Abstract Author: Edward Berry and Klaus Weickmann
Abstract Title: The Great Floods of 1993 and 2008: The Roles of ENSO and the MJO/GWO
Abstract: Excessive precipitation during the springs and early summers of 1993 and 2008 brought record flooding to the central USA. The periods have been dubbed “the great floods”. Locations hardest hit were Iowa and surrounding states with the precipitation pattern having a similar small scale structure that favored Mississippi River flooding. The purpose of this work is to examine the interannual-subseasonal relationships of the global atmospheric circulation that may have contributed to the “great floods”. Subseasonal indices representing the Madden-Julian oscillation (MJO) and a global wind oscillation (GWO) help depict the state of the global atmosphere and are used as a starting point to telescope to the USA Great Plains. An important result is that even though the tropical convection was displaced toward El-Nino during 1993 and La-Nina in 2008, both climate regimes supported the regional scale record flooding and other high impact weather for an extended period of time.
The positive precipitation anomalies during 1993 are consistent with the El Nino composite signal while the positive anomalies during 2008 are opposite to the La Nina dry signal. The asymmetric outcomes are partially linked to subseasonal atmospheric variability. Specifically, the GWO and MJO phase space plots both show substantially greater subseasonal variability during 2008 compared to 1993. Further, the 1993 circulation was nearly steady state with fast, weak subseasonal variations. During 2008 slow, strong variations occurred and contributed to a ~1 standard deviation increase in global atmospheric angular momentum from March to May. The synoptics that contribute to these spring-summer floods are apparently similar during a decaying El Nino or during a large subseasonal oscillation.
Real-time monitoring of the subseasonal events during 2008 offered several opportunities for successful week 2-3 forecasts, especially during May 2008. Composites and phase space plots for both the GWO and the MJO are presented to demonstrate the predictive potential.