Doldrums

Technically known as the intertropical convergence zone (ITCZ) or the intertropical front, monsoon trough, or equatorial convergence zone, the doldrums are a belt of low pressure surrounding the Earth at the equator. It is formed by the vertical ascent of warm, moist air from the latitudes above and below the equator. The air is drawn into the ITCZ by the action of the Hadley cell, a macroscale atmospheric feature that is part of the Earth’s heat and moisture distribution system. It is transported aloft by the convective activity of thunderstorms.

The position of the ITCZ varies over time throughout the year. Over land, it moves back and forth across the equator following the sun’s zenith. Over the oceans, where the ITCZ is better defined, the seasonal cycle is more subtle, as convection is limited by the relative stability of ocean temperatures. Occasionally, 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. The location of the ITCZ can have dramatic effects on rainfall in equatorial locations, resulting in wet and dry seasons in the tropics, rather than the cold and warm seasons characteristic of higher latitudes. Regions in the ITCZ receive precipitation more than 200 days per year. 

Within the ITCZ surface, winds are slight, unlike the zones north and south of the equator, where the trade winds blow. For this reason, sailors named this belt of calm the “doldrums” because of the stagnation they found their ships in after days of no wind, high heat, and humidity. The ITCZ’s role in the formation of tropical storms depends upon the low-level vorticity as one of its six requirements, as it is a zone of wind change and speed, otherwise known as horizontal wind shear. As the ITCZ migrates more than 300 mi. (500 km) from the equator during the respective hemisphere’s summer season, the increasing Coriolis force enhances the formation of tropical storms within this zone. In the north Atlantic and the northeastern Pacific oceans, tropical waves move along the axis of the ITCZ, causing an increase in thunderstorm activity; under weak vertical wind shear, these clusters of thunderstorms can become tropical hurricanes.