Intertropical convergence zone

As business aircraft continue to expand their range and ceilings, more pilots based in middle-latitude locations are finding themselves planning intercontinental flights that cross the equator or reach deep into tropical latitudes.

Unfortunately, the region is one that can fill the sky with lines of towering cumuli. Even pilots cruising the stratosphere may be unable to avoid them, as the tops of these clouds often top out well above FL500. Understanding the behavior and hazards of the intertropical convergence zone (ITCZ), can help pilots find a smooth ride when traversing the tropics.

What is the ITCZ?

Earth’s atmosphere is driven by global differences in solar heating. While the tropics receive regular insolation throughout the year, the high latitudes lose more heat to space than they receive from the sun.

Visible satellite image of ITCZ convective clouds over the eastern tropical Pacific Ocean. Although the ITCZ is a band of surface air convergence circling the Earth, cloud coverage is often intermittent or even absent due to localized conditions. However, pilots crossing the ITCZ should remain vigilant, as turbulence and developing storms can occur anywhere along the ITCZ axis.

Because temperature and air density are inversely related – meaning as temperature rises, air density drops – and air flows from areas of high density to low density, there is a net movement of surface air from the poles toward the equator. At the same time, the surface air flowing into the tropics from both the north and south poles converges in the tropics.

Not being able to continue horizontally, and being heated by the surface, the air must rise. At the top of the troposphere it is forced to diverge and return toward the poles, completing the density circuit.

The ITCZ is the term for this region of surface convergence and corresponding region of convection in the tropics. It is often described as a global belt of convective storms that migrate north and south through the tropics, depending on the position of the sun through the year and on whether it is over land or ocean. However, it is often much wider than just a single line of storms. In some places, the actual convergence zone may be hundreds of miles wide.

On average, and particularly over the tropical oceans, which don’t vary too much in temperature through the year, the ITCZ can normally be found between the equator and around 10° N, moving to the north during the northern hemisphere summer, and to the south during the winter. Over the land, however, the ITCZ may migrate to as much as 25° latitude due to the additional heating during the warm months.

This seasonal movement of the ITCZ is why many locations in the tropics and subtropics experience distinct dry and wet (monsoon) seasons. Closer to the equator, most places experience either year-round wetness or 2 wet seasons as the ITCZ moves back and forth over the area.

When the ITCZ is in the vicinity of a location, the result can be weeks of storms and heavy rain showers. Some of the challenges this presents to pilots are decreased visibility, flooding, and embedded thunderstorms. Heavy rain and the precipitation fog that can occur when the lower atmosphere becomes saturated from rainfall both can restrict visibility and depth perception.

In rising terrain, precipitation fog blends in with low ceilings to obscure dangerous terrain. Flooding on runways can reduce traction and braking effectiveness, and may even cause hydroplaning. In many places, monsoon rains can block access roads and even cause water damage to aircraft on the tarmac.

Although most tropical thunderstorms aren’t considered strong or severe, they still contain internal forces that can damage or destroy an aircraft. The same rules for flying near thunderstorms hold in the tropics as they do elsewhere. Avoid flying within 20 miles of a storm, and never fly beneath a storm.

One situation where the ITCZ does produce severe storm conditions is in its formative role in tropical cyclones. One of the factors needed for a tropical cyclone to develop is low-level vorticity, which the horizontal windshear present in the ITCZ provides. In fact, most tropical cyclones begin their life as a simple, disorganized cluster of ITCZ thunderstorms, with a easterly wave traveling along the ITCZ axis serving as a catalyst.

Icing and turbulence

Aloft, the towering cumuli of the ITCZ pose different hazards. Naturally, wherever there is convection, there is also a good chance of encountering turbulence. Although turbulence from ITCZ convection is most often light to moderate, severe turbulence is not uncommon.

While stronger turbulence is normally found within the clouds, turbulence is also found in the areas between convective cells, and, frequently, the unlimited water vapor above the tropical ocean can mean that any convective cells are embedded. At night or in IMC at any altitude, pilots should make use of onboard radar to avoid active cells and areas of indicated turbulence.

January and July average tracks of the ITCZ. The ITCZ follows the position of the sun through the year, resulting in monsoon or wet and dry seasons in many parts of the world.

In addition, ITCZ storms can top out at above 40,000 ft, with some reaching heights of around 60,000 ft. Not only do these heights eclipse the service ceilings of most aircraft – they also place abundant amounts of water well above the freezing level. Any time the temperature is below freezing and water is present in liquid form, pilots should expect icing and activate any anti-ice systems.

Given the energy contained in one of these ITCZ cells, pilots should not be surprised to find supercooled liquid droplets at incredibly high altitudes, with no way to climb over the clouds that contain them. In fact, it was an encounter with supercooled droplets in the upper reaches of ITCZ towering cumuli that brought down an Airbus A330 over the Atlantic in 2009. This high-profile accident spurred debate over potential overreliance on fly-by-wire control and decision systems, particularly when flying through potentially hazardous weather conditions.

Planning for the ITCZ

Fortunately, with its connection to the position of the sun throughout the year, and the fact that the ITCZ is a region of more-or-less continuous convergence and convection, forecasting the ITCZ is relatively straightforward. Depending on the time of year, and whether it is over land or ocean, pilots can anticipate where they are likely to encounter ITCZ storms or monsoon conditions at the surface.

Pilots can also locate the ITCZ easily on satellite images. Hint: Look for the ribbon of clouds around the equator. On significant weather charts, the tropical thunderstorm and turbulence regions will point the way. Particularly over the ocean, where few observations exist, even if your aircraft is producing routine automated reports, extra weather information around the ITCZ is always helpful.