North American Winter-Spring Storms: Modeling Investigation on Tropical Pacific Sea Surface Temperature Impacts

An increased frequency and intensity of winter and spring storms have recently manifested over a broad area of North America especially along the east and west coast of the U.S., although global mean storm tracks are suggested to shift northward. To understand these changes, we have conducted modeling experiments using NCAR Community Atmosphere Model 3.1_p2 to examine the response of North American storm activity to the elevated tropical Pacific sea surface temperature associated with El Niño. The model experiments were conducted for a period of December to May and configured at a resolution of T85. A storm identification and tracking algorithm was employed to analyze the 6 hourly model output. The results indicate that, when tropical Pacific SST increases, there are more numerous intense storms over southwestern, southeastern, and northwestern North America, but fewer weaker storms over the northeast. Transient eddy analysis of the general circulation demonstrates consistent changes, suggesting systematic changes from large-scale general circulation to synoptic-scale storms. These changes can be attributed to enhanced lower tropospheric baroclinicity, to which the southward shift and an intensification of extratropical jet streams make a major contribution.