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VOLUME 21
Understanding the Effects of Convective Momentum Transport on Climate
Simulations: The Role of Convective Heating
XIAOLIANG SONG
AND
XIAOQING WU
Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa
GUANG JUN ZHANG
Center for Atmospheric Sciences, Scripps Institution of Oceanography, La Jolla, California
RAYMOND W. ARRITT
Department of Agronomy, Iowa State University, Ames, Iowa
(Manuscript received 13 August 2007, in final form 5 February 2008)
ABSTRACT
A simplified general circulation model (GCM), consisting of a complete dynamical core, simple specified
physics, and convective momentum transport (CMT) forcing, is used to understand the effects of CMT on
climate simulations with a focus on the role of convective heating in the response of circulation to the CMT
forcing. It is found that the convective heating dominates the meridional circulation response and dynamical
processes dominate the zonal wind response to the CMT forcing in the tropics; the simplified model
reproduces some of the key features of CMT-induced circulation changes observed in the full GCM in the
tropics. These results suggest that the CMT-induced zonal and meridional circulation changes in the tropics
in the full GCM are dominated by dynamical processes and the convective heating, respectively. Inclusion
of the CMT in the model induces a marked change in convective heating, which negatively correlates with
the change in vertical velocity, indicating the existence of CMT-induced convective heating–circulation
feedback. The sensitivity experiment with the removal of mean convective heating feedback demonstrates
that the convective heating affects the response of the meridional circulation to the CMT forcing through
the CMT-induced convective heating–circulation feedback.
1. Introduction
Atmospheric convection not only releases latent heat
of condensation and vertically transports heat and
moisture, but it also transports momentum. Numerous
observational studies have demonstrated the importance of convective momentum transport (CMT) to the
atmospheric momentum budget (Houze 1973; Sanders
and Emanuel 1977; Stevens 1979; LeMone et al. 1984).
Numerical studies also demonstrated that the realistic
simulation of tropical circulation requires cumulus friction to be included in the momentum equations (Stone
et al. 1974; Stevens et al. 1977). Several CMT param-
Corresponding author address: Dr. Xiaoliang Song, Climate,
Atmospheric Science, and Physical Oceanography Division,
Scripps Institution of Oceanography, La Jolla, CA 92093.
E-mail: [email protected]
DOI: 10.1175/2008JCLI2187.1
© 2008 American Meteorological Society
eterization schemes were proposed to represent the effect of CMT in large-scale numerical models. The earlier mixing-type CMT schemes assumed that the incloud momentum is modified only by lateral
entrainment of momentum from outside the clouds
(Ooyama 1971; Schneider and Lindzen 1976; Shapiro
and Stevens 1980; Sui et al. 1989). Recognizing the important impact of convection-induced pressure gradient
on the in-cloud momentum and the CMT (e.g., Moncrieff 1981, 1992; LeMone 1983; Schlesinger 1984; Flatau and Stevens 1987; LeMone and Moncrieff 1994),
Zhang and Cho (1991a), Wu and Yanai (1994), and
Gregory et al. (1997) developed more comprehensive
CMT schemes that include the effect of convectioninduced pressure perturbations on momentum transport. Studies using observational data and cloudresolving model (CRM) simulations showed that these
schemes were able to reproduce the observed and
CRM-simulated apparent momentum sources (Zhang
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SONG ET AL.
and Cho 1991b; Wu and Yanai 1994; Mapes and Wu
2001; Zhang and Wu 2003; Gregory et al. 1997), and
demonstrated improvement compared to simple mixing-type CMT schemes. This suggests that these comprehensive schemes can capture the essential features
of convective momentum transport.
Several studies incorporated the CMT scheme in
general circulation models (GCMs) to examine the effects of CMT on global climate simulations. Using
simple mixing-type CMT parameterization scheme proposed by Schneider and Lindzen (1976), Helfand
(1979) found that the winter Hadley circulation was
enhanced and the meridional wind field was closer to
observations when cumulus friction was included in
January simulations using the Goddard Laboratory for
Atmospheric Sciences model. Zhang and McFarlane
(1995) investigated the effects of CMT on climate simulation by incorporating the Zhang and Cho (1991a)
CMT scheme in the Canadian Climate Centre (CCC)
GCM. Seasonal simulations showed that by including
CMT the summer Hadley circulation was enhanced and
the wind field was closer to observations. Implementing
their CMT scheme to the Hadley Centre Climate
Model, Gregory et al. (1997) found that the CMTinduced changes in both the zonal and meridional wind
were similar to those described by Zhang and McFarlane (1995). Because long-term climate statistics are
more appropriate for evaluating the effect on climate
simulation, Wu et al. (2003) conducted a 20-yr simulation in which the Zhang and Cho scheme was implemented in the National Center for Atmospheric Research (NCAR) Community Climate Model, version 3
(CCM3). They found a secondary meridional circulation, characterized by strong upward motion along the
strongest ascending belt of the Hadley circulation and
downward motion north and south of the belt, within
the ascending branch of the Hadley circulation when
the CMT was included. Further analysis (Wu et al.
2007) showed that the CMT significantly modified not
only the tropical circulation but also the precipitation,
cloud, and radiation.
On the other hand, the dynamical mechanism by
which CMT affects climate simulation is rarely addressed. Although Helfand (1979) suggested that the
change of the Hadley circulation is a response of the
meridional wind to the zonal CMT forcing based on the
good correlation between zonal CMT forcing and the
change in Coriolis force, the analysis was largely qualitative. To date, the physical picture of CMT influencing
climate simulation remains unclear, which is, to a large
extent, because of the complex nonlinear interactions
among dynamical, thermodynamic, and physical processes in GCMs as illustrated in Fig. 1a in which each
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FIG. 1. Sketch of the interaction between the CMT forcing and
GCM for (a) full GCM, (b) ID-CCM3, and (c) IDT-CCM3. Convective heating–circulation feedback is indicated by bold arrows
and rectangles. Note that plates (a) and (b) identify the study of
Song et al. (2008) in which the dynamical effects of CMT were
isolated, while plate (c) identifies the present paper’s contribution
of the effects of convective heating: namely, temperature tendency from the Zhang and McFarlane (1995) convection scheme
is passed through to the temperature equation. [The water vapor
remains specified as in Song et al. (2008).]
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loop represents a feedback process. In particular,
numerous feedback processes (e.g., convection–
evaporation–wind feedback, condensation heating–
circulation feedback, etc.) in the climate system can be
triggered by the CMT forcing. For example, when wind
fields are perturbed by the CMT forcing, the adjustment of dynamical advection results in the modification
of temperature fields and hence convection (convective
heating), which in turn can affect the temperature. By
affecting geopotential height, the change in temperature leads to an extra wind change, which in turn can
induce an extra convection (convective heating) change
and hence wind change. This forms the CMT-induced
convective heating–circulation feedback as indicated by
the bold arrows and rectangles in Fig. 1a. In addition,
both changes in convection and wind fields can affect
the CMT forcing, that is, there is a convection–CMT–
wind feedback. Because the effects of all the processes
are intermingled in a climate response after long-term
integration, it is difficult to identify the contribution of
each process to the response of circulation to the CMT
forcing in full GCMs.
To understand the climate response of sufficiently
complex GCMs, a simplified climate model containing
clear dynamical mechanism is a useful tool and is able
to provide more insight. Song et al. (2008) proposed a
simplified GCM framework appropriate for investigating the climate impact of CMT to which the core
sources of complexity of the full GCM can be sequentially added. Thus, the role of each component of the
full GCM in the response of the circulation to the CMT
forcing can be identified from the circulation difference
between the simulations with and without that component. The dynamical core and convection scheme of the
simplified GCM are based on the NCAR CCM3, while
the other physical processes (e.g., radiation, boundary
layer, etc.) commonly included in comprehensive
GCMs are represented by specified idealized physics
suggested by Held and Suarez (1994). The Zhang and
Cho (1991a) CMT scheme is incorporated into the convection scheme to calculate the CMT forcing that is
used to force the momentum equations, while convective temperature (moisture) tendencies are not passed
into the model calculations in order to remove the
physical feedback between convective heating (drying)
and wind fields. Excluding complex physical processes,
the simplified model consists of a complete dynamical
core, simple specified physics, and the CMT forcing. As
shown in Fig. 1b, it contains only the responses of dynamical processes to the CMT forcing. This simplified
GCM (hereafter also referred to as idealized GCM) can
be integrated stably and produces reasonable climate
mean state and CMT forcing in the tropics. Using this
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simplified GCM, Song et al. (2008) investigated the dynamical effects of CMT on climate simulations in a
clean manner. They found that when the CMT was
included the easterlies decreased in the lower troposphere over the equatorial region and increased above,
and the Hadley circulation became stronger. Further
analysis showed that the zonal wind change was a direct
response of zonal wind to the zonal CMT forcing, and
that the strengthening of the Hadley circulation was an
indirect response of the meridional wind to the zonal
CMT forcing through the Coriolis effect.
It is worth noting that, as a key simplification, neglecting parameterized convective heating in the model
has two advantages. First, it excludes the influence of
convective heating–circulation feedback so that dynamical effects of CMT can be isolated. Second, in the
absence of atmospheric stabilization from convective
heating, the model produces much stronger convection
and hence much stronger CMT forcing because convection does not consume convective available potential
energy. The amplification of CMT forcing results in
more notable circulation response so that dynamical
effects of CMT are more easily identified. A disadvantage is that the amplification of dynamical effects of
CMT makes it difficult to quantitatively evaluate the
contribution of dynamical processes to the total response of circulation to the CMT forcing. To further
understand the effects of CMT on climate simulations,
the convective heating needs to be included in the
model. On the other hand, condensation heating–
circulation feedback is an important physical process in
tropical atmospheric dynamics, and how and to what
extent it affects the response of circulation to the CMT
forcing is not yet clear. The present study will focus on
this physical process, that is, the convective heating, to
investigate its role in the response of circulation to the
CMT forcing by incorporating the convective heating in
the simplified GCM framework proposed by Song et al.
(2008).
The organization of the paper is as follows. A brief
description of the model and experimental design is
presented in section 2. Impacts of the convective heating on the responses of circulation to the CMT forcing
are evaluated in section 3. The mechanism of convective heating affecting the response of meridional circulation to the CMT forcing is examined in section 4.
Section 5 gives the summary of results and conclusions.
2. Model and experimental design
a. Model
The simplified GCM used in this study is an idealized
model (hereafter referred to as ID-CCM3) proposed by
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SONG ET AL.
Song et al. (2008). The dynamical core of the model is
based on the NCAR CCM3 (Kiehl et al. 1998). It is a
global spectral model with triangular truncation at
zonal wavenumber 42 (approximately 2.8° ⫻ 2.8° latitude–longitude) in the horizontal and 18 levels in the
vertical. The top of the model is at 2.9 mb. Deep convection is parameterized using the Zhang and McFarlane (1995) scheme. Other physical processes (e.g., radiation, boundary layer, etc.) commonly included in
comprehensive GCMs are represented by specified idealized physics suggested by Held and Suarez (1994).
The diabatic forcing term in the thermodynamic equation is expressed as the Newtonian relaxation of temperature to a prescribed zonally symmetric state.
Boundary layer forcing in the momentum equations is
expressed as Rayleigh friction. The model has no land–
sea contrast, no topography, and no heat or momentum
flux at the surface boundary. Water vapor is included in
the model to initiate moist convection. However, the
time change of water vapor is not considered in order to
remove the effect of water vapor change on simulations. The Zhang and Cho (1991a) CMT parameterization scheme, in which the cloud-scale pressure gradient
is parameterized in terms of the interaction between
convective updraft/downdraft and large-scale vertical
wind shear, is incorporated into the Zhang and McFarlane (1995) convection scheme to calculate the CMT
forcing that is used to force the momentum equations.
However, the temperature tendency predicted from the
convection scheme is set to zero in order to eliminate
the thermodynamic interaction between convection
heating and the large-scale fields. Detailed description
of model can be found in Song et al. (2008).
b. Experimental design
To investigate the role of convective heating in the
response of circulation to the CMT forcing, one modification is made to the ID-CCM3, that is, the temperature tendency predicted by the Zhang and McFarlane
(1995) convection scheme is passed into the model’s
thermodynamic equation so that convection can feed
back to the temperature field. The ID-CCM3 with this
modification is hereafter referred to as IDT-CCM3. It
should be pointed out that water vapor in the IDTCCM3 remains specified and is not coupled to the temperature change because our purpose is simply to examine how convective heating can influence the responses of circulation to the CMT forcing. Thus, as
shown in Fig. 1c, IDT-CCM3 contains the response of
dynamical processes and the CMT-induced convective
heating–circulation feedback.
A pair of long-term integrations is conducted with
the IDT-CCM3. In the simulation referred to as IDT-
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CMT, the CMT forcing is included in momentum equations, while in the simulation IDT-CTL, the CMT forcing is excluded. The IDT-CTL is taken as the control
run to which IDT-CMT is compared in order to assess
the impacts of CMT in the IDT-CCM3. Another two
pairs of simulations from the full CCM3 (hereafter FCCM3) and the ID-CCM3 are also presented here for
comparison. The simulations with CMT from the FCCM3 and ID-CCM3 are referred to as F-CMT and
ID-CMT, respectively, while the simulation without
CMT are referred to as F-CTL and ID-CTL, respectively. (Note that the ID-CMT and ID-CTL experiments are identical to the IDCMT and IDCTL carried
out in Song et al. (2008, respectively). Thus the CMT
impacts in the F-CCM3 and ID-CCM3 can be evaluated
by comparing F-CMT and ID-CMT with F-CTL and
ID-CTL, respectively. Because the only difference between the ID-CCM3 and IDT-CCM3 is convective
heating forcing, the influence of convective heating on
the CMT impacts is readily identified from the difference of the CMT impacts between those two models. In
addition, contributions of the convective heating and
dynamical processes to the total effects of CMT on the
climate simulations can also be evaluated by comparing
the CMT impacts in the IDT-CCM3 and ID-CCM3 to
those in the F-CCM3. All simulations presented in this
study start from 1 December, with initial conditions
taken from results of a previous model simulation and
run for 2221 days. The initial specific humidity distribution on 1 December is the same as that used in Song
et al. (2008), which has a peak just north of the equator,
as shown below, resulting in slightly zonally asymmetric
distribution of convection (CMT forcing) and circulation in the ID-CCM3 and IDT-CCM3. The statistics
from the last 1825 days (5 yr) are used to represent the
model climate.
3. Impacts of convective heating on circulation
response to the CMT forcing
a. Climate of the IDT-CTL experiment
The climate of the IDT-CTL experiment, as represented by the zonally averaged zonal wind, meridional
wind, vertical velocity, and temperature, is shown in
Fig. 2. In general, inclusion of the convective heating in
the idealized GCM produces a more realistic zonal
mean circulation, which is similar to the observed annually averaged circulation in many aspects. In the
zonal wind field (Fig. 2a) westerlies prevail throughout
the troposphere in the midlatitude, with well-defined
westerly jet streams located at 150 mb near 45° latitudes. Easterlies appear over the equator and near the
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FIG. 2. Zonal average of (a) zonal wind (m s⫺1), (b) meridional wind (m s⫺1), (c) vertical velocity (mb day⫺1), and (d) temperature (K)
from the IDT-CTL. The contour intervals are (a) 5 m s⫺1, (b) 0.5 m s⫺1, (c) 5 mb day⫺1, and (d) 10 K. Negative values are shaded.
poles, as well as in the subtropical boundary layer. The
meridional wind (Fig. 2b) and vertical velocity (Fig. 2c)
together clearly show the three-cell circulation on the
meridional plane. The Hadley circulation lies between
approximately 30°S and 30°N, with strong rising motion
centered on the equator. The maximum equatorward
flow associated with the Hadley circulation is located
below 850 mb and the maximum poleward flow is located between 200 and 100 mb. Because temperature is
relaxed to the prescribed zonally symmetric state, the
temperature distribution (Fig. 2d) is similar to the prescribed radiative–convective equilibrium temperature.
In general, the climate of the IDT-CTL is in good
agreement with the annual mean climate produced by
the F-CTL (Fig. 3), although the meridional circulation
and subtropical jets are a bit stronger. The strengthening of circulation in the IDT-CTL can be attributed to
higher temperature in the lower troposphere between
10°S and 10°N, which produces more convective instability and hence stronger convection, resulting in an
enhanced Hadley circulation and thereby more intense
subtropical jets. In addition, the climate of the IDTCTL shows generally zonal and hemispheric symmetry
because symmetric thermal forcing is used. Thus, the
maximum rising motion of the Hadley circulation occurs on the equator, while the climate produced by the
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SONG ET AL.
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FIG. 3. Same as Fig. 2, but from the F-CTL.
F-CTL shows a slight asymmetry with the strongest upward motion located at about 8°N.
b. CMT forcing
Figures 4a,b show the zonal average of zonal and
meridional CMT forcing from the IDT-CMT. Similar to
that in the ID-CMT (Figs. 4c,d), the CMT forcing in the
IDT-CMT is also confined mainly between 10°S and
10°N, because convection is active mostly in that region
in the idealized GCM (Song et al. 2008). Although major CMT forcing also occurs between 10°S and 10°N,
the F-CMT (Figs. 4e,f) produces considerable CMT
forcing outside of that region because of the occurrence
of convection associated with seasonal shift of the solar
radiation. Because convection in the intertropical con-
vergence zone (ITCZ) plays a pivotal role in driving
tropical atmospheric circulation, this study will focus on
the effect of CMT forcing over the ITCZ on climate
simulations and will not adjust the prescribed moisture
and reference temperature fields to get stronger CMT
forcing outside the 10°S–10°N tropical belt.
Comparison of Figs. 4a,b with Figs. 4c,d shows that
the CMT forcing in the IDT-CMT is much weaker than
that in the ID-CMT (with the contour interval of 0.2
m s⫺1 day⫺1 for IDT-CMT and 2 m s⫺1 day⫺1 for IDCMT), while it is comparable in magnitude to that in
the F-CMT (Figs. 4e,f). This indicates that convective
heating stabilizes the atmosphere and hence results in
much weaker convection and CMT forcing in the IDTCMT. Between 10°S and 10°N, the zonal CMT forcing
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FIG. 4. Zonal average of zonal CMT forcing from (a) IDT-CMT, (c) ID-CMT, and (e) F-CMT, and meridional
CMT forcing from (b) IDT-CMT, (d) ID-CMT, and (f) F-CMT. Units are m s⫺1 day⫺1. The contour intervals are
0.2 m s⫺1 day⫺1 for (a), (b), (e), and (f), and 2 m s⫺1 day⫺1 for (c) and (d). Negative values are shaded.
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SONG ET AL.
5041
in the IDT-CMT shows a positive tendency below 800
mb and a negative tendency between 800 and 450 mb,
similar to the pattern observed in the ID-CMT and
F-CMT. This indicates that the convective heating
mainly affects the intensity of zonal CMT forcing
through alteration of the convection intensity. Thus, if
the dynamical processes are dominant in the response
of zonal wind to the CMT forcing, the zonal wind
change induced by the CMT forcing in the IDT-CMT
should be similar in structure to that in the ID-CMT,
but of a much smaller magnitude. Compared to the
zonal CMT forcing (Fig. 4a) in the IDT-CMT, the meridional CMT forcing (Fig. 4b) is smaller in magnitude.
The same is true for the ID-CMT and the F-CMT in the
10°S–10°N tropical belt. However, the distribution of
the meridional CMT forcing in the IDT-CMT (Fig. 4b)
shows a different dipole pattern to that in the ID-CMT
(Fig. 4d). Northerly and southerly accelerations occur
north and south of the equator between 850 and 600
mb, and an opposite pattern occurs between 600 and
250 mb in the IDT-CMT.
c. Response of large-scale circulation
The response of large-scale circulation to the CMT
forcing is readily identified from the zonally averaged
difference of circulation between the simulations with
and without the CMT. Then, the influence of convective heating on the impacts of CMT can be identified
from the difference of the CMT-induced circulation
changes between the IDT-CMT and ID-CMT. We can
also evaluate the contribution of dynamical processes
and convective heating to the total effects of the CMT
on climate simulations by comparing the CMT impacts
in the IDT-CMT with those in the ID-CMT and FCMT. Figure 5 shows the zonally averaged difference
of circulation between the IDT-CMT and IDT-CTL.
Because the circulation change is confined mainly in
the tropics, only the difference of circulation between
35°S and 35°N is shown. For comparison, Figs. 6 and 7
show the same difference fields for the ID-CCM3 pair
(ID-CMT–ID-CTL) and the F-CCM3 pair simulations
(F-CMT–F-CTL), respectively. Clearly, the CMTinduced circulation change in the IDT-CMT is much
smaller than that in the ID-CMT (Fig. 6), but it is comparable to that in the F-CMT (Fig. 7), which is consistent with the much weaker CMT forcing in the IDTCMT than in the ID-CMT.
The most pronounced changes in zonally averaged
zonal wind (Fig. 5a) between the IDT-CMT and IDTCTL occur in the 10°S–10°N tropical belt, where tropical easterlies are significantly enhanced above 700 mb
and reduced below when the CMT is parameterized.
This feature is similar to that of the CMT-induced zonal
FIG. 5. Zonal average of the difference of (a) zonal wind
(m s⫺1), (b) vertical velocity (mb day⫺1), and (c) meridional wind
(m s⫺1) between IDT-CMT and IDT-CTL. The contour intervals
are (a) 0.5 m s⫺1, (b) 1 mb day⫺1, and (c) 0.05 m s⫺1. Negative
values are shaded.
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FIG. 6. Same as Fig. 5, but between ID-CMT and ID-CTL.
The contour intervals are (a) 5 m s⫺1, (b) 5 mb day⫺1, and (c)
0.5 m s⫺1.
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FIG. 7. Same as Fig. 5, but between F-CMT and F-CTL.
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SONG ET AL.
wind change in the ID-CCM3 (ID-CMT–ID-CTL, Fig.
6a), which mainly contains the dynamical processes of
the F-CCM3. It indicates that the convective heating
has little influence on the pattern of zonal wind response to the CMT forcing and dynamical processes
dominate the zonal wind response in the 10°S–10°N
tropical belt in the IDT-CMT. The noticeable decrease
of westerly wind at about 25°N is not observed in the
ID-CMT, indicating that it may be attributed to the
inclusion of convective heating. Comparing Fig. 5a with
Fig. 7a, it is shown that the CMT-induced zonal wind
change in the IDT-CMT is similar in pattern and magnitude to that in the F-CMT between 10°S and 10°N.
This indicates that physical processes, except for the
convective heating in the F-CCM3, have little influence
on the zonal wind response to the CMT forcing in this
tropical belt. Therefore, it suggests that dynamical processes in the F-CCM3 dominate the zonal wind response to the CMT forcing in the 10°S–10°N tropical
belt.
Zonally averaged meridional wind change from the
IDT-CTL to IDT-CMT (Fig. 5c) shows strong convergence below 700 mb and divergence above 250 mb between about 5°S and 5°N, and opposite changes between about 5°N(S) and 10°N(S). The meridional wind
change from the ID-CTL to ID-CMT (Fig. 6c) shows
strong convergence below 700 mb and divergence between 700 and 400 mb from about 8°S to 8°N, and
opposite changes between 8°N(S) and 25°N(S). Correspondingly, the vertical pressure velocity difference between the IDT-CMT and IDT-CTL (Fig. 5b) shows
strong upward motion in the troposphere from about
5°S to 5°N, and weaker downward motion from about
5°N(S) to 10°N(S). The vertical pressure velocity difference between the ID-CMT and ID-CTL (Fig. 6b)
shows strong upward motion below 400 mb between
about 8°S and 8°N, and downward motion between
about 8°N(S) and 25°N(S). The meridional wind and
vertical velocity changes together clearly show that the
CMT induces a secondary meridional circulation within
the ascending branch (10°S–10°N) of the Hadley circulation in the IDT-CMT, which is characterized by
strong upward motion along the strongest ascending
belt (5°S–5°N) of the Hadley circulation and downward
motion north and south of the belt, while the CMT
induces the strengthening of the whole Hadley circulation in the ID-CMT, which is characterized by the enhanced and concentrated Hadley cell’s ascending
branch and broadened descending branch. Because the
only difference between the IDT-CMT and ID-CMT is
the inclusion of convective heating in the IDT-CMT,
these results indicate that the convective heating has a
strong impact on the response of meridional circulation
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to the CMT forcing and produces secondary meridional
circulation in the IDT-CMT.
Zonally averaged vertical velocity change from the
F-CTL to F-CMT runs (Fig. 7b) shows strong upward
motion in the troposphere within the strongest ascending belt of the Hadley circulation from about 5° to
10°N, and downward motion north and south of the
belt. The meridional wind difference from the F-CTL
to F-CMT (Fig. 7c) shows consistent convergence and
divergence in the corresponding regions. Thus, the
CMT-induced meridional circulation changes in the
IDT-CMT and F-CMT show the similar secondary meridional circulation with strong upward motion along
the strongest ascending belt of the Hadley circulation
and downward motion north and south of the belt (Wu
et al. 2003), indicating that the IDT-CMT contains the
dominant processes that affect the meridional circulation response to the CMT forcing in the F-CMT. Because the convective heating is the key physical process
in the generation of the secondary meridional circulation in the IDT-CMT, it suggests that the convective
heating dominates the response of meridional circulation to the CMT forcing in the F-CMT.
4. Mechanism of convective heating affecting
meridional circulation response to the CMT
forcing
The foregoing analysis shows that the convective
heating plays an important role in the response of meridional circulation to the CMT forcing. However, the
mechanism by which the convective heating affects the
response of meridional circulation to the CMT forcing
is still obscure. A comparison between Fig. 4b and Fig.
4d shows that the inclusion of convective heating results
in significant changes in the meridional CMT forcing. It
indicates that the convective heating may affect the meridional circulation response by altering the meridional
CMT forcing. However, a comparison between Fig. 4b
and Fig. 5c shows noticeable differences between the
meridional wind change and meridional CMT forcing
below 900 mb and between 200 and 125 mb. In addition, the meridional momentum budget diagnostics
show that the meridional CMT forcing is much smaller
than the pressure gradient force and Coriolis force
terms. This suggests that altering meridional CMT forcing is not the dominant mechanism of convective heating affecting the meridional circulation response.
On the other hand, as indicated by the bold arrows
and rectangles in Fig. 1c, the CMT-induced convective
heating–circulation feedback can also affect the meridional circulation. It is noted that a key feature of con-
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FIG. 8. Zonal average of the difference of convective heating rate (K day⫺1) between
IDT-CMT and IDT-CTL.
vective heating–circulation feedback is that the convective heating is markedly changed because of the interaction between convective heating and wind fields.
Figure 8 shows the zonally averaged convective heating
difference between the IDT-CMT and IDT-CTL.
Clearly, the convective heating is changed remarkably
in the IDT-CMT, with heating in the troposphere between about 5°S and 5°N and cooling between about
5°N(S) and 10°N (S). Moreover, the change in convective heating negatively correlates with the change in
vertical velocity (Fig. 5b), that is, more heating corresponds to stronger upward motion. The correlation coefficient between the changes in convective heating and
vertical velocity above 850 mb from 15°S to 15°N is
⫺0.929. It indicates that the convective heating–
circulation feedback actually exists in the IDT-CMT
and suggests that the convective heating affects the meridional circulation response in the IDT-CMT through
the CMT-induced convective heating–circulation feedback.
To further support this argument, an experiment,
IDT-CMT_NF, is conducted in which the setup is the
same as the IDT-CMT, except that the 5-yr mean difference of the convective heating between the IDT-
CMT and IDT-CTL (Fig. 8) is subtracted from the thermodynamic equation. Here the 5-yr mean difference of
convective heating between the IDT-CMT and IDTCTL is used to represent the mean convective heating
feedback in the IDT-CMT. Thus, in the IDT-CMT_NF,
the mean convective heating–circulation feedback is removed but the convective heating itself remains in the
model. Comparing IDT-CMT_NF and IDT-CMT, we
can evaluate the effects of the convective heating feedback on the circulation response. Comparing IDTCMT_NF with IDT-CTL, we can also evaluate the effects of dynamical processes and no-feedback convective heating on the circulation response.
The zonally averaged circulation changes from the
IDT-CMT_NF to IDT-CMT and from the IDT-CTL to
IDT-CMT_NF are shown in Fig. 9, respectively. Because the zonally averaged meridional wind and vertical velocity changes are related through zonally averaged mass continuity equation, only vertical velocity is
shown here to represent the changes of meridional circulation. Comparison of the change of vertical velocity
from the IDT-CTL to IDT-CMT_NF (Fig. 9a) and that
from the IDT-CTL to IDT-CMT (Fig. 5b) shows that
the simulation neglecting mean convective heating–
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SONG ET AL.
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FIG. 9. Zonal average of the difference of vertical velocity (mb day⫺1) (a) between IDT-CMT_NF and IDT-CTL, and (b) between
IDT-CMT and IDT-CMT_NF. The contour intervals are 1 mb day⫺1. Negative values are shaded.
circulation feedback cannot reproduce the secondary
meridional circulation observed in the IDT-CMT, indicating that no-feedback convective heating and dynamical processes are not the main mechanism for the
formation of secondary meridional circulation. On the
other hand, the vertical velocity difference between the
IDT-CMT_NF and IDT-CMT (Fig. 9b) is almost identical to that between the IDT-CTL and IDT-CMT (Fig.
5b) in pattern, but of a larger magnitude. The rootmean-square difference between the changes in vertical
velocity from the IDT-CMT_NF to IDT-CMT and that
from the IDT-CTL to IDT-CMT between 15°S and
15°N is about 0.837 mb day⫺1. Because the only difference between the IDT-CMT and IDT-CMT_NF runs is
that mean convective heating feedback is included in
the IDT-CMT, it demonstrates that the CMT-induced
convective heating–circulation feedback is the main
mechanism by which the convective heating affects the
response of meridional circulation to the CMT forcing.
Because the convective heating dominates the response
of meridional circulation to the CMT forcing in the
F-CMT, it suggests that the CMT-induced convective
heating–circulation feedback is the main mechanism for
the formation of secondary meridional circulation in
the F-CMT.
In addition, the meridional CMT forcing in the IDTCMT_NF (Fig. 10) is almost identical to that in the
IDT-CMT (Fig. 4b). With similar CMT forcing, however, the IDT-CMT_NF cannot reproduce the secondary meridional circulation observed in the IDT-CMT.
This demonstrates that altering meridional CMT forc-
ing is not the dominant mechanism of convective heating affecting meridional circulation response to the
CMT forcing.
5. Summary and conclusions
The role of convective heating in the response of
circulation to the CMT forcing is investigated in this
study. Because understanding the climate response of a
sufficiently complex GCM is a very difficult task because of complex nonlinear interactions among dynamical, thermodynamic, and physical processes, the
ID-CCM3, a simplified GCM containing clear dynamical mechanism, is used in this study. The ID-CCM3
consists of a complete dynamical core, simple specified
physics, and the CMT forcing so that it only contains
the dynamical processes’ response to the CMT forcing.
To evaluate the role of convective heating, a model of
intermediate complexity, the IDT-CCM3, is set up by
incorporating parameterized convective heating into
the ID-CCM3. The simulation with the IDT-CCM3
produces reasonable zonal mean circulation and CMT
forcing, indicating that the IDT-CCM3 is appropriate
for this study.
The CMT forcing produced by the IDT-CCM3 is
much weaker than that produced by the ID-CCM3 such
that it is comparable in magnitude to that produced by
the F-CCM3. This indicates that convective heating stabilizes the atmosphere and hence results in much
weaker convection and CMT forcing. As a result, the
CMT-induced circulation change in the IDT-CCM3 is
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VOLUME 21
FIG. 10. Zonal average of meridional CMT forcing (m s⫺1 day⫺1) from IDT-CMT_NF. The
contour intervals are 0.2 m s⫺1 day⫺1. Negative values are shaded.
much smaller than that in the ID-CCM3 and more comparable to that in the F-CCM3.
The influence of convective heating on the impacts of
CMT is identified from the difference of the CMTinduced circulation changes between the IDT-CCM3
and ID-CCM3. The CMT-induced zonal wind change
in the IDT-CCM3 is similar in structure to that in the
ID-CCM3 between 10°S and 10°N. It suggests that dynamical processes dominate the zonal wind response to
the CMT forcing in this tropical belt in the IDT-CCM3.
However, the CMT-induced meridional circulation
change in the IDT-CCM3 is very different in structure
to that in the ID-CCM3, suggesting that convective
heating dominates the meridional circulation response
to the CMT forcing in the IDT-CCM3. Because the
IDT-CCM3 reproduces some key features of CMTinduced zonal wind change in the tropics and the secondary meridional circulation observed in the F-CCM3,
it suggests that the CMT-induced zonal wind and meridional circulation changes in the F-CCM3 are dominated by dynamical processes and convective heating,
respectively.
The mechanism by which convective heating affects
meridional circulation response to the CMT forcing is
examined. Because the change of convective heating is
marked and negatively correlates with the change of
vertical velocity when the CMT is parameterized, it indicates that the CMT-induced convective heating–
circulation feedback exists and is a possible mechanism
of convective heating affecting the meridional circulation response to the CMT forcing. A sensitivity experiment is conducted in which the mean convective heating feedback is removed. The results show that the
model without convective heating feedback cannot produce the secondary meridional circulation observed in
the IDT-CCM3. It demonstrates that convective heating affects the meridional circulation response to the
CMT forcing through the CMT-induced convective
heating–circulation feedback.
Acknowledgments. This research was supported by
the Biological and Environmental Research Program
(BER), U.S. Department of Energy, Grants DE-FG0204ER63868, DE-FG02-04ER63865, and DE-FG0203ER63532.
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