Abstract Author: H. Mark Helfand
Abstract Title: Relationship between mean winds and large 10-day precipitation anomalies over the continental U.S.
Abstract: To better understand the role of surface boundary forcing upon interannual and decadal variability of droughts and precipitation excesses over the continental U. S., an observational study has been carried out to better investigate the dynamical linkages between surface boundary forcing and droughts/floods. This study examines the relationship during the warm season between observed mean wind patterns on the 10-15 day time scale and anomalies in precipitation on that same time scale over the U. S.
Each of the 7 cases of significant 10-day, warm-season drought over the U.S. since 1948 that has been investigated was accompanied by significant continental-scale ridging of the mean 10-day 250 mb wind pattern over the continental U.S. and a mid-Pacific relative maximum of 250 mb wind speed of at least 30 m/sec within 20 degrees of the International Date Line. This perhaps suggests the presence of a wave-guide phenomenon over the Pacific. The drought signal for each of these cases was still quite evident when averaged over the monthly time scale.
While peak magnitudes of the precipitation anomalies were much larger for the 7 chosen cases of significant of precipitation excess were than for the drought cases none of the patterns of precipitation excess were as unambiguous or wide-spread as the drought cases. In only 2 to 3 cases was there evidence of a continental trough in the 250 mb winds. In fact, one case showed slight ridging over Canada and the extreme northern U.S. Strong southerly low-level jets, however, were evident in the 925 mb winds over south-central Texas or the nearby Gulf of Mexico in virtually all of the excessive precipitation cases, while the Great Plains LLJ was weak or displaced to the north of Texas in most of the drought cases.
We will further investigate the asymmetry between droughts and precipitation excesses in their horizontal homogeneity and extent, in their temporal evolution, and in how the are forced by the combined effects of the mean 250 mb westerlies and the mean 925 mb southerly winds.