Stratospheric response to the 11-year solar cycle: Breaking planetary waves, internal reflection and resonance

Breaking planetary waves (BPWs) affect stratospheric dynamics by reshaping the waveguides, causing internal wave reflection and preconditioning sudden stratospheric warmings. This study examines observed changes in BPWs during the northern winter due to enhanced solar forcing and the consequent effect on the seasonal development of the polar vortex. During the period 1979-2014, solar-induced changes in BPWs first observed in the uppermost stratosphere. High solar forcing was marked by sharpening of the potential vorticity (PV) gradient at 30-45°N, enhanced wave absorption at high latitudes and a reduced PV gradient between these regions. These anomalies instigated an equatorward shift of the upper stratospheric waveguide and enhanced downward wave reflection at high latitudes. The equatorward refraction of reflected waves from the polar upper stratosphere then led to enhanced wave absorption at 35-45°N, 7-20 hPa, indicative of a widening of the middle stratospheric surf zone. The stratospheric waveguide was thus constricted at ~45-60°N, 5-10 hPa in early Boreal winter; reduced upward wave propagation through this region resulted in a stronger upper-stratospheric westerly jet. From January, the regions with enhanced BPWs acted as “barriers” for subsequent upward and equatorward wave propagation. As the waves were trapped within the stratosphere, zonal wavenumber 2-3 anomalies were reflected poleward from the stratospheric surf zone. Resonant excitation of some of these reflected waves resulted in rapid growth of wave disturbances and a more disturbed polar vortex in late winter. These results provide a process-orientated explanation for the observed solar cycle signal. They also highlight the importance of nonlinearity in the processes that drive the stratospheric response to external forcing.

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