Present Weather and Climate: Evolving Conditions reveals the variability and trends of the region’s climate using data from weather instruments and paleoclimate records (such as tree rings), comparing the climate of the last 100 years with that of the last 1,000 years, and linking patterns of past climatic conditions to the variability of river flows in the region’s major water basins.
Citation:
Hoerling, M. P., M. Dettinger, K. Wolter, J. Lukas, J. Eischeid, R. Nemani, B. Liebmann, and K. E. Kunkel. 2013. “Present Weather and Climate: Evolving Conditions.” In Assessment of Climate Change in the Southwest United States: A Report Prepared for the National Climate Assessment, edited by G. Garfin, A. Jardine, R. Merideth, M. Black, and S. LeRoy, 74–100. A report by the Southwest Climate Alliance. Washington, DC: Island Press.
Martin P. Hoerling (NOAA, Earth System Research Laboratory)
Michael Dettinger (U.S. Geological Survey and Scripps Institution of Oceanography)
Klaus Wolter (University of Colorado, CIRES)
Jeff Lukas (University of Colorado, CIRES)
Jon Eischeid (University of Colorado, CIRES)
Rama Nemani (NASA, Ames)
Brant Liebmann (University of Colorado, CIRES)
Kenneth E. Kunkel (NOAA Cooperative Institute for Climate and Satellites, North Carolina State University, and National Climate Data Center)
Key Findings
The areal extent of drought over the Southwest during 2001–2010 was the second largest observed for any decade from 1901 to 2010.
The period since 1950 has been hotter in the Southwest than any comparable period in at least 600 years.
Streamflows in the four major drainage basins of the Southwest were 5–37% lower during 2001–2010 than their average flows in the 20th century.
Fewer cold waves and more heat waves occurred over the Southwest during 2001–2010 compared to their average occurrences in the 20th century.
Figure Gallery
Figure 5.1: Trends in temperature and precipitation, 1901–2010. Shown in the first three (upper) panels are the pattern and intensity of 110-year trends in the Southwest in annually averaged daily temperature (TAVG), and daily maximum temperature (TMAX) and daily minimum temperature (TMIN), respectively, as estimated from station data for which there are at least 90 years of available data during the period. The magnitude of trends is indicated by a station circle’s size, with warming (cooling) trends denoted by red (blue) shades. Bottom panel shows trend in annual averaged precipitation for 1901- 2010, showing data for individual stations plotted with outlines of four major basins in the Southwest (Sacramento-San Joaquin, Great Basin, Upper Colorado, and Rio Grande). Units are the total change expressed as percent of annual climatology, and positive (negative) trends are shown in green (orange). Larger circle sizes denote greater magnitude trends. Filled stations denote statistically significant trends at 95% confidence.
Figure A5.1: The 1901–2010 trends in annual averaged daily temperature (TAVG, top), daily maximum temperature (TMAX, second panel), and daily minimum temperature (TMIN, third panel). Units are the change in °C/110yrs, and stronger positive (negative) trends shown in red (blue). Bottom panel is the 1901–2010 trend in annual averaged precipitation. Units are the total change expressed as percent of annual climatology, and positive (negative) trends are shown in green (orange). Larger circle sizes denote greater magnitude trends. Trends are computed at station locations using the GHCN (Version 3) raw monthly (unadjusted) data. Results are shown only for locations having at least 90 years of available data. Filled stations denote statistically significant trends at 95% confidence based on a parametric t-statistic. Stations with temperature (precipitation) changes less than 0.5°C (5%) are denoted with a + symbol. The fraction of stations with significant trends is 61%, 40%, and 63% for TAVG, TMAX, and TMIN respectively.
Figure 5.2: Growing season anomalies for 1900 to 2010. Departure from the normal number of days in the growing season are shown as number of days per year. Length of the growing season is defined as the period between the last freeze in spring and the first freeze in the subsequent fall. Source: NOAA National Climatic Data Center for the Cooperative Observer Network (http://www.ncdc. noaa.gov/land-based-station-data/cooperative-observer-network-coop).
Figure 5.3: Occurrence of cold waves and heat waves, 1901–2010. Cold waves (top) are defined as four-day periods that are colder than the threshold of a one-in-five-year occurrence; heat waves (bottom) are four-day periods warmer than the same threshold. The thresholds are computed for the entire 1901–2010 period. Source: NOAA National Climatic Data Center for the Cooperative Observer Network (http://www.ncdc.noaa.gov/land-based-stationdata/cooperative-observer-networkcoop).
Figure 5.4: Occurrence of extreme precipitation events, 1900–2010. Extreme precipitation events are defined as five-day totals that are wetter than the threshold of a one-in-five-year occurrence. Source: NOAA National Climatic Data Center for the Cooperative Observer Network (http://www.ncdc. noaa.gov/land-based-station-data/cooperative-observer-network-coop).
Figure 5.5: Areal coverage of drought in the Southwest United States, 1900–2010. Data is for drought during summer (June through August). Graph based on the Palmer Drought Severity Index (PDSI) and monthly data from the NOAA National Climatic Data Center for 1901–2010.
Figure 5.6: Changing streamflow timing 2001–2010 compared to 1950–2000. Differences between 2001–2010 and 1950–2000 average date when half of the annual streamflow has been discharged (center of mass) for snowmelt-dominated streams (Stewart, Cayan and Dettinger 2005).
Figure 5.7: Proxy and modeled data for the past 2000 years. (A) Solar variability (solar irradiance) shown in watts per square meter (Bard et al. 2000), (B) Multi-proxy reconstructions of Northern Hemisphere temperatures, two estimates (Mann et al. 2008), (C) ECHO-G GCM simulations of temperatures for the U.S. Southwest-Intermountain region (Stevens, Gonzalez-Ruico and Beltrani 2008), (D) Temperature reconstruction for the Colorado Plateau from tree rings (Salzer and Kipfmueller 2005). The lighter shading indicates the period of the Medieval Climate Anomaly and the darker shading indicates the mid-12th century megadrought in the Colorado River Basin. Adapted from Woodhouse et al. (2010).
Figure 5.8: History of drought in the West. Percent area affected by drought (PDSI)