WACCM Simulations of Zonal Mean Gravity Wave Drag Short-term Variability in the Southern Hemisphere Mesosphere

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Friday, March 16, 2018

Whole Atmosphere Community Climate Model (WACCM) simulations are used to investigate the short-term (30-day) temporal variability in the mid- and high-latitude Southern Hemisphere mesosphere.

gravity wave drag anomaly image
The figure shows the standard deviation of the gravity wave drag anomaly in the (a) Northern and (b) Southern Hemisphere mid-latitudes (45-75 degrees). The results are based on specified dynamics Whole Atmosphere Community Climate Model (WACCM) simulations from 1979-2014. The standard deviation is calculated over a 30-day moving window, and represents the short-term variability in gravity wave drag. The figure shows that the short-term gravity wave drag variability in the Southern Hemisphere during August-November is nearly as large as in the Northern Hemisphere during December-February. This is a somewhat unexpected result since Northern Hemisphere is generally considered to be more dynamically variable compared to the Southern Hemisphere.

We focus primarily on the short-term variability in zonal mean gravity wave drag, and its influence on the mesosphere circulation and chemistry. The seasonal climatology from 36 years (1979-2014) of specified dynamics WACCM simulations reveals that the short-term variability of zonal mean gravity wave drag, which is quantified by the standard deviation over a 30-day window, in the Southern Hemisphere mid-latitude mesosphere maximizes during December to March and September to November. The December to March enhancement is related to inter-hemispheric coupling during Northern Hemisphere sudden stratospheric warming events, and the enhancement during September to November is attributed to variability in the stratosphere and lower mesosphere zonal mean zonal winds that is driven by planetary wave activity in the Southern Hemisphere. Analysis of the simulations demonstrates that the short-term variability in mesospheric gravity wave drag leads to changes in the residual circulation, which subsequently introduces short-term variability in atomic oxygen (O) and nitrogen oxides (NOx). Comparison with the Northern Hemisphere reveals that the zonal mean short-term variability in the Southern Hemisphere during September to November is nearly as large (~80-85%) as the maximum short-term variability in the Northern Hemisphere, which occurs during December to March. This demonstrates that although the Southern Hemisphere middle atmosphere is generally considered to be less disturbed than the Northern Hemisphere, there is still substantial short-term variability in the Southern Hemisphere mesosphere, especially during late winter to early spring.

Publication Name: Journal of Geophysical Research—Atmospheres

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