by Tim Vasquez
After the crash of Air France Flight 447 in the Atlantic Ocean on June 1, 2009, the meteorological term ITCZ (Intertropical Convergence Zone) rose to prominence as the latest buzzword of the week. Newspapers, talk shows, and news programs around the world availed themselves of this terminology, explaining the factors that downed the Paris-bound Airbus A330. Google Insights for Search showed that Internet keyword searches for “ITCZ” jumped by an order of magnitude in the week following the crash. Unfortunately, the quick rise of a technical scientific concept in popular media brings many inaccuracies and omissions, and indeed many news outlets referred to the ITCZ simply as a line of strong storms in the tropics.
Ever since large sailing fleets reached the tropics during the eighth century, mariners have studied the strong “trade winds” which blow from subtropical latitudes toward the deep tropics. The subtropical regions where the trade winds start, at about 30° latitude, are known as the horse latitudes. The zone where the trade winds end, meeting the opposing wind flow from the opposite hemisphere, is known as the doldrums. Both the horse latitudes and the doldrums have long been recognized as harboring dreaded zones of light winds where sailing ships occasionally find themselves becalmed for days or weeks. With intercontinental commerce rapidly expanding in the sixteenth century, a scientific understanding of these areas became a subject of intense interest.
Caption: Clouds along the intertropical convergence zone in the western hemisphere, as captured in the very first GOES-14 image on July 27, 2009.
In 1735, English scientist George Hadley attempted to explain the trade winds, building upon a theory developed by astronomer Edmond Halley in 1686. This explained how the winds in mid-latitudes were drawn south by strong heating and also proposed a poleward return current of air in the upper atmosphere. Though some of the assumptions were incorrect, this model of circulation became known as the Hadley cell. Other scientists during the next two centuries built upon the Hadley model, showing that the Coriolis effect causes the upper-level return flow to “accumulate” in subtropical latitudes with great eastward momentum, forcing sinking motion, fair weather, and high pressure to occur. This produces all of the expected weather in the horse latitudes, including subtropical highs like the Bermuda high and the extensive deserts found at these latitudes in the continental interiors.
As the science of meteorology matured in the nineteenth century, explanations for the unsettled and sometimes stormy weather in the doldrums began to trickle forth. The nature of this area was not well-understood, largely due to the lack of dense tropical observation networks, but climatological studies clearly showed that the doldrums were associated with an equatorial pressure trough and moved north and south with the sun's seasonal movements. Research also revealed complexities and discontinuities in the equatorial trough, many of which were not well understood until the advent of geostationary satellites.
Interestingly, the revolution in understanding weather fronts and the development of storms, brought by Norwegian scientists in the 1920s led much of the meteorological community to view the tropics in terms of air mass contrasts and weak fronts. This led to the equatorial trough being reinvented as the “intertropical front”. During World War II, real-time surface weather observations from the tropics became commonplace and weather analysis techniques not only revealed the lack of temperature contrasts but began heavily emphasizing streamline analysis owing to the weak pressure fields. As a result, the ideas of tropical fronts were abandoned and the axis of trade wind convergence agreed upon to be the key analysis feature. The name Intertropical Convergence Zone (ITCZ) was coined.
What is the ITCZ?
By definition, the ITCZ is the axis of confluence of the trade winds; in other words, where the trade winds from the north and south hemispheres meet. Trade winds are mainly an oceanic feature, driven by subtropical high pressure areas, so the ITCZ is found over the oceans. In continental areas of the tropics, such as in India, the Amazon Basin, and central Africa, monsoon wind systems are driven largely by uneven heating of the earth's surface, in combination with heat lows, thunderstorm outflow areas, and other disturbances, and this adds substantial variability to the wind field. The influence of the oceanic subtropical highs are also much weaker in these continental areas. As a result, the trade winds do not meet and might not even be present in the region.
Granted, large-scale weather maps do show a vague convergence of the winds in these continental areas due to the strong equatorial heating at latitudes where the sun is near its zenith. This is what inspired meteorologists in the post-World War II years to draw a continuous ITCZ circling the globe, crossing continental regions in one unbroken chain. However by the 1960s, some researchers were beginning to question this model and warning about the perils of depending on the ITCZ for a forecast. It was demonstrated that trade wind convergence is primarily an oceanic feature, and that the mechanisms for weather vary significantly across the tropics, with or without an ITCZ.
Caption: Captured by the NASA Aqua satellite, a patchwork of convective clouds near Nekemte in western Ethiopia, July 21, 2009, in the tropical rain belt produces showers, thunderstorms, and soft cirrus debris. This marks the location of monsoon rains locally known as the kiremt.
Caption: Mean seasonal positions of the intertropical convergence zone in August (boreal summer) and February (austral summer). The dashed lines indicate secondary tropical convergence zones. Abbreviations are SPCA for Southern Pacific convergence zone, SITCZ for Southern ITCZ, SACZ for South American convergence zone, and IOCZ for Indian Ocean convergence zone.
“This has long been problematic, particularly over Africa,” said Dr. Sharon Nicholson, a tropical weather researcher at Florida State University. “It is often assumed that the wind convergence, cloudiness and rainfall maxima are co-located, and that this defines the ITCZ. My recent work shows that this is absolutely not the case.”
Nicholson explained that this ambiguity causes problems tracking tropical rainfall: “A general misconception is that, over Africa, the northward progression of the ITCZ is a good indicator of rainfall, so that by predicting its position you can predict rainfall. The relationship is not very strong at all with wind convergence. While the meteorological community has, to some extent, come to understand this, many in geology, agriculture, and other sciences still try to interpret climate variability in the context of movements of the ITCZ.”
Meteorology now holds that a true ITCZ is found only over the oceans where a distinct convergence of trade winds occurs. Furtherrmore, “ITCZ” refers to the convergence of the winds, not to the pressure troughs and rain bands themselves. The word “tropical rainbelt” is considered a suitable term for belts of rainy weather that are found near the equator, and they may have more to do with the position of pressure troughs, old outflow boundaries, or cool upper-level temperatures than with convergence of trade winds.
Weather in the ITCZ
Following the crash of Air France Flight 447, newspapers began painting the ITCZ as a feature that harbors the most dangerous thunderstorms on the planet. Though the crash did indeed occur right at the location of a classic Atlantic ITCZ, strong instability and intense surface heating, which are factors in most violent thunderstorms, do not occur over ocean waters since the waters are very efficient at absorbing the sun's radiation. Organized weather systems can occasionally develop on the ITCZ and produce strongly unstable environments, but this occurs over the warmest ocean currents or in areas well away from the equator where the Coriolis effect can allow organized systems to deepen. Neither of these ingredients was present near the crash location.
Furthermore, there is a growing body of evidence that cumulonimbus clouds over oceanic areas may not actually produce any significant thunder or lightning. Even the tallest towers with ample instability might not produce any lightning. The reasons for this are not clear, but meteorologists account for this by avoiding use of the word “thunderstorm” and adopting the phrases “deep convection” and “showers”.
Showers can form where the ITCZ is found, but the ITCZ does not have a monopoly on rain production. Showers can develop anywhere there is strong heating, moisture, and sources of lift, and rain can be quite heavy. Organized bands of rain may be found anywhere in the tropics, not just on an ITCZ. The sheer depth of the troposphere in the tropics allows the highest cloud tops to reach 50,000 ft and sometimes even more than 70,000 ft MSL.
The great vertical dimensions of tropical showers were cited as a factor in the disappearance of Air France Flight 447 on June 1, 2009. The plane crashed about halfway between Brazil and west Africa after crossing through a cluster of deep convection, with satellite imagery indicating a storm top of 55,000 ft. In spite of this, spaceborne and ground-based lightning detection networks indicated no electrical activity. But lightning is not necessary for a storm to have strong turbulence, icing, or even hail at flight level. Preliminary investigation work suggests that the weather in the cumulonimbus clouds were a factor and that the plane's airspeed sensors succumbed to some combination of equipment failure, icing, or precipitation contamination.
Caption: Idealized cross-section of the tropics in July, showing the wind flow and the structure of cloud layers. Over the ocean the rain belt is often co-located with the ITCZ and precipitation is weaker, but over the continents the winds and boundaries are much weaker and nondescript and precipitation is heavier. Abbreviations are: Cb, cumulonimbus; Ci, cirrus; Ac, altocumulus; As, altostratus; Cu, cumulus; and Sc, stratocumulus. Blue wind arrows indicate relatively cool, dry air arriving from the winter hemisphere.
The ITCZ and Climate in the Tropics
When a seasonal wind shift occurs, it is often referred to as a “monsoon”. Monsoon wind shifts are strongly tied to the season of peak heating and might coincide with the arrival of equatorial pressure troughs and the ITCZ, both of which follow the sun's seasonal migrations north and south. All of these factors bring changes in wind, instability, and moisture, as well as additional mechanisms for low-level wind convergence, which favor the development of rain and thunderstorms. In subtropical Africa and Asia, which depend on the monsoon for annual crop harvests, the rainy season tends to arrive in June and July.
Tropical cyclones, including hurricanes and typhoons, are popularly associated with easterly waves, which are disturbances in the trade wind belt that migrate westward. But they are more often the result of disturbances within the ITCZ, including those portions in proximity to an easterly wave. During most of the year the ITCZ is so close to the equator that the lack of Coriolis force prevents lows from deepening. But portions of the ITCZ that lie at 15° latitude or farther poleward can be potent breeding grounds for hurricanes and typhoons. In such cases, the ITCZ can form a wavelike pattern and undergo a process known as “breakdown”, producing one or more weak cyclones which might then intensify further if wind shear, sea-surface temperature, and Coriolis force are favorable.
Due to the weak winds in the tropics and the lack of observations, science has been slow to understand the dynamics and mechanics of the ITCZ, the monsoon, and tropical rainbelts in general. For example, observational understanding does not fully address why the ITCZ prefers to remain north of the equator or separates into multiple bands. During the past 20 years, however, research with numerical modelling has led to great advances in our knowledge of this feature.
The feature is tracked daily by the Climate Prediction Center (CPC) in Camp Springs, Maryland.
In the Sahel, a minor variation in the position and intensity of these features makes the difference between prosperity and famine, factors which are momentous enough to change the political landscape of Africa. In Central America, a stalled ITCZ might bring about landslides, spoiled crops, and the emergence of epidemics. And for the United States, a minor change in the nature of the Atlantic ITCZ has an enormous effect on what takes place in the hurricane breeding grounds off west Africa. Tropical climatology, the tropical rainbelt, and land-sea interactions bring consequences that hape not only the day-to-day forecast but the content of history books, and understanding them will be one of the most important issues in twenty-first-century meteorology.
Caption: National Centers for Environmental Prediction surface chart for 2 a.m. EDT, August 2, 2009, showing the ITCZ (dashed red line) as defined by the convergence of the northern and southern trade winds. In continental areas, the ITCZ disappears and wind is dominated mostly by variations in solar heating. Isopleths are stream function lines, which are roughly parallel to the wind direction.