Border Climate Summary Resumen del Clima de la Frontera Issued: February 18, 2010 Tree Rings and the Monsoon in the Southwestern U.S. By Daniel Griffin, School of Ge- To better grasp ography and Development and The the full range of 1871 1872 1873 1874 Laboratory of Tree-Ring Research, spatial and tem- University of Arizona poral variabil- ity that is pos- The monsoon is a major component of sible under natu- southwestern North America’s climate ral (non-human) regime and arguably one of the most conditions, re- anticipated regional climate events of searchers at the the year, delivering varying doses of life- University of Ar- giving rains to the U.S.-Mexico border re- izona are study- EW LW EW LW EW EW LW gion each summer. On average, the mon- ing the long- Dry Dry Dry soon brings three-fourths of northwestern term climate Wet Wet Wet Wet Winter Winter Winter Winter Mexico’s annual rainfall and up to half of history of the Summer Summer Summer Pre-Monson? Pre-Monsoon? the annual rain in the U.S. Southwest. monsoon using Dry Summer annual growth Figure 1. This photomicrograph illustrates a sequence of four Douglas- fir tree rings (1871–1874) from southwestern New Mexico. Each annual Variability in monsoon rainfall in time rings from long- growth ring is composed of light-colored earlywood (EW) and dark-colored and location increases northward with lived, moisture- latewood (LW). Both 1871 and 1872 contain intra-annual density variations distance from the “core region” in west- stressed trees. known as “false rings,” which are probably related to reduced soil moisture ern and northwestern Mexico and is in the May–June pre-monsoon period. Note the variability of EW-width, a Tree Rings and function of cool-season precipitation, and the independent variability of LW notably dramatic along the border and width, which corresponds to warm-season precipitation. into the southwestern U.S. These year- the Monsoon to-year changes influence ecosystems, Our understanding of the monsoon excellent records for reconstructing en- rangelands, agriculture, public health, climate system has improved great- vironmental history, including natu- water resources, and water demand, ly in recent years, largely through the ral climate variability at time-scales yet the mechanisms behind the chang- cooperative efforts of research associ- of years to decades to centuries. es are not completely understood. ated with the North American Mon- soon Experiment (http://www.eol. Southwestern North America has a ucar.edu/projects/name/) and im- dense network of tree-ring collec- proved seasonal forecasts. For lon- tions that extend back more than 400 Table of Contents: ger time scale climate change projec- years. Historically, scientists have used 1 Tree-Rings and the Monsoon in tions, global climate models robust- these records to learn about the long- the Southwestern U.S. ly predict that the region’s cool season term history of drought and wet- 3 A look back at the 2009 summer will be drier in the future, but predic- ness in the winter season, but these monsoon tions of future North American Mon- tree-ring samples also can be used soon precipitation, in response to hu- to study moisture history associated Recent Conditions man-caused climate change, are varied. with the North American Monsoon. 6 Temperature 7 Precipitation Natural proxy records in the form The annual growth rings in many south- 8 North American Drought Monitor of tree rings can help put these pro- western conifer species, such as pine 9 Sonoran Reservoir Levels jections and future changes in long- and fir trees, are composed of light col- Forecasts term perspective. As described in ored “earlywood” that forms in the 10 Streamflow Forecast the July 2009 Border Climate Sum- spring and dark colored “latewood” that 11 Precipitation Forecast mary (available online: http://www. forms in the summer (Figure 1). In the 12 ENSO climas.arizona.edu/forecasts/bor- Southwest U.S. and northwest Mexico, der/archive.html), tree rings provide continued on page 4 The information in this packet is available on the web: http://climas.arizona.edu/forecasts/border/summary.html 2 | Executive Summary Disclaimer–This packet contains official and non-official forecasts, as well as other information. Executive Summary While we make every effort to verify this informa- tion, please understand that we do not warrant the accuracy of any of these materials. The user assumes the entire risk related to the use of this data. In General – El Niño is expected to influence the winter storm track and bring an CLIMAS disclaims any and all warranties, whether increased chance of above-average precipitation to the U.S. Southwest and north- expressed or implied, including (without limitation) any implied warranties of merchantability or fitness western Mexico in the February–April period. Servicio Meteorológico Nacional pre- for a particular purpose. In no event will CLIMAS dicts well above-average precipitation for the Baja California peninsula and north- or The University of Arizona be liable to you or to any third party for any direct, indirect, incidental, western Sonora in March and April. Northwestern Mexico and the southwestern consequential, special or exemplary damages or lost United States received above-average precipitation during the second half of January. profit resulting from any use or misuse of this data. Temperature – In northwestern Mexico and the southwestern U.S., temperatures were mostly average to above average during June–November 2009. Precipitation – Thanks to several tropical storms in September and October, Baja California Sur and Sonora received above-average fall precipitation. November was mostly a dry month for the entire region. Precipitation Forecast – SMN predicts well above-average precipitation for the Baja California peninsula and northwestern Sonora in March and April, and well above- average precipitation for Baja California Norte and northwestern Sonora in May. ENSO – The chance of El Niño persisting through at least April stands at more than 90 percent, according to the latest IRI ENSO forecast. Neutral conditions are expect- ed to return rapidly in the May–July period, which is typical of El Niño events. Climate Assessment for the Southwest Staff: Aaron Banas, UA Graduate Research Assistant Jason Criscio, UA graduate research assistant Stephanie Doster, Institute of the Environment Associate Editor Luis Farfan, CICESE Research Scientist Gregg Garfin, Institute of the Environment Deputy Director for Outreach David Gochis, NCAR Research Scientist Rebecca Macaulay, Graphic Artist Funding for the Border Climate Summary/Resumen del Clima de la Frontera was provided Andrea Ray, NOAA Research Scientist by Inter American Institute for Global Change Research (IAI) and the NOAA Sector Applications Research Program. Border Climate Summary 3 | Articles A look back at the 2009 summer monsoon By David J. Gochis, National Observed Accumulated Precipitation (mm) Center for Atmospheric Research 140 140 120 120 On May 21, 2009, the monsoon mes- 100 100 sage was clear. During an online briefing, 80 80 Below Normal or webinar, forecasters and researchers 60 60 outlined their forecasts and expectations 40 40 Above Normal for the 2009 North American Mon- 20 20 soon (NAM) season (http://ag.arizona. 0 0 edu/climate/ws/052109.htm): a wet 1 Jul 6 Jul 11 Jul 16 Jul 21 Jul 26 Jul 1 Aug 6 Aug 11 Aug 16 Aug 21 Aug 26 Aug 1 Sep 6 Sep 11 Sep 16 Sep 21 Sep start to the monsoon tempered by un- Daily Precipitation and Normal Precipitation (mm/day) 6 6 certainty beginning in August. Yet as daily normal the season progressed, conditions took 5 5 an unexpectedly severe turn as the rains 4 4 all but disappeared. So, what happened during the 2009 NAM season, and 3 3 how accurate were those predictions? 2 2 1 1 The consensus statement from the we- binar was for an early and robust on- 0 1 Jul 6 Jul 11 Jul 16 Jul 21 Jul 26 Jul 1 Aug 6 Aug 11 Aug 16 Aug 21 Aug 26 Aug 1 Sep 6 Sep 11 Sep 16 Sep 21 Sep 0 set of monsoon rains in the Southwest Figure 1. Summer monsoon precipitation in 2009 for northwest Mexico and the Southwest U.S. U.S. and northwest Mexico, with con- Top: green indicates cumulative precipitation greater than the 1979–1995 average, and brown ditions becoming very uncertain by indicates cumulative precipitation below the average. By the end of the monsoon season, the av- erage cumulative deficit for the region was around 40 mm. Bottom: green bars indicate the daily mid-season due to the development of precipitation rate (mm/day) for the region; the black curve shows the average daily precipitation an El Niño event (warmer-than-aver- rate. Note the substantial decline in precipitation rate between July 5 and August 10, and the near age ocean temperatures) in the tropi- cessation of precipitation between July 27 and August 10. cal eastern Pacific Ocean. This consen- sus statement was based on indepen- (departures from average conditions) for As suspected at the time of the sea- dent forecasts made by the U.S. and the period ending September 24. Ar- sonal forecast webinar, El Niño ap- Mexican national weather services, as eas of northeast and eastern Mexico pears to have played a significant role well as analyses by climate researchers. (Coahuila, Nuevo León, Tamaulipas, in suppressing monsoon rainfall, par- and Veracruz) and portions of south- ticularly over southern and eastern To a certain degree, this general fore- ern Mexico (Yucatán, Chiapas, Guer- Mexico. Figure 1c shows the aver- cast turned out to be accurate for a rero, and Michoacán) were drier than age sea surface temperature anoma- limited region of the NAM, includ- average nearly all summer and faced lies for June 2b through September 16.