In the Doldrums

Storm’s Brewing: The ITCZ is a cause of much danger at sea

They’re the most treacherous seas on the planet. At those that lie beneath the Intertropical Convergence Zone (ITCZ), known by sailors as the ‘doldrums’ or ‘unwise tantrums’ for their dangerous tendency to lurch between extremes, almost absolute calm can unexpectedly degrade into raging thunderstorms and hurricanes. Many a sailing ship has become caught on the still waters, with no wind for propulsion for days or weeks on end and the crew slowly dying of hunger and thirst while they wait for a storm to brew. On land, the zone, which is usually centred between  and N, is associated with the heaviest rainfall and some of the most severe droughts on Earth. This is the force that drives the West African monsoon and is a key component in the East Asian equivalent that dumps vital torrents of rain on India and China every year. Any changes to its strength or behaviour can have profound effects on societies and ecosystems. It’s therefore essential for us to understand what drives this curious and dangerous phenomenon.

Close to the equator, trade winds from the north east and south east meet and merge together. This convergence pushes warm air masses upwards, carrying vast quantities of water vapour up to high altitudes, where it cools and condenses into a thick band of cloud, pregnant with rain. Most of the time, conditions under this cloud are eerily calm. But every couple of weeks, the water erupts in a deluge of rain and thunderstorms. But because the band does not remain in a fixed position, the region over which this rain falls changes over time. Its location is determined by the flow of heat between Earth’s two hemispheres. Because an atmospheric flow known as the Atlantic Meridional Overturning Circulation (AMOC) moves heat northwards, the northern hemisphere is always slightly warmer than the southern. To balance this out, warm air flows southwards across the equator, and the convergence zone lies across the latitude at which this flow begins. If the northern hemisphere warms relative to the southern hemisphere, the flow of heat is stronger and begins at a higher latitude, so the ITCZ moves northwards. If the southern hemisphere warms relative to the northern, it moves southwards.

Over the course of a year, this behaviour causes the region about the equator to experience a cycle of draught and rain: as the ITCZ moves northwards in the northern hemisphere summer and southwards in its winter, land at any particular latitude switches from being hot and dry when the ITCZ is absent to extremely wet when returns. That’s why in the tropics there are ‘wet’ and ‘dry’ seasons rather than ‘warm’ and ‘cold’ ones. Countries a few degrees south of the ITCZ’s mean latitude experience their wet season in the months surrounding December when it moves to its southernmost point; those a few degrees north have their wet season closer to June. Countries very close to the equilibrium ITCZ position experience two rainy seasons as it moves back and forth across them.

But the zone’s overall north-south range can also change, over longer timescales. Over the past few hundred thousand years, there have been times when the northern hemisphere has been warmer or cooler relative to the southern hemisphere than it is today. These shifts in relative temperature have been shown to match very closely with the amount of rainfall recorded in basin sediment records, showing that the entire ITCZ cycle moved southwards when the northern hemisphere was relatively cooler. Furthermore, the monsoon was much weaker in these episodes. For the last few thousand years, Earth’s perihelion – that is, the point in its orbit at which it is closest to the sun – has shifted from the northern hemisphere to the southern hemisphere summer (currently it occurs in February). That’s reduced the hemispheres’ temperature difference and caused the ITCZ to slowly move southwards, and is the primary reason why the Sahara, which a few millennia ago was dotted with lakes and oases, is now so dry and desolate.

In recent decades, this trend has been sped up, probably because of emissions of aerosols over the industrialised northern hemisphere, which reflect the sun’s light and cool it down. The resulting failure of the African Monsoon to penetrate very far northwards caused drought and crop failure. Now, anthropogenic emissions of greenhouse gasses, which tend to warm the northern hemisphere more rapidly than the southern, may be beginning to offset this effect. Either way, the message is clear: the ITCZ is dangerous at sea, but essential to human societies on land. It is nothing if not wild and temperamental. But by polluting our atmosphere, we can inadvertently create dramatic shifts in its position, shifts that could have devastating consequences for life in the tropics.