15 MAY 2023

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MASUNAGAThe Edge Intensification of Eastern Pacific ITCZ Convection
HIROHIKO MASUNAGAaaInstitute for Earth-Space Environmental Research, Nagoya University, Nagoya, Japan(Manuscript received 18 May 2022, in ﬁnal form 28 January 2023)

ABSTRACT: Tropical precipitation is climatologically most intense at the heart of the intertropical convergence zone
(ITCZ), but this is not always true in instantaneous snapshots. Precipitation is ampliﬁed along the ITCZ edge rather thanat its center from time to time. In this study, satellite observations of column water vapor, precipitation, and radiation as
well as the thermodynamic ﬁeld from reanalysis data are analyzed to investigate the behavior of ITCZ convection in lightof the local atmospheric energy imbalance. The analysis is focused on the eastern Paciﬁc ITCZ, deﬁned as the areas wherecolumn water vapor exceeds 50 mm over a speciﬁed width (typically 400–600 km) in the domain of 208S–208N, 1808–908W.
The events with a precipitation maximum at the southern and northern edges of the ITCZ are each averaged into composite statistics and are contrasted against the reference case with peak precipitation at the ITCZ center. The key ﬁndings are
as follows. When precipitation peaks at the ITCZ center, suppressed radiative cooling forms a prominent positive peak inthe diabatic forcing to the atmosphere, counteracted by an export of moist static energy (MSE) owing to a deep vertical advection and a large horizontal export of MSE. When convection develops at the ITCZ edges, to the contrary, a positive
peak of the diabatic forcing is only barely present. An import of MSE owing to a shallow ascent on the ITCZ edges presumably allows an edge intensiﬁcation to occur despite the weak diabatic forcing.
KEYWORDS: Atmosphere; Intertropical convergence zone; Convection; Satellite observations

1. Introduction
The intertropical convergence zone (ITCZ) is an extensive
area of enhanced moisture and precipitation, characterizing thelarge-scale patterns of low-latitude weather. The ITCZ has been
known since the early years of modern meteorology to occur
preferentially in certain geographical regions (e.g., Crowe 1951).
The eastern Paciﬁc ITCZ is particularly striking in that a narrow
band of the ITCZ extends from east to west all the year aroundon the northern side of the equator. This meridional asymmetryof the eastern Paciﬁc ITCZ has motivated numerous studies tounveil the mechanisms behind it (e.g., Mitchell and Wallace
1992; Xie and Philander 1994; Philander et al. 1996; Li 1997; Xie
2004; Kang et al. 2008; Masunaga and L’Ecuyer 2011). All these
theories are built upon air–sea interactions, while otherwise different from one another, in explaining the climatological patternof the eastern Paciﬁc ITCZ.
The ITCZ morphology on short time scales, on the other
hand, is affected primarily by day-to-day weather disturbances.
Figure 1 presents selected satellite snapshots of column water
vapor (CWV) and surface precipitation from Advanced Microwave Scanning Radiometer for the Earth Observing System(AMSR-E) observations. The eastern Paciﬁc ITCZ forms aband of moist air in all cases, but varies in width and latitudinallocation in a case dependent manner as it extends to the east of
the date line. Another notable feature in Fig. 1 is that the areasof heavy precipitation, marked by reddish colors, are occasionally conﬁned to along the edge of ITCZ rather than at the heart

Denotes content that is immediately available upon publication as open access.
Corresponding author: Hiro Masunaga, masunaga@nagoya-u.jp

of it. Edge intensiﬁcation events are found on the northernedge of the ITCZ in some cases (1608–1408W on 14 July 2006 and
1308–1158W on 1 September 2008) and on the southern edgein others (1558–1408W on 17 December 2006 and 1108–958W on
27 June 2009). The edge intensiﬁcation of convection may be
counterintuitive in that the ITCZ is exposed to dry subtropical air
at the edges, where convection is more likely to be suppressedthan invigorated (e.g., Neggers et al. 2007). The cases presented
here were chosen subjectively from the daily snapshots includingedge intensiﬁcation events, but in fact similar events are not rareas will be shown later in section 2b.
The edge intensiﬁcation of ITCZ convection has not been
well documented in the literature. A careful survey of radiative–
convective equilibrium (RCE) simulations, nonetheless, reveals
that convection can intensify near the edge of convective envelopes (Windmiller and Hohenegger 2019, see also their introduction for review). An enhancement of convective activity
is sometimes found along the margins of the moist tropics
from satellite observations (Mapes et al. 2018). Windmiller
and Hohenegger (2019) argued that the edge intensiﬁcation
in idealized RCE simulations arises from a collective effectof consecutively forming cold pools. This study explores themechanism behind the edge intensiﬁcation of ITCZ convection in the eastern Paciﬁc.

2. Data and method
a. Data
The present analysis is based on A-Train satellite observations and reanalysis data. The A-Train constellation comprises asuite of satellites ﬂying in formation including Aqua, CloudSat,
and CALIPSO. Column water vapor (CWV), surface precipitation, sea surface temperature (SST), and near-surface wind at
10-m height (U10) are obtained from the Aqua AMSR-E daily

DOI: 10.1175/JCLI-D-22-0382.1
Ó 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright
Policy (www.ametsoc.org/PUBSReuseLicenses).
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FIG. 1. AMSR-E snapshots of CWV (green-to-blue shading) overlaid by surface precipitation(yellow-to-red shading) over the eastern Paciﬁc for selected dates. PM and AM in the title ofeach panel refers to the ascending (1330 local time) and descending (0130 local time) tracks,
respectively. The date line is marked by a thick dotted line for reference.

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15 MAY 2023

MASUNAGA

data products produced by Remote Sensing Systems (RSS)(Wentz 2013; Hilburn and Wentz 2008). Latent heat ﬂux (LHF)
from sea surface is evaluated from SST and U10 using the bulk
equation (Large et al. 1994), combined with an empirical formula determining the near-surface water vapor mixing ratio as afunction of CWV and SST (see the appendix of Masunaga and
L’Ecuyer 2010). The CWV, SST, and U10 estimates are unavailable in heavily raining areas and are each interpolated fromneighboring measurements wherever missing due to precipitation. Microwave measurements of SST and U10 tend to be missing in the ocean surfaces covered by signiﬁcant rainfall, while
CWV is affected only by extreme cases. As a result, LHF may
rely to a certain extent on the interpolated ﬁeld around the precipitation peak, but CWV does so only to a very minor degree.
The column radiative heating is obtained from the net radiative
ﬂuxes at the top and bottom of the atmosphere provided by the
Aqua CERES Level-2 Single Scanner Footprint (SSF) Edition
4A dataset (Su et al. 2015). The vertical structure of cloud radiative effect (CRE) is provided by the CALIPSO, CloudSat,
CERES, and MODIS (CCCM) Release D1 product (Kato et al.
2011).
The vertical proﬁles of air temperature (T), water vapor
mixing ratio (qy ), horizontal wind (u and y ), and vertical pressure velocity (v) are taken from the ERA5 data (Hersbach et al.
2020). The ERA5 variables, originally stored as hourly time series, are interpolated over time to the 0130 and 1330 local time
A-Train ground tracks from the two temporal neighbors. All
the satellite and ERA5 data are matched together on a 0.258
grid where AMSR-E and CERES data are both available. As
an exception, the CCCM composite plots are constructed separately with nadir tracks only.
The data period spans 4.5 years from 1 July 2006 to
31 December 2010. The target region is the tropical eastern Paciﬁc as deﬁned by 208S–208N, 1808–908W with islands excluded.
The portion of the Isthmus of Panama and the Caribbean Sea
that falls in this domain is also discarded from the analysis. The
ERA5 air temperature is averaged at each pressure level overthe same period and region as above to obtain the climatologicalstability proﬁle required for the vertical normal mode transform.

b. Compositing method
A composite analysis is carried out to explore the statisticalcharacteristics of the edge intensiﬁcation. The methodology is
described below.
Different ways have been proposed to objectively identify
the ITCZ from observations (e.g., Wodzicki and Rapp 2016,
and references therein). The current study adopts a new, simple approach based on a ﬁxed CWV threshold of 50 mm. Thisparticular value was inspired by the 48-mm margin separating
the moist tropics from the dry subtropics (Mapes et al. 2018),
which has a physical basis in that a CWV of 48 mm roughly
corresponds to the neutrality in the atmospheric energy budget (Masunaga and Mapes 2020). The ITCZ is required to bemoister than 50 mm, visualized as blue-colored areas to contrast with the surrounding, green-colored regions in Fig. 1.
Sequences of A-Train measurements are sampled in the direction parallel to the subsatellite track across the full width3471

of the 1450-km-wide AMSR-E swath, with the exception of thenadir-only CCCM measurements. Since the Aqua satellite is apolar orbiter with an orbital inclination of about 988, each sampled sequence follows a quasi-north–south path. The ITCZ isdeﬁned by the along-track segments where CWV continuously
exceeds 50 mm. The southern (northern) edge of the ITCZ isthe point where CWV increases northward (southward) across
50 mm, with the midpoint between the southern and northernedges being the ITCZ center. In case where multiple bands of
ITCZ are identiﬁed along the same satellite track, they aretreated as individual samples, whether they are genuinely separate (e.g., double ITCZs) or different portions of the same
meandering system.
A composite ITCZ is constructed by averaging a large
number of the ITCZ segments together. Composite statistics
are broken down into three categories depending on where
the precipitation maximum is located. If the ITCZ precipitation maximum is found within 100 km from the southern edgeof the ITCZ, the observations are stored for the south-edge
composite category. Similarly, the north-edge category is deﬁned with the precipitation peaks detected within 100 km
from the northern ITCZ edge. As a reference, the centerpeaked composite is constructed when the precipitation maximum falls within 650 km about the ITCZ center. As such, all
the three cases share the same search radius of 100 km, ensuring that the likelihood of ITCZ occurrence is not artiﬁciallyskewed among the cases. Changing the 100-km margin or the
50-mm threshold to a larger value would push the “edge”
deeper into the ITCZ, blurring the distinction between theedge and center-peaked composites. It is reiterated that the
50-mm threshold is intended as a physically based measure of
the ITCZ that the net input of thermal energy to the atmosphere via radiation and surface heat ﬂuxes is positive inside
the ITCZ while negative outside (Masunaga and Mapes
2020).
The south-edge, north-edge, and center-peaked composite
groups are each partitioned further into subcategories divided
by different ITCZ widths of 300–400, 400–600, 600–800, and
800–1000 km. ITCZ samples narrower than 300 km are discarded from the analysis because otherwise there would beedge-intensiﬁcation cases that cannot be unambiguously separable from the center-peaked case.
Figure 2 shows the number of detected ITCZ events that
constitute each composite category. It is noted that an “ITCZ
event” is meant to be each ITCZ segment sampled by a single
satellite track. The satellite tracks are projected on a 0.258
global grid in the current analysis, so that a group of neighboring 0.258-wide satellite tracks parallel to one another constituteeach satellite swath as shown in Fig. 1. As such, a single bandof ITCZ precipitation is counted more than once if intersected
multiple times by adjacent satellite tracks, so that an elongated
ITCZ feature is weighted more than a shorter one in the histogram. As may be expected, center-peaked composite events
are observed more often than each of south- and north-edge
events for all ITCZ widths. The edge-intensiﬁcation cases are,nevertheless, far from rare in that a precipitation peak occurs
near the southern or northern edge of the ITCZ roughly or
more than half as frequently as at the ITCZ center.

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Number of occurrences

30000
300-400 km
400-600 km
600-800 km
800-1000 km

25000
20000
15000
10000
5000

uth
Ce
nte
No r
rth

So

uth
Ce
nte
No r
rth

So

uth
Ce
nte
No r
rth

So

So

uth
Ce
nte
No r
rth

0

FIG. 2. Histogram of ITCZ events for different ITCZ widths as
indicated by legend. “South,” “north,” and “center” on the abscissa
refer to the south-edge, north-edge, and center-peaked cases, respectively. See text for the precise deﬁnition of “ITCZ events.”

3. Resultsa. Precipitation and moisture convergence
The precipitation distribution across the ITCZ for different composite categories is plotted in Fig. 3. The abscissa isthe distance along the A-Train ground tracks (northward positive) relative to the ITCZ midpoint in the center-peaked caseand to the corresponding ITCZ edge in the south- and northedge cases. Composite precipitation by construction rises to a

maximum exactly at distance zero in center-peaked cases, withthe peak height increasing with the ITCZ width. The southedge (north-edge) composite precipitation has a peak slightly
shifted northward (southward) from the edge. It follows thatthe edge intensiﬁcation of convection occurs on the moist sideof the 50-mm boundary of the ITCZ. The composite precipitation peak is weaker when occurring at the ITCZ edges than at
the ITCZ center.
To verify the validity of using a ﬁxed CWV threshold to deﬁne
the ITCZ, column-integrated moisture convergence is examinedin composite space (Fig. 4). The center-peaked composite curvein Fig. 4a (300–400-km width) implies that moisture convergencestays positive over a distance of 6200 km about the ITCZ midpoint. In the edge intensiﬁcation cases of Fig. 4a, moisture convergence is positive over a width of ;400 km with its peak shiftedto the negative (southern) side for the north-edge composite andto the positive (north) side for the south-edge composite. This
conﬁrms that a moist band identiﬁed by the 50-mm thresholdoverall agrees with an independent (and more authentic) deﬁnition of the ITCZ based on moisture convergence. It is notedthat the CWV threshold has been adopted instead of moisture
convergence for the event-by-event detection of the ITCZ because the moisture convergence ﬁeld is rather noisy on an instantaneous basis.
Figures 4b–d shows the same plots but for different widthsof 400–600, 600–800, and 800–1000 km. The span of positive
moisture convergence increases roughly in accordance withincreasing ITCZ width, although apt to be near (or slightly

Composite Precipitation (300-400 km width)

Composite Precipitation (400-600 km width)
80

(a)
60

Center
North
South

40
20

Precipitation [mm/d]

Precipitation [mm/d]80

0-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]

(b)
60
40
20
0
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]Composite Precipitation (600-800 km width)

Composite Precipitation (800-1000 km width)
80

(c)
60
40
20
0
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]Precipitation [mm/d]

80
Precipitation [mm/d]

VOLUME 36

(d)
60
40
20
0
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]FIG. 3. Composite precipitation for different ITCZ widths of (a) 300–400, (b) 400–600, (c) 600–800, and(d) 800–1000 km. The center-peaked, north-edge, and south-edge cases are shown in solid, dashed, and dottedcurves, respectively. See text for the deﬁnition of the relative distance (abscissa).

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Moisture Convergence [10-4kg/s/m2]
Moisture Convergence [10-4kg/s/m2]

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Composite Moisture Convergence (300-400 km width)
4
Center
North
3
South

Composite Moisture Convergence (400-600 km width)
4

2

2

1

1

0

0

-1
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]-1-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]Composite Moisture Convergence (600-800 km width)

Composite Moisture Convergence (800-1000 km width)

4

4

(a)

3

(c)

3

3

2

2

1

1

0

0

-1
-800 -600 -400 -2000

200 400 600 800

Relative distance [km]

(b)

(d)

-1
-800 -600 -400 -2000

200 400 600 800

Relative distance [km]FIG. 4. As in Fig. 3, but for moisture convergence integrated over height.

short of) the lower end of each range (e.g., ;800 km for thecase of 800–1000 km).
It is shown in Figs. 3 and 4 that the ITCZ is approximately“scale-free” in that the horizontal structure of precipitationand moisture convergence is qualitatively similar regardless of
the ITCZ width. Only the composite category of 400–600 km,
having the largest sample size and hence statistically most robust (Fig. 3), will be discussed in the remainder of this paper
except where otherwise noted. Results for other ITCZ widths
do not alter any main conclusion of the current paper.

b. MSE budget breakdown
A moist static energy (MSE) budget analysis is carried out
in composite space for illustrating the key processes at work
behind the edge intensiﬁcation of ITCZ convection. MSE per
unit mass, deﬁned as cpT 1 f 1 Lqy , where cp is the speciﬁcheat of air at constant pressure, f is geopotential, and L is the
speciﬁc latent heat of vaporization, is conserved in moist adiabatic processes in a hydrostatic atmosphere, and therefore is a
useful measure of large-scale convective dynamics. The evolution of vertically integrated MSE satisﬁes the MSE budget
equation:h­t hi 1 hu ? =hi 1 hv­p hi 5 FL 1 FS 1 hQR i,(1)where u is horizontal wind, FL is LHF, FS is sensible heat ﬂux(SHF), QR is radiative heating rate, and h???i denotes the vertical integral over the whole thickness of the atmosphere. Forease of reference, the MSE budget terms are hereafter denoted ash­t hi 5 HADV 1 VADV 1 NDF,

(2)

where HADV, VADV, and NDF refer to the vertically integrated horizontal advection of MSE, the vertically integrated
vertical advection of MSE, and the net diabatic forcing,respectively:
HADV ; 2hu ? =hi,
VADV ; 2hv­p hi,
NDF ; FL 1 FS 1 hQR i:
As described above in section 2a, FL and hQRi are estimatedfrom satellite measurements while h, HADV, and VADV are
derived from the ERA5 datasets matched in space and timewith A-Train observations. SHF stays small in magnitude compared to LHF and column radiative heating over tropicaloceans, so FS is ﬁxed at a typical value of 10 W m22 in the current analysis.
The three terms on the rhs of (2) each have essential roles intropical dynamics. NDF collectively represents the rate of thermal energy input to (or, if negative, output from) the atmosphere,
acting as the “external” forcing that drives the atmospheric circulation. Here NDF is referred to as external in that NDF has no explicit reference to the large-scale meteorological state (T, qy , u,and v), but is likely to be implicitly intertwined with these variables through various processes such as the anvil cloud effects on
hQRi.

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JOURNAL OF CLIMATE
Composite heat fluxes (South)

Composite heat fluxes (Center)

Composite heat fluxes (North)

150

0
-50
-100
-150
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]

(b)

50

2

50

100

100

0
-50
-100
-150
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]

(c)

50

2

LHF
P

150
Heat fluxes [W/m ] and P [mm/d]<QR>

Heat fluxes [W/m ] and P [mm/d]

DF (-)(a) netnet DF (+)

2

Heat fluxes [W/m ] and P [mm/d]

150
100

VOLUME 36

0
-50
-100
-150
-800 -600 -400 -200 0 200 400 600 800
Relative distance [km]FIG. 5. Diabatic forcing terms (W m22) in composite space, that is, column radiative heating hQRi (solid), LHF FL (dashed), and the
NDF or FL 1 FS 1 hQRi (shaded) for (a) south-edge, (b) center-peaked, and (c) north-edge composites. Precipitation (mm day21) isshown as a thin curve for reference.

VADV, quantifying the import (export) of MSE into (from)an atmospheric column owing to the internal dynamics within
the column, is a variable sensitive to the depth of circulationand hence to details in the moist convective processes as will bedemonstrated later. Changes in the column-integrated MSE
due to the large-scale horizontal transport are summarized by
HADV. Since free-tropospheric temperature is nearly horizontally uniform in the low-latitude atmosphere, HADV is primarily governed by the horizontal gradient of moisture and thehorizontal wind ﬁeld.

c. Diabatic forcing
The composite spatial structure of NDF is ﬁrst investigated.
Figure 5 shows the composite spatial structure of column radiative heating (solid), LHF (dashed), and NDF. LHF is largely
counteracted by column radiative cooling (i.e., negative hQRi),
leading to a weakly negative NDF except near distance zero.
In the center-peaked composite (Fig. 5b), NDF has a distinct
maximum of 150 W m22 around the precipitation peak at the
ITCZ center, tapering off nearly symmetrically on both sides
to a negative value at distances beyond 6200 km. This spatial
Net CRE [K/day] (South)
16

Net CRE [K/day] (Center)
1.5

14

16

1

Net CRE [K/day] (North)
1.5

14

16

1

8

0

6

-0.5

4

0.5

10
8

0

6

-0.5

4
-1

2

0
-1.5
-800-600-400-200 0 200 400 600 800

112Altitude [km]

0.5

10

1.5

14

12
Altitude [km]12
Altitude [km]variability in the diabatic forcing arises principally from that ofcolumn radiative heating, while LHF has little horizontal
variability.
NDF reaches a maximum roughly in phase with the precipitation peak in the edge intensiﬁcation cases as well (Figs. 5a,c),
whereas the peak height is only barely positive. The spatial variation of hQRi is asymmetric about the peak, accompanied by amore gradually decreasing shoulder to inside the ITCZ than tooutside.
The spatial structure of column radiative heating is mainly
accounted for by the anvil cloud effects as illustrated by thevertical proﬁle of the net CRE (Fig. 6). The net CRE is enhanced in the upper troposphere (8–13 km in height), which is
a combined effect of shortwave and longwave heating by anvil
clouds. The horizontal distribution is approximately symmetricin the center-peaked case (Fig. 6b) while asymmetric in thesouth- and north-edge cases (Figs. 6a,c) just as seen in column
radiative heating (Fig. 5). The anvil cloud heating is partially
offset to a minor extent by a negative CRE observed at altitudes of 13–15 km, attributable to the cloud-top longwave
cooling associated with deep convection. A narrow layer of

0.5

10
8

0

6

-0.5

4
-1

2

0
-1.5
-800-600-400-200 0 200 400 600 800

Relative distance [km]Relative distance [km]

-1

2

0
-1.5
-800-600-400-200 0 200 400 600 800
Relative distance [km]21

FIG. 6. The vertical structure of the net CRE (K day ) in composite space for (a) south-edge, (b) center-peaked, and (c) north-edge
composites.

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Composite p-velocity [hPa/h] 1st mode (Center)

Composite p-velocity [hPa/h] 1st mode (North)

100

4

100

4

100

4

(a)
150

3

(b)
150

3

(c)
150

3

250
300

0

400

-1
500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]Composite p-velocity [hPa/h] 2nd-4th (South)
100
4

(d)

200

3
2

250
300

1

400

-1

0

500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]

1

250
300

0

400

-1
500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]Composite p-velocity [hPa/h] 2nd-4th (Center)
100
4
150

Pressure [hPa]

150

2

200

Pressure [hPa]

1

(e)

3
2

200
250
300

1

400

-1

0

500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]

2

200

1

250
300

0

400

-1
500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]Composite p-velocity [hPa/h] 2nd-4th (North)
100
4
150

Pressure [hPa]2

200

Pressure [hPa]

Pressure [hPa]Composite p-velocity [hPa/h] 1st mode (South)

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MASUNAGA

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(f)

3
2

200
250
300

1

400

-1

0

500
-2
600
700
-3
850
1000
-4
-800-600-400-200 0 200 400 600 800
Relative distance [km]FIG. 7. The vertical structure of v (hPa h21) and its vertical mode decomposition in composite space for (a) south-edge, (b) centerpeaked, and (c) north-edge composites. The deep mode or the ﬁrst mode is shaded in color and the original (undecomposed) v proﬁle iscontoured at an interval of 1 hPa h21 for ascent (negative v) only. (d)–(f) As in (a)–(c), but for the shallow mode or the second to fourthmodes summed together (color).

longwave cooling at shallow cloud tops is visible at a height of
2 km, which would be largely canceled out by a layer of cloudlongwave heating immediately below when integrated vertically into hQRi.

d. Vertical mode decomposition
VADV can change not only in amplitude but in sign dependingon the vertical structure of v. Given a typical vertical proﬁle of
MSE over tropical oceans, a top-heavy v proﬁle exports MSE
out of the atmospheric column while a bottom-heavy one could
import MSE into the column (e.g., Back and Bretherton 2006;
Raymond et al. 2009; Inoue and Back 2015). To quantify the bottom-heaviness, a vertical mode decomposition is applied to the
ERA5 v proﬁles. The mode decomposition is carried out by thevertical normal mode transform based on the eigenvalue solution
to the vertical component of the linearized primitive equations (Fulton and Schubert 1985; Haertel and Johnson 1998).
The results of vertical mode decomposition are presented inFig. 7. Ascent motion is concentrated around the center as revealed by the v proﬁles (contoured). The area of ascent is somewhat displaced to the north in the south-edge case and to thesouth in the north-edge case, which is in line with the precipitation peak (Fig. 3). The v proﬁles are consistently bottom-heavy
with its peak located at a level near 800 hPa. The top row ofFig. 7 shows the ﬁrst normal mode in color, accounting for the

deep component of the ascent extending throughout the troposphere. On the other hand, the sum of the second to fourthmodes as colored in the bottom row is comprised of a lowertropospheric ascent, characterizing the bottom-heavy component. The ﬁrst mode is hereafter called the deep mode and thesecond to the fourth modes in combination are referred to as
the shallow mode. The individual effects of the deep and shallow
modes on VADV are assessed in the subsequent subsection.

e. Vertical and horizontal advections of MSE
VADV is expanded into a series of normal modes:
VADV 5 VADVd 1 VADVs 1 VADVr ,with
VADVd 5 2hv1­p hi,
4

VADVs 5 2 ∑ hvi­p hi,
i52‘

VADVr 5 2 ∑ hvi­p hi,
i55

where vi refers to the ith normal mode and the subscripts d, s,
and r denote the deep mode, shallow mode, and residual,

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Composite VADV (South)

Composite VADV (Center)-<hdw/dp> [W/m 2]

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Relative distance [km]Relative distance [km]FIG. 8. (a)–(c) Column-integrated vertical MSE advection or VADV (W m22; shaded) and its vertical mode decomposition (solid curvefor VADVd, dashed for VADVs, and dotted for VADVr) for (a) south-edge, (b) center-peaked, and (c) north-edge composites.
(d)–(f) As in (a)–(c), but for column-integrated net DSE advection (shaded), the vertical mode decomposition of vertical DSE advection (black solid and dashed curves), and precipitation in the units of W m22 (orange dashed curve). Note that the sign of the advection terms is deﬁned to be positive (negative) for an import (export) of MSE and of DSE.

respectively. Figures 8a–c shows the vertical mode decomposition of VADV. In the area of ascent near distance zero,
VADVd exhibits a striking negative peak, implying that MSE
is exported in the upper troposphere to a greater extent thanimported near the surface by a deep overturning circulation
represented by the deep mode. In contrast, a shallow ascent associated with VADVs results in a net import of MSE, accounting for the positive peak of VADVs. VADVd and VADVs
almost precisely cancel each other out at distance zero in thecenter-peaked case, while the deep mode is not so intense as toentirely counteract the shallow mode in the edge intensiﬁcation
cases. The residual term stays close to zero where VADVd and
VADVs are each enhanced in magnitude, suggesting that the
deep and shallow modes well account for the large-scale dynamics inside the ITCZ. The residual is not negligible in theareas of subsidence outside the ITCZ.
For comparison, the leading terms of dry static energy (DSE,deﬁned by cpT 1 f) budget are plotted in Figs. 8d–f. The net
DSE advection, practically identical to its vertical component
since the horizontal component is negligibly small, is nearly amirror image of precipitation but with the opposite sign. This

conﬁrms that the condensation heating (or precipitation) islargely counteracted by vertical DSE advection or the adiabatic
cooling due to large-scale ascent, primarily accounted for by the
deep (ﬁrst baroclinic) mode. The precipitation peak is somewhat sharper than the corresponding peak of DSE advection. It
is unclear whether this discrepancy is a real signal or due to an
artifact in the reanalysis data.
Figures 9a–c shows HADV with its zonal and meridional
breakdowns. HADV has a large negative value in the righthalf of the south-edge composite plot (Fig. 9a), in the left halfof the north-edge composite (Fig. 9c), and symmetrically about
distance zero in the center-peaked case (Fig. 9b). As such, horizontal winds export MSE out of the atmospheric column deep
inside the ITCZ, while the export of MSE rapidly decays outward across the ITCZ edge. This behavior of HADV is ascribed principally to the meridional component, with thecontribution of the zonal advection being secondary.
To understand the origins of meridional MSE advection, thevertical structures of meridional wind (y ) and the meridional
gradient of MSE (­yh) are plotted in Figs. 9d–f. It is noted thatthe composite means of y and ­yh are not directly combined

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Composite HADV (Center)

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Composite v [m/s], dh/dy [J/kg/km] (Center)

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FIG. 9. (top) Column-integrated horizontal MSE advection or HADV (W m22; shaded) along with its zonal (dashed curve) and meridional (solid curve) components for (a) south-edge, (b) center-peaked, and (c) north-edge composites. (bottom) Vertical structure of ­yh(shaded) and meridional wind (contoured) for (d) south-edge, (e) center-peaked, and (f) north-edge composites. The contour interval is
1 m s21 with negative (northerly) contours dashed and zero contours thickened. White arrows indicate the meridional wind direction.

into the mean meridional advection because of nonlinearity(y ­y h Þ y­y h) but are nonetheless beneﬁcial for an intuitive
illustration. For the center-peaked composite (Fig. 9e), boundary
layer inﬂows from both north and south converge at the ITCZ
center. Since the lower tropospheric air is substantially drier outside the ITCZ than in the ITCZ center, the inward transport ofthe boundary layer MSE is guaranteed to reduce MSE in the
ITCZ, accounting for the negative HADV deep inside the ITCZ.
On the other hand, a weak return ﬂow immediately above theboundary layer, signifying a shallow meridional circulation typicalof the eastern Paciﬁc ITCZ (Zhang et al. 2004), has the effect ofmoistening the ITCZ edges by carrying the midlevel humid air
from the ITCZ center. This midlevel moistening effect is efﬁcient,
as implied by the prominent meridional gradient of MSE(shaded) on both sides of the ITCZ, in offsetting the low-level
drying when integrated over height. As a result, HADV rapidly
diminishes in magnitude from the ITCZ center toward theedges. A close inspection of Fig. 9 reveals that the HADV variability is not precisely symmetric between the south- and northedge cases: the net HADV almost vanishes or is slightly positiveat negative distances in the south-edge composite (Fig. 9a),while HADV stays negative entirely in the north-edge composite (Fig. 9c). This asymmetry leads to discernible consequenceson the total MSE advection as discussed later.

The net effect of column-integrated MSE advection(VADV 1 HADV) is shown in Fig. 10. A conﬁned peak of the
net MSE advection stands out at distance zero, ﬂanked by valleys of a near-zero or negative MSE on both sides for thesouth-edge composite (Fig. 10a). The peak is attributed to
VADV, while the valleys result from a competing effect between VADV and HADV being opposite in sign. In contrast,
the center-peaked composite plot gives no hint of a positive
peak (Fig. 10b), for which the double-peaked MSE import by
VADV is more than offset by the signiﬁcant export of MSE byhorizontal advection and hence HADV practically dominates
the net MSE advection. The north-edge composite (Fig. 10c)
resembles the south-edge case, although the peak is much
weaker. This muted magnitude of advection peak in the northedge case is ascribed to a negative HADV that works against
the VADV peak, which would otherwise give rise to a peak ofcomparable strength to the south-edge composite. VADV is
more or less symmetric between the north- and south-edge
cases, while HADV is less so as noted earlier.
The composite plots shown in Figs. 5–9 are constructed forthe ITCZs having moderate widths of 400–600 km. For comparison, the composite VADV and HADV for broader
ITCZs of 600–800 km are plotted in Figs. 10d–f. The peakof the net MSE advection in the north-edge composite is

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Composite net ADV (South, 400-600 km)
150

net ADV (-)
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200 400 600 800

Relative distance [km]

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200 400 600 800

Relative distance [km]FIG. 10. Column-integrated net MSE advection (VADV 1 HADV) (W m22; shaded) along with VADV (solid curve) and HADV(dashed curve) for (a) south-edge, (b) center-peaked, and (c) north-edge composites. (d)–(f) As in (a)–(c), but for broader ITCZs(600–800 km) instead of 400–600 km.

more salient for broad ITCZs (Fig. 10f) than for modest
ITCZ widths (Fig. 10c), but otherwise there is no qualitative
difference.
The key ﬁnding here is that the column-integrated net MSE
advection exhibits contrasting behaviors between the edge andcenter intensiﬁcation cases. The vertical advection of MSE gives
rise to an import of MSE sharply conﬁned near the precipitation
peak in the edge intensiﬁcation cases, whereas the precipitation
peak is accompanied by an export of MSE when occurring at
the ITCZ center. This fundamental difference, along with thecontrast in NDF as discussed earlier, suggests that an edge intensiﬁcation is likely driven by a mechanism distinctly different
from the one governing the convective intensiﬁcation at theheart of the ITCZ.

4. Conclusions and discussion
This study explores the possible mechanism of ITCZ convection with focus on the edge intensiﬁcation cases. Observations from a suite of A-Train satellite instruments were
employed to identify the ITCZ as a band of moist air with
CWVs greater than 50 mm. The internal structure of the
ITCZ sampled along satellite tracks is averaged together into

a composite diagram around the ITCZ edge or the center,
separated by ITCZ width (400–600 and 600–800 km, etc.) andthe location of precipitation maximum (south-edge, centerpeaked, and north-edge). The edge intensiﬁcation cases are
not as frequent as the typical case where convection is mostactive in the middle of the ITCZ, but nonetheless are far fromrare.
The MSE budget terms were projected onto composite
space to reveal the behavior of NDF to the atmosphere andthe MSE advection (VADV and HADV). When convection
peaks at the ITCZ center, column radiative cooling is strongly
suppressed owing to the extensive anvil clouds accompanying
deep convection, forming a prominent positive peak in NDF.
The total MSE advection in this case is negative as the combined effects of a strong deep-mode VADV and the transportof dry air from outside the ITCZ, both resulting in an exportof MSE. When convection peaks at the ITCZ edges, in contrast, a positive peak of NDF is present but substantiallyweaker than in the center-peaked case. Instead, shallowmode VADV leads to a net import of MSE on the ITCZ
edges, which presumably allows an edge intensiﬁcation to occur despite the weak NDF. The shallow-mode VADV plays a
key role in both the south- and north-edge cases, while theUnauthenticated | Downloaded 04/28/23 01:14 PM UTC

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net advection peak is weaker in the north-edge composite because HADV differs in magnitude between the two cases.
Shallow convection has been known to be instrumental for
promoting a subsequent development of deeper convection. The
deepening of shallow cumulus clouds to deep convection is often
observed in association with a low-level accumulation of moisture in the growing phase of the MJO (Lin and Johnson 1996a,b;
Kemball-Cook and Weare 2001). Such a “pre-conditioning” effect has been argued in the literature to occur through a variety
of processes including the detrainment of moisture from congestus clouds (Takayabu et al. 2006; Waite and Khouider 2010) andlarge-scale moisture convergence (Hohenegger and Stevens
2013; Masunaga 2013). A shallow overturning (or bottom-heavy)
circulation coupled with moist processes has the potential of dynamically amplifying itself under a negative gross moist stability(GMS), marking a sharp contrast to deep (top-heavy) convection, which has a positive GMS and is self-destructive by nature(Back and Bretherton 2006; Raymond et al. 2009; Back andBretherton 2009; Masunaga and L’Ecuyer 2014; Inoue and Back2015). Inoue et al. (2021) showed that convective dynamics ismaintained over tropical oceans through regionally dependent
feedback processes: a negative GMS due to shallow circulation isa primary driver of convection in some regions while a feedback
involving the interplay of NDF with deep convection plays a major role in others. The current analysis implies that such different
processes could coexist within the eastern Paciﬁc ITCZ, with the
diabatic feedback being at work in its center and a negative
GMS driving convection at the edges.
Other processes potentially accounting for the edge intensiﬁcation of convection include a “super cold pool,” or an ensembleof sequentially generated cold pools pushing convection toward
the edge of a convectively active region. The super-cold-pooltheory for the edge intensiﬁcation of convection, elaborated byWindmiller and Hohenegger (2019) based on their numericalsimulations, may also contribute to the observed enhancementof convection along the ITCZ edges, although difﬁcult to proveor disprove in the current analysis approach because cold pools,
particularly when concealed by overlying clouds, are hard to
track from satellite observations and reanalysis data.
It is possible that two contrasting convective events, one at
the ITCZ center and the other at the edge, can belong to thesame convective system at different stages of the life cycle.
The super-cold-pool theory predicts that convection initiated at
the center of a moist patch would propagate to the edge as a series of cold pools crawl forward. To the contrary, a Lagrangian
air parcel carried along the lower branch of the Hadley cellwould ﬁrst give rise to an edge intensiﬁcation of convection
before eventually arriving in the ITCZ center (Masunaga andMapes 2020). The evolution from one convective event to another is not easy to explore in the current analysis because of
the inability to continuously observe the same event from lowEarth-orbiting satellites but is an interesting subject worth investigating in the future possibly with the aid of geostationary
infrared imagery.
The meridional overturning circulation associated with ITCZ
convection differs in depth between the edge- and centerintensiﬁcation cases, potentially imposing different consequences on the atmospheric energy transport (AET). A

shallow circulation as found typically with the edge intensiﬁcation has a poleward ﬂow in the midtroposphere instead of theupper troposphere, and thus would have a moderate AET compared to the deep Hadley cell, all else being equal, since DSE
and MSE are lower in the midtroposphere than at higher levels.
It should be noted, however, that edge-intensiﬁcation eventsare unlikely to signiﬁcantly affect the AET on a global scale because an area of enhanced ITCZ convection is so fragmentaryon an instantaneous basis (see Fig. 1) that the edge effects
would be averaged out in the zonal mean.
The present analysis strategy is in theory equally applicable toglobal tropical oceans, except that the existence of islands (inthe western Paciﬁc Ocean) and of continents to the immediate
north or south of the ITCZ (in the Indian and Atlantic Oceans)
somewhat challenges the ability to unambiguously detect the
ITCZ edges with microwave measurements of CWV. Future
work comparing similarities and differences across different
regions would potentially offer a more comprehensive insighton the ITCZ dynamics in light of how moist convection is
maintained.
Acknowledgments. This work is supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for
Scientiﬁc Research (KAKENHI) 19H01966 and 19H05704.
The comments by the anonymous reviewer and Margaret
Duffy were helpful in revising the paper.
Data availability statement. The RSS AMSR-E data may
be downloaded from https://www.remss.com/missions/amsr/.
The CERES SSF Edition 4A data are provided by https://ceres.
larc.nasa.gov/data/#ssf-level-2. The CCCM Release D1 product isavailable from https://asdc.larc.nasa.gov/project/CERES/CER_
CCCM_Aqua-FM3-MODIS-CAL-CS_RelD1 (NASA/LARC/
SD/ASDC 2011). The ERA5 dataset is available at https://cds.
climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressurelevels (Hersbach et al. 2018).

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