The Monthly ITCZ Central Latitude

By Andy May

The Intertropical Convergence Zone or ITCZ is where the trade winds from the Northern and Southern Hemisphere converge and where the column-integrated meridional (north-south) circulation and the “near-surface meridional mass flux” vanishes according to Adam et al., 2016. For a history of the discovery of the ITCZ see Nicholson, 2018. The ITCZ is not the solar equator, the latitude where the Sun is directly overhead at noon, but it is closely related to it, and they move in a coordinated fashion. The ITCZ is an oceanic phenomenon and doesn’t really exist over land in the same way as described here, except in coastal areas (Nicholson, 2018).

The Sun is almost directly overhead at noon each day in the ITCZ, and it delivers over 940 W m^-2 of power to that location at noon, which is enough to raise the water temperature to 86°C or 187°F on a clear day, absent a cooling mechanism. The solar radiation causes intense evaporation which carries away a lot of latent heat. When the heat carried away by convection and conduction is added to the escaping latent heat, the surface water is cooled to a more reasonable 20° to 30°C (Sud et al., 1999). Since water vapor is much less dense than dry air, the moist air rises. The result is deep convection that can sometimes reach the upper troposphere and even the stratosphere (Gettleman et al., 2025). When the rising air hits the more stable stratosphere it spreads horizontally, with a significant poleward component in both hemispheres.

My new paper (May, 2025) shows that the ITCZ location affects the distribution of atmospheric properties and moves the location of the peak altitude of the molar density intersection with it throughout the year. The monthly extremes of the ITCZ are shown in figure 1.

As noted in (Nicholson, 2018), the ITCZ is an oceanic phenomenon, so the extreme positions shown in figure 1 in Asia, Africa, and South America are probably not real. I didn’t use these positions in my study but approximately identified central ITCZ latitudes for each month using several measures.

The location of the ITCZ

A summary of the latitudes and the evidence used is given in Table 1. These recent (~1990 to 2025) locations are approximate and vary from year to year, century to century, and millennia to millennia (Yuan et al., 2023). Yuan, et al. document that the ITCZ moved dramatically northward from ~3500BC to ~2000BC and then dramatically southward from ~500BC to ~500AD when it reached its southernmost position since the last glacial period.

Month	Waliser, 1993, figure 4	Zero v_i (N-S wind speed)	Minimum Total mass flux	Near zero N-S Mass flux	ITCZ wind	ITCZ Latitude used
January	-6	8	-10	-10	-4.0	-5.0
February	-5	10	-10	-6	-2.0	-3.5
March	-4	2	-10	-5	-4.3	-4.2
April	0	0	0	-2	-0.7	-0.3
May	4	0	0	0	0.0	2.0
June	5	10	0	-2	2.7	3.8
July	8	3	20	-8	5.0	6.5
August	9	10	20	-1	9.7	9.3
September	8	10	10	11	10.3	9.2
October	5	0	0	0	0.0	2.5
November	1	-1	0	-10	-3.7	-1.3
December	-2	3	-10	-6	-4.3	-3.2

Defining the ITCZ

In both model studies and studies of historical data there is uncertainty about the location of the ITCZ and the distribution of tropical rainfall (Jung et al., 2023). There are many definitions of the ITCZ and none of them are universally accepted, further they often conflict with one another. It has been called the climatic equator or the meteorological equator and represents a narrow, tropical band of low pressure, rising air, and heavy precipitation. One cause of confusion is that the area of lowest tropical pressure and the lowest meridional mass flux is not always the location of the most intense precipitation and the heaviest clouds (Nicholson, 2018). Another source of confusion is that the tropics sometimes seems to contain two ITCZ bands of clouds and storms, this happens often in the month of April, see slide 8 here.

While the definition of the ITCZ is unclear it is one of the central features of the global water cycle and yet modeling it is problematic and very sensitive to model resolution (Yuan et al., 2023) & (Jung et al., 2023). This includes global circulation models as well as weather reanalysis models. This is probably the result of the complexity of the Hadley circulation and the multiple ITCZ definitions used. Is the ITCZ the location where meridional (north-south) mass flux ceases or where the column integrated meridional energy flux vanishes (Adam et al., 2016)? Is it the location where the north-south wind speed vector reaches zero? Is it the location where tropical precipitation is maximal (Jung et al., 2023) and (Marshall et al., 2014)? Or is it the location where the meridional stream function as computed from air pressure is the lowest (Byrne et al., 2018)? All these definitions have been used, and they do not all happen at the same location (Adam et al., 2016) & (Nicholson, 2018).

About one-third of Earth’s precipitation falls within the ITCZ (Clark et al., 2024) and (Kang et al., 2018). All models create an ITCZ-like precipitation feature near the equator due to the intense insolation at that location (Jung et al., 2023). However, the details of the ITCZ vary dramatically as the model’s resolution changes. Increasing the model cell size (decreasing the horizontal resolution), while reducing convection, increases rainfall in the ITCZ, while decreasing the cell size decreases the ITCZ rainfall (Clark et al., 2024). Larger cells produce more low-level clouds, accelerate the Hadley circulation, as well as increasing precipitation. Smaller cells increase energy transport efficiency, weakening the Hadley circulation, and reducing precipitation. This serves as a warning when interpreting model results, many critical weather processes can only be modeled at very small scales, that is at very high resolution.

While the ITCZ is usually thought of as a narrow band of deep convective clouds and intense precipitation that circles the Earth (Adam, et al., 2016), I prefer to think of it as the point where the north-south wind component (meridional transport) reaches zero. Because the radiosonde data is mostly over land this definition is hard to use because the presence of land confuses wind direction and speed measurements. Thus, the ITCZ location is hard to detect with radiosonde measurements. Also, due to land, its location is not a circular path around the earth, but wavy between the extremes shown in Fig. 1 (Britannica & Chmielewski, 2025).

Locating the ITCZ

Exactly why the distinctive tropical band of clouds surrounding the Earth is so complex on average is not completely understood and often hotly debated (Nicholson, 2018). The constantly moving zonal-mean ITCZ central latitude is the origin of the ascending branch of the Hadley circulation (Adam et al., 2016). The cartoon drawings of the so-called northern and southern “Hadley Cells” are oversimplified (Connolly et al., 2021) and (Connolly, 2025), because they do not have a fixed location and their mean air flow is not north and south, but a rather complex constantly moving three-dimensional pattern with a significant meridional (north-south) component.

Besides the mean ITCZ latitudes provided by Waliser, et al. (column 2 in Table 1), the vector-average north-south wind speed (v_i), the minimum total mass flux (in any direction), and the north-south mass flux were all examined by latitude. An example illustration is shown in Fig. 2 for the month of September. The upper left plot in Fig. 2 is the east-west (zonal) wind speed vector (“u”) plotted by latitude. Positive values indicate the u wind component is blowing toward the east, the opposite of the meteorological convention. This measure was initially examined but ultimately rejected as an ITCZ latitude indicator because it disagreed with the other measures.

The top-right plot in Fig. 2 is the north-south wind speed vector (v), with positive values indicating the v wind component is blowing toward the north. The v component crosses zero wind speed at about 10° N, a latitude marked by the vertical line. The lower left plot shows that the north-south total mass flux for July reaches a minimum between the equator and 20° N, the central value at 10° N is lowest so it is the value chosen. The plot on the lower right is the north-south vector average mass flux for September. According to Adam, et al. this is the value that should be zero at or near the ITCZ location. The value reaches very close to zero at about 11° N. Table 1 lists all the values for all months.

How to define and locate the ITCZ is a subject of debate (Nicholson, 2018). But it is usually detected globally by following either the deep convective cloud pattern near the equator or the heavy precipitation that accompanies it (Marshall, et al., 2014) & (Clark et al., 2024). The ITCZ doesn’t really exist in any organized way over land, except in coastal regions, so it may not be detectable in land-based radiosonde data (Nicholson, 2018).

While using tropical cloud patterns and maximum tropical precipitation to locate the ITCZ can be debated, I used these as two of the factors to consider when picking the ITCZ latitudes used in this study (Nicholson, 2018) & (Adam et al., 2016). Further complicating the picture is that the location of the ITCZ does not move smoothly, but mostly in two jumps (Hu et al., 2007). These jumps are from the Southern Hemisphere to the Northern Hemisphere around mid-April and from the Northern Hemisphere to the Southern from around mid-December. This inter-hemispheric movement is accomplished in about 10 days and are the most abrupt jumps in the ITCZ during each year (Hu et al., 2007).

Figure 3 is based only on precipitation and cloud cover, which are only two of the criteria for the ITCZ, but they are important. The precise reason why the ITCZ jumps abruptly across the equator is unknown but probably related several factors: cooler temperatures at the equator due to equatorial upwelling of deeper waters in the Pacific, monsoons, and enhanced convection off the equator (Hu et al., 2007). Convection always favors warmer SSTs, so the ITCZ jumps from one warmer region to the other and skips the cooler temperatures at the equator (Hu et al., 2007).

One known multi-year-scale driver of the ITCZ location is ENSO (Adam et al., 2016), but there may be others. The central latitudes in Table 1 are rough, but act as a guide on the maps. September, August, July, and April have a fairly well-behaved and flat ITCZ in the Pacific, but other months do not. The ITCZ is hard to locate over southeastern Asia due to the South Pacific Convergence Zone northeast of Australia. This zone of converging winds trends southeast to northwest (see Fig. 4) complicates locating the ITCZ over Indonesia.

As described by Adam, et al. (2016), the ITCZ is often considered to be the location of the precipitation maximum in the tropics, the darker colors in Fig. 4. It is also the location of the densest cloud cover in the tropics and the place where the near surface meridional (north/south) mass flux disappears and low-density moist air rises to great heights, sometimes reaching the stratosphere. Further, as Adam, et al. write, it is also the location of the energy flux equator where the column integrated meridional energy flux vanishes. To choose the central ITCZ latitudes shown in Table 1 we examined NOAA’s precipitation maps for 2024 and 2025, our computed North/South wind speed vector (“v”), the total mass flux, and the North/South mass flux vector component.

Plots like figures 2 and 4 for all months can be downloaded and viewed here. The various methods used to pick the central latitude do not always agree, in choosing the central latitude I emphasized the mean north-south mass flux and north-south wind speed vector over the other estimates since these are the most direct measures. The 2024-2025 NOAA precipitation maps were secondary and often the chosen latitude does not look very good on them in the Northern Hemispheric winter, especially in the Pacific, although from March through September they look OK.

September is usually the final month of the Indian monsoon, a period of intense upwelling of moist air in the Indo-Pacific Warm Pool. The monsoon location is easily identified in Fig. 4 as where the ITCZ and the South Pacific Convergence Zone meet north of central Australia. September is also the month with the highest relative humidity in the tropics at the molar density intersection, as shown in Fig. 5, which also shows the molar density intersection relative humidity for all the monsoon months.

The highest molar density intersection is between 20° S and 10° N, and is above 14 km. It is not easy to get water vapor this high, since it normally precipitates out long before reaching this altitude. As you can see in Fig. 5, the highest monthly mean relative humidity seen in the tropics is in September at 51 %. The other monsoon months, June, July, and August are also high in the tropics. To a lesser extent, so are December, February, January, November, and October. While the ITCZ is constantly moving (see the SSEC geostationary satellite Image animations available here), it does lift (through evaporation and updrafts) moisture into the stratosphere (Raymond, 2017). Once the moisture has frozen out of the air, the cooler and drier air descends toward the surface helping to form subtropical deserts and the ocean gyres, see figures 3 & 5 here or (May, 2025).

Discussion

It is clear that the ITCZ moves north and south seasonally, however the precise monthly location is unknown. Since the ITCZ location changes continuously and is the root of the Hadley circulation, it is important to global weather. It can only be approximately located by studying the atmospheric mass flux, wind speed and direction, and the zone of maximum precipitation and cloudiness, thus it is very hard to pin down.

Works Cited

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