Tropospheric QBO-ENSO Interactions and Differences between the Atlantic and Pacific

This study investigates the interaction of the quasi-biennial oscillation (QBO) and the El Nino-Southern Oscillation (ENSO) in the troposphere separately for the North Pacific and North Atlantic region. Three 145-yr model simulations with NCAR's Community Earth System Model Whole Atmosphere Community Climate Model (CESM-WACCM) are analyzed where only natural (no anthropogenic) forcings are considered. These long simulations allow the authors to obtain statistically reliable results from an exceptional large number of cases for each combination of the QBO (westerly and easterly) and ENSO phases (El Nino and La Nina). Two different analysis methods were applied to investigate where nonlinearity might play a role in QBO-ENSO interactions. The analyses reveal that the stratospheric equatorial QBO anomalies extend down to the troposphere over the North Pacific during Northern Hemisphere winter only during La Nina and not during El Nino events. The Aleutian low is deepened during QBO westerly (QBOW) as compared to QBO easterly (QBOE) conditions, and the North Pacific subtropical jet is shifted northward during La Nina. In the North Atlantic, the interaction of QBOW with La Nina conditions (QBOE with El Nino) results in a positive (negative) North Atlantic Oscillation (NAO) pattern. For both regions, nonlinear interactions between the QBO and ENSO might play a role. The results provide the potential to enhance the skill of tropospheric seasonal predictions in the North Atlantic and North Pacific region.