The interannual variability of the Haines Index over North America and its relationship to indicators of large-scale circulation anomalies

The Haines Index (HI), also known as the Lower Atmospheric Severity index, is a fire-weather index commonly included in operational fire weather forecasts as an indicator of the potential for dry, low-static stability air in the lower troposphere to contribute to erratic fire behavior or large fire growth. The index is comprised of two equally important components: one for humidity and another for stability in the lower-to-mid troposphere, and higher values of HI indicating higher potential. In this study, we have examined the interannual variability of the number of days during warm season with high HI values over North America and explored the relationship between the interannual variability of HI and indicators of large-scale circulation anomalies. The gridded North America Regional Reanalysis (NARR) data set is used for computing the HI for the North America domain while the NCEP-DOE global reanalysis II data set is used for deriving large-scale circulation patterns. Empirical Orthogonal Function (EOF) analyses are performed and the results show that the first three HI EOF modes, which account for approximately 23%, 15%, and 13% of the total variance, are related, respectively, to the El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), and the interdecadal Sea Surface Temperature (SST) variation over the tropical Pacific Ocean. These connections are explained by an examination of how the particular large-scale circulation anomaly might influence the warm season weather pattern over North America.