Simulating Multi-Season Past Droughts
Abstract

Abstract Author: Scott Weaver

Abstract Title: Warm season variations in the low-level circulation and precipitation over the central United States in idealized SST experiments

Abstract: The central U.S. is a hydroclimatically and economically sensitive region given its agricultural prominence and significant warm season precipitation variability. The proximity of this region to the Rocky Mountains, Gulf of Mexico, and Atlantic and Pacific Oceans provide a unique combination of potential climate influences. As such, the central U.S. is prone to drought and pluvial, highlighted most recently by the flooding during the spring of 2008. An important feature governing a large portion of the warm season precipitation variations is the Great Plains low-level jet (GPLLJ). Reanalysis based studies of the GPLLJ suggest connectivity to hemispheric wide atmospheric circulation anomalies emanating over the adjoining ocean basins and a midsummer correlation of ~ 0.7 to central U.S. precipitation. Given the recent evidence for SST variability in producing precipitation anomalies over the Great Plains on seasonal to interannual timescales, it is necessary to understand the role of slowly varying SST modes in generating GPLLJ variability.

The recently completed U.S. CLIVAR Drought Working Group idealized SST simulations are employed to study the evolution and modulation of the low-level moisture bearing circulation over the central U.S. under various configurations of Atlantic and Pacific SST anomalies. The seasonal cycle of low-level winds and their impacts on moisture fluxes and precipitation is analyzed in the NSIPP-1 (NASA), GFS (NOAA), CAM 3.5 (NCAR), AM 2.1 (GFDL), and CCM 3 (Lamont-Columbia) idealized SST-forced AGCM integrations, and supplemented with AMIP, reanalysis, and observationally based comparisons where available. Interesting features include varying seasonal preference for SST-influenced low-level circulation, with CAM 3.5 and AM 2.1 showing weak SST linkages, while NSIPP-1 (late summer), CCM 3 (late summer), and GFS (spring) display marked sensitivity among the varying polarities of prescribed SSTs. All 5 models show reduced (enhanced) precipitation over the northern central U.S. with coincident weakening (strengthening) of the GPLLJ (save GFS) under a cold (warm) Pacific and warm (cold) Atlantic scenario. Possible mechanisms for the SST links to central U.S hydroclimate are investigated.