The monsoon is not simply a giant sea breeze after all
For a long time the monsoon was explained as a sea breeze on a continental scale: land heats faster than water, so moist ocean air rushes inland and rains out. That picture is no longer thought to be the cause. Scientists now tie monsoons to the yearly migration of a planet-wide rain belt.
That belt is the Intertropical Convergence Zone, where trade winds from both hemispheres meet near the equator and air rises in towering storms. Over the year it swings north and south, dragging seasonal rains with it. In West Africa it moves up from the equatorial Atlantic in February, reaches the region around 22 June and retreats by October. While it is overhead, rain-bearing southerly winds interrupt the dry northeasterly harmattan, and the Sahel and Sudan get most of their yearly rainfall.
Land and sea still matter, because they set how far the belt can travel. Water holds heat well, roughly 4 joules per gram for every degree, and mixes warmth down as far as 50 metres. Soil, sand and rock hold only a tenth as much or less and pass heat downward only slowly, so their temperature swings more. In summer, air above warm land expands, pressure drops and moist ocean air flows in, rises, cools and drops its water. In winter the pattern reverses, and winter monsoons tend to bring drought.
Geography helps too. The high Tibetan Plateau is thought to shape how far north the South Asian monsoon reaches, and the monsoon's strengthening has been linked to the plateau rising after India collided with Asia around 50 million years ago. Estimates of when the modern monsoon became strong vary, from about 8 million years ago to 15 to 20 million, based on dust deposits, plant fossils and seabed sediments. Its strength has swung ever since with the ice ages, and rainfall was markedly lower in glacial periods than in warm ones like today.
The word travelled with sailors. English monsoon came through Portuguese monção, and partly through early Dutch, from the Arabic mawsim, meaning season. It first described the great southwesterly winds that brought rain to British India. Whether weaker patterns in the Americas deserve the name is debated, since their winds do not fully reverse, but Arizona residents know the North American monsoon well: from late June into September, desert thunderstorms build within half an hour's drive of sunny Southern California beaches.
Source: Monsoon
