Investigation of the 2006 drought and 2007 flood extremes at the Southern Great Plains through an integrative analysis of observations

Hydrological years 2006 (HY06; October 2005 to September 2006) and 2007 (HY07; October 2006 to September 2007) provide a unique opportunity to examine hydrological extremes in the central United States because there are no other examples of two such highly contrasting precipitation extremes occurring in consecutive years at the Southern Great Plains (SGP) in recorded history. The HY06 annual precipitation in the state of Oklahoma, as observed by the Oklahoma Mesonet, is around 61% of the normal (92.84 cm, based on the 1921-2008 climatology), which results in HY06 as the second-driest year in the record. In particular, the total precipitation during the winter of 2005-2006 is only 27% of the normal, and this winter ranks as the driest season. On the other hand, the HY07 annual precipitation amount is 121% of the normal, and HY07 ranks as the seventh-wettest year for the entire state and the wettest year for the central region of the state. Summer 2007 is the second-wettest season for the state. Large-scale dynamics play a key role in these extreme events. During the extreme dry period (11/2005-02/2006), a dipole pattern in the 500 hPa geopotential height anomaly existed where an anomalous high was over the southwestern U.S. region and an anomalous low was over the Great Lakes. This pattern is associated with inhibited moisture transport from the Gulf of Mexico and strong sinking motion over the SGP, both contributing to the extreme dryness. The precipitation deficit over the SGP during the extreme dry period is clearly linked to significantly suppressed cyclonic activity over the southwestern United States, which shows a robust relationship with the western Pacific teleconnection pattern. The precipitation events during the extreme wet period (May-July 2007) were initially generated by active synoptic weather patterns, linked with moisture transport from the Gulf of Mexico by the northward low-level jet, and enhanced the frequency of thunderstorms and their associated latent heat release. Although the drought and pluvial conditions are dominated by large-scale dynamic patterns, we have found two possible positive feedback processes during the extreme dry and wet periods in this study that play key certain roles to maintain and reinforce the length and severity of existing drought and flood events. For example, during the extreme dry period, with less clouds, liquid water path, precipitable water vapor, precipitation, and thinner Cu cloud thickness, more net radiation was absorbed and used to evaporate water from the ground. The evaporated moisture, however, was removed by low-level divergence. Thus, with less precipitation and removed atmospheric moisture, more absorbed incoming solar radiation was used to increase surface temperature and to make the ground drier.