Influence of ENSO on the characteristics of extreme ridges over Pacific-North America

Atmospheric ridges across the Pacific-North America region are associated with a range of surface extremes such as heat waves, cold waves, droughts, and wildfires. There is limited research on spatial characteristics of extreme ridges in this region and on the influence of El Niño-Southern Oscillation (ENSO) on their characteristics. Better understanding the ENSO influence can improve the predictability of extreme ridges and their surface impacts. Using ERA5 reanalysis data, we identified ridge objects as spatially contiguous regions showing positive 500 hPa geopotential height zonal anomalies from 1950-2022. We develop a climatology detailing the frequency, extent, intensity, and number of ridge days for the Pacific-North America region. Additionally, we distinguished two types of extreme ridges—large and intense—based on the highest 10th percentile of extent and intensity, respectively, and analyzed the influence of ENSO’s warm and cold phases on these ridges. The largest ridges occur across the higher latitudes of our domain while the most intense ridges are concentrated over the Gulf of Alaska. ENSO differentially influence the location of extreme ridges across seasons as well as their characteristics. For instance, large ridges center over Alaska during La Niña winters and El Niño summers. Large ridges in the fall have higher spatial extent and intensity during La Niña relative to El Niño. In contrast, large ridges during El Niño have higher intensity in the summer. El Niño is associated with more large ridge days over land. Similarly, La Niña winters and El Niño summers are linked to more intense ridges. These intense ridges exhibit larger extents and greater intensities, particularly during La Niña winters. Additionally, La Niña winters and El Niño summers feature a higher number of intense ridge days, highlighting a seasonal pattern influenced by ENSO phases.