Mechanisms Limiting the Northward Extent of the Northern...

406 VOLUME 16J O U R N A L O F C L I M A T E

q 2003 American Meteorological Society

Mechanisms Limiting the Northward Extent of the Northern Summer Monsoons overNorth America, Asia, and Africa*

CHIA CHOU

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan

J. DAVID NEELIN

Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

(Manuscript received 11 October 2001, in final form 22 July 2002)

ABSTRACT

Mechanisms determining the poleward extent of summer monsoon convergence zones for North America,Asia, and Africa are examined in an intermediate atmospheric model coupled with a simple land model and amixed layer ocean. Observations show that thermodynamical factors associated with the net heat flux into theatmospheric column provide favorable conditions for the monsoon convergence zone to extend farther polewardthan actually occurs. To understand the discrepancy, a series of experiments are designed to test the importanceof mechanisms previously examined in the South American case by the authors, namely, soil moisture, ventilation,and the interactive Rodwell–Hoskins mechanism. The latter refers to the interaction between baroclinic Rossbywave dynamics and convective heating. In North America, experiments suggest that ventilation by both tem-perature and moisture advection is a leading effect. The interactive Rodwell–Hoskins mechanism tends to favoreast coast rainfall and west coast dryness. In Asia, ventilation by moisture advection is particularly importantand the interactive Rodwell–Hoskins mechanism tends to favor interior arid regions and east coast precipitation.Overall, these dynamical factors are crucial in setting the poleward extent of the convergence zone over NorthAmerica and Asia. Africa differs from the other continents because of the high surface albedo over much ofnorthern Africa. Because there is less positive net flux of energy into the atmospheric column, convection isless thermodynamically favored and the dynamical factors, ventilation and the interactive Rodwell–Hoskinsmechanism, have a weaker impact on preventing poleward extent of the convergence zone.

1. Introduction

Land–ocean heating contrast is considered funda-mental to summer monsoon circulations (Webster 1987;Young 1987). One might conjecture that the thermo-dynamics of ocean–atmosphere–land interaction couldbe the controlling factor in the forcing of monsoons.However, examining the relation between solar heatingof the continent and the associated monsoon rainfall,the rain zone does not extend as far poleward as themaximum heating would seem to indicate. This suggeststhat other mechanisms besides the land–sea heating con-trast determine the northward extent of the summermonsoon. Many studies (Lofgren 1995; Meehl 1994;Xue and Shukla 1993; Yang and Lau 1998; Nicholson

* Institute of Geophysics and Planetary Physics, University of Cal-ifornia, Los Angeles, Contribution Number 5692.

Corresponding author address: J. David Neelin, Department ofAtmospheric Sciences, University of California, Los Angeles, LosAngeles, CA 90095-1565.E-mail: [email protected]

2000) discuss the importance of land processes, such assoil moisture and surface albedo, in affecting the mag-nitude and position of the monsoon. Topography, suchas the Tibetan Plateau, also affects the monsoon cir-culation (Flohn 1957; Murakami 1987; Meehl 1992;Yanai and Li 1994; Wu and Zhang 1998). This is thethird in a series of papers in which we examine somelarge-scale dynamical mechanisms that mediate land–sea contrasts and affect the poleward extent of mon-soons, and compare them with land processes.

Chou et al. (2001, hereafter CNS) discuss mecha-nisms that affect the seasonal movement of continentalconvergence zones for the summer monsoon on an ide-alized, rectangular continent. Because of strong sum-mertime insolation, thermodynamic conditions for es-tablishing a convective zone appear favorable far pole-ward in the continent. The poleward extent of conti-nental convection is largely determined by dynamicaleffects, such as ocean heat transport, ventilation, andRossby wave dynamics (the interactive Rodwell–Hos-kins mechanism) as well as by soil moisture feedbacks.The ventilation is defined as the import into the conti-nents of low moist static energy air from the ocean

1 FEBRUARY 2003 407C H O U A N D N E E L I N

regions or the export of high moist static energy out ofthe continents. The term ‘‘interactive Rodwell–Hos-kins’’ (IRH) mechanism was coined by CNS to describethe way Rossby wave–induced subsidence to the westof monsoon heating interacts with the convergencezone. Rodwell and Hoskins (1996, 2001) examined theimportance of baroclinic Rossby wave dynamics in cre-ating subsidence to the west of specified heating regions.The adjective ‘‘interactive’’ in IRH stresses that the spa-tial pattern of the monsoon heating itself is determinedinteractively with the subsidence regions.

CNS conducted a series of numerical experiments thatshowed impacts of all of these processes, especially ven-tilation and the IRH mechanism, on the idealized con-tinent. The conclusions might be modified and the rel-ative importance of various mechanisms might differ inapplication to more realistic geography because of dif-ferences between continents. For instance, the width,shape, latitude, relation to surrounding oceans, and al-bedo of the particular continents may play a role.

The South American case has been discussed in Chouand Neelin (2001, hereafter CN). Examining variousmechanisms, ventilation has a dominant impact in lim-iting the poleward extent of the South American summermonsoon. Here we would like to compare the impactof the various mechanisms on the Northern Hemispheresummer monsoons in North America, Asia, and Africa.There are many studies of detailed aspects of each ofthe monsoon systems: North American (e.g., Douglaset al. 1993; Higgins et al. 1997; Barlow et al. 1998; Yuand Wallace 2000); African (e.g., Palmer 1986; Xue andShukla 1993; Lare and Nicholson 1994; Rowell et al.1995; Eltahir and Gong 1996; Cook 1997; Semazzi andSun 1997; Janicot et al. 1998); and Asian (Ramage1971; Riehl 1979; Luo and Yanai 1984; Murakami 1987;Ding 1992; Yasunari and Seki 1992; Lau and Yang1996; Li and Yanai 1996; Webster et al. 1998; Anna-malai et al. 1999; and references therein). What we hopeto add is a particular moist dynamical perspective onthe large-scale aspects of these systems. No topographyis used in this study, so we can examine the dynamicalmechanisms in a situation that has the realistic land–ocean contrast and other geographic factors, but isslightly simpler to analyze. The Tibetan Plateau has asignificant impact on the evolution of the Asian summermonsoon (Yanai and Li 1992), so the assumption of notopography implies caveats on the discussion of theAsian summer monsoon. Nevertheless, we should beable to get some insight into mechanisms in an Asia-like continent that possibly affect the Asian monsoon.

Second, we slightly refine some of the experimentaldesign used in CNS and CN. To examine the ventilationeffect, CNS and CN suppressed the horizontal advectionterms that mediate it, namely, v · =T and v · =q, whereT is temperature, q is moisture, and v is horizontal wind.This is useful for hypothesis testing, but the remainingtransport terms, such as T= ·v and q= ·v do not integrateexactly to zero across the domain (although cancellation

occurs to a large extent) so the energy and moisturebudgets of the model are not perfectly conservative inthese experiments. To avoid this problem, the irrota-tional (purely divergent) part of wind (vx) is used tocalculate the advection instead of suppressing the entireadvection v · = terms. This also cleanly separates effectsof the nondivergent and purely divergent flow. Detailsare discussed in section 2. To test the impacts of Rossbywave dynamics, CN and CNS used experiments thatsuppress the beta effect in part of the domain. We modifythe design of this experiment to help distinguish be-tween the effects of conventional Rossby wave dynam-ics and possible effects that might be associated with a‘‘local Hadley circulation.’’

A detailed description of experiment design and re-view of the model is given in section 2. The northernsummer climate of the simulation and observations arecompared in section 3. Estimates of an important var-iable, the net energy input into the atmospheric column(F net), are presented and discussed. In sections 4 and 5,the North American monsoon and the Asian monsoonare examined, respectively. The most different case, thenorthern African monsoon, is examined in section 6,followed by a discussion.

2. The model and experiment design

a. The model

An atmospheric model of intermediate complexity(Neelin and Zeng 2000), a simple land surface model(Zeng et al. 2000) and a mixed layer ocean model (CNS)are used here. The atmospheric model uses analyticalsolutions for deep convection derived by Neelin and Yu(1994) and Yu and Neelin (1994) from the Betts andMiller (1986, 1993) deep convection scheme as leadingbasis functions for a Galerkin expansion in the vertical.We refer to this quasi-equilibrium tropical circulationmodel with a single vertical structure of deep convectionfor temperature and humidity as QTCM1. QTCM1 mim-ics a general circulation model (GCM) with the Betts–Miller deep convection scheme over deep convectiveregions, but it is roughly equivalent to a two-layer modelfar from deep convective regions. A cloud-radiationscheme is used to calculate effects of deep cirrocumulus/cirrostratus (CsCc), cirrus, and stratus clouds. This pa-rameterization package includes a linearized longwaveradiation parameterization (Chou and Neelin 1996)based on a full general circulation model scheme(Harshvardhan et al. 1987), a simplified shortwave ra-diation parameterization (Zeng et al. 2000) derived fromFu and Liou (1993) with prescribed marine aerosol, andan empirical parameterization for deep and cirrocu-mulus/cirrostratus cloud fraction (Chou and Neelin1999). A modified version of QTCM1 version 2.2 wasused in these experiments. The main differences be-tween this and model version 2.1 used in CNS and CNare in the treatment of vertical advection of momentum


and diffusion terms, and small improvements to thetreatment of cloud-radiative processes. These changesare documented as a subset of the version 2.3 release.

To simulate the interaction between the atmosphereand land surface, an intermediate land surface modelwith a single land surface layer for both energy andwater budget is used. There are three land surfacetypes—forest, grass, and desert—differing in soil mois-ture, field capacity, roughness length, and evapotrans-piration characteristics (see Zeng et al. 2000). Prescribedsurface albedo from the Darnell et al. (1992) climatol-ogy is used. The version of the model used here doesnot include topography.

A mixed layer ocean model (50-m fixed depth) withprescribed divergence of ocean heat transport, known asthe Q flux (Hansen et al. 1988), is used to examine in-teraction between the atmosphere and the ocean. Theseasonal Q flux is estimated from a 10-yr model run withclimatological SST. With the prescribed Q flux, SST isdetermined by the energy balance between latent heat,sensible heat, surface radiative flux, and Q flux. The re-sulting surface heat fluxes are discussed in section 3.

b. Refinement of the experiments for ventilation

In order to examine the ventilation effect, CNS sup-press the advection terms v · =T and v · =q in the tem-perature and moisture equations. Adiabatic cooling andmoisture convergence associated with the divergenceare retained. This approach does indeed show the im-portance of these terms in the transition from a regimetypical of tropical dynamics to a different dynamicalbalance that limits the extent of the convection zones.However, suppressing these terms causes errors in theconservation of energy and moisture, the magnitude ofwhich is hard to estimate in advance. Furthermore, it isdesirable to separate more clearly the effects of the di-vergent (irrotational) part of the flow from those of thenondivergent part. We present a refinement of these ex-periments by suppressing the part of the advection as-sociated with the nondivergent wind. Both the energybudget and moisture budget thus retain conservationproperties in certain cases. The moisture equation canbe written

] q 1 v · =q 1 v · =q 1 v] q 5 · · · , (1)t c x p

where vc is nondivergent part of horizontal wind, vx isirrotational part of horizontal wind, v is vertical velocityin pressure level, and q is specific humidity. By defi-nition,

= ·v 5 0,c (2)

so

v · =q 5 = · (v q).c c (3)

Taking the area-weighted integral of the advection byvc through the entire domain,

v · =q dA 5 0, (4)E c

Domain

using periodic boundary conditions in longitude andvc · n 5 0 at the poleward boundaries, where n is thenormal direction at the boundary. Thus droppingvc · =q for the testing purposes does not affect domain-integrated conservation. Similar results hold for tem-perature. Usually vx is much smaller than vc, so usingvx to calculate the advection terms reduces the advectionvery substantially. This experiment thus largely sup-presses the ventilation mechanism that is mediated byv · = terms. The climatology of vc and vx will be dis-cussed in section 3a.

In order to limit the impacts on the climatologythrough effects outside the region of interest, we definetarget regions over which the experiments are appliedfor each continent. These target regions are specified insection 2d. When applying the compression of vc · =qexperiment to target regions, (4) becomes instead

v · =q dA 5 v q · n dl, (5)E c cRRegion

where 6( ) dl is around the boundary of the target region.Suppressing vc · =q thus creates an experiment equiv-alent to stopping the moisture flux by vc at the bound-aries of the target region and otherwise conserves mois-ture (and enthalpy in the T equation). The experimentthus has a simple physical interpretation. Furthermore,we have some freedom in choosing boundaries to reducethe size of this term. As the target region area becomeslarge, the right-hand side of (5) divided by the domainsize becomes small relative to average precipitation inthe region. For instance, for the no-ventilation experi-ment in the North American case (section 4) the averageprecipitation over the target region is 6.3 mm day21

whereas the omitted moisture sink due to (5) is 0.3 mmday21. For the Asian case in section 5, the omitted mois-ture source due to (5) expressed as a domain averageis 0.3 mm day21, compared to an average precipitationof 8.8 mm day21. Corresponding effects in the temper-ature equation are of similar magnitude.

In all these experiments the irrotational (purely di-vergent) part of the wind is consistently kept, that is,both the terms involving the divergence itself, and thevx · = terms. The vertically integrated moisture equationmay be written equivalently as

] q 1 v · =q 1 v] q 5 E 2 P 1 · · · , or (6) t x p] q 1 = · v q 5 E 2 P 1 · · · , (7)t x

using v 5 0 at top and bottom of the model whereis a mass-weighted vertical integral, E is evaporation,( )

and P is precipitation.We remark that the nondivergent component of flow

cannot increase the maxima of moisture and cannot in-crease moisture beyond values in the upstream flow. The


TABLE 1. List of experiment types. The motivation for each experiment is discussed in the text. The target region is defined for eachcontinent: North America, Asia, and Africa. A related set of experiments for the African region with modified albedo is described insection 6.

Experiment name Description

ControlSaturated soilNo ventilationNo bNo ventilation and no bNo ventilation and partial bNo T ventilationNo q ventilation

Soil moisture is set to field capacity in target regionvc · =T and vc · =q suppressed in target regionA constant value of f is used in target regionvc · =T and vc · =q suppressed and a constant f is used in target regionvc · =T and vc · =q suppressed and b effect acts only on zonal mean wind across target regionvc · =T suppressed in target regionvc · =q suppressed in target region

divergent component of the flow can increase the max-ima or decrease the minima of the moisture field. Di-agnostics of the moisture budget that display vq as amoisture transport tend to be misleading in that they aredominated by vcq.

c. Testing the importance of Rossby wave dynamicsby suppressing b

The momentum equation can be written

Dv 1 f k 3 v 1 ( f 2 f )k 3 v 1 ( f 2 f )k 3 v90 0 0Dt

5 2=f 1 · · · , (8)

where f 0 is a reference value of the Coriolis parameter,is zonally averaged across a target region, v9 is thev

departure from the zonal average in a target region, kis a vertical unit vector, and f is geopotential. The beffect is contained in the f 2 f 0 terms. In previousexperiments (CNS, CN), we suppressed effects asso-ciated with Rossby wave dynamics by dropping the f2 f 0 terms in the momentum equation within the targetregion. However, one might consider that for a casewhere this is applied across the whole hemisphere, italso impacts the effects of variable f on conservationof angular momentum in the Hadley cell. Even for atarget region of smaller longitude extent, one might wor-ry that in addition to affecting Rossby wave dynamics,these experiments also impact the dynamics of merid-ional overturning circulations similar to the Hadley cir-culation within the continental target region.

Suppose we define local Hadley circulation as anoverturning circulation involving the zonal mean acrossthe target region. We caution that the concept of a localHadley circulation is hazily defined from a dynamicalpoint of view: at zonal scales much larger than the targetregion, a local overturning circulation is simply part ofa large-scale stationary Rossby wave. Any rising orsinking motion may be partially compensated to the eastor west of the region, rather than purely meridionally.Nonetheless, the terminology is used in the field andwe would like to address whether making the distinctionbetween zonal mean dynamics across the region andRossby wave dynamics within the region is important.

More precisely, we use the following means of sep-arating the dynamics of such a local Hadley circulationfrom the dynamics of Rossby waves within the targetregion. In (8) ( f 2 f 0)k 3 represents Coriolis effectsvon the local Hadley circulation while ( f 2 f 0)k 3 v9is associated with Rossby wave dynamics within thetarget region. By suppressing the latter term, while keep-ing ( f 2 f 0)k 3 , an experiment with ‘‘partial-b ef-vfect,’’ that is, having the b effect on zonally averagedwind in the target region, is defined. This maintains theb effect of a local Hadley circulation while suppressingRossby waves on scales smaller than the domain size.(For zonal scales much larger than domain, the targetregion can still act like a coarse-grained section of alarger Rossby wave.) In the partial-b experiments, thereis no east–west asymmetry within the domain arisingfrom Rossby wave dynamics. If the partial-b experi-ments and the no-b experiments behave similarly, thenit removes the question of whether one need worry aboutdistinguishing local Hadley cell dynamics from Rossbywave dynamics more generally.

There are edge effects associated with the change inf at the domain boundaries as noted in CN. Rossbyedge waves can occur and the anticipated decay scaleinside the region is less than the radius of deformation,on the order of a few hundred kilometers. Choice oftarget region boundaries is discussed below.

d. Experiments

A control run for seasonal climatology from a 10-yraverage is used to compare to other experiments. Be-sides the control run, seven different experiments aredesigned to examine various effects that limit the north-ward extent of northern summer convection zones. Asummary is given in Table 1. The experiments are ap-plied, in turn, to each of the target regions describedbelow. An additional set of experiments using a mod-ified albedo specification is conducted for the Africancase, as described in section 6. Results are for 10-yraverages after a spinup period (10 yr in most cases) toensure statistical equilibrium has been reached.

The first experiment, ‘‘saturated soil moisture,’’ ex-amines the effect of land hydrology; soil moisture in


a target region is set to field capacity, so water supplyto the atmosphere by evapotranspiration is not a lim-iting factor. The second experiment, ‘‘no ventilation,’’is designed to examine the ventilation effect by sup-pressing the contributions of the nondivergent flow vc

to the horizontal advection of moisture and temperatureas described in section 2b. A pair of experiments withvc · =T suppressed (‘‘no T ventilation’’) and vc · =qsuppressed (‘‘no q ventilation’’) are used to examinethe importance of the energy transport and the moisturetransport, respectively.

To examine the effects of Rossby wave dynamics andthe local Hadley cell, three experiments are conducted.The ‘‘no-b’’ experiment, uses only the f 0k 3 v termof (8), so no b effect acts within the target region. Witha combination of constant f 0 and no ventilation (‘‘noventilation and no b’’), the dynamical effects on deter-mining the position of the monsoon rainfall disappear,so only the thermodynamical factor affects the monsoonand it can be examined here. Using f 0 changes the cir-culation thus the ventilation. In order to examine theeffect of the local Hadley cell, the no-b and no-venti-lation experiment is used to compare with the ‘‘no-ven-tilation and partial-b’’ experiment where ( f 2 f 0) 3v9 is suppressed.

The model modifications for each experiment are ap-plied in a target region for North America, Asia, andAfrica, respectively. A value of f 0 5 f (7.58N) is usedin the North American and African cases, while f 0 5f (12.58N) is used in the Asian case. The size of eachtarget region is chosen to cover the continent or sub-section of continent and large enough to minimize edgeeffects on the forcing region. For the North Americancase, the target region is between 7.58–52.58N and 1808–458W. For the Asian case, the target region is between12.58–52.58N and 628–1468E. For the African case, thetarget region is between 7.58–42.58N and 22.58W–518E.Edge effects are largest near the target region bound-aries, but can have some impacts within the target re-gion. In addition to Rossby edge waves in the no-b andpartial-b experiments, other experiments may have po-tential edge effects. There is a trade-off in determiningthe target region size for such experiments. A largerdomain for the target region places more distance fromthe edge, but impacts the model climate over a largerscale. We have chosen the domains with this compro-mise in mind, and then have conducted many sensitivitytests (not shown) with variants of these target regionsto ensure the edge definition does not affect the con-clusions. We have also conducted experiments (notshown) with the entire Northern Hemisphere as the tar-get region to verify that results are qualitatively similar.

3. Northern summer climate

a. Precipitation and wind fields

Figures 1a and 2a show July climatology of precip-itation from the model and the observations, respec-

tively. In the Tropics, the model precipitation is similarto the Xie–Arkin precipitation (1996). The North Amer-ican summer monsoon is a little weak and does notextend far enough northward compared to observations.It compares favorably with some GCM simulations(Yang et al. 2001). Considering that both topographyand mesoscale effects are known to affect Mexican mon-soon precipitation (Berbery 2001), this result is reason-able. The Asian monsoon is simulated by the model,although the model does not have distinct maximumprecipitation over the Bay of Bengal. We emphasize thatthe absence of the Tibetan Plateau in the model certainlyimpacts particular aspects especially at local scales.However, the large-scale aspects of the rainfall pattern,including the northward extent along the east coast tendto be simulated. The African monsoon is well estab-lished, although the precipitation is slightly strongerthan observed. The northward limit of the equatorialconvergence zone occurs in the Sahel region as ob-served. The model precipitation associated with stormtracks in the Atlantic and Pacific is located farther eastthan in observations, possibly associated with weakerstorm development in the summer season. This also af-fects the precipitation along the east coast of NorthAmerica.

The model wind fields at 850 (Fig. 1b) and 500 mb(Fig. 1c) are similar to the National Centers for Envi-ronmental Prediction (NCEP) reanalysis (Figs. 2b and2c). At 850 mb, the trade winds are slightly strongerthan the NCEP data. The cross-equatorial flow over theIndian Ocean is simulated by the model, but the north-ward component is weak compared to the observations.This is associated with strong precipitation over thewestern part of the Indian Ocean, south of the corre-sponding maximum precipitation from the observation.This northeastward flow joins the trade winds from thewestern Pacific flowing into east Asia. Distinct sub-tropical anticyclonic circulations, such as over the north-ern Pacific and the Atlantic are generally well simulatedby the model. The Atlantic subtropical high is weak.The anticyclonic circulation associated with the Pacificsubtropical high in the model (Fig. 1b) is farther eastthan the observation in Fig. 2b. The strong northwardflow near Arizona at 850 mb, which transports low-levelmoisture into this region is not simulated, perhaps con-tributing to weaker North American monsoon rainfall.At 500 mb the lower portion of the subtropical jet maybe seen (the 300-mb view may be seen in Fig. 3). Thelocation in both hemispheres is reasonable compared toNCEP data in Fig. 2, although in the Northern Hemi-sphere the flow extends slightly southward over the Pa-cific–North American sector, which may affect theNorth American monsoon.

Over the African continent, the model simulates low-level westerlies (Fig. 1b) between the equator and 158N,which are a main component of the African monsoon.North of the westerlies, the Harmattan easterlies closethe circle of a cyclonic flow associated with a thermal


FIG. 1. Jul climatology from the model for (a) precipitation (shaded over 2 mm day21), (b)winds at 850 mb, and (c) winds at 500 mb (length scale in m s21).

low centered over Saharan Africa. At 500 mb, distincteasterlies, the African easterly jet (AEJ) from the trop-ical easterly jet, and the Harmattan easterlies are found.However, the AEJ in the model is weaker and the al-titude of the AEJ is a little higher than in observations.At 600 mb this feature is weak in the model. The AEJis caused by positive meridional temperature gradientsat the surface associated with strong meridional soilmoisture gradients and a reversal of the meridional tem-perature gradient in the lower troposphere, around 600mb (Cook 1999; Thorncroft and Blackburn 1999). In

the model, the reversal of the temperature gradient isnot simulated. Thus the AEJ is not distinguished fromthe tropical easterlies at the upper troposphere.

The nondivergent and irrotational parts of wind at300 mb are shown in Fig. 3. The irrotational part of thewind is plotted at a scale 1/6 that of the nondivergentpart of wind. Maps of total 300-mb wind (not shown)look very similar to the nondivergent wind in Fig. 3b.Because the nondivergent wind is stronger, using onlythe irrotational part of wind to calculate the advectionterms of T and q can significantly reduce the ventilation


FIG. 2. As in Fig. 1 but from observations.

effect, as described in section 2b. The upper-level di-vergent flow is closely related to low-level convergence.The strongest features are associated with the convec-tion zones, especially in the tropical western Pacific andIndian Oceans.

b. The net energy input into the atmosphere, Fnet

Here F net is defined as the net energy input into theatmospheric column:

net ↓ ↑ ↓ ↑ ↑ ↓ ↑F 5 S 2 S 2 S 1 S 2 R 2 R 1 Rt t s s t s s

1 E 1 H, (9)

where subscripts s and t on the solar (S↓ and S↑) andlongwave (R↑ and R↓) radiative terms denote surface andmodel top, and ø 0 has been used. The E is evaporation↓Rt

and H is sensible heat flux. In general, Fnet must be bal-anced by divergence of the moist static energy transportin the vertically integrated moist static energy equation.This includes a term associated with the purely divergentcomponent of the circulation, where h is moist statich] vp

energy and v the vertical velocity, and advection terms

and In the Tropics, horizontal gradients ofv · =T v · =q. temperature and thus v · =T are small and v · =q is rel-atively small. Positive Fnet must be balanced by rising


FIG. 3. Jul climatology from the model control run for (a) the irrotational part of the wind at300 mb and (b) the nondivergent part of the wind at 300 mb (scale in m s21).

motion and divergent circulation (including low-level con-vergence). A number of theoretical studies have made useof this balance, including evidence that the effectivenessof the Fnet forcing is determined by the gross moist stabilityof the troposphere (Neelin and Held 1987; Neelin and Yu1994) which is relatively constant within convective zones(Yu et al. 1998). The rising motion is associated with low-level convergence and moisture convergence. Zeng andNeelin (1999) examine how Fnet drives moisture conver-gence in deep convective regions over land. Sobel andBretherton (2000) show that a constant temperature ap-proximation captures general features of convection zonesequatorward of 158 latitude by a balance of Fnet and thedivergent circulation (although v · =q terms improve thesimulation). It is thus surprising that the spatial distributionof Fnet has been discussed relatively little based on ob-servations.

Figure 4 shows F net and the net heat flux into thesurface (Fs 5 2 1 2 2 E 2 H) for July,↓ ↑ ↓ ↑S S R Rs s s s

as estimated from observations. The top-of-atmosphereradiative fluxes are from the Earth Radiation BudgetExperiment (ERBE; Ramanathan 1987). The surface

fluxes are from the Comprehensive Ocean–AtmosphereData Set (COADS; Woodruff et al. 1987), except thatthe Darnell et al. (1992) estimate for the surface short-wave is used. Over land, a condition of zero net surfaceflux is used to obtain F net. This assumption holds fortimescales longer than a day. Since some of the observedcomponents of Fs, E, H, , , , and , are difficult↓ ↑ ↓ ↑S S R Rs s s s

to measure, errors are larger in Fs than in the top-of-atmosphere data. Because the zero Fs condition is evenmore accurate, the estimate of F net is considerably moreaccurate over land than over the ocean.

In Fig. 4, F net is positive in most of the regions ofoceanic convective zones in the Tropics including thePacific intertropical convergence zone (ITCZ) and SouthPacific convergence zone (SPCZ). An exception is anarrow strip near the equator in the western Pacific thathas a slightly negative value of F net while the convectionis strong. This is likely due to errors in the surface fluxdata over the region—the net surface flux Fs impliedby the COADS and Darnell datasets is over 60 W m22

into the oceans, whereas other estimates suggest it issmaller (Cronin and McPhaden 1997). This would imply


FIG. 4. Jul climatology from observations for (a) the net flux of energy into the atmosphericcolumn, F net (net surface and top-of-atmosphere shortwave and longwave radiation, sensible heat,and latent heat fluxes); (b) the net surface flux Fs. Both are in W m22, shaded for positive values.

a substantial positive F net in this region. Nonconvectiveregions in the Tropics tend to coincide with stronglynegative F net in this estimate. If surface solar is over-estimated (e.g., Arking 1999), the pattern would remainessentially the same while the zero line would shift. Theoverall tendency would remain for the net flux forcingof the atmospheric column to be positive in convectionzones and to be negative in dry regions.

Turning to the more accurate estimates of F net overcontinental regions, the winter hemisphere continentshave strongly negative F net outside the Tropics, as ex-pected. During the summer season, North America andAsia both have strong positive F net . This positive ther-modynamic forcing of the atmospheric column extendsvery far poleward, and is roughly of similar magnitudefrom the subtropics to the Poles. This is because thesummer insolation has a large positive input extendingall the way poleward. The part of outgoing longwaveradiation (OLR) associated with temperature has largespatial scales due to dynamical effects on upper-tro-pospheric temperature and does not fully compensatefor the large summer insolation into the atmosphere–land column. Cloud effects can modify both longwaveand shortwave but do not change the basic picture.

This evaluation of summer continental F net raises thequestion regarding the summertime convection zones:

what sets the poleward edge of the convection zone?Positive thermodynamic forcing of the tropospheric col-umn exists over the whole summer continent. In a trop-ical dynamical regime, this would be balanced by risingmotion with associated low-level convergence. Low-level convergent motions supply moisture so unlesssome other factor intervenes, a convection zone is es-tablished. Since the convergence zones and associatedstrong convection do not move nearly so far polewardas the region of positive F net, clearly a transition to someother dynamical regime must occur in the summer con-tinents. Comparing the summertime precipitation (Fig.2a) to F net over summer hemisphere continents makesthis clear. The region of strong precipitation has a sharpedge not associated with changes in the smoothly vary-ing F net . This implies some dynamical mechanism mustset the edge of the convergence zone, not the simplerthermodynamic balance of positive net energy inputwith rising/divergent motions. Chou and Neelin showthe Southern Hemisphere summer counterpart of Fig.4a, and discuss similar considerations in southern sum-mer where positive F net likewise extends far polewardover southern continents.

Northern Africa differs from Asia and North America.In Fig. 4, most of equatorial and northern Africa haspositive F net but over desert regions its magnitude is


FIG. 5. As in Fig. 4 but from the model control run.

considerably smaller than typical summer continentalvalues. This is principally due to large surface albedo,which exceeds 0.4 in some parts of the Sahara. Albedoeffects in northern Africa have long been considered acontributor to regional deserts (Charney 1975), so wecan anticipate differences between the African case andthe other continents.

The zero surface heat flux condition makes land–at-mosphere interaction very different from ocean–atmo-sphere interaction where heat storage and heat transportby the ocean create nonzero surface heat flux. This isquantified in Fig. 4b, which shows the net surface heatflux. Over the Northern (summer) Hemisphere, a largeheat flux into the ocean exceeds 100 W m22 over mostof the subtropical to higher-latitude oceans. The leadingcontributor to this would be ocean heat storage. Theheat flux into the ocean in the deep Tropics tends topersist in the annual average and thus must be substan-tially associated with ocean heat transport. The com-bination of net heat flux into the ocean and zero heatflux over land (Fig. 4b) produces a strong land–oceancontrast. The consequence for the atmospheric heatingis positive F net over land and negative F net over ocean(see Fig. 4a) in the summertime subtropical and higherlatitudes.

These observations of F net and the implications out-lined above appear not to have been discussed in theliterature, although there have been a number of studies

that estimate zonal averages of related quantities (e.g.,Carissimo et al. 1985). Trenberth and Solomon (1994)use the European Centre for Medium-Range WeatherForecasts (ECMWF) analyses to present the divergenceof the atmospheric energy transport, which must beequal to F net for a system that conserves energy. Theirresults tend to agree qualitatively for large regions, al-though they are contaminated by spatially noisy signals.Since the ECMWF analysis does not maintain energyconservation there is good reason to believe that theanalysis in Fig. 4 is the more accurate, especially overland regions where the accuracy is that of the ERBEdataset.

The model simulation of F net and Fs is shown in Fig.5. Comparing Fig. 5a to Fig. 4a, the pattern of F net fromthe model is similar overall to the observations. Deepconvection regions over the oceans in the Tropics havea positive value of F net. Over land, positive F net overnorthern continents and negative F net over southern con-tinents are found. The simulated F net tends to be about15 W m22 stronger than the observations. Over regionsthat have small cloud cover, such as the Sahara desertand the cold tongue of the Pacific Ocean, a contributingfactor is that the OLR from the model (not shown) isless than the observations over clear-sky regions. None-theless, the model results still show relatively low F net

over the Sahara desert as observed. In Fig. 5b Fs ispositive over the summer hemisphere oceans (except for


a small region in the Indian Ocean where simulatedcloud fraction is too large). This creates a strong ocean–atmosphere contrast in F net in subtropical to high lati-tudes, as observed.

4. North American case

Figure 6 shows the results for the control run and theseven experiments discussed in section 2d and listed inTable 1 applied to the North American target region.The control run in Fig. 6a as discussed in section 3 hasa weaker North American monsoon than observed: therain zone is a little too far south and the east coast doesnot receive enough rain. This may be due in part to thelack of topography (Broccoli and Manabe 1992) and inpart to the slight shift of the storm track eastward fromthe coast. Figure 6b with saturated soil moisture showssome impacts. There is greater northward extension ofthe North American monsoon rainfall over the south-west region of the United States compared to Fig. 6a.The precipitation over the eastern part of the continentin Fig. 6b also increases. In the saturated soil moisturecase, the corresponding evaporation increases and en-hances local precipitation. The increase of evaporationalso reduces surface temperature over North Americaand produces anticyclonic circulation anomalies. Theseanomalies transport extra moisture from the Gulf ofMexico into the south and southeast of the United States.Thus the movement of the monsoon and the increase ofthe rainfall is the result of both local evaporation andlow-level moisture convergence. The saturated soilmoisture results here may overestimate the effect of soilmoisture, perhaps because of the weaker North Amer-ican monsoon in the control. Other soil moisture ex-periments (Fennessy and Shukla 1999; Zeng et al. 2000)show smaller sensitivity of the summer rainfall thoughthe tendency is consistent.

To test the ventilation mechanism via suppressing thehorizontal advection of T and q, only the irrotationalcomponent of the wind (vx) is used to calculate theadvection as described in section 2b. The precipitation(Fig. 6c) becomes much more intense, especially overthe south and east of the continent, and the rain zoneextends northward over much of the continent. Clearly,the ventilation has a very strong impact on the extentof the rain zone (much stronger than soil moisture). Theintervention of turning off ventilation is rather drastic,so the resulting high precipitation is not something oneexpects to occur in realistic climate situations, but theexperiment does show that ventilation is a leading ordereffect.

Some east–west asymmetry remains even where ven-tilation is suppressed. In Fig. 6c, the southwestern Unit-ed States is the only area with small precipitation. Fordifferent versions of the model, the size of this less rainyarea varies, but the east–west asymmetry is very con-sistent and it is consistent with the idealized monsooncase in CNS. It is thus likely associated with Rossby

wave effects, suggesting these are important to estab-lishing arid regions in the Southwest.

A test of the effect of Rossby wave dynamics is pre-sented in Fig. 6d for the no-b experiment, that is, usingthe reference value of the Coriolis parameter, f 0 overthe region. Almost the entire continent is covered byconvective rain over 2 mm day21. Bearing in mind thecaveat that suppressing b is a rather heavy interventionin the dynamics, the experiment does show that dynam-ical factors related to b have a significant impact. Theprecipitation in Fig. 6d does not have much east–westasymmetry, suggesting that the b effect is important increating the east–west asymmetry of the convectionzone. Unfortunately, these results cannot be compareddirectly to Fig. 6c to examine whether ventilation orRossby wave effects have a stronger impact because themodification of f greatly alters the circulation and thuschanges the ventilation as well. To further examine theeffects of ventilation and the b effect, the no-ventilationand no-b experiment is shown in Fig. 6e. The east–westasymmetry of the precipitation has disappeared. Withthe dynamical factors suppressed, such as the ventilationand the Rossby wave effect, the thermodynamical fac-tors (shown by F net) control the convection zone, so thedistribution of precipitation follows F net extendingnorthward.

Figure 6f shows the results of an experiment aimedat distinguishing the effects of Rossby wave dynamicsfrom affects that could be described as a local Hadleycell. Of the two terms in (8) associated with the b effect,this experiment has ( f 2 f 0)k 3 included and ( f 2vf 0)k 3 v9 suppressed over the North American region.Ventilation is also suppressed, so this no-ventilation andpartial-b experiment may be compared with Fig. 6ewhere the b effect is suppressed entirely, as is venti-lation. Figure 6f shows that precipitation covers the en-tire North American continent. The convergence zonesin both experiments extend far poleward; the case inFig. 6e in fact does not extend as far as Fig. 6f althoughwe do not attribute much significance to the differencesin the far northern continent. Including the b effect onthe local Hadley cell clearly does not prevent the con-vection zone from extending northward. Thus descentin a local Hadley cell can be eliminated as a significantmechanism limiting the extent of the convection zone.The northward edge is affected by the Rossby wavedynamics, which concurs with the IRH mechanism asdescribed in CNS.

A pair of experiments designed to examine the effectsof vc · =T and vc · =q, respectively, on the ventilationmechanism are shown in Figs. 6g and 6h. From theenergy budget of the control run, (not shown) vc · =Thas a cooling tendency exceeding 60 W m22 over mostof the continent north of the monsoon region, whereasvc · =q has magnitude less than 30 W m22 except insmall regions of moistening or drying. Thus it wouldappear that vc · =T is more important for balancing pos-itive F net over the continent than vc · =q, as is discussed


FIG. 6. Jul precipitation for experiments over the North American region: (a) control run, (b)saturated soil moisture, (c) no ventilation, (d) no b, (e) no ventilation and no b, (f ) no ventilationand partial b, (g) no T ventilation, and (h) no q ventilation. Contour interval is 2 mm day21. SeeTable 1 and section 2d for details of the experiments.


in CNS. When suppressing the effect of vc · =T, a strongnorthward extension of the rain zone might be expected.However, the result in Fig. 6g does not show any north-ward movement of the rain zone. Without vc · =T, thecontinent cannot cool down efficiently, so the surfaceand air temperature increase significantly. The value oflow-level moisture required to create convective avail-able potential energy (CAPE) and initiate convectionthus increases. The oceanic moisture values importedby vc · =q are lower than what is required to initiateconvection, so most of the continent remains without asustained convergence zone. When suppressing the ef-fect of vc · =q, the moisture of the atmosphere increasesrelative to the control in the southern continent whilethe temperature remains about the same. Figure 6hshows a very slight northward extension of the rainzone. However, the positive F net is balanced by vc · =T,so rising and convergent motions are not generated overmost of the continent and the change in the convectionzones remains small.

Comparing Figs. 6g and 6h to Fig. 6a, either advec-tion term is enough to prevent the convergence zoneextending northward. A strong nonlinear effect is thusinvolved in the ventilation mechanism. When suppress-ing one advection term, the effect of the other term oncooling and drying is enhanced. The results of the ex-periments are thus very different from attributions basedon the budget analysis in which both advection termsappear to contribute to preventing northward extensionof the convergence zone. One should thus use cautionwhen employing budget analysis to understand mech-anisms.

A similar nonlinearity is noted in the impact of sup-pressing the ventilation mechanism and that of sup-pressing the b effect. Suppressing either creates largechanges in the continental precipitation so ranking theirimportance must be done with caution.

We have repeated the experiments shown in Fig. 6with different model versions and find the general con-clusions to be robust. There is considerable sensitivityin the quantitative distribution of precipitation becauseof the drastic changes that the experiments produce. Theresults here are used only to qualitatively estimate theimpact of these mechanisms on determining the north-ward extent of the convergence zone.

5. Asia-like continent case

We reiterate that the Tibetan Plateau is important toproducing a realistic Asian monsoon. Using a continentthat has the geography of Asia without topographic ef-fects can, however, give us a first glimpse of mecha-nisms that can affect the Asian monsoon. GCM simu-lations that remove this plateau (Hahn and Manabe1975; Zhisheng et al. 2001) do retain many large-scaleaspects of the monsoon convection zones. Caveats onthe climatology in Fig. 7a include the following: it doesnot have a distinct feature of maximum precipitation

over the Bay of Bengal, the precipitation in south andSoutheast Asia is confined in a smaller region, and theprecipitation along the northeast coast is a little higherthan observed. Any of these might be affected by theabsence of the Tibetan Plateau, though GCMs that havefull topography have similar problems (Sperber andPalmer 1996).

Experiments in the Asia-like continent case, overall,have many similarities to the North American case. Sat-urated soil moisture (Fig. 7b) has a similar impact onextending the convergence zone modestly northwardand enhancing the precipitation over the east coast.When ventilation associated with vc is suppressed, thepoleward extension of the convergence zone is sub-stantial (Fig. 7c). The convergence zone has east–westasymmetry favoring convection over the eastern part ofthe continent and descent to the west. In the interior ofthe continent, reduced precipitation may be partially dueto the albedo effect, which reduces F net by reflectingsolar radiation. The no-b experiment (Fig. 7d) showssignificant poleward extension and no obvious east–west asymmetry. With both the ventilation associatedwith vc and the b effect suppressed, again polewardextension and little east–west asymmetry are seen, sim-ilar to the no-ventilation and partial-b experiment.

A difference between Figs. 7e and 7f, which is alsofound in the North American case, is that the conver-gence zone in Fig. 7e does not extend as far north asin Fig. 7f. The whole region becomes much warmer andhigher OLR allows F net to approximately balance. Theconvergence zone is terminated at high latitudes as itextends toward regions of lower moisture. For the no-ventilation and partial-b experiment, the target regionmight feel the effect of Rossby wave dynamics for scalesat the domain size and larger, which allows descent tobe communicated westward. However, in all these ex-periments, the small differences are less important thanthe result that including b effects on a domain-averaged‘‘local Hadley cell’’ is clearly not a barrier to polewardextension of the convergence zone. Thus earlier con-clusions regarding the impact of the IRH mechanismseem to hold.

A difference between the Asian case and the NorthAmerican case comes when turning to the no T and noq ventilation experiments seen in Figs. 7g and 7h, re-spectively. When vc · =T is suppressed in the no-T ven-tilation case (Fig. 7g), the vc · =q term is able to preventthe convection zone from extending poleward and thesimulation remains similar to the control. Suppressed qventilation alone has an effect comparable to the no-ventilation case, at least for a region in and somewhatpoleward of the control convection zone in central Asia.Still, the convection zone in Fig. 7h does not extend asfar poleward or into the continental interior as in theno-ventilation experiment (Fig. 7c). This may be con-trasted to the North American case where keeping eithervc · =T or vc · =q could prevent poleward extension ofthe convection zone. In the Asian case, vc · =q alone


FIG. 7. As in Fig. 6 but over Asia.


FIG. 8. Jul precipitation for experiments over the African region: (a) control run, (b) saturatedsoil moisture, (c) no ventilation, (d) no b, (e) no ventilation and no b, (f ) no ventilation andpartial b. The 0.3 contour of surface albedo has been added to (c) for reference.

can maintain a climate similar to the control, but vc ·=T alone is less effective at preventing rainfall increase.Conjectures are the following: (i) T ventilation is lessefficient because the wind has traveled across greaterdistances of continental region in the Asian case; (ii)there is a considerable northerly flow component in thewind in parts of the region north of the Asian monsoon,and this may be particularly effective at bringing lowq air to disfavor convection over the continent.

Understanding the circulation involved in ventilationadmittedly is more complicated in Asia than in othercontinents because of the wider continent (even withoutthe Tibetan Plateau). The path of air masses responsiblefor ventilation and the relation to oceanic effects areless clear. The effect of the Tibetan Plateau obviously

needs to be addressed to understand the actual impactof these mechanisms on the extent of the Asia monsoon.The effects on east Asia in these results are more re-alistic than those in the immediate vicinity of the pla-teau. However, the results here suggest that ventilationdoes tend to be a cooling and drying effect, akin to theNorth American and South American (CN) cases. Over-all, high moist static energy air is exported from thecontinental region and by one path or another is replacedby lower moist static energy air.

6. African case

In experiments for the African case (Fig. 8), one en-counters major differences from North America and the


Asia-like continent. Saturated soil moisture (Fig. 8b)does create a region of low but nonzero precipitationover much of the descent region. Saturated soil moistureis a more drastic assumption for Africa than for othercontinents because of climatological low soil moistureover northern Africa, so this result might be somewhatunrealistic. The sensitivity is higher than found in Dou-ville et al. (2001). The implied evaporation (not shown)over the artificially saturated desert is very large, rough-ly similar to neighboring ocean regions. Ventilation isonly able to remove about half this moisture input, theremainder of which precipitates. Further examination ofthe results in Fig. 8b shows that the region of intenserainfall in the equatorial continental convection zoneextends farther northward compared to the control run(Fig. 8a). Increases in evaporation tend to increaseboundary layer moisture and the resulting increase inthe boundary layer moist static energy (see, e.g., Paland Eltahir 2001) tends to favor convection (when it issufficient to meet convective instability criterion). Theincrease in low-level moisture is partly due to localevaporation and partly to increased evaporation upwindaffecting moisture advection.

The no-ventilation experiment (Fig. 8c) has much lesseffect than for other continents. Convection is still verytropically confined over most of Africa. Suppressingventilation does allow poleward extension up the eastside of the region, which creates rainfall over Arabia.Rossby wave descent due to the Asian monsoon is notvery effective at preventing this increase of precipitationover Arabia—which suggests caveats on the originalRodwell and Hoskins (1996) hypothesis for this region.Experiments with an enhanced Asian summer monsoon,such as the no-ventilation experiment in Fig. 7c, alsoshow only small southward movement of the Africansummer monsoon. This suggests that the specific re-gional aspects of the Rodwell and Hoskins (1996) hy-pothesis may be the leading effects, even though thegeneral mechanism is found to be important in otherregions. The obvious limitation on poleward migrationof convection in the African region experiments is thealbedo, as already noted in the discussion of F net insection 3b, especially over the Sahara. Precipitation con-tours actually tend to follow albedo contours, as maybe seen from the reference contour of albedo added toFig. 8c. The high surface albedo over the Sahara pre-vents the convergence zone from extending northward.Suppressing ventilation does allow poleward extensionup the east side of the region, which creates rainfallover Arabia.

The no-ventilation and no-b (Fig. 8e), and no-ven-tilation and partial-b (Fig. 8f) experiments encounter asimilar effect to the no-ventilation experiment (Fig. 8c).The convergence zone cannot move very far polewardover northern Africa due to the albedo effect. Theseexperiments do affect the regions to the immediate northof the control-run convection zone, so dynamical mech-

anisms have some effect in Sahel. But they are not near-ly as important as on other continents.

To confirm the importance of the albedo effect innorthern Africa, a set of constant surface albedo ex-periments is conducted. Albedo is set to a constant valueof 0.26 over the African continent in the entire set ofexperiments including the control. The results are shownin Fig. 9. We note that the difference between the con-stant albedo control run (Fig. 9a) and the observed al-bedo control run (Fig. 8a) has a dipole pattern in pre-cipitation (not shown), which is consistent with Xue andShukla (1993).

Qualitatively, the results for the dynamical-factor ex-periments in the constant albedo case (Fig. 9) are similarto other continents. Suppressing the ventilation mech-anism (Fig. 9c) permits the convection zone to expandover almost the whole of northern Africa including theSahara. This is also the case for experiments with noventilation and no b or partial b (Figs. 9e and 9f). Sup-pressing b alone also permits a substantial, though less-er, expansion. Thus, once spatial variations in albedoare removed as a factor, the dynamical factors becomethe controlling mechanisms as on other continents. Thismay have implications for paleoclimate modeling of theAfrican monsoon, in which albedo feedback is believedto be a major—but not sole—factor (e.g., Claussen etal. 1999; Joussaume et al. 1999). In a situation wherealbedo is reduced over the Sahara, such as the constantalbedo control in Fig. 9a, the dynamical factors set thepoleward limit of the convection zone. It is reasonableto hypothesize that they would enter similarly in analbedo feedback case.

Some differences between the dynamical factor ex-periments over Africa and over other continents mayalso be noted, in particular for the no-ventilation ex-periment (Fig. 9c). There is little east–west asymmetrycaused by Rossby wave interaction with the heatingwithin the African continent. The Rossby wave effectassociated with the convection over southern Asia (i.e.,the Rodwell–Hoskins hypothesis) may play a role inthis. The Asian influence would tend to disfavor theeastern part of the region, while regional Rossby waveeffects would disfavor the western side and the two maytend to balance out. Finally, we note that albedo doesenter as a factor on other continents as well but theeffects are modest compared to the dynamical mecha-nisms, as may be anticipated from F net (Fig. 4).

7. Discussion

Motivated by observational estimates of the net en-ergy flux into the atmosphere over summer continents,a series of numerical experiments are conducted forNorthern Hemisphere summer monsoons for each of theNorth American, Asian, and African regions. The ex-periments are aimed at testing mechanisms that poten-tially limit the poleward extent of the summer seasonconvergence zones. The mechanisms, outlined in CNS


FIG. 9. As in Fig. 8 but with constant surface albedo over Africa for all experiments including(a) the control run for this case.

and CN, are soil moisture feedback, the ventilationmechanism, and the IRH Rossby wave mechanism.From a methodological point of view, we have verifiedconclusions of CNS and CN for a variant of the no-ventilation experiments that maintains conservation andmore clearly separates purely divergent and nondiver-gent components of wind. The experiments designed totest Rossby wave effects do so by suppressing b, thatis, by setting the Coriolis parameter to a constant valuein a target region. However, one might worry that theoriginal experiment could also impact effects of a ‘‘localHadley circulation.’’ By including the b effect on thezonal mean across the target region, we can examineeffects of a local Hadley cell and Rossby wave dynamicsseparately. Qualitatively similar results are found for the‘‘no-ventilation and no-b’’ experiment and the ‘‘no-ven-

tilation and partial-b’’ experiment: the convection zoneextends much farther poleward and less east–west asym-metry is found in either case. Thus it supports the pre-vious focus on Rossby wave dynamics as responsiblefor the east–west asymmetry of the precipitation withinthe continent.

The net flux of energy into the atmosphere, F net issimilar for South America (CN), North America, andAsia for their respective summer seasons: positive F net

extends far poleward in the continents in observationsand simulation. For Africa, positive F net extends less farpoleward because of the strong surface albedo. In theprevious studies for an idealized continent and for theSouth American monsoon, the thermodynamical factorassociated with positive F net is a necessary conditionfor establishment of a convergence zone over conti-


nents, but the dynamical factors determine how far pole-ward the convergence zones can extend. Overall, theimportance of dynamical factors in setting poleward ex-tent of the convergence zone seems to apply in NorthAmerica and Asia as in South America.

Although it is difficult to assign relative importanceto ventilation versus the IRH mechanism, experimentshere suggest that ventilation is very important for eachcase except for Africa with its strong surface albedo.The Rossby wave effect is important in favoring eastcoast rainfall and west coast dryness in North America.Likewise in Asia, Rossby wave dynamics favor in-creased rainfall toward the east and dryness in the in-terior. Because of the drastic changes in each experimentand the nonlinearity of the response, care must be takenwhen interpreting the results. For instance, in NorthAmerica suppressing either ventilation associated withv · =T alone or ventilation associated with v · =q alonehas little effect on extending northward of the conver-gence zone. Either advection term alone can prevent thepoleward extension of the convergence zone. Over Asia,ventilation associated with moisture advection appearsto be more important than the cooling associated withv · =T.

Africa is clearly different than other continents be-cause of the high surface albedo found in large regions.This reduces the positive value of F net over much ofnorthern Africa. Thus, even very modest values of moiststatic energy transport by the nondivergent componentof the flow can balance this energy input and preventconvergence. Even when the dynamical factors, such asventilation and Rossby wave dynamics, are suppressed,little northward extension of the convergence zone isfound. Over the high albedo regions, even a small warm-ing of the land–atmosphere column can cause F net tobecome negative and thus limit poleward extension ofthe convergence zones. Contrasting with the other con-tinents, a first requirement for the summertime polewardmovement of the convection zones is that the simplethermodynamical factor associated with F net should fa-vor convection. In Africa this requirement ceases southof the Sahara. If this first requirement is met, which isthe case even far poleward in Asia and North America,then the dynamical factors determine the poleward ex-tent of the convergence zone.

These experiments suggest a modification to the viewof monsoon systems. Moisture availability is typicallynot the leading order effect in limiting monsoon extent,if other factors favor establishment of a low-level con-vergent circulation. The irrotational (purely divergent)part of the flow is very effective at increasing moisturesufficiently to meet convective criteria, if the moist staticenergy budget terms are such as to sustain the conver-gence. Except for northern Africa, the extent of mon-soon systems is thus set by the transition from a tropicaldynamical regime, in which positive net energy inputinto the column is balanced by rising motion and low-level convergence, to a midlatitude dynamical regime,

in which advection terms vc · =T and vc · =q are ofleading importance in energy transport (where vc is thenondivergent component of the wind). Aspects of thistransition have been previously noted in analysis da-tasets by Barlow et al. (1998) who discuss the transitionfrom a balance of diabatic heating with adiabatic coolingin the monsoon region, and with v · =T in the regionpoleward of the North American monsoon. The resultshere also make clear that vc · =q tends to act to opposepoleward extension of the convection zone. This is be-cause the nondivergent component of the flow cannotincrease moisture above values that occur in the up-stream region, and values over cooler oceans tend to besmaller than the values that would be required to initiateconvection over the warm continent.

A summary of this view of summer monsoon con-vection zones is a follows. Unless the albedo is veryhigh, there is a positive net energy flux into the atmo-spheric column extending far poleward in the continent.This favors low-level converging motions that supplysufficient moisture for a convergence zone up until suchlatitude as ventilation by the nondivergent componentof the flow is able to balance the energy input. Thedivergent circulation is subject to baroclinic Rossbywave dynamics, so the spatial pattern of the convergencezone and descent region are linked. This favors greaterpoleward extension of the convergence zone on the east-ern side of the continent and arid regions west of that.The geometry of a particular continent can have a con-siderable quantitative impact on the interplay of thesemechanisms, but for the large scale this scenario seemsto hold for each of the North American, Asian, andSouth American summer convection zones.

Acknowledgments. This work was supported underNational Science Foundation Grant ATM-0082529, Na-tional Oceanic and Atmospheric Administration GrantsNA86GP2003 and NA86GP2004, National Aeronauticsand Space Administration Grant NAG5-9358, and Na-tional Science Council Grant NSC89-2811-M-034-0001.

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