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Climate Change, Atmospheric Rivers and Floods in California

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Climate Change, Atmospheric Rivers and Floods in California JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 47, No. 3 AMERICAN WATER RESOURCES ASSOCIATION June 2011 CLIMATE CHANGE, ATMOSPHERIC RIVERS, AND FLOODS IN CALIFORNIA – A MULTIMODEL ANALYSIS OF STORM FREQUENCY AND MAGNITUDE CHANGES1 Michael Dettinger2 ABSTRACT: Recent studies have documented the important role that ‘‘atmospheric rivers’’ (ARs) of concentrated near-surface water vapor above the Pacific Ocean play in the storms and floods in California, Oregon, and Wash- ington. By delivering large masses of warm, moist air (sometimes directly from the Tropics), ARs establish con- ditions for the kinds of high snowlines and copious orographic rainfall that have caused the largest historical storms. In many California rivers, essentially all major historical floods have been associated with AR storms. As an example of the kinds of storm changes that may influence future flood frequencies, the occurrence of such storms in historical observations and in a 7-model ensemble of historical-climate and projected future climate simulations is evaluated. Under an A2 greenhouse-gas emissions scenario (with emissions accelerating through- out the 21st Century), average AR statistics do not change much in most climate models; however, extremes change notably. Years with many AR episodes increase, ARs with higher-than-historical water-vapor transport rates increase, and AR storm-temperatures increase. Furthermore, the peak season within which most ARs occur is commonly projected to lengthen, extending the flood-hazard season. All of these tendencies could increase opportunities for both more frequent and more severe floods in California under projected climate changes. (KEY TERMS: climate variability ⁄ change; meteorology; atmospheric rivers; flooding.) Dettinger, Michael, 2011. Climate Change, Atmospheric Rivers, and Floods in California – A Multimodel Analy- sis of Storm Frequency and Magnitude Changes. Journal of the American Water Resources Association (JAWRA) 47(3):514-523. DOI: 10.1111/j.1752-1688.2011.00546.x INTRODUCTION increased standards for urban flood protection. Many Californians face unacceptable risks from flooding, both from where they live and work and from where Major floods are a recurring theme in California’s they derive water supplies. In response to the risks climatology and hydrology, and have a long history of and conflicts posed by flooding, the California Depart- being an important cause of death and destruction in ment of Water Resources Water Plan Updates in both California (Kelley, 1998). Even today, California’s 2005 and 2009 strongly recommend that water supply aging water supply and flood protection infrastruc- management and land-use development be much ture, including more than a thousand kilometers more fully integrated with flood management in the of levees, is challenged by punishing floods and State (DWR, 2005, 2009). The Delta Vision Task 1Paper No. JAWRA-10-0049-P of the Journal of the American Water Resources Association (JAWRA). Received April 16, 2010; accepted December 20, 2010. ª 2011 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the USA. Discussions are open until six months from print publication. 2Research Hydrologist, U.S. Geological Survey, Scripps Institution of Oceanography, 9500 Gilman Drive, Dept. 0224, La Jolla, California 92093 (E-Mail ⁄ Dettinger: [email protected]). JAWRA 514 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION CLIMATE CHANGE,ATMOSPHERIC RIVERS, AND FLOODS IN CALIFORNIA –AMULTIMODEL ANALYSIS OF STORM FREQUENCY AND MAGNITUDE CHANGES Force has identified improvements in floodplain and express’’ storms in California are just one version of a flood emergency management among its key recom- more common feature of the midlatitude atmosphere mendations for the future of California’s Delta (Delta (Dettinger et al., 2011). It has been estimated that Vision Blue Ribbon Task Force, 2008). Perhaps most about 90% of all the water vapor transported toward convincingly, the people of California passed Proposi- the poles across the midlatitudes is transported tions 1E and 84 in 2006 to fund bonds intended to within narrow, intense filamentary bands of moist provide over $4.5 billion specifically for flood manage- air, called atmospheric rivers (ARs) (Zhu and Newell, ment programs in the State. 1998), that together typically span less than about Although uncertainties abound, a significant part 10% of the Earth’s circumference at any given lati- of this focus has been motivated by the risks that flood tude. Ralph et al. (2006) recently noted that every may occur more frequently or become more extreme ‘‘declared’’ flood on the Russian River near Guerne- with climate changes due to increasing greenhouse- ville, California, during the past 10 years has been gas concentrations in the global atmosphere. Current associated with the arrival of an AR. Dettinger (2004) climate-change projections for 21st Century California showed that, during the past >50 years, flows in the uniformly include warming by at least a couple of Merced River near Yosemite Valley have typically degrees, and, although great uncertainties remain risen by about an order of magnitude more following about future changes in long-term average precipita- the pineapple express form of ARs than following tion rates in California (e.g., Dettinger, 2005; Cayan other winter storms. From these and other examples, et al., 2008), it is generally expected that extreme- AR storms are now increasingly understood to be the precipitation episodes may become more extreme as source of most of the largest floods in California, and the climate changes (Trenberth, 1999; Jain et al., an evaluation of the future of floods must attempt to 2005; Cayan et al., 2009). As a step toward better understand their future. understanding of the risks and as an example of anal- The long thin band of high water-vapor amounts ysis of frequency changes in a specific storm and flood (yellow and orange) between roughly Hawaii and cen- mechanism as a way of understanding likely overall tral California in Figure 1a, and the southwesterly flood frequency changes (Logan and Helsabeck, 2009), band of (white) clouds in Figure 1b, is the AR associ- this paper summarizes a preliminary analysis of the ated with the New Year’s 1997 storm (which yielded 21st Century future of a particularly dangerous sub- the flood of record on many California rivers) and set of flood-generating storms – the pineapple express gives a sense of the scope and scale of these features. or atmospheric-river storms – from seven current cli- The other panels show other ways of visualizing the mate models. same episode. Investigations by Ralph et al. (2004, 2006), Neiman et al. (2008a,b), and others have shown that, as they approach the west coast of North America, ARs are typically 2,000 or more kilometers CHARACTERISTICS OF ATMOSPHERIC-RIVER long but only a few hundred kilometers wide (Ralph STORMS et al., 2006). The air column within a typical AR will contain more than 2 cm of water vapor, with most of that vapor contained in the first 2.5 km above the sea Although warming alone may be expected to alter surface and with a jet of intense and moist winds flood regimes in many snowfed settings by raising centered near about 2 km above the surface (Neiman snowlines and increasing basin areas receiving rain- et al., 2008b). When the AR is oriented so that these fall in many storms (e.g., Knowles et al., 2006; intense winds carry their moist air directly up and Dettinger et al., 2009), changes in California’s storm over the mountains of California (i.e., in directions types, frequencies, or magnitudes may provide more nearly perpendicular and upslope into the mountain direct and pervasive opportunities for change. Histor- ranges), intense storms of orographically enhanced ically, the most dangerous storms in California have precipitation result (Neiman et al., 2002; Andrews been warm and wet storms that strike in winter, pro- et al., 2004). ducing intense rains over large areas and unleashing The presence or absence of ARs along the Pacific many of the State’s largest floods. The most com- coast can be detected by monitoring the strength of monly recognized of these storms have been described the water-vapor transports across the region. The as ‘‘pineapple express’’ storms because of the way likely impact of such storms depends both on how that they are observed (in weather satellite and other much vapor the AR contains and how fast that vapor imagery, e.g., Figure 1) to steer warm, moist air from is being transported across coastal mountains where the tropics near Hawaii northeastward into Califor- orographic uplift can extract the vapor as more or nia (Weaver, 1962; Dettinger, 2004). More recently, less intense precipitation. Intense pineapple express studies have highlighted the fact that ‘‘pineapple storms can be identified in daily general-circulation JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 515 JAWRA DETTINGER FIGURE 1. Various Approaches to Visualizing Atmospheric-River Conditions. SSM ⁄ I integrated water-vapor imagery from GMT morning on 2 January 1997 (a; warm colors for more vapor, cool colors for less), infrared weather-satellite imagery of the Pacific Ocean basin (GOES- West) from 18:00 hours GMT on 1 January 1997 (b; light colors are cloud bands, coasts indicated in green), corresponding daily weather map (c) and vertically integrated water-vapor transport directions and relative rates (d) from NCAR-NCEP Reanalysis fields; arrow at bottom indicates length of a 1,000 kg ⁄ m ⁄ s vapor-transport vector. model (GCM)-scale atmospheric fields by tracing back the 925-mb pressure level (about 1 km above the sur- to sources of intense water-vapor transport plumes to face) for a GCM grid cell just offshore from the cen- determine which begin in the subtropics or tropics tral California coast. In this study, these variables near Hawaii (Dettinger, 2004). However, this can be were determined for each day from the periods 1961- a cumbersome algorithm to apply to current projec- 1980, 1981-2000, 2046-2065, and 2081-2100, and for a tions of climate change, because of the large fields model-grid cell offshore from central California.
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