1 MARCH 2001

LIETZKE ET AL.

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Evolutionary Structure of the Eastern Pacific Double ITCZ Based onSatellite Moisture Profile Retrievals
CHRISTOPHER E. LIETZKE*
Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

CLARA DESER
Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado

THOMAS H. VONDER HAAR
Department of Atmospheric Science, Cooperative Institute for Research in the Atmosphere,
Colorado State University, Fort Collins, Colorado(Manuscript received 18 June 1999, in final form 23 November 1999)
ABSTRACTFor about a month near the boreal vernal equinox, the eastern Pacific intertropical convergence zone (ITCZ)
is observed to form two troughs quasi-symmetrically situated about the equator near 58–78 latitude during years
when an equatorial sea surface cold tongue is present (e.g., La Niña years). The three-dimensional structure andtemporal evolution of the eastern Pacific double ITCZ is documented using weekly cloud liquid and ice water
fields and relative humidity profiles retrieved from Special Sensor Microwave/Temperature-2 measurements. The
depth of convection in the southern branch of the double ITCZ, as determined by the coincident presence ofcloud liquid and ice as well as by upward motion inferred from the relative humidity field, is observed to besensitive to both the underlying SST and subsidence from the northern branch. The equatorial sea surface cold
tongue appears to be the determining factor regulating the formation of a double ITCZ in the eastern Pacific.
Areas of deep convection within the double ITCZ are accompanied by surface wind convergence maxima.
However, the coincident maxima in deep convection and surface convergence are located several degrees oflatitude equatorward of the highest sea surface temperatures.

1. Introduction
The earth’s climate system is strongly influenced byocean–atmosphere interaction in the Tropics. The thermally direct Hadley circulation over the tropical oceans
is driven by the near-equatorial convergence zones, principally the intertropical convergence zone (ITCZ) andthe South Pacific convergence zone. The former is typically a single band of convection located north of theequator for much of the year. During the boreal spring,
however, the ITCZ has been observed, in some years,
to form in two distinct bands, quasi-symmetrically situated about the equator in the tropical eastern Pacific.

* Current affiliation: ITT Aerospace/Communications, Fort Wayne,
Indiana.
Corresponding author address: Christopher E. Lietzke, ITT Aerospace/Communications, 1919 W. Cook Road, P.O. Box 3700, Fort
Wayne, IN 46801-3700.
E-mail: chris.lietzke@itt.com

q 2001 American Meteorological Society

Because the southern branch of the double ITCZ occurs
in a notoriously data-poor region of the oceans, it was
not until the launch of the first experimental meteorological satellites in the 1960s by the Environmental
Space Services Administration (ESSA) that its existence
was clearly established (Kornfield et al. 1967). Further
studies using the ESSA satellites (Hubert et al. 1969;
Winston 1971; Gruber 1972; Musk 1976) confirmed thatthe double ITCZ structure as observed in the cloudinessfield, was only present in the eastern Pacific for a limited
time during the boreal spring. More recently, Waliser
and Gautier (1993) used the Highly Reflective Cloud(Garcia 1985) dataset to document the frequency andintensity of convection in the global ITCZ, including
the double ITCZ in the tropical eastern Pacific. Verticalprofiles of moisture and temperature in the eastern Pacific ITCZ have been documented by Fernandez-Partagas and Estoque (1985), but only during summer whenthe ITCZ is located north of the equator. The temporal
evolution and vertical structure of the double ITCZ inthe eastern Pacific have received little observational
study due to the paucity of data.

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Numerical modeling studies directed at understanding
the spatial structure of tropical convection and hence
the position of the ITCZ began in earnest soon after the
ESSA observations. Charney (1971) proposed that conditional instability of the second kind (CISK) is responsible for tropical cyclogenesis and the ITCZ. Holton et al. (1971) proposed that the position of the ITCZ
is determined by easterly wave propagation, while Lindzen (1974) used wave-CISK theory to assign a latitudinal preference to the ITCZ. Pike (1971) employed anaxially symmetric coupled atmosphere–ocean model toinvestigate the latitudinal preference of the ITCZ basedon meridional sea surface temperature (SST) profiles.
More recently, Waliser and Somerville (1994) demonstrated, using an axially symmetric primitive equation
model, that the ITCZ has a latitudinal preference regardless of where the meridional SST maximum is located. The presence of a double ITCZ year-round is acommon error produced by many coupled atmosphere–
ocean general circulation models (GCMs; Mechoso etal. 1995). This problem is so pervasive that it is referred
to as the coupled GCM ‘‘double ITCZ syndrome’’ (Maet al. 1996).
The purpose of this study is to document the threedimensional structure and evolution of the double ITCZ
in the eastern tropical Pacific based upon satellite-derived cloud water information and moisture profiles. We
also examine the relationship between the double ITCZ
structure and the underlying sea surface temperature andsurface wind fields. Based on the results, we propose aconceptual model for the mechanism of the double
ITCZ. Section 2 describes the data and methods, section
3 presents an analysis of these data, and section 4 provides a discussion of the basic mechanisms that determine the structure of the ITCZ in the eastern Pacific.
Finally, a summary of the results is given in section 5.
2. Data and methods
For use in climate analyses, the Special Sensor Microwave/Temperature-2 (SSM/T-2) is the only satelliteborne sensor capable of being used to retrieve soundingsof tropospheric moisture in cloudy regions. The SSM/
T-2 is a five-channel total power microwave radiometer
aboard the Defense Meteorological Satellite Program(DMSP) polar-orbiting satellites. This sensor measuresthe upwelling radiation near the 183.31-GHz water vapor absorption line. The sensor characteristics are listed
in Table 1. Many researchers have shown that moistureprofiles can be retrieved using radiances measured near
the 183-GHz line (Kakar 1983; Lutz et al. 1991; Kuo
et al. 1994; Wang et al. 1995). Analysis of cloud waterand moisture profile fields retrieved from SSM/T-2 observations (e.g., Wilheit 1990) is extremely limited. The
Advanced Microwave Sounding Unit (AMSU), which
has been in orbit since 1998, has a sensor (AMSU-B)
that is very similar to the SSM/T-2. For climate analysis,
its time record (approximately 1 yr at present) is too

VOLUME 14

TABLE 1. SSM/T-2 sensor characteristics. The noise equivalentchange in brightness temperature (NEdT) is the nominal sensor
noise.
Centerfrequency(GHz)

Side band(GHz)

Half-band width(MHz)

NEdT (K)91.655150.000
183.310183.310183.310

1.25
1.25
1.00
3.00
7.00

750.0
750.0
250.0
500.0
750.0

0.4
0.5
0.4
0.5
0.6

short and the instrument on board the National Oceanicand Atmospheric Administration’s (NOAA) NOAA-15
is largely unusable, as it has very large scan-dependent
biases.
The presence of atmospheric convection, and hence
the ITCZ, is usually inferred by the presence of cloud.
Many different types of remote sensing techniques havebeen used to observe the presence of cloud: solar reflectivity, infrared cloud-top temperature, and liquid water absorption. From these observations, products likeoutgoing longwave radiation (OLR), precipitation rate,and cloud liquid water (CLW) path are also created,from which many have inferred the presence of convection. None of these observations or products is able
to consistently identify the depth of convection. For
example, a cold cloud top (low values of OLR) does
not necessarily indicate deep convection: it may just be
thick cirrus blowoff from nearby deep convection. Similarly, the presence of precipitation or high values of
CLW does not necessarily indicate deep convection (theconvection may in fact be quite shallow). In order to
identify deep convection, the remote sensing techniquemust account for the presence of high cloud water values
throughout the entire column. This is possible if thesensor can be used to retrieve cloud liquid water alongwith cloud ice water coincident with a nearly saturated
column of moist air. The advantage of retrieving bothcloud liquid and cloud ice from the same sensor eliminates collocation error as would occur if one were to
combine OLR and CLW datasets from different sensors.
Furthermore, the presence of cloud liquid and cloud ice
retrieved in a single profile ensures that the retrieved
profile is radiatively consistent with sensor observations.
A dataset of cloud water path (liquid and ice) andcoincident relative humidity profiles produced according to the methods described in Lietzke (1998) was used
for this work. The results presented here should not be
considered in any way to be validation of the retrieval
algorithm used. A comparison to nine radiosonde sites
in the Caribbean, over a 2-month period, was completed
before application of the algorithm to the eastern Pacific.
Table 2 shows the relative humidity profile comparison
statistics with the Barbados site where a total of 41
profiles were used. Although this is insufficient to beconsidered algorithm validation, it is apparent that the1 MARCH 2001

LIETZKE ET AL.

TABLE 2. Relative humidity (%) comparison with the Barbados
synoptic radiosonde at retrieved levels.

7451995) on 1.08 lat 3 1.08 long grids for the years 1982–
97 were also used.

0.0 km 1.5 km 3.0 km 5.5 km 7.5 km 9.5 km
Rms (RH%)
Bias (RH%)

7
22

14
23

13
1

15
0

19
21

17
2

3. Resultsa. Time evolution

retrieved profiles agree with the radiosonde profiles towithin about 15%. This is consistent with a retrieval
error analysis using simulated radiances that suggested
that the rms error was in the 10%–15% range. The analysis of the double ITCZ presented in this paper alsorelies on the retrieved cloud liquid and ice water paths.
It is nearly impossible to validate such retrievals due tothe lack of in situ measurements and volume-sampling
error. Therefore, although the cloud water plots are given in the appropriate path units of grams per square
meter, they should be regarded only as an index of cloudamount, from which relative differences, not absolute
quantities, may be inferred.
The SSM/T-2 derived water vapor profiles and cloudwater were retrieved twice daily at the sensor’s reportinginterval (;50 km) and averaged to a 1.08 lat 3 1.08
long grid. Since the SSM/T-2 data were obtained fromthe DMSP F12 satellite, the earliest observations of theboreal spring are in 1995. The retrievals were limited
to the tropical eastern Pacific (158N–158S, 758–1658W)
for all reported observations from 1 March to 31 May
for the years 1995–97. Surface zonal and meridional
winds were obtained from the European Remote Sensing satellite (ERS-1 and ERS-2) scatterometer series.
The wind fields were averaged over 6-day periods on agrid of approximately 2.88 lat 3 2.88 long (T. Hoar 1998,
personal communication). In order to document a longer
record of the ITCZ, monthly mean Microwave Sounding
Unit (MSU) derived precipitation fields (Spencer 1993)
on a 2.58 lat 3 2.58 long grid for the years 1982–96
were used. Weekly SST fields (Reynolds and Smith

Figure 1 shows a latitude–time plot of monthly mean
SST and precipitation averaged over the eastern Pacific(908–1508W) for the years 1982–97. Consistent withprevious studies (e.g., Mitchell and Wallace 1992; Waliser and Gautier 1993), the main precipitation maximum
occurs from 58 to 108N, while a secondary maximum
appears briefly, early in the year, near 58S. We shallrefer to the simultaneous occurrence of precipitation
maxima on either side of the equator as a ‘‘double
ITCZ.’’ The double ITCZ undergoes considerable interannual variability; appearing during the boreal springof some years but not during others. The double ITCZ
has been observed to appear as early as late February
and disappear as late as early May. However, the mostcommon time period for occurrence is during Marchand April, which we shall nominally refer to as theboreal spring throughout this paper. The double ITCZ
is accompanied by off-equatorial SST maxima (Fig. 1,top), although the position of the southern maximum(58–108S) extends a few degrees farther south than its
precipitation counterpart. It is apparent that the El Niño–Southern Oscillation cycle, characterized by interannual
SST changes in the equatorial eastern Pacific, modulates
the double ITCZ structure. The departure from the longterm monthly mean SST averaged over the Niño-3 region is shown in Fig. 2. When SSTs in this region areabove normal during the boreal spring, as in 1983, 1987,
1992, and 1997, the double ITCZ is absent; whereas,
when they are below normal, as in 1985, 1986, 1989,
and 1996, the double ITCZ is prominent.
To help quantify the relationship between the meridional SST profile and the double ITCZ, an index was

FIG. 1. Latitude–time (Hovmöller) plots of zonally averaged (908–1508W) monthly mean sea
surface (top) temperature and (bottom) precipitation. The SST field is contoured every 0.58C
between 26.08 and 28.08C with values in excess of 27.58C shaded. The precipitation field is
contoured every 20 mm between 40 and 120 mm with values in excess of 100 mm shaded.

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TABLE 3. List of months used to composite meridional profiles ofprecipitation and sea surface temperature for double ITCZ years andnon–double ITCZ years.

FIG. 2. Niño-3 mean SST departure from long-term monthly mean.
Vertical dotted lines denote 1 Apr.

developed to measure the degree of equatorial symmetry
present in the precipitation field during any given
month. The double ITCZ index, dI is defined as
1
dI 5 [H(58N) 1 H(58S)] 2 [H(08N) 1 1]
2
where
1


H 5 0
21( p $ 100 mm),
(70 , p , 100 mm),
( p # 70 mm),and p is the zonally averaged (908–1508W) monthly
mean precipitation. Using this index, the months whena well-defined double ITCZ was present (dI 5 1) andthe months when no double ITCZ was present (dI #21) were identified and are listed in Table 3. Double
ITCZ and ‘‘non–double ITCZ’’ composites were formedby averaging the SST and precipitation data for all double ITCZ and non–double ITCZ months, respectively.
These composites are shown in Fig. 3. During double
ITCZ periods (Fig. 3, solid curves), the equatorial minimum in SST is pronounced (SST less than 268C), whileduring times when the double ITCZ is absent (Fig. 3,
dashed curves), the equatorial minimum is suppressed(SST greater than 278C). A time series of equatorial
SST (not shown) indicates that the SST never exceeds
26.58C during double ITCZ years, but always exceeds

DoubleITCZ
years 1982

1984

1986

1988

1989

Mar
Apr

X
X

X

X
X

X

1987

1992

X
X

X
X
X

X

Non– 1983
double
ITCZ
years
Jan
X
Feb
X
Mar
X
Apr
X
May
X

1993

1994

1996

X

X
X

X
X

this threshold during years when the double ITCZ isabsent. The double ITCZ composite also shows that the
SST maxima are located about 38 poleward of the precipitation maxima in both hemispheres. For both setsof composites, the maximum SST is greater in the
Northern Hemisphere than in the Southern Hemisphere,
while the precipitation profiles are roughly symmetricwith respect to the equator. Exclusion of the months
January, February, and May from the non–double ITCZ
composite has no significant effect on the results (notshown).
Having established the general features of the annual
cycle and interannual variability of the eastern Pacificdouble ITCZ and its relation to SST, we now consider
in more detail the time evolution of the double ITCZ
during the boreal spring. Figure 4 shows weekly meansof cloud ice water, cloud liquid water, and SST averaged
over the region 908–1508W, as a function of latitude andtime for March–May of 1995 and 1996, both double
ITCZ years (recall Fig. 1). Using the retrieved cloudliquid and ice water together, we are able to infer thedepth of convection and its dependence on SST. For
example, the simultaneous occurrence of cloud liquid
and ice water indicates deep convection, while the pres-

FIG. 3. (a) Composite precipitation and (b) SST for double ITCZ index equal to 1.0 (solid) and for double ITCZ index less than or equal
to 21.0 (dash).

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FIG. 4. Latitude–time (Hovmöller) plots of zonally averaged (908–1508W) weekly mean (top) cloud ice water, (middle) cloudliquid water, and (bottom) sea surface temperature for (left) 1995 and (right) 1996. The cloud ice water field is contoured every
30 g m22 between 10 and 100 g m22 with values in excess of 10 g m22 shaded. The cloud liquid water field is contoured every
40 g m22 between 30 and 150 g m22 with values in excess of 30 g m22 shaded. The SST field is contoured every 0.58C between
26.58 and 28.58C with values in excess of 26.58C shaded.

ence of cloud liquid water alone (cloud ice water alone)indicates low clouds or shallow convection (thick cirrus
blowoff from nearby deep convection). During 1995 and1996, the northern and southern branches of the double
ITCZ are marked by the simultaneous occurrence ofcloud ice and liquid water. During 1995, the southernbranch appears only briefly (10 March–14 April) whileduring 1996, it is longer lived (3 March–5 May). The
appearance of the southern ITCZ during both 1995 and1996 coincides with the occurrence of a southern SST
maximum that is similar in magnitude to the northern
SST maximum. The demise of the southern branch of
the double ITCZ occurs as the northern branch intensifies with increasing SST.
b. Horizontal structure
The horizontal structure of the double ITCZ as depicted by the cloud liquid and ice water paths is shownin Fig. 5 for the 2-week period immediately following
the boreal vernal equinox (23 March–6 April) when the
double ITCZ is near its most symmetric state during
1995 and 1996. We will refer to this time interval as
the ‘‘double ITCZ period.’’ Again, by observing thepresence of cloud liquid and ice, we are able to infer
the type of convection present. Figure 5 indicates thatthe southern branch of the double ITCZ in both 1995
and 1996 is of large zonal extent, spanning nearly 708
longitude in both the cloud liquid and ice water fields.
In 1997, a partial double ITCZ appears to be present in

the ice water field over the western portion of the region,
however, the liquid water field does not confirm the
double ITCZ structure east of 1408W.
The mean SST and surface wind divergence fields
for the double ITCZ period during 1995, 1996, and1997, are shown in Fig. 6. Comparing the surface winddivergence fields with the cloud ice and liquid water
fields (Fig. 5), it is evident that areas of deep convection
are accompanied by surface wind convergence maxima(2–5 3 1026 s21 ). The low-level convergence maxima
associated with the double ITCZs in 1995 and 1996 tend
to occur on the equatorward flanks of the SST maxima
in both hemispheres, consistent with the results shownin Figs. 1 and 3. The surface winds overlying the strong
equatorial cold tongue in 1996 are divergent. Similar
surface wind divergence maps were presented by Zheng
et al. (1997) based on ERS-derived winds for 1992–94.
c. Vertical structure
The vertical structure of the relative humidity field
can be used to infer vertical motion. In the absence of
local moisture sources and sinks and horizontal advection, elevated humidity indicates upward motion (convection), whereas suppressed humidity indicates downward motion (subsidence). The vertical structure of therelative humidity field at 1108W for the double ITCZ
period during 1996–97 is shown in Fig. 7. As inferred
earlier from the cloud water fields in Fig. 5, the mean
latitude and strength of the postequinoctial ITCZ is quite

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FIG. 5. Time mean (24 Mar–7 Apr) (left) cloud liquid water and (right) cloud ice water for (top–bottom) 1995–97. The
cloud liquid water field is contoured every 60 g m22 between 30 and 210 g m22 with values in excess of 30 g m22 shaded.
The cloud ice water field is contoured every 60 g m22 between 10 and 190 g m22 with values in excess of 10 g m22 shaded.

different between 1996 and 1997. In 1996, the relative
humidity surfaces indicate that upward motion (convection) is present throughout the lower and midtroposphere on either side of the equator, with subsidence
at the equator and poleward of 108 latitude. The comparable strength of the inferred upward motion in bothbranches of the double ITCZ is consistent with the symmetry in the cloud ice and liquid water fields and in thesurface wind convergence field shown earlier. The
height–latitude relative humidity cross section in 1997shows no double ITCZ: a single ITCZ is located between 58N and 58S with suppressed humidity values oneither side (a narrow region of upward motion is also
indicated near 128N).
4. Discussion
The results suggest the following conceptual modelof the double ITCZ in the eastern tropical Pacific. The
subtropical highs and equatorial trough dominate thelarge-scale sea level pressure distribution in the eastern
tropical Pacific (Fig. 8). During most of the year, theSouth Pacific high is stronger than its Northern Hemisphere counterpart, extending across the equator anddisplacing the equatorial trough into the Northern Hemisphere. For a brief period during boreal spring, however,the subtropical highs are symmetrically positioned about
the equator. This symmetry produces a region of lowlevel convergence in the eastern equatorial Pacific at atime when the sea surface is generally warm. Easterly
surface winds along the equator produce upwelling that,in the presence of a shallow thermocline, cools the ocean
surface mixed layer. The resulting cold tongue along theequator suppresses deep atmospheric convection andsplits the convergence zone in two, resulting in a double
ITCZ. This phenomenon is observed only during non–
El Niño years, and is quite well defined during cold
events (La Niña years) when the thermocline is sufficiently shallow to allow the effects of upwelling to impact the heat budget of the oceanic mixed layer. A schematic illustration of an equatorially symmetric double
ITCZ circulation with its associated SST profile isshown in Fig. 9a. This situation was present during thecold event of 1996 (recall Figs. 5, 6, and 7). When thesea surface is warmer to the north of the equator than
to the south (e.g., in the spring of 1995), convection in
the northern branch of the double ITCZ is stronger thanthat in the south (Fig. 9b). The numerical modelingstudy of Hack et al. (1989) suggests that the northern
ITCZ may act to suppress convection in the southern
ITCZ through large-scale subsidence. During El Niñoevents when the equatorial cold tongue is suppressed(e.g., in 1997), deep convection occurs over the warm
sea surface throughout the equatorial region (Fig. 9c).
5. Summary
The present work illustrates how simultaneous retrievals of cloud liquid and ice water and relative humidity
profiles from SSM/T-2 radiances can be used to observe
the strength and vertical extent of atmospheric convection
and large-scale vertical motion. These remotely sensed

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FIG. 6. Time mean (24 Mar–7 Apr) (left) sea surface temperature and (right) surface wind vectors and divergence for (top–bottom) 1995–
97. The SST field is contoured every 18C between 268 and 298C with values in excess of 268C shaded. The surface wind divergence field
is contoured every 2 3 1026 s21 between 24 3 1026 s21 and 4 3 1026 s21 with values less than 22 3 1026 s21 shaded.

observations are particularly useful in data-sparse regions
like the tropical eastern Pacific. To the best of our knowledge, this study represents the first detailed examinationof the three-dimensional structure and temporal evolutionof the double ITCZ in the eastern tropical Pacific. The
results may be summarized as follows.

1) The double ITCZ, defined by the simultaneous occurrence of deep convection on either side of theequator, is a short-lived phenomenon of approximately 6–8 weeks’ duration, present during Marchand April.
2) Each convective branch of the double ITCZ, located

FIG. 7. Time mean (24 Mar–7 Apr) relative humidity height–latitude cross section at 1108W
for (left) 1996 and (right) 1997.

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FIG. 8. Comprehensive Ocean–Atmosphere Data Set mean sea level pressure for Mar–Apr and Sep–Oct.

near 58–78 latitude, extends across approximately 708
longitude.
3) The convection and associated upward motion in the
double ITCZ is deep, as indicated by the simultaneous occurrence of cloud ice and liquid water aswell as by the vertical displacements of relative humidity surfaces throughout the lower and midtroposphere (surface–7 km).
4) Areas of deep convection within the double ITCZ
are accompanied by maxima in surface wind convergence. The coincident maxima in convection andsurface wind convergence are located several degrees equatorward of the highest sea surface temperatures.
5) The conditions necessary for the formation of a double ITCZ appear to be (a) a general region of lowlevel wind convergence with a warm sea surface inthe deep Tropics, and (b) a narrow sea surface equatorial cold tongue. The former condition occurs every year during March and April, while the lattercondition is present during non–El Niño years andis strongest during La Niña years.
Although beyond the scope of this study, it remains to
be understood why boreal spring is favored over theboreal autumn for the occurrence of an equatoriallysymmetric circulation (e.g., a double ITCZ). It is hopedthat the present work will stimulate further research of
convection and atmospheric vertical motion based onmoisture retrievals from the SSM/T-2.
FIG. 9. Illustration of atmospheric motion in the meridional plane
for the eastern Pacific ITCZ during the boreal spring of (a) a symmetric meridional SST La Niña year, (b) an asymmetric meridional
SST non–El Niño year, and (c) an El Niño year.

Acknowledgments. We are grateful to the following
sources for providing data: NASA Marshall Space
Flight Center (SSM/T-2), NOAA Climate Diagnostics
Center (MSU Precipitation), and NOAA Climate Pre-

1 MARCH 2001

LIETZKE ET AL.

diction Center (SST). The ERS surface winds were provided courtesy of T. Hoar (NCAR). This research was
funded by the Center for Geosciences, Phase II at CIRA/
CSU under DoD Grant DAAH04-94-G-0420 and by
NASA Grant NAG5-3449.
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