A Modeling Study of the Causes and Predictability of the Spring 2011 Extreme US Weather Activity

Area of Science:

• Atmospheric Science
• Climate Dynamics
• Meteorology

Background:

• Extreme weather events in the U.S. pose significant risks.
• Understanding the drivers of precipitation anomalies is crucial for predictability.
• The interplay of various factors influences large-scale weather patterns.

Purpose of the Study:

• To investigate the causes and predictability of the spring 2011 U.S. extreme weather.
• To assess the impact of sea surface temperature (SST) anomalies, land conditions, and atmospheric modes on precipitation.
• To determine the influence of initial atmospheric conditions on April precipitation forecasts.

Main Methods:

• Utilized Modern-Era Retrospective analysis for Research and Applications (MERRA) reanalyses.
• Employed Goddard Earth Observing System, version 5 (GEOS-5) Atmospheric General Circulation Model simulations.
• Conducted stationary wave model experiments to isolate forcing impacts.

Main Results:

• April 2011 record precipitation in the Ohio River Valley was primarily driven by an unforced, high-amplitude North Atlantic Oscillation (NAO)-like mode.
• Sea surface temperature (SST) forcing (La Niña) influenced continental precipitation patterns, causing drying in the southern plains and wet anomalies in the northeast.
• Realistic North American land conditions enhanced precipitation in the upper Midwest and caused deficits in the southeast.
• March initial atmospheric conditions significantly influenced April precipitation over the eastern U.S., suggesting links to SST forcing and polar vortex behavior.
• SST-forced base states modestly enhanced the amplitude of the NAO response.

Conclusions:

• Unforced internal atmospheric variability, like the NAO, can be a dominant factor in extreme regional precipitation events.
• Predictability of regional precipitation is limited when internal variability is the primary driver.
• Sea surface temperature and land conditions play roles in modulating continental-scale precipitation patterns and influencing internal modes.
• Coupling between SST, land, and atmospheric initial conditions, including stratospheric influences, impacts extended-range predictability.