The spring transition of the North Pacific jet and its relation to deep stratosphere-to-troposphere mass transport over western North America

Stratosphere-to-troposphere mass transport to the planetary boundary layer (STT-PBL) peaks over the western United States during boreal spring, when deep stratospheric intrusions are most frequent. The tropopause-level jet structure modulates the frequency and character of intrusions, although the precise relationship between STT-PBL and jet variability has not been extensively investigated. In this study, we demonstrate how the North Pacific jet transition from winter to summer leads to the observed peak in STT-PBL. We show that the transition enhances STT-PBL through an increase in storm track activity which produces highly amplified Rossby waves and more frequent deep stratospheric intrusions over western North America. This dynamic transition coincides with the gradually deepening PBL, further facilitating STT-PBL in spring. We find that La Nina conditions in late winter are associated with an earlier jet transition and enhanced STT-PBL due to deeper and more frequent tropopause folds. An opposite response is found during El Nino conditions. El Nino-Southern Oscillation (ENSO) conditions also influence STT-PBL in late spring or early summer, during which time La Nina conditions are associated with larger and more frequent tropopause folds than both El Nino and ENSO-neutral conditions. These results suggest that knowledge of ENSO state and the North Pacific jet structure in late winter could be leveraged for predicting the strength of STT-PBL in the following months.