87th AMS Annual Meeting

Thursday, 18 January 2007: 11:30 AM
Drought and wel spells over the United States and Mexico
211 (Henry B. Gonzalez Convention Center)
Kingtse Mo, NOAA/NWS/NCEP, Camp Springs, MD; and J. K. E. Schemm
The wet and dry extreme events as measured by the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI) have preferred regions to occur and persist. The interior United States west of 90-95 W and the northwestern Mexico are more prone to droughts and persistent wet spells. In contrast, the extreme events are less persistent over the eastern United States and California. The physical mechanisms for the regional preference are examined by classifying extreme events into three categories: Multi-decadal, interannual and intraseasonal time scales. The extreme events modulated by multi-decadal SST forcing are located over the northwestern Mexico and the western Mountain regions. The extreme events which have interannual time scales are located over the Great Plains and the Southwest. The SST forcing which persists over the raining season has consistent influence on rainfall over these areas. Over the Southwest there is little rainfall in spring and soil moisture reaches a minimum. Once the monsoon starts, daily rainfall continues for one to two months. Rainfall is influenced largely by the large scale atmospheric conditions with little local modulation from soil conditions. Over the Great Plains, the coupling between soil moisture is strong and is not limited to the local feedbacks. Over the southern Plains, soil moisture anomalies over Texas in spring are associated with summer rainfall anomalies over the Great Plains. The location and pattern of soil moisture anomalies in spring influence the precipitation pattern. The extreme events are less likely to occur and persist over the central eastern United States, the East Coast and the Ohio Valley. These areas have very weak seasonal cycle. Rainfall from many seasons can contribute to the SPI or the PDSI. Rainfall is influenced by ENSO events which often last from winter to summer. However, ENSO often has the opposite impact on winter and summer rainfall over the same region.

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