paper is to provide a description of the modern climate setting of theSouthwestern United States that includes consistent climate patterns and thefactors that affect them. The following analysis will include a discussion onmulti-year droughts, seasonal climate trends, and a focus on outside mechanismsthat cause variations in these trends. Temperature Temperaturein the southwest follows a normal four season cycle with maximums in the summerand minimums in the winter, decreasing with higher elevations. The mean annualtemperature of New Mexico, at 12°C, is lower than that of Arizona, at17°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 atlower altitudes. (Sellers & Hill, 1974) Droughts and the Effects of GlobalWarming TheSouthwest owes much of its aridity to a quasi-permanent subtropicalhigh-pressure ridge over the region. Other contributing factors include hightemperatures, high levels of evapotranspiration, and rainshadow effects frommountain ranges (Scott, 1991).Droughts in the Southwest can be particularly intense, sometimes spanningseveral decades. (Carrillo, Castro, Chang, & Luong, 2017) In analyzing thepast century of drought data using tree ring data complimented with theTwentieth-Century Reanalysis product (20CR) and a long-term simulation thatdynamically downscaled this data, (Carrillo, Castro, Chang, & Luong, 2017) demonstrated thatmost multi-year droughts can be linked to the El-Niño Southern-Oscillation,with the exception of the 1892-1899 drought, which was related to inneratmospheric variability.
Figure 1. The JA MFC (July-AugustMoisture Flux Convergence) anomaly time-series. The convergence and divergenceof the moisture flux are pointed out with vectors. The dotted bars show annualchanges and the solid line shows the running mean over 10-years. (Carrillo, Castro, Chang, & Luong, 2017) As reportedby the National Climate Assessment in 2014, the Southwest is alreadyexperiencing the effects of climate change.
The period since the 1950’s hasseen the highest temperatures in the last 600 years (Garfin, et al., 2014). A higher frequencyin droughts, combined with human caused temperature increases, has led toinsect outbreaks, as well as increased forest fire frequency and treemortality. (Garfin, et al.
, 2014) The El Niño Southern-Oscillation andSpring/Autumn Climate Variations The El NiñoSouthern Oscillation (ENSO) is the largest cause for variation in theSpring/Autumn dry seasons of the Southwestern climate. (Kiladis & Diaz, 1989) While it’s effectsare largely minimized in the winter by mid-latitude storm systems (Andrade & Sellers, 1988),large ENSO events can have an effect on cold season precipitation (Kiladis & Diaz, 1989).The effects of ENSO on summer precipitation are also minimized, primarily dueto the fact that the North American Monsoon System (NAMS) provides a fairlyconstant influx of moisture (Sheppard, et al., 1999). In thesouthwest region, the spring and autumn seasons tend to be affected greatly byENSO. Both seasons see a pronounced increase in precipitation due to unusuallywarm water off the West Coast.
This leads to the development of strongwest-coast troughs and weakens the tradewind inversion (Andrade & Sellers, 1988). In the autumn, thiscauses stronger, more frequent development of tropical storms and hurricanesoff the West Coast. (Smith, 1986) Cold Season Climatic Setting andVariations TheSouthwest, being located south of the typical winter westerly storm tracks, isgenerally not subject to the substantial precipitation brought to the northernstates (Sheppard, et al., 1999). Rather, thesewesterly storms result in high winds and cloudy skies throughout the region (Sheppard, et al., 1999). When precipitationdoes occur in the winter, it is often caused by large cyclones spanning a fewthousand kilometers that enter North America via California (Sellers & Hill, 1974).
Heavy winter rainsin Arizona tend to coincide with unusually dry periods in northwestern states (Sellers & Hill, 1974). Largevariations in winter climate in the Southwest can be attributed to 4 majorcauses: the Pacific/North American pattern, southwestern troughing, the ElNiño-Southern Oscillation, and the Pacific Decadal Oscillation (Sheppard, et al., 1999). The Pacific/NorthAmerican (PNA) pattern (Simmons, Wallace, & Branstator, 1983) can be linked toabove average Southwest precipitation depending on the east-west position ofthe high pressure ridge, where a reverse PNA pattern results in below averageprecipitation. Southwestern troughing, a phenomenon in which meridional flow isdisplaced westward, the winter circumpolar vortex expands and displaces Pacificstorms to the south, causing them to absorb more moisture and contributing toover sixty percent of January precipitation totals in southern Arizona (Sellers & Hill, 1974). StrongEl-Niño events, as discussed in further detail in a different section of thispaper, result in the largest variation in annual climate in the troposphere (Kiladis & Diaz, 1989). Large warm ENSOevents (El Niño) result in cool and wet winters, with southwestern desertsexperiencing winter flooding (Kiladis & Diaz, 1989). Strong cold ENSOevents (La Niña), on the other hand, result in a reverse PNA pattern, resultingin warmer and drier conditions in the southwest (Kiladis & Diaz, 1989).
Summer Climate Setting andVariations Thedominant feature of summers in the southwest is the North American MonsoonSystem (NAMS) (Sheppard, et al., 1999). Monsoon systemsdevelop from the thermal contrast between continents and oceans in low-latituderegions, and have a large effect on warm season precipitation patterns (Higgens, Yao, & Wang, 1997). The North AmericanMonsoon System affects much of the western United States and northwestern Mexico(Higgens, Yao, & Wang, 1997). The onset of NAMStypically occurs in Mexico in June, and later in higher latitudes such as inNew Mexico and Arizona (Higgens, Yao, & Wang, 1997). Variationsin the North American Monsoon System are controlled by factors such as ENSO, soilmoisture, snow cover, and most importantly, Sea Surface Temperature Anomalies(SSTA), (Higgins & Shi, 2000).
(Higgins, Mo, & Yao, 1998) demonstrated thatwet summer monsoons in the southwest were usually preceded by dry winters inthe northwest and vice versa; the primary cause of this was attributed toPacific SSTA. In a study by (Higgins & Shi, 2000), it was concludedthat the mechanism relating SSTA to the summer monsoon was the impact ofchanges in the Pacific jet on West Coast precipitation regimes from precedingwinters. Figure 2. SSTA (8C) for NH winter(January, February, March) for the 50’s (a) and 80’s (b). Computed as comparedto monthly means from the base period from 1961 to 1990. (Higgins & Shi, 2000) Discussion The modernclimate of the southwest can classified as primarily arid, sometimes enduringmulti-year droughts (Carrillo, Castro, Chang, & Luong, 2017), and can be affectedby a number of outside factors throughout the year. The spring and autumn dryseasons see a dramatic increase in precipitation during ENSO events (Kiladis & Diaz, 1989).
The winterprecipitation is primarily varied by large cyclones from the West Coast (Sellers & Hill, 1974). The summers, beingcharacterized by NAMS, sees variations primarily due to Sea Surface TemperatureAnomalies (SSTA). Some of the questions I seek to answer in later assignmentsinclude how we can use paleoclimate reconstruction techniques to understand howchanges in these weather systems have affected the climatic past of the Southwest. Works Cited Andrade, E. R., & Sellers, W. D. (1988).
El Niño and its effect of precipitation in Arizona and Western New Mexico. Journal of Climatology, 403-410. Carrillo, C.
M., Castro, C. L., Chang, H.-I., & Luong, T.
M. (2017, March 21). Multi-year climate variability in the Southwestern United States within a context of a dynamically downscaled twentieth century reanalysis. Climate Dynamics, 4217-4236. Garfin, G., Franco, G.
, Blanco, H., Comrie, A., P., G., Piechota, T., . . .
Waskom, R. (2014). Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program. Washington D.C.
: U.S. Global Change Research Program. Higgens, R., Yao, Y., & Wang, X. (1997). Influence of the North American Monsoon System on the U.
S. Summer Precipitation Regime. Journal Of Climate, 10, 2600-2622. Higgins, R., & Shi, W. (2000).
Dominant Factors Responsible for Interannual Variability of the Summer Monsoon in the Southwestern United States. Journal Of Climate, 759-766. Higgins, R., Mo, K., & Yao, Y. (1998). Interannual variability of the United States summer precipitation regime with emphasis on the southwestern monsoon.
Journal Of Climate, 11, 2582–2606. Kiladis, G., & Diaz, H. (1989). Global climate anomalies associated with extremes of the Southern Oscillation. Journal of Climate(2), 1069-1090. Scott, R. (1991).
Essentials of physical geography. New York: West Publishing Co. Sellers, W., & Hill, R. (1974). Arizona Climate 1931-1972 (2nd ed.
). Tucson: University of Arizona Press. Sheppard, P. R.
, Comrie, A. C., Packin, G. D., Angersbach, Kurt, & and Hughes, M. K. (1999). The Climate of the Southwest.
The University of Arizona. Tucson: Institute for the Study of Planet Earth. Simmons, A., Wallace, J., & Branstator, G. (1983). Barotropic wave propagations and instability, and atmospheric teleconnections.
Amos Sci(40), 1363-1393. Smith, W. (1986). The Effects of Eastern North Pacific Tropical Cyclones on the Southwestern United States. Silver Spring: National Weather Service.