Widening of ITCZ and Its Relation to the Tropical Broadening
from AIRS Measurements
Yibo Jiang, Hartmut H. Aumann, Yuk L. Yung
Jet Propulsion Laboratory, Californian Institute of Technology, Pasadena, CA
Abstract
We have analyzed the tropical ocean high cloud location distribution from the
measurements of the AIRS (Chahine, 2006) satellite between 2003 and 2010. It shows
the evidence of the widening of ITCZ of about 0.95o/decade in the northern hemisphere
and almost no trend in the southern hemisphere. This trend is consistent with the tropical
belt broadening rate. Because of the complex relationship between ITCZ and the Hadley
circulation, this raises the question about the mechanism of the widening of tropical belts.
Introduction
Of all the various phenomena that characterize the tropical region is the intertropical
convergence zone (ITCZ). The ITCZ lies in the equatorial trough and constitutes the
ascending branch of the Hadley circulation. This permanent low pressure feature marks
the meteorological equator where the trade winds, laden with heat and moisture from
surface evaporation and sensible heating, converge to form a zone of increased mean
convection, cloudiness, and precipitation. The rising air eventually spreads poleward
when it reaches the top of the troposphere, and it sinks to the surface in the subtropics
about 30N and 30S due to radiation emission.
The Hadley circulation is the largest circulation system on the Earth. The ITCZ and
Hadley cells are not stationary, and they move over the year with the position of the sun.
It is essential to understand its past and future in order to improve our understanding of
long-period climate variability.
Recent studies found that in climate model simulations the jet streams and the associated
wind and precipitation patterns tend to move poleward under global warming. As the jet
streams are indicators of the poleward limits of the tropics, this implies that the tropics
will expand as the Earth warms. Based on these studies, it appears that climate forcings
over the twenty-first century would be expected to lead to an expansion of the tropics by
as much as 2 degrees latitude. The observed widening appears to have occurred faster
than climate models predict in their projections of anthropogenic climate change. Indeed,
considerable uncertainty surrounds the mechanisms causing the observed widening of the
tropics and, specifically, of the Hadley circulation.
Analysis indicates that the connections between the ITCZ and Hadley circulation are
complex. The expansion of the tropical belt will definitely result in the expansion or the
migration of the ITCZ or vice verse.
Our interest in the latitude changes of high cloud is motivated by several recent research
on the tropical belt broading from different factors such as total column ozone [Hudson et
al., 2006], the zero crossing of the mean meridional mass stream function [Frierson et al.,
2007; Johanson and Fu, 2009], and the frequency of the high tropopause levels [Seidel
and Randel, 2007; Birner, 2010]. The tropical high clouds represent visually the locations
of the ITCZ. As the starting points of the Hadley circulation, the high cloud analysis
should present more information on the mechanism of the tropical belt broading.
Data & Method
AIRS is a grating spectrometer was launched on the EOS-Aqua satellite in May 2002 and
it measures spectra from 650cm-1 to 2700cm-1 by 2378 channels which are separated into
17 modules of detectors which do not overlap. AIRS scans to ±49.5o cross track as the
satellite moves forwards taking 90 spectra each with an instantaneous field of view of
1.1o in a row perpendicular to the direction of motion of the satellite. This results in a
ground footprint of 13km diameter at nadir but closer to 41km by 22.4km at the largest
zenith angles.
We used both day time and night time high cloud data from AIRS ClimSubset. The high
cloud is defined as. The 3σ outliers in both data sets are filtered out.
Figure 1 shows the typical distribution of the tropical high clouds over oceans between
southern 30o latitude and northern 30o latitude as observed by AIRS satellite in October,
2009. The ITCZ is clearly shown. Figure 2 shows the distribution of tropical high clouds
in percentage of measurements for the same time period as Figure 1. The locations of the
highest percentages in both northern and southern parts clearly show the locations of the
ITCZ cores, with about 6.5% for northern ITCZ and 3% for southern ITCZ. It indicates
the location of the ITCZ we defined in this paper. The two red lines show the locations of
northern hemisphere ITCZ, one line shows the location of the core (max distribution) and
the other line shows the location of the wing or boundary (defined as the latitude where
there are total 16% of measurements beyond this latitude). The two red dashed lines
represent the locations of southern hemisphere ITCZ locations and are defined in the
same way as that of the northern hemispheric ITCZ.
Figure 1. Locations (blue dots) of high clouds observed by AIRS satellite in October,
2010 over tropical ocean between south 30 degree and north 30 degree.
Figure 2. High clouds distribution (black line) observed by AIRS satellite in October,
2010 over tropical ocean. The red lines and red dot lines represent the locations of
northern ITCZ and southern ITCZ respectively.
Results
Figure 3 shows the monthly averaged ITCZ core and wing latitude for both northern and
southern hemispheres from 2003 to 2010 as observed by AIRS satellite over tropical
ocean. The averaged ITCZ core and wing locations are around 6o and 12o for northern
hemisphere, and -6o and -12o for southern hemisphere. Both hemispheres show similar
seasonal variability. The northern ITCZ core moves away from the equator in the summer
and move back to close to the equator in other season. The southern ITCZ core follows
the northern ITCZ core seasonally. The locations of the core seem to have more features
as compared to the location of the wing.
The latitude difference between ITCZ core and wing is defined here as the ITCZ width
for both hemispheres. Figure 4 shows the monthly averaged ITCZ width from 2003 to
2010. The linear trend is estimated using standard linear regression analysis, and the
estimated trends and their 95% confidence intervals for each hemisphere are summarized
in Table 1. The ITCZ width in both hemispheres has a trend around 1o per decade which
is consistent with the tropical belt broadening trends from other analysis.
The ITCZ expansion maybe related to the tropical belt broadening. This indicates that the
tropical belt broadening may be caused by the ITCZ expansion that the Haldley
circulation is push polewards in both hemispheres. Previous study [Aumann et al.] clearly
shows the direct link between the formation of tropical high clouds and sea surface
temperature. Therefore, the sea surface temperature play the key role for the expansion of
the ITCZ and tropical belt which contradicts to the model study by Lu at al. [2009] whos
experiments shows the narrowing of the tropics by sea surface temperature forcing alone.
The model study is needed to find out the direct connection between ITCZ expansion and
tropical belt broadening.
Figure 3. Monthly averaged northern core latitude (black line), northern wing latitude
(black dash line), southern core latitude (red line) and southern wing latitude (red dash
line) from 2003 to 2010 as observed by AIRS satellite over tropical ocean.
Figure 4. Monthly averaged northern belt width (black line) and its linear fit (black dash
line), southern belt width (red line) and its linear fit (red dash line) from 2003 to 2010 as
observed by AIRS satellite over tropical ocean.
Summary
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