Sources, sinks and propagation characteristics of the quasi 6-day wave and its impact on the residual mean circulation

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Saturday, August 25, 2018

This study, by Pedatella and others, employs a troposphere to lower thermosphere reanalysis dataset generated by the Whole Atmosphere Community Climate Model with data assimilation provided by the Data Assimilation Research Testbed (WACCM+DART).

Amplitudes of the Q6DW image
Amplitudes of the Q6DW as a function of latitude and time in zonal winds at 90 km and temperatures at 105 km simulated by the WACCM+DART (a, b) and observed by TIDI and SABER (c, d). Write region denotes data missing or the wave amplitude is not reliable.

We explore the sources, sinks, and propagation characteristics of the quasi 6-day wave (Q6DW) in the year 2007. WACCM+DART reproduces the burst-like Q6DW and compares well with direct Q6DW diagnostics of SABER and TIDI observations. The most prominent Q6DW took place in later February and mid- October, while the Q6DW was absent during solstice conditions in 2007. The occurrence of a large Q6DW in the equinoctial mesosphere and lower thermosphere (MLT) is highly dependent on wave amplification and over-reflection processes associated with barotropic/baroclinic instabilities and wave critical layers defined by the zonal mean zonal winds. During solstices, the winter hemisphere wave guide quantified by the square of the refractive index is negative and prevents the vertical wave propagation from the source region into the MLT. Meantime, the critical layer for the Q6DW encloses the unstable region in the summer hemisphere and thus blocks the energy conversion from the mean flow to the wave. WACCM+DART allows one to derive the residual circulation induced by the westward forcing on the background winds due to the Q6DW momentum deposition. The resulting circulation pattern is upward and poleward in both hemispheres and thus weakens the residual mean circulation in the Northern Hemisphere but strengthens it in the Southern Hemisphere. While not the focus of this study, the Q6DW impact on the residual mean circulation also points to broader implications for the mean state of the upper atmosphere, e.g., the thermospheric O/N2 ratio due to upward constituent transport and related changes in the ionospheric plasma.

Publication Name: Journal of Geophysical Research - Atmospheres

First HAO Author's Name: Nick Pedatella