Investigation of the 2006 drought and 2007 flood extremes at the SGP through an integrative analysis of observations

Description

Hydrological years 2006 (HY06, 10/2005–09/2006) and 2007 (HY07, 10/2006–09/2007) provide a unique opportunity to examine climate 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) site in recorded history. The HY06 annual total precipitation amount 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 being the second-driest year in the record. In particular, the total precipitation (3.7 cm) during the winter of 2005–06 is only 27% of the normal, and this winter ranks as the driest season. On the other hand, the HY07 annual mean precipitation over Oklahoma 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 with a total precipitation of 40.8 cm, 68% higher than the normal. Precipitation is positively correlated with cloud fraction (corr=0.579), cumulus cloud thickness (0.654), cloud liquid water path (0.784), longwave flux (0.602), and atmospheric precipitable water vapor (0.64), but negatively correlated with shortwave flux (-0.645). The onsets of the SGP droughts and floods are triggered by persistent large-scale flow anomalies. For example, the winter precipitation deficit over the SGP is clearly linked to significantly suppressed cyclonic activity over the southwestern United States, which shows a robust relationship with the Western Pacific (WP) teleconnection pattern. From the Atmospheric Radiation Measurement (ARM) SGP observations, two positive feedback processes have been identified, which provide a physical basis for interpreting the observation that a drought or flood tends to “feed upon itself.” These observational results can serve as a baseline for future modeling studies that aim at simulating the onsets/demises of droughts and floods over the SGP and the multiple feedback processes involved in the formation of these hydrological extremes.