ENSO modulation of the QBO effects on the polar vortex in a GCM simulation

The QBO and ENSO effects on the polar vortex have already been analyzed separately. A weaker polar vortex accompanied by a warmer polar stratosphere appears during the easterly QBO phase or during an El Niño event. However, the joint effect of both phenomena and the influences of one on the other are still difficult to assess mainly due to the different timing of QBO phases and strong ENSO events. In this work we focus on the ENSO effects on the polar vortex response to the QBO using simulations from the Middle Atmosphere ECHAM5 general circulation model, which is capable of simulating a realistic QBO.

Based on a 100 year simulation with climatological SST boundary conditions, two ensembles have been defined consisting of periods starting in July and ending in September of the following year with their QBO phase being in either strong westerly (QBO/W) or strong easterly (QBO/E) phase during the winter. The initial states of the selected periods were then used for experiments using the prescribed SST of June 1997 to September 1998, thus generating additional ensembles of simulations, which combine the selected QBO phase, inherited from the initial condition, and the strong El Nino SST anomalies that peaked in November 1997. A total of 14 realizations for westerly QBO phases (QBO/W) and 17 for easterly QBO phases (QBO/E) have been performed. The ensemble means and deviations have been analyzed and compared with their corresponding ensembles in the simulations without El Niño.

Results in zonal mean zonal wind and zonal mean temperature show that the effects of El Niño are detected later if combined with QBO/E than with QBO/W. In addition, both the anomalous warming and weakening in the polar vortex generated by El Niño are larger and take longer during a QBO/W phase, while in QBO/E the downward propagation of the QBO anomalies is accelerated by including El Niño. The analysis of the vertical wave propagation through EP flux diagrams suggests that the effects of El Niño and QBO/E, despite being in the same sense (a more intense upward propagation), are not additive and seem to compete with each other when occurring simultaneously.