Development of the Extratropical Response to the Stratospheric Quasi-Biennial Oscillation

Using the Model of an Idealized Moist Atmosphere (MiMA) capable of spontaneously generating a quasibiennial oscillation (QBO), the gradual establishment of the extratropical response to the QBO is explored. The period and magnitude of the QBO and the magnitude of the polar Holton-Tan (HT) relationship is simulated in a free-running configuration of MiMA, comparable to that in state-of-the-art climate models. To isolate mechanisms whereby the QBO influences variability outside the tropical atmosphere, a series of branch experiments are performed with nudged QBO winds. When easterly QBO winds maximized around 30 hPa are relaxed, an Eliassen-Palm (E-P) flux divergence dipole quickly forms in the extratropical middle stratosphere as a direct response to the tropical meridional circulation, in contrast to the HT mechanism, which is associated with wave propagation near the zero wind line. This meridional circulation response to the relaxed QBO winds develops within the first 10 days in seasonally varying and fixed-seasonal experiments. No detectable changes in upward propagation of waves in the midlatitude lowermost stratosphere are evident for at least 20 days after branching, with the first changes only evident after 20 days in perpetual midwinter and season-varying runs, but after 40 days in perpetual November runs. The polar vortex begins to respond around the 20th day, and subsequently a near-surface response in the Atlantic Ocean sector forms in mid-to-late winter runs. These results suggest that the maximum near-surface response observed in mid-to-late winter is not simply due to a random seasonal synchronization of the QBO phase, but is also due to the long lag of the surface response to a QBO relaxation in early winter and the short lag of the surface response to a QBO relaxation in mid-to-late winter.