Wave Driving in the Tropical Lower Stratosphere as Simulated by WACCM.
Part I: Annual Cycle
MASAKAZU TAGUCHI
Department of Earth Sciences, Aichi University of Education, Kariya, Aichi, Japan
(Manuscript received 11 June 2008, in final form 9 December 2008)
ABSTRACT
This study explores the climatological annual cycle of temperature, circulation, and wave driving distribu-
tions in the tropical lower stratosphere as produced in a 50-yr simulation of the Whole Atmosphere Com-
munityClimateModel (WACCM). The simulation is forcedwith a climatological sea surface temperature and
sea ice condition. The present diagnoses verify the primary balances of the annual cycle in this region, con-
sistent with lower temperatures, stronger residual circulation (upwelling and local meridional outflow), and
nearby stronger wave driving forNorthernHemisphere (NH)winter.An in-detail analysis on thewave driving
further reveals that the stronger driving, occurringmostly in the northern tropics and subtropics, is contributed
by northward and upward propagation (associated with meridional and vertical fluxes of zonal momentum,
respectively) of equatorial Rossby waves forced by convective heating, and also by equatorward propagation
of NH extratropical planetary and synoptic waves. The results are used to discuss implications about possible
factors that may affect the different observations of the wave driving. The present framework and results will
be extended to investigate ENSO-induced changes in this region during NH winter in a forthcoming paper.
1. Introduction
The thermal structure near the tropical tropopause is
vital to the stratospheric climate; for example, it controls
stratospheric humidity to a high degree over various time
scales that affect stratospheric radiation and chemistry
(Kley et al. 2000; SPARC 2000). This study focuses on
the climatological annual cycle of temperature, circula-
tion, and wave-driving distributions in the tropical lower
stratosphere as simulated by Whole Atmosphere Com-
munity Climate Model (WACCM), whereas we will in-
vestigate El Nin˜o–Southern Oscillation (ENSO)-induced
changes during Northern Hemisphere (NH) winter in
a future paper (Taguchi 2009, manuscript submitted to
J. Atmos. Sci., hereafter TAG).
It is well known that temperatures in the tropical
lower stratosphere exhibit a notable annual cycle: the
region is coldest during NH winter and warmest during
Southern Hemisphere (SH) winter (Yulaeva et al. 1994;
Holton et al. 1995). The observed annual cycle in trop-
ical temperatures is strongly coupled to the annual cycle
in stratospheric water vapor (Mote et al. 1996). It has
been long held that the annual cycle in temperatures in
the tropical lower stratosphere arises from the so-called
‘‘extratropical stratospheric pump’’ mechanism (Holton
et al. 1995). The zonal body force due to dissipating
planetary and gravity waves drives the mean meridional
(Brewer–Dobson) circulation in which air is drawn up-
ward from the tropical troposphere and then poleward,
especially in the winter hemisphere. Because the plane-
tary wave drag, playing a dominant role in the strato-
sphere, is much stronger in the NH winter stratosphere
than in the SH counterpart, it drives stronger upwelling
and hence leads to lower temperatures in the tropical
lower stratosphere during NH winter. Plumb (2002)
noted that the effects of synoptic-scale tropospheric
disturbances, present throughout the year, are probably
responsible for inducing the strong two-cell circulation
in the lower stratosphere and upper troposphere.
There have been also a few indications, however, that
the influence of the extratropical planetary wave drag
would be limited to the extratropics on the annual time
scale (e.g., Shepherd 2002). Plumb and Eluszkiewicz
(1999) found in their numerical experiments that the
extratropical wave drag alone is unable to produce trop-
ical upwelling as observed, whereas the equatorward
edge of the drag is required to be unrealistically close to
the equator for realistic tropical upwelling.
Corresponding author address: Masakazu Taguchi, 1 Hirosawa,
Igaya-cho, Kariya, Aichi 448–8542, Japan.
E-mail: mtaguchi@auecc.aichi-edu.ac.jp
JULY 2009 TAGUCH I 2029
DOI: 10.1175/2009JAS2854.1
 2009 American Meteorological Society

Recently, Kerr-Munslow and Norton (2006) performed
a diagnosis of the heat budget near the tropical tropo-
pause with the 15-yr European Centre forMedium-Range
Weather Forecasts (ECMWF) Re-Analysis (ERA-15)
data to suggest that the annual cycle in tropical tropo-
pause temperatures is driven by the seasonal contrast of
equatorial Rossby waves, with the vertical flux of the
zonal momentum playing a large role. Here, ‘‘equatorial
Rossby waves’’ means stationary large-scale waves in
low latitudes, especially over Indonesia for NH winter
and India for NH summer (part of the Asia monsoon),
that are forced by convective heating in the troposphere
and are prevalent in monthly and seasonal means (e.g.,
Newell et al. 1972; Gill 1980; Highwood and Hoskins
1998). The suggestion is further supported in simple
numerical experiments in which localized distribu-
tions of tropical convective heating are applied to a
primitive equation model (Norton 2006). The experi-
ments demonstrated that the tropical tropopause is
colder in an experiment for a NH winter-like situation
when the convective heating placed on the equator forces
equatorial Rossby waves in the deep tropics, thus in-
ducing stronger equatorial upwelling. When the heating
is placed off the equator at 158N to mimic a NH sum-
merlike condition, the response is characterized by a
dominant monsoon pattern in the subtropics.
In contrast, a further observational analysis by Randel
et al. (2008) using the National Centers of Environmental
Prediction (NCEP)–National Center for Atmospheric
Research (NCAR) and 40-yr ECMWF Re-Analysis
(ERA-40) data stresses the importance of the meridional
flux of the zonal momentum of extratropical waves
(synoptic waves and, in the NH, planetary waves) and
also equatorial waves in subtropical wave driving, with-
out supporting the role of the vertical momentum flux.
Boehm and Lee (2003) also claim that equatorial Rossby
waves play an important role through the meridional
momentum flux in driving the upwelling of the Brewer–
Dobson circulation near the tropical tropopause.
The purpose of this study is to explore the thermo-
dynamics and dynamics (especially wave driving) of
the climatological annual cycle in the tropical lower
stratosphere as simulated byWACCM. This study takes
a different approach of utilizing a 50-yr simulation of
WACCM, one of stratosphere-resolving general circu-
lation models (GCMs), for a re-examination of the
tropical and subtropical wave driving, with the observed
discrepancy kept in mind (Kerr-Munslow and Norton
2006; Randel et al. 2008) as stated above. Our analysis
of the WACCM simulation benefits from temporal/
spatial homogeneity and physical consistency of the
data, whereas observational analyses may suffer some
data uncertainties. The WACCM simulation is also
advantageous in that it is forced with climatological
conditions and hence can provide a robust picture of the
annual cycle, whereas observational results on the an-
nual cycle may be affected by interannual variability.
Furthermore, the good representation of the strato-
sphere in WACCM contributes to reliability of our re-
sults. The present GCM-based approach is also useful
for our better understanding of this vital region, as we
discuss implications of the results in comparison to some
existing studies. Such a re-examination of the annual
cycle, also serving as a model evaluation, is indispensable
before we proceed to study ENSO-induced changes in
the tropical stratosphere during NH winter using per-
petual WACCM experiments in a future paper (TAG).
The rest of this paper is organized as follows: Section 2
briefly describes the WACCM simulation as well as the
analysis framework used in this study. Section 3 presents
the diagnostic results on the climatological annual cycle
in the simulation, including details of wave driving. Fi-
nally, section 4 provides a summary and discussion.
2. Simulation and analysis framework
a. Simulation
This study diagnoses the climatological annual cycle,
especially in the tropical lower stratosphere simulated
by WACCM (version 1b). WACCM is a version of the
NCAR Community Climate Model (CCM) that is ex-
tended to include the middle and upper atmosphere up
to about 140 km altitude. The details of the model are
documented in Sassi et al. (2002). The horizontal reso-
lution is T63 (model output on 2.88 3 2.88 grids), which
can reasonably simulate synoptic and planetary scales.
Small-scale waves, which are not resolved in the model,
may have a substantial role in the tropics, as suggested,
for example, for the quasi-biennial oscillation (QBO;
Baldwin et al. 2001). The model has 66 vertical levels,
corresponding to grid spacing dz ; 1.2 km in the upper
troposphere and lower stratosphere. The QBO in the
tropical stratosphere is not simulated or included in the
run used here.
The present analysis on the mean annual cycle makes
use of a 55-yr simulation forced with a climatological sea
surface temperature and sea ice distribution that cor-
respond to the climatology of observed distributions
from 1950 to 1999 (cf. the NCEP Reynolds dataset at
http://podaac.jpl.nasa.gov/PRODUCTS/p118.html). The
first 5 yr are discarded as an initial spinup period, leaving
50 yr for the analysis. The 50-yr dataset is useful for
the present analysis on the annual cycle in the tropical
lower stratosphere, where interannual variability is not
large in the absence of the QBO.
2030 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 66