The Effects of El Niño-Southern Oscillation on Lightning Variability over the United States McArthur “Mack” Jones Jr. 1, Jeffrey M. Forbes 1, Ronald L. Holle 2 ENSO Effects on the Weather of the US Motivation from a Previous Study Research Objectives and Questions Lightning and ENSO Data Used Statistically Significant Correlations between ENSO and Lightning Anomalies Summary, Conclusions, and Future Work 1 (1) Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA (2) Vaisala Inc., Tucson, Arizona, USA Monday, July 21, FESD-ECCWES Meeting

ENSO Effects on Synoptic Scale Meteorology over the Continental US 2 Figures from Mayes et al., (2008) It is now widely recognized that the El Niño/Southern Oscillation (ENSO) influences the synoptic-scale atmospheric dynamics leading to anomalous temperature, precipitation, and severe weather distributions across the North American continent. Monday, July 21, FESD-ECCWES Meeting

ENSO Influence on Lightning in the Gulf Coast Region (Laing et al., [2008]) 3 Laing et al., [2008] performed statistical study correlating CG lightning flash density with ENSO from in Gulf Coast Region *ENSO-lightning relationship appeared throughout most of year, especially in winter months* Monday, July 21, FESD-ECCWES Meeting

Research Objective and Science Questions 4 Research Objective Understand and quantify (i.e., through a statistical correlative study) the influence of the ENSO on lightning activity over the United States. Science Questions 1.Do similar results to Laing et al. [2008] persist throughout our longer lightning time series? 2.What geographic regions across the United States reflect the best statistically significant correlations between ENSO and lightning anomalies? *To answer these questions we use a new lightning dataset from the National Climatic Data Center (NCDC) which provides an unprecedented chance to perform long-term correlative study between the lightning variability over the US and ENSO.* Monday, July 21, FESD-ECCWES Meeting

Lightning Data Utilized in this is from the US National Lightning Detection Network 5 The NCDC now provides publicly available lightning climatologies that include the number of CG lightning strikes calculated for each day, hour, month, and year from 1986 to the present, on a 4 km Albers Equal Area grid, fit to the U.S. In this study, we consider regridded (60 x 60 km resolution) lightning data from due to the sparsity of data before 1992 and instrument upgrades. Monday, July 21, FESD-ECCWES Meeting

Detection Efficiency Corrections 6 Detection efficiency corrections must be applied to 1992 through 2004 lightning data as the sensors and coverage of the NLDN has been incrementally upgraded. US Lightning Distribution (1995: Uncorrected) US Lightning Distribution (1995: Corrected) Monday, July 21, FESD-ECCWES Meeting

ENSO Data utilized in this study is from the Multivariate ENSO Index (MEI) 7 The MEI is the first principle component (PC) calculated from the six combined variables (i.e., P SLP, U and V winds, T SURF_AIR, SST, and total cloud fraction) in the equatorial Pacific each bi-monthly season (e.g., Jan/Feb, …). *Potential teleconnections between ENSO and the lightning variability across the US are identified by computing Pearson correlations between concurrent monthly MEI and lightning flash rate anomalies.* Monday, July 21, FESD-ECCWES Meeting

MEI – Lightning Anomaly Monthly Correlations Results: 1992 – 2012 Monthly Correlations 8 FebruaryMay OctoberDecember Monday, July 21, FESD-ECCWES Meeting

MEI – Lightning Anomaly Monthly Correlations Results: 1997 – 2001 El Niño/La Niña Event 9Monday, July 21, FESD-ECCWES Meeting DecemberFebruary Statistically significant positive correlations in the southern US appear to be related to the strong El Niño/La Niña Event in the 1997 – 2001 time frame with correlation coefficients in this time period of ~0.95.

MEI – Lightning Anomaly Monthly Correlations Results: 2009 – 2012 El Niño/La Niña Event 10Monday, July 21, FESD-ECCWES Meeting AprilFebruary Statistically significant negative correlations in the midwest/central plains appear to be related to the strong El Niño/La Niña Event in the 2009 – 2012 time frame with correlation coefficients in this time period ranging from to -0.5.

MEI – Lightning Anomaly Monthly Correlations Results: Combined El Niño/La Niña Events 11Monday, July 21, FESD-ECCWES Meeting DecemberFebruary Combining the two intense El Niño/La Niña transitions further reinforces that intense El Niño/La Niña events are most responsible for altering the lightning distribution across the U.S in winter and spring months.

Summary, Conclusions, and Future Work 12 Summary and Conclusions Statistically significant correlations between ENSO and lightning anomalies mainly occur during the fall, winter, and early spring months in the southern and midwest/central plains regions of the US from 1992 – Positive correlations between ENSO and lightning anomalies may result from an amplified storm track in the southern portion of the U.S during El Niño conditions in the winter months (Mayes et al. 2008). Negative correlations between ENSO and lightning anomalies more moist conditions in the midwest/central plains of the U.S during La Niña conditions in the winter and early spring months (Mayes et al. 2008). Quick transitions from El Niño to La Niña conditions appear to be most responsible for changes in the lightning distribution across the US. Statistically significant correlations between ENSO and lightning anomalies also occur during the spring and summer months in the Rocky Mountain regions stretching from Arizona to Montana from 1992 – The cause of the above statement is the subject of future work. Monday, July 21, FESD-ECCWES Meeting