The Contribution of the NAO and AO to Anomalous Precipitation across the Midwest during the Warm Season 1899-2015

The summer of 2015 was one of the wettest periods on record for the Midwest region. The states of Illinois, Indiana, and Ohio recorded their wettest June on record (1895-2015), and Kentucky recorded its wettest July. Five Midwest states (Illinois, Indiana, Ohio, Missouri, and Kentucky) experienced their third wettest two-month rainfall for June - July 2015. Overall, the Midwest had its third wettest June-July and six Midwest states ranked among the ten wettest June-Julys on record, resulting in a vast array of climate impacts. Property damage due to flooding, record algae blooms, flash flooding, agricultural loss, and delayed barge transportation on the Illinois River resulted in millions of dollars of reported loss.

This study places the 2015 observed June and July rainfall in historical context based on climatology developed from the National Oceanic and Atmospheric Administration’s (NOAA) Climate Divisional Database (nClimDiv). Additionally, the study seeks to determine if the prolific rainfall in summer 2015 was attributable to identifiable atmospheric signals. Statistical analyses and reanalysis were used to investigate common modes of climate variability impacting United States weather patterns – the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Pacific North American Pattern (PNA) and El Niño-Southern Oscillation (ENSO). These were performed with the goal of improving operational seasonal forecasts of summertime precipitation to better prepare for historic events such as June and July 2015. Results suggest that of the teleconnections, the NAO and AO had the largest influence on the wet climate pattern over the Midwest in summer 2015 by enhancing Gulf moisture transport into the region, especially during the month of July. A moderate El Niño and neutral PNA during a majority of the period favored a zonal flow pattern, allowing the continual progression of midlatitude weather systems across the region that provided the lift needed for continuous rainfall generation. This presentation is of research that evolved out of the presentation “The Midwest’s Wet Summer of 2015: The Impacts and Comparisons to Past Midwest Summers” by Bryan Peake and Olivia Kellner, presented at the 22^nd Conference on Applied Climatology in New Orleans, 14 January 2016.