Description:
<jats:p>Abstract. Measurements of long-lived trace gases (SF6, CFC-11, CFC-12, HCFC-22,
CCl4, N2O, CH4, H2O, and CO) performed with the Michelson
Interferometer for Passive Atmospheric Sounding (MIPAS) have been used to infer
the stratospheric and mesospheric meridional circulation. The MIPAS data set
covers the time period from July 2002 to April 2012. The method used for this
purpose was the direct inversion of the two-dimensional continuity equation
for the concentrations of trace gases and air density. This inversion predicts
an “effective velocity” that gives the best fit for the evolution of the
concentrations on the assumption that an explicit treatment of Fickian diffusion
can be neglected. These effective velocity fields are used to characterize the
mean meridional circulation. Multiannual monthly mean effective velocity fields
are presented, along with their variabilities. According to this measure, the
stratospheric circulation is found to be highly variable over
the year, with a quite robust annual cycle. The new method allows us to track the
evolution of various circulation patterns over the year in more detail than
before. According to the effective velocity characterization of the circulation,
the deep branch of the Brewer–Dobson circulation and the mesospheric
overturning pole-to-pole circulation are not separate but intertwined phenomena.
The latitude of stratospheric uplift in the middle and upper stratosphere is
found to be quite variable and is not always found at equatorial latitudes.
The usual schematic of stratospheric circulation with the deep and the shallow
branch of the Brewer–Dobson circulation and the mesospheric overturning
circulation is an idealization which best describes the observed atmosphere
around equinox.
Sudden stratospheric warmings and the quasi-biennial oscillation cause a
pronounced year-to-year variability of the meridional circulation.
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