An Unusual Train of Waves in the Southern Hemisphere Stratosphere

Authors: Lawrence Coy, William Putman, Steven Pawson

The Southern Hemisphere (SH) winter of 2019 is proving to be one of the most dynamically active in the stratosphere since 1980, the beginning MERRA-2 record. The normally strong SH stratospheric polar vortex has been displaced off the pole by a strong polar warming event beginning in late August. The displaced polar vortex has continued to pull additional tropical air towards the pole while at the same time, air is being stripped from the polar vortex, weakening the displaced polar vortex. This interaction and mixing between the polar vortex and more tropical air can be described as “wave breaking,” analogous to waves breaking at a beach but on a much large scale. A dynamical tracer such as EPV (Ertel’s Potential Vorticity) provides one way to visualize atmospheric wave breaking.

EPV, a scalar field based on winds and temperatures, is well conserved as it is advected by the atmospheric flow. EPV has a natural gradient in latitude so that latitudinal mixing can be readily discerned. Here we examine EPV at 7 hPa (~34km), near the middle of the stratosphere during September 2019 taken from an experimental high resolution version of the NASA Global Modeling and Assimilation Office global circulation model, GEOS (Goddard Earth Observing System).

The animation above, taken from a 10-day forecast initialized on 16 September, 2019, reveals wave breaking on two scales. The global scale that deforms the polar vortex (darkest color) and moves air from the tropics (green and yellow) to the pole, and a wave of smaller scale vortices that mix tropical and high latitude air near 25°S. The high latitude air near 25°S originated from near the edge of the polar vortex and was stripped off the vortex earlier as described above. Typically, in the Northern Hemisphere, the air stripped from the vortex edge rolls up into one or two independent, smaller vortices. Here, however, a wave structure consisting of a chain of six smaller vortices has formed, and these appear to reinforce each other as a coherent wave. Such behavior has not been noted, previously.

The distance between the wave crests, the wavelength, is ~450-500 km. These waves are most likely created by a latitudinal wind shear instability (Kelvin-Helmholtz Instability, KHI) associated with the (dark blue) filament of polar EPV brought to about 25°S by the large scale wave breaking. They closely resemble billow clouds (also created by shear instability) but on a much larger scale. In addition to the forecast results shown here, the data assimilated, analyzed, EPV fields verify these existence of this unusual chain of waves.

The sudden stratospheric warming (SSW) of 2019 is the largest disruption of the SH stratosphere since the major SSW of 2002. A careful analysis of the 2019 SH winter coupled with insights from modeling and theory should provide significant improvements in understanding and future prediction of stratospheric dynamics and ozone variability. While not a major component of the 2019 SSW, the wave breaking at 25°S showcases an extreme example of the dissipation and mixing of air stripped from the vortex edge that weakens the polar vortex.

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