Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra Institute of Chemistry and Dynamics of the Geosphere (ICG-1), Forschungszentrum Jülich, Jülich, Germany
Received: 19 Jan 2009 – Published in Atmos. Chem. Phys. Discuss.: 03 Mar 2009Abstract. The quasi-biennial oscillation (QBO) of the zonal mean zonal wind
is a dynamical phenomenon of the tropical middle atmosphere.
Influences of the QBO can even be found at mid and high latitudes.
It is widely accepted that the phase descent of alternating tropical
easterlies and westerlies is driven by atmospheric waves of both
global scale (equatorial wave modes like Kelvin, equatorial Rossby,
Rossby-gravity, or inertia-gravity waves), as well as mesoscale gravity waves.
However, the relative distribution of the different types of waves
to the forcing of the QBO winds is highly uncertain.
This is the case because until recently there were no high resolution
long-term global measurements in the stratosphere.
In our study we estimate Kelvin wave momentum flux and
the contribution of zonal wind forcing by
Kelvin waves based on
space-time spectra determined from both
Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)
temperature measurements as well as temperatures from
European Centre for Medium-Range Weather Forecasts (ECMWF)
Peak values of total Kelvin wave zonal wind forcing found
are about 0.2 m/s/day.
There is good agreement between SABER and ECMWF results.
Altitude-time cross sections are shown and
the results are compared to the total wave forcing required to balance the
Sometimes Kelvin wave forcing is sufficient to explain almost the whole
total wave forcing required for the momentum balance
during the transition from QBO easterly to westerly winds.
This is especially the case
during the periods of strong westerly wind shear
when the zonal wind is between −20 and 10 m/s
at the equator
in the altitude range 20 to 35 km.
During other parts of the phases of strong westerly wind shear, however, the
contribution of Kelvin waves can be comparably low and
the missing wave forcing, which is often attributed to mesoscale
gravity waves or intermediate scale waves, can be the by far dominant
contribution of the QBO forcing.
It is also found that seasonal variations of Kelvin wave accelerations
could play an important role for the maintenance of the QBO westerly wind jets
in the lower stratosphere.
Revised: 28 May 2009 – Accepted: 06 Jun 2009 – Published: 17 Jun 2009
Citation: Ern, M. and Preusse, P.: Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra, Atmos. Chem. Phys., 9, 3957-3986, doi:10.5194/acp-9-3957-2009, 2009.