Journal of the Meteorological Society of Japan, Vol. 88, No. 1, pp. 79--90, 2010.

79

DOI:10.2151/jmsj.2010-106

A Peculiar Feature of the Seasonal Migration of the South American Rain Band

Youichi TANIMOTO and Takushi KAJITANI1
Faculty of Environmental Earth Science and Graduate School of Environmental Science,
Hokkaido University, Sapporo, Japan

Hideki OKAJIMA
Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology,
Yokohama, Japanand
Shang-Ping XIEInternational Paciﬁc Research Center and Department of Meteorology, University of Hawaii, Honolulu, USA(Manuscript received 14 April 2009, in ﬁnal form 28 October 2009)

Abstract
Seasonal migration of tropical rainfall is examined in the South American sector and compared with that overAfrica, using satellite and reanalysis data. While the African rain band moves continuously back and forth acrossthe equator following the seasonal march of the sun, the South American one displays a peculiar asymmetry between its northward and southward migration. The rain band moves gradually northward from October to Julyfrom the Amazon toward the Caribbean Sea, while its return to the Amazon is an abrupt event, with convection
developing rapidly in October around 10 S without going through the equator. Over the equatorial Amazon during July–October, equivalent potential temperature (ye ) is kept low by the easterly advection of low temperatureand humidity air from the equatorial Atlantic, coinciding with the seasonal development of ocean upwelling anda cold tongue in sea surface temperature. The low ye values prevent convection from developing in the equatorial
Amazon while warm SST supports convection on the north coast of South America during August–October.
Meanwhile solar radiation continues to heat up the land surface to the south, eventually triggering the onset ofdeep convection there in October.
Atmospheric general circulation model experiments were conducted to examine the e¤ect of the Atlantic cold
tongue on tropical rain bands. Without the seasonal development of the Atlantic cold tongue, surface ye remains
high in September, and rainfall in the equatorial Amazon switches to a pronounced semi-annual cycle with a peakin each equinox. These results illustrate the role of the Atlantic cold tongue in the peculiar meridional migrationof the observed South American rain band.

1. Introduction
Seasonal migration of tropical rainfall in themeridional direction displays di¤erent characterisCorresponding author: Youichi Tanimoto, Faculty ofEnvironmental Earth Science, Hokkaido University,
N10 W5, Kita-ku, Sapporo 060-0810, Japan.
E-mail: tanimoto@ees.hokudai.ac.jp
1 Present a‰liation: Shimanto City O‰ce
6 2010, Meteorological Society of Japantics over the ocean and land in the hemisphereof 140 W–40 E (Mitchell and Wallace 1992). In
this West Hemisphere including the eastern Paciﬁcand Atlantic, the oceanic intertropical convergencezone (ITCZ) is locked onto the meridional maximum of sea surface temperature (SST), both displaced north of the equator. Only during a brief
period of March–April, SST south of the equator
rises to high enough values for convection. In re-80

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sponse to the seasonal SST warming in the Southern Hemisphere, a double ITCZ astride the equator
develops in the eastern Paciﬁc (Waliser and Gautier
1993; Lietzke et al. 2001), while the Atlantic ITCZ
reaches its southernmost location and in the western basin, connects with the rain band on northeast
South America (Mitchell and Wallace 1992). The
development of the climatic asymmetry over theeastern Paciﬁc and Atlantic is a result of oceanatmosphere interactions triggered by latitudinal
asymmetry in continental geometry (Xie and Philander 1994).
Over continents, by contrast, tropical rain bands
tend to follow the seasonal march of the sun because of the small heat capacity of soil layers involved in the seasonal cycle. Combining observations over the American and African continents,
Mitchell and Wallace (1992) show that deep convection over land moves back and forth across theequator between 15 north and south. This equatorially symmetric migration of the continental ITCZ
is conﬁrmed by more recent observations based on
a combined satellite-rain gauge data set (Kodama
and Tamaoki 2002). This symmetric meridional
movement of the rain band is pronounced only inthe tropical ITCZ within 25 N/S, but not associated with other rain bands such as the South Paciﬁcand South Atlantic convergence zones (Kodama
1992, 1993).
South America and Africa feature very di¤erent
geometries. In 10–30 E, the African Continent
occupies the entire tropics on both sides of theequator, allowing the rain band to move across theVol. 88, No. 1

equator in this land strip. The South American
Continent, on the other hand, faces the Atlantic
Ocean to its north. Because of large di¤erencesin heat capacity between South America and theocean to its north, the meridional migration of thetropical rain band may di¤er between South America and Africa. Indeed, based on in-situ rain gauge
and OLR data sets, previous studies (Horel et al.
1989; Marengo et al. 2001; and references therein)pointed out a rapid southward migration during
August–October before the onset of the rainy season in the central Amazon, while a northwardmigration of the rain band is gradual during
February–April. Though some discrepancies between in-situ and OLR data in location and intensity of the tropical rain band are reported as in
Marengo et al. (2001), we conﬁrm this peculiar
movement of the rain band over the South America
by using new satellite observations and by contrasting these continents that face each other across the
Atlantic Ocean. Rainfall in the South American
sector displays highly asymmetric migration acrossthe equator, moving gradually from the continent
toward the North Atlantic Ocean but featuring a
discontinuous jump in the other direction (Fig. 1a).
As a mechanism for an abrupt onset of the rainyseason in the Amazon during austral summer(November–January), Williams and Renno (1993)
and Fu et al. (1999) showed, based on the in-situ
soundings, that a local increase of equivalent potential temperature (ye ) in the planetary boundary
layer (PBL) is necessary for the seasonal onset of
convection. As stated in Fu et al. (1999), increases

Fig. 1. Latitude-time sections of rainfall (contour at 50 mm month1 intervals) based on the TRMM 3B43
product and JRA-25 equivalent potential temperature at the surface (colors in K) in (a) the South American(60–50 W), (b) Atlantic (20–10 W), and (c) African (20–30 E) sectors. Note that two annual cycles arepresented. The longitudinal bands are chosen to avoid high orography of the Andes and African Rift Valley. The Atlantic (African) sector is all ocean (land).

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Y. TANIMOTO et al.

of land surface temperature alone cannot initiate
the convection, unlike the increases in SST over theocean. Given lack of soil moisture before the onset,
the small heat capacity of land surface enhances
land surface warming, but cannot increase moist
static energy near the surface, probably due to insu‰cient moisture supply from the surface. Fu
et al. (1999) suggested that changes in large-scaleatmospheric circulation are responsible for increased ye in the PBL by moisture transport into
the southern Amazon before the onset, while local
latent and sensible heat ﬂuxes play minor roles inmoistening the lower atmosphere. Wang and Fu(2002) showed that intraseasonal northerly winds
in the lower atmosphere intermittently transportmoisture into the southern Amazon before the onset. Meanwhile, Fu et al. (1999) suggested that thesurface heat ﬂuxes are indirectly responsible forthe onset by forming strong land-sea contrast in
the lower atmosphere that drives large-scale surface
ﬂow toward land. Indeed, Fu and Li (2004) and Li
and Fu (2004) showed, based on reanalysis data,that surface heat ﬂuxes over the Amazon begin to
increase about 2 months before the large-scale
northerly winds initiate the transition from the dry
to wet season. They also showed, even before thewet season (rain rate > 8 mm day1 ), that the latent heat ﬂux (80–100 W m2 ) is larger than sensible heat ﬂux (30–40 W m2 ) because of weak rainfall (@3 mm day1 ) in the Amazon. Several studies
pointed out that intense subtropical rainfall east of
the Andes is provided by the passage of several synoptic frontal systems before its onset (Garreaud
and Wallace 1998; Li and Fu 2006; Raia and Cavalcanti 2008), accompanied by intrusions of theextratropical cold air. Over the subtropical SouthAmerica, the cold air intrusion in the entire troposphere tends to increase relative humidity and thebuoyancy in the lower troposphere, and the associated rainfall helps increase the soil moisture and latent heat ﬂux. These features can help establish an
increased ye in the PBL as well as the large-scalesurface ﬂow.
These previous studies revealed that land surface
processes contribute to forming favorable conditionof convection over the Amazon basin in two di¤erent ways. One is a local e¤ect in an increase of ye inthe lower atmosphere by heat ﬂuxes. Another is indirect. A strong land surface warming forms a large
gradient of temperature in the lower atmospherebetween land and ocean. This surface temperature
gradient thermally drives large-scale circulation

81

that provides moisture into the inland southern
Amazon. Although these two e¤ects set a necessary
condition for the rapid onset of convection over the
Amazon basin, they do not fully explain how themeridional distribution of ye in the lower atmosphere is related to the peculiar meridional movement of rain band over the entire South America.
The present study examines how large-scale ye inthe meridional direction over the South America
modulates the seasonal migration of the rain band
based on observational data. Our analysis leads toa hypothesis that the equatorial asymmetry in themeridional migration of South American rainfall isclosely related to meridional gradients of ye , which
are a¤ected by the seasonal cold tongue in theequatorial Atlantic Ocean as well as land-sea contrast in heat capacity. We will further examine theinﬂuences of the Atlantic cold tongue on ye overSouth America by evaluating atmospheric generalcirculation model (AGCM) experiments with andwithout the seasonal modulation of the Atlantic
cold tongue.
The rest of the paper is organized as follows. Section 2 describes the datasets and methods employedin the present study. Sections 3 and 4 present theresults of our observational analysis and AGCM
experiment, respectively. Section 5 is summary anddiscussion.
2. Method
2.1 Observational analysis
We use the Tropical Rain Measuring Mission(TRMM) satellite monthly accumulated rainfall
dataset (3B43 product) from January 1998 to December 2007, available on a 0.25 grid. To examine
thermal structures in the lower atmosphere, we also
use the monthly Japanese reanalysis datasets (hereafter JRA-25, Onogi et al. 2007), available on 2.5
grid, for the same period as the TRMM rainfall.
The variables include temperature, water vapor
mixing ratio, geopotential height and wind velocity.
Based on these data, monthly climatologies for the
10-year period of 1998–2007 are calculated for thefollowing observational analysis.
2.2 AGCM experiment
To seek key processes involved in the meridionalmigration of the tropical rain band, we use the
AGCM jointly developed at Center for Climate
System Research (CCSR) of The University ofTokyo and National Institute for Environmental
Studies (NIES). The model resolution is a triangu-

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lar truncation at 42 wave numbers (T42) in the horizontal and 20 sigma-levels in the vertical. Realistic
continental distribution and topography are given
to the model. Standard physical processes such as
the modiﬁed Arakawa-Schubert cumulus parameterization (Arakawa and Schubert 1974), radiativetransfer (Nakajima and Tanaka 1986), and abucket model for land surface processes (Manabe
et al. 1965) are incorporated. Further description is
documented in Numaguti et al. (1995), Shen et al.
(1998), and Okumura and Xie (2004).
To capture the e¤ect of the Atlantic cold tongue
on the rain band over the South American and African continents, we conduct two experiments. One
is a control experiment run with the atmosphericmodel intercomparison project (AMIP I) monthly
SST and sea ice (Gates 1992). The other is a ‘‘noequatorial annual cycle experiment’’, with the Atlantic equatorial annual cycle in SST reduced to its
annual mean. Both experiments start from a same
initial condition. Further details of this experiment
are later documented in Section 4. To calculate theclimatology in each of the experiments, output for
the last 16 out of 20 years is employed.
3. Observational ﬁndings
3.1 Rainfall
Figures 1b, 1c contrast the seasonal cycle of rainfall between the all-ocean Atlantic (20–10 W) andall-land African (20–30 E) sectors, based on the
TRMM monthly climatology for the 10-year period of 1998–2007. Conﬁned to the north of theequator year round, the rainfall maximum in the
Atlantic sector migrates slowly and smoothly between 2–3 N and 10 N, reaching the southernmost
and northernmost latitudes in March and August,respectively. The African rain band, on the other
hand, displays much larger swings in the meridional direction, with its maximum moving backand forth across the equator between 15 S and
5 N following the sun (the 100 mm month1 contour encompasses a larger latitudinal range of20 S–15 N). The northern reach of the African
rainfall is much smaller than the southern one presumably because of dry land surface conditions in
the Sahara desert. In a broad sense, the seasonal
cycle of the African rain band is symmetrical between its northward and southward migrations1,
smoothly crossing the equator. The above characteristics of the seasonal migration of tropicalrainfall over land and ocean are consistent with
Mitchell and Wallace (1992).

Vol. 88, No. 1

The seasonal cycle of rainfall in the South American (60–50 W) sector is very di¤erent, highly
asymmetric between its northward and southwardmigrations (Fig. 1a). The rain band moves gradually northward from October to July, shifting therainfall maximum from 12 S to 10 N across theequator. The equator experiences the maximum
rainfall in March–April. This gradual northwardmigration of rainfall over South America is similar
to that over Africa. What happens in and after August is very di¤erent between the two continents.
From August to October, the marine ITCZ isweak but a large rainfall maximum develops suddenly around 10 S over South America (Fig. 1a),the latter corresponding to the abrupt onset of theSouth American monsoon (Gan et al. 2004; Zhou
and Lau 1998; Horel et al. 1989). On horizontal
maps, rainfall is found on the northern edge ofSouth America in August (Fig. 2a) but spreads
abruptly southward to 10–20 S in October (Fig.
2b). This South American rainfall maximum continues to intensify and begins moving northward
during October–December. North of the equator,
there is a secondary rainfall maximum that originates from the marine ITCZ in August, shows atendency to move southward from August to January, and eventually merges near the equator in
January with the northward-migrating major rainband (Fig. 1a). The lack of a gradual southwardmigration of the major rain band causes an asymmetry between equinoxes: rainfall is at its maximum in March–April but minimum in September–
October near the equator. In the Amazon basinsouth of the equator, rainfall displays a pronounced
annual cycle with the rainy season during October
to April. Between 5 S and the equator on the African Continent, by contrast, rainfall peaks twice ayear associated with the northward and southwardmigrations of the rain band.
We have examined TRMM rainfall in individualyears and the above di¤erences between the Africanand South American sectors are present every year
during 1998–2007 (not shown), characterized by
the presence and lack of a gradual southwardmigration of the major rain band, respectively. The
August-to-October abrupt jump in the major rainfall maximum in the South American sector is in1

The African rain band also shows a weak asymmetry
in its meridional migration, probably related with theWest African Monsoon, but the present study focuseson larger asymmetry in the South American sector.

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Y. TANIMOTO et al.

Fig. 2. Equivalent potential temperature(colors in K) at the surface based on the
JRA-25 and TRMM rainfall (contours at
100 mm month1 intervals) in (a) August,(b) October, and (c) their di¤erences.

sensitive to the choice of longitudinal span for average and is still present in a wider longitudinal average of 70–40 W band (not shown). The next section examines the factors giving rise to theasymmetry between the northward and southwardmigrations of the South American rain band.
3.2 Equivalent potential temperature
The marine ITCZ over the Atlantic tends to follow the meridional SST maximum with its seasonal
march. The continental rain band, however, does
not always form over the local maximum of land

83

surface temperature because soil moisture is also afactor. Often increased land surface temperature isa result of lack of rainfall and soil moisture to evaporate water from the ground and balance solar
radiation heating. Here we examine equivalentpotential temperature (ye ) at the surface, which is afunction of both temperature and humidity and agood measure of atmospheric moist stability. Highye favors deep convection. Indeed in Fig. 2, meridional maxima in ye derived from the JRA-25
monthly climatology for the 10-year period of
1998–2007 tend to be collocated with rainfall maxima, over both land and ocean.
In Fig. 1, the latitude-time diagrams of ye in theAmerican, Atlantic and African sectors are superimposed on TRMM rainfall in contours. In the Atlantic sector with relative humidity around 80%,
SST dictates near-surface ye . The marine ITCZ
tends to follow the meridional maximum of ye(Fig. 1b).
In the South American sector, there is an asymmetry between the northward and southward migrations of the ye maximum (Fig. 1a). From December to June, there is one single meridional
maximum in ye and it moves gradually northward
together with the rainfall maximum. The situation
becomes complicated from July to November, during which the ye maximum hardly ever moves
southward as the sun does. The northern maximum
is associated with the marine ITCZ and has greater
values than the southern one until the latter develops in November. The intensiﬁcation of the southern ye maximum is due to the seasonal increase insolar radiation in the Southern Hemisphere andthe resultant increase in surface temperature. The
southern ye maximum eventually exceeds the northern maximum under the marine ITCZ, triggering
deep convection that signals the onset of the SouthAmerican monsoon in November. During this transition period of July–November, the northern ye
maximum remains nearly constant at 348 K due tolarge thermal inertia of the ocean. Meanwhile thesouthern maximum rapidly increases from below
339 to 344 K.
On horizontal maps, maximum ye at the surface
in August is found near the north coast of SouthAmerica and the Caribbean Sea, with ye decreasing
rapidly southward in the Amazon basin (Fig. 2a).
While varying little in the tropical ocean and northern South America, ye increases rapidly from August to October over the Amazon (Fig. 2b). The
October–August di¤erence ﬁeld (Fig. 2c) shows84

Journal of the Meteorological Society of Japanthat large rainfall increase over the region from theAmazon to the subtropical South America east of
the Andes is associated with a local increase in ye
but there is little change in ye near the northerncoast of South America and the Caribbean Sea. In
October when land convection begins to develop
around 10 S, surface ye there is still much smaller
than over the Caribbean in monthly mean. The diurnal cycle is much larger over land than ocean,likely to play a role in initiating land convection
south of the equator in October. Indeed, Kikuchi
and Wang (2008) showed a large diurnal variation
and an afternoon peak in rainfall over South America. Our analysis of JRA-25 data shows that daytime maximum in land surface ye is 6–9 K above
the daily mean at 10 S in October. In addition to
the daytime increase in land surface ye , the daytime
PBL is much deeper over land than ocean. Increased ye in the well-mixed deep PBL over land
leads to an increase in Convective Available Potential Energy (CAPE), which favors October convection over South America around 10 S.
During the South American monsoon from December to June, the ye maximum in and south ofthe continental rain band is well above 344 K (Fig.
1a). The slight poleward displacement of the ye
maximum relative to the continental rain band
may reﬂect that the interaction between soil moisture and inland-advancing rain band is not in equilibrium due to slow adjustments of ground hydrology. Dry ground limits local moisture supplies fordeep convection. The gradual wetting of the groundslows the rain band’s inland advance behind thesun’s seasonal march (Xie and Saiki 1999).
Near equatorial South America, ye remains low
below 344 K from July through November, preventing the rain band from migrating southward
across the equator (Fig. 1a). Then what maintains
low ye values near equatorial South America? To
answer this question, we need to consider variationsin the east-west direction. Figure 3 shows the seasonal cycle of ye and wind velocity at 925 hPa meridionally averaged in 5 S–5 N from South America
all the way to the African Coast. The 925 hPa levelis within the planetary boundary layer from which
deep convection originates, and it is above most orography to allow us to see zonal variations of ye
without the topographic bias. Over the equatorial
Atlantic, ye reaches an annual minimum in July because of intensiﬁed ocean upwelling in response to
the increased southeast trades (Mitchell and Wallace 1992; Okumura and Xie 2004). During June–

Vol. 88, No. 1

Fig. 3. A longitude-time section of JRA-25
equivalent potential temperature (colors inK) and wind velocity (arrows in m s1 ) at
925 hPa averaged over the 5 S–5 N band.

October, ye is much lower over the ocean (low intemperature and speciﬁc humidity) than over land,
and the inland advection by the prevailing easterlies
keeps ye low over equatorial South America. During February–April, the land-sea ye contrast ismuch relaxed, with ocean upwelling suppressed
and ye increasing over the equatorial Atlantic. The
annual cycles in upwelling cooling and ye appear to
be a key to the di¤erences between the March and
September equinoxes in ye over equatorial SouthAmerica. In March, ye is high enough to support
deep convection over equatorial South America
but it is not in September. The next section examines the e¤ect of the Atlantic cold tongue on theSouth American rainfall with AGCM experiments.
4. AGCM experiment
4.1 Experimental design
The observed annual cycle of tropical Atlantic
SST is characterized by the equatorial cold tongue(below 26 C) development east of 25 W during
June–October (Fig. 4a). Similar to the SST annual
cycle, ye at 925-hPa in the ocean east of 40 W
shows the cooling in the cold tongue season (Fig.
3). We conduct the control AGCM simulation(CTRL) with prescribed monthly climatological
SST. We then conduct an AGCM experiment
forced by the same SST outside of the tropical Atlantic but with no equatorial annual cycle (NEAC)
in the Atlantic. Speciﬁcally, we apply a meridional

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Y. TANIMOTO et al.

85

Fig. 4. (a) A longitude-time section of the climatological annual cycle in AMIP SST (contours at 0.5 C intervals) in the equatorial Atlantic (4 N–4 S) as the prescribed SST ﬁeld for the CTRL run. (b) A latitudinal
plot of the annual mean factor applied to the tropical Atlantic SST. This factor is applied for the No-Equatorial-Annual-Cycle. (c) As in (a), but for the NEAC SST (contours at 0.5 C intervals). Shades with white
contours at 0.3 C intervals are di¤erences of SST between the CTRL and the NEAC experiments. (d) As in(c), but for a latitude-time section zonally averaged for 50 W–10 E.

weight function (Fig. 4b) to tropical Atlantic SST
in order to reduce the equatorial annual cycle. The
weight function, að yÞ, is set as below:að yÞ ¼ exp½ð y=10Þ 2 ; 30 < y < 30;
where y is a latitude in degree. The SST for the
NEAC experiment, Tsneac , is set as below:
Tsneac ðtÞ ¼ ½1  að yÞTsctrl ðtÞ þ aðyÞTsctrl ;where Tsctrl ðtÞ is the AMIP I monthly climatology
of SST for the CTRL experiment and Tsctrl is theannual mean of Tsctrl ðtÞ. For example, Tsneac ðtÞ ¼
Tsctrl at the equator where aðyÞ ¼ 1, andTsneac ðtÞ ¼ Tsctrl ðtÞ in the mid- and higher latitudes where að yÞ ¼ 0. With this modiﬁed SST ﬁeld
in the tropical Atlantic, the annual cycle is strongly
reduced in the equatorial Atlantic (4 S–4 N, Figs.
4c and 4d) but remains largely unchanged polewardof 10 N/S on either side of the equator (Fig. 4d).
To see the cold tongue’s inﬂuence on ye and rainfall
in September equinox near equatorial South America, we examine the di¤erence between the CTRL
and NEAC experiments. Both experiments startfrom the same, isothermal, and static initial condition, and the last 16-year results out of 20-year runs
are analyzed for model climatology.
4.2 E¤ect of Atlantic cold tongue on Amazon
rainfall
The CTRL experiment successfully reproduces
the observed meridional migration of the tropicalrain band in South America, Atlantic, and African
sectors. Over equatorial South America (Fig. 5a),the simulated rainfall maximum is located overhigh surface ye (354 K@) in March equinox and its
minimum over low surface ye (@344 K) in Sep-

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Vol. 88, No. 1

Fig. 5. Latitude-time sections of simulated (CTRL experiment) equivalent potential temperature at the surface (colors in K) averaged over the (a) South American (60–50 W), (b) Atlantic (20–10 W), and (c) African (20–30 E) sectors. Superimposed are of the 100, 150, 200 and 250 mm month1 contours for rainfall.

tember equinox. As a result, the annual cycle—
strong northward and weak southward movementsof the rain band and surface ye in particular—iswell reproduced in the CTRL simulation.
The year-round, northward displacement of therain band over the tropical Atlantic (Fig. 5b) is
well simulated in the CTRL. The African rain bandin the CTRL is of comparable intensities for thenorthward and southward movements throughthe annual cycle as observed (Figs. 1c, 5c), while
the rainfall maximum during austral summer(November–January) in 5–15 S is 2–3 times larger
than in TRMM observations. These rain bands
mostly follow the local maximum of surface ye ,but surface ye values over the land in the CTRL
tend to be higher by 2–6 K than in the JRA-25,
probably due to an unrealistic representation ofsoil moisture in a bucket model. High surface ye
destabilizes the atmosphere and causes excessive
rainfall.
In the NEAC experiment, the rain bands over
the Atlantic and African sectors mostly retain theircharacteristic features as mentioned in the CTRL
experiment, albeit with slightly enhanced local
maximum (Figs. 6b, c). The only major di¤erence
over the Atlantic sector is the reduced seasonal
cycle of surface ye due to suppressed SST variations(Figs. 6b, e). As a result, the maximum in March–
April over the equator is much reduced (Figs. 6b,e). The reduced annual cycle over the Atlantic
causes no major di¤erence in the rain band overAfrican between the experiments (Figs. 6c, f ).
These results show that the Atlantic cold tongue’s
inﬂuence does not extend far eastward.

By contrast, the meridional migration of theSouth American rain band in the NEAC experiment is very di¤erent from that in the CTRL experiment. Although SST modiﬁcation is limited to theequatorial Atlantic, the annual cycle in surface
ye nevertheless weakens in NEAC over equatorial
South America (Figs. 6a, d). This illustrates astrong inﬂuence of equatorial Atlantic SST on theannual cycle in surface ye over equatorial SouthAmerica, probably due to the advection by themean easterly trades. Speciﬁcally, surface ye inMarch–May (September–November) over equatorial South America in the NEAC experiment islower (higher) by 3 K, than in the CTRL (Fig. 6d),comparable in magnitude to surface ye di¤erence
over the equatorial Atlantic between the experiments (Fig. 6e). The maximum (minimum) in surface ye di¤erences in the March (September) equinox leads to an increase (decrease) in rainfall by
50–75 mm month1 (Fig. 6d). In the NEAC experiment, these changes enhance (weaken) discontinuity in the South American rain band in the March(September) equinox. As a result, the South American rain band encompassed by 100 mm month1
contours in Fig. 6a is not so substantially di¤erent between the northward and southward migrations. Rainfall in 5–12 N decreased (increased) in
March–May (August–September) in the NEAC experiment, compared to the CTRL experiment (Fig.
6d). This o¤-equatorial di¤erence in rainfall is not
fully associated with changes in local surface ye .
Therefore, correlation between ye and rainfall is
weaker in Fig. 6a than in Fig. 5a, particularly in
the Northern Hemisphere. The changes in equato-February 2010

Y. TANIMOTO et al.

Fig. 6. (a–c) As in Fig. 5, but for the NEAC experiment. (d–f ) As in (a–c), but for the di¤erence of equivalent potential temperature at the surface (colors in K) and rainfall (contours at 25 mm month1 intervals).

rial conditions can modify the strength of the local
Hadley circulation and bring about remote impacts
on the o¤-equatorial rainfall. Gan et al. (2004)
showed the inﬂuence of the local Hadley circulation on the rainy season onset over western-central
Brazil.
Very low values of surface ye (< 344 K) overequatorial South America in September–October
are found in the CTRL but not in the NEAC experiment, while a local maximum of surface ye over
the Caribbean Sea (5–20 N) for the same period isfound in both experiments. These results suggestthat large thermal inertia over the Caribbean Sea
is not directly responsible for the local minimum of
surface ye and resultant dry spell over the Amazon
in the September equinox. Rather, the Atlantic cold
tongue is a source of low surface ye , which a¤ects
equatorial South America in September–October
via advection. Figure 7 displays the di¤erence ﬁelds
in the surface ye , wind velocity and rainfall averaged in 5 S–5 N between the CTRL and NEAC
experiments. The cold tongue in the far eastern Atlantic begins to form in July with the onset of theFig. 7. A longitude-time section of di¤erences equivalent potential temperature(colors in K), wind velocity (arrows inm s1 ) and rainfall (contours at 25 mmmonth1 intervals) at the surface averaged
latitudinally for 5 S–5 N between the
CTRL and the NEAC experiments.

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Journal of the Meteorological Society of Japan

African Monsoon and propagates westward by airsea interaction (Okumura and Xie 2004). The thermal advection by the mean easterly trade begins toform anomalous low ye at the surface in August
over 45–65 W, a¤ecting the annual cycle of SouthAmerican rainfall. This negative di¤erence in thesurface ye remains until the following February.
Intense solar radiation in the March equinox increases surface ye and destabilizes the overlying atmosphere, initiating the wet spell over the Amazon.
In April–July, the thermal advection from theAtlantic helps increase surface ye over equatorial
South America (Fig. 7), giving rise to a discontinuity in the northward migration of the rain band in
the NEAC experiment (Fig. 6a).
Fu et al. (2001) drew a similar conclusion thatseasonal variations in tropical Atlantic SST are thekey to the dry season of eastern equatorial Amazon
at the September equinox, but they based the conclusion on di¤erent experiments that allow the SST
seasonal cycle over the broad tropical Atlantic between 30 S and 30 N, and set SST elsewhere at theannual mean values. Our experimental design enables the isolation of the seasonal development of
the equatorial cold tongue as the cause of suppressed Amazon convection during September–
October.
5. Summary and discussion
We have examined the seasonal migration of
rainfall in the South American, Atlantic and African sectors. While the African rain band moves
gradually both north- and southward following theseasonal march of the sun, the South American rainband displays a unique asymmetry between thenorth- and southward transitions. While its northward migration is a gradual process as over Africa, the South American convection center jumps
abruptly southward from August to October, signaling the onset of the South American monsoon.
Over both land and ocean, atmospheric convection
centers are found over regions of large equivalentpotential temperature (ye ) at the surface. During
July to October, the surface ye over equatorial
South America is kept low by the easterly advection of low ye air cooled by intense ocean upwelling over the equatorial Atlantic. This low ye overthe equatorial Amazon appears to be critical topreventing deep convection from migrating fromthe Atlantic ITCZ southward across the equator.
Meanwhile, the continuing increase in solar radiation south of the equator keeps raising ground tem-Vol. 88, No. 1

perature and ye , eventually triggering deep convection around 10 S in October.
To identify the e¤ect of ocean upwelling on theSouth American rain band, we conducted a pairof AGCM experiments with and without the annual cycle in equatorial Atlantic SST. The results
support the hypothesis that the advection of low
surface ye from the equatorial Atlantic by meaneasterly trades causes the dry spells during
September–October in the equatorial Amazon in
the CTRL simulation. There may be interactions
between equatorial Amazon rainfall and the Atlantic cold tongue. The present study shows inﬂuencesof the cold tongue on South American rainfall during the equatorial cold season, and recent analysesof coupled model simulations indicate that Amazon
rainfall during the warm season (March–May) affects the subsequent development of the equatorial
cold tongue (Chang et al. 2007; Richter and Xie
2008).
Factors other than equatorial SST may also contribute to, such as the land-sea contrast in heat capacity. The role of thermal contrast between SouthAmerica and the Caribbean/North Atlantic needs
to be further explored. Let us compare March and
September equinoxes. In March, the South American rain band is located near the equator and inno hurry to jump northward as Caribbean andNorth Atlantic SST to the north is still cool. As aresult, the northward migration of rainfall can be
gradual and slow. In September, by contrast, Caribbean and North Atlantic SST is at its annual
maximum preventing convection from taking placepre-maturely over the continent to the south and allowing ye to build up for an abrupt onset of land
convection over South America. This may contribute to the abrupt onset of land convection overSouth America, aided by cold air intrusions fromthe extratropics (Garreaud and Wallace 1998; Li
and Fu 2006). While our focus is on the climatological annual cycle, Grimm et al. (2007) and Grimm
and Zilli (2009) showed, in the context of interannual variability, that previous conditions in austral
spring in terms of soil moisture and near surface
temperature may inﬂuence the peak monsoon rainfall over South America.
Acknowledgments
The authors wish to thank Drs. H. Matsuyama,
M. Watanabe, F. Hasebe, K. Yamazaki, and H.
Hatsushika for useful discussion, and CCSR/NIES
model developers for the model code. Model ex-February 2010

Y. TANIMOTO et al.

periments were performed on IPRC SGI servers
and JAMSTEC NEC SX-8R. This work was supported in part by the Global Environment Research Fund (S-5) of the Ministry of the Environment, Japan, Grand-In-Aid for ScientiﬁcResearch defrayed by the Ministry of Education,
Culture, Sports, Science and Technology of Japan(18204044), NOAA, NASA, and JAMSTEC. This
is IPRC publication 655 and SOEST publication
7850.
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