Being stuck in the doldrums is now used in everyday speech to mean being in a state of inactivity, mild depression, listlessness or stagnation. However, ‘the Doldrums’ is originally a name given to the Intertropical Convergence Zone, or ITCZ.
The Intertropical Convergence Zone, is the region that circles the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together. The water in the equator is warmed by the intense sun which in turn heats the air in the ITCZ, raising its humidity and making it buoyant.
Aided by the convergence of the trade winds, the buoyant air rises. As the air rises it expands and cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms.
The Dreaded Belt of Calm
Early sailors named this belt of calm “the Doldrums” because of the inactivity and stagnation they found themselves in after days of no wind. In an era when wind was the only effective way to propel ships across the ocean, finding yourself in the Doldrums could mean death.
Today, we often hear about the ITCZ or Doldrums in relation to sailing and races such as the Transat Jacques Vabre. Sailing through this belt greatly impacts the racers – as it is more difficult to move quickly through the water, and frequent thunderstorms make the conditions even more difficult.
Location of the ITCZ
The Doldrums are generally located between 5 degrees latitude north and south, but they can extend as far as 18, depending on the season.
The location of the intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun’s zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures.
Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other.
When was it identified?
The ITCZ was originally identified from the 1920s to the 1940s as the “Intertropical Front” (ITF), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term “ITCZ” was then applied.