What is the impact of ENSO Cycle? Suzanne Fortin

Recent Annual Severe Weather Climatology Distribution of of Severe Events During the Cold Season Analysis of Severe Events and ENSO Cycle Significance of ENSO Cycle to Severe Incidence

TypeNumber Events Significant Reports Hail Wind Tornadoes All MonthTotal January3 February4 March57 April112 May164 June135 July87 August63 September43 October11 November15 December6

February 28, 2007; January 8, 2009 Previous studies focused on yearly activity and ENSO phase (Mayes et al., 2006, Agee and Zurn-Birkhimer, 1998, Browning, 1998). Situational awareness Initial study conducted on CWA only, study expanded to encompass area roughly defined by 36-41N and 92-96W Initial study analyzed 50+ years

Warm (red) and cold (blue) episodes based on a threshold of +/- 0.5 o C for the Oceanic Niño Index (ONI)*

WINDHAILTORNADO

Used compositing methodology developed by NWSH, CPC and NWS field Test of statistical significance performed on statistical analysis to determine if deviations from normal climatology are significant (90%)

Concurrent 5 Months Prior Overall severe event incidence is more prevalent during cold phase of ENSO (La Nina) Weak signal that severe is more likely during early cold season in La Nina phase Mixed signal during late winter, with severe events occurring during both cold and warm phases of ENSO Signal that severe incidence in late winter greater when cold phase (1) or neutral phase (2) during late summer/early fall

Compared ONI with previous years Found similarities with ONI during lead in to winters of *, , *, , * No noticeable increase in tornado incidence throughout warm phase of ENSO cold season During and analogs, severe events started about one month earlier than normal start of late March

Scaled analysis to the past 30 years due to limited dataset prior to 1970s Statistical analysis also showed severe weather frequency greater during warm phase (El Nino) of ENSO for certain periods Initial study area encompassed only CWA, expansion to larger domain (36-40N, 92-99W) resulted in even more mixed signals Signal that severe weather frequency greater during cold season during concurrent cold phase of ENSO Signal that severe weather frequency during later part of cold season greater if cold/neutral phase of ENSO during late summer/early fall

Compositing Further redefining of domain? Analysis of specific severe type frequency