Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3The 1993 floodranks among thetop five weatherdisasters in theUnited States.large in scope and persistent, such as those thatoccurred during 1993 in the Upper MississippiValley (Kunkel et al., 1994; Anderson et al.,2003) (and again in 2008), regionally extensiveepisodes of flooding can occur. When climatesignificantly changes, as it has in the past(Knox, 2000), and will likely do in the future,changes in the overall flood regime, includingthe frequency of different size floods and theareas affected, will also occur (Kundzewicz etal., 2007).6.1 The 1993 Mississippi ValleyFloods—Large-Scale Controls andLand-Surface FeedbackThe flooding that occurred in the Upper MississippiValley of central North America in thelate spring and summer of 1993 provides a casestudy of the control of a major flood event bylarge-scale atmospheric circulation anomalies.Significant feedback from the unusually wetland surface likely reinforced the wet conditions,which contributed to the persistence of thewet conditions. The 1993 flood ranks among thetop five weather disasters in the United States,and was generated by the frequent occurrenceof large areas of moderate to heavy precipitation,within which extreme daily total rainfallevents were embedded. These meteorologicalevents were superimposed on an above-normalsoil-moisture anomaly at the beginning ofJune of that year (Kunkel et al., 1994). Theseevents were supported by the occurrence of alarge-scale atmospheric circulation anomalythat featured the persistent flow of moisturefrom the Gulf of Mexico into the interior of thecontinent (Bell and Janowiak, 1995; Trenberthand Guillemot, 1996). The frequency of seasonal(90-day long) excessive (i.e., exceeding a20-year return period) precipitation anomalieshas generally been increasing over time inthe United States (Kunkel et al., 2008 (CCSPSAP 3.3, Sec. 2.2.2.3, Fig. 2.9)).The atmospheric circulation features thatpromoted the 1993 floods in the MississippiValley, when contrasted with the widespreaddry conditions during the summer of 1988,provide a “natural experiment” that can be usedto evaluate the relative importance of remote(e.g., the tropical Pacific) and local (over NorthAmerica) forcing, and of the importance offeedback from the land surface to reinforcethe unusually wet or dry conditions. For example,Trenberth and Guillemot (1996) used acombination of observational and “reanalysis”data (Kalnay et al., 1996), along with somediagnostic analyses to reveal the role of largescalemoisture transport into the midcontinent,with dryness occurring in response to less flowand flooding in response to greater-than-normalflow. Liu et al. (1998) used a combination ofreanalysis data and simple models to examinethe interactions among the different controlsof the atmospheric circulation anomalies inthese 2 years.Although initial studies using a regional climatemodel pointed to a small role for feedback fromthe wet land surface in the summer of 1993 toincrease precipitation over the midcontinent(Giorgi et al., 1996), subsequent studiesexploiting the 1988/1993 natural experimentusing both regional climate models and generalcirculation models point to an important rolefor the land surface in amplifying the severityand persistence of floods and droughts (Bonanand Stillwell-Soller, 1998; Bosilovich and Sun,1999; Hong and Pan, 2000; Pal and Eltahir,2002). These analyses add to the general patternthat emerges for large moisture anomalies(both wet and dry) in the midcontinent ofNorth America to have (a) local controls (i.e.,atmospheric circulation and moisture fluxover North America), (b) remote controls (e.g.,Pacific SST anomalies), and (c) a significantrole for feedback that can reinforce the moistureanomalies. The 1993 floods continue to bea focus for climate model intercomparisons(Anderson et al., 2003).6.2 Paleoflood HydrologyThe largest floods observed either in theinstrumental or in the paleorecord have avariety of causes (O’Connor and Costa, 2004),for the most part related to geological processes.However, some the largest floods aremeteorological floods, which are relevant forunderstanding the nature of abrupt climatechanges (Hirschboeck 1989; House et al. 2002)and potential changes in the environmentalhazards associated with flooding (Benito etal., 2004; Wohl, 2000). Although sometimesused in an attempt to extend the instrumentalrecord for operational hydrology purposes (i.e.,fitting flood-distribution probability density110