Monsoon

Somewhat less than half of Earth’s population lives in monsoon zones and is highly dependent on monsoon precipitation for agricultural production. Religious festivals are timed to the onset of the wet and dry monsoons. Failure of monsoon rains or flooding from monsoon rains raise the specter of famine. Monsoon is a commonly used but misunderstood term. One might hear an acquaintance talk about “today’s monsoon” out of a Great Plains thunderstorm, but the term was not originally used to convey the sense of prodigious wetness. The word is derived from the Arabic mausam meaning “season” and referring to the seasonal reversal of wind directions in the arid Arabian Sea and western Indian Ocean. Sailors were able to reliably time their trading trips by this expected feature of the atmosphere. Pronounced seasonal wind reversals are present in much of the world, but the term monsoon now most properly refers to a climatic regime of reversal of seasonal winds bringing alternating seasons of wetness and dryness. The monsoon regions of the planet encompass wide swaths of the tropics and subtropics and are, classically, experienced in Asia and parts of central Africa. From December through February the primary wind direction is northeast to southwest, while June through September brings the reversal of this flow. The former period is quite dry while the latter is prodigiously wet.

Monsoons have their roots in the tropical and subtropical parts of the global wind and pressure belts. The shifting declination of the vertical sun causes parts of the global circulation to expand, contract, and change latitudes. Monsoons are features in the tropical and subtropical portions of the planetary circulation. Differential heating of land versus water is the simplest explanation of the wind reversals. Continents heat more dramatically in the summer than do oceans and these temperature gradients set up wind gradients transporting air from the oceans to the continents. In the winter, the thermal gradients reverse and cause the surface air to reverse flow. Continental air is dry while oceanic air is moist and associated with great amounts of precipitation. The monsoon is most prominent around Asia because of the immense size of the Asian land mass and the complexities of its coasts.

The air flow high in the troposphere is known as a partial cause of monsoons. The subtropical jet stream is most active in the winter season because of the great temperature difference between equator and poles. With its mean winter position to the south of the Himalaya Mountains and Tibetan Plateau this strong upper flow complements the cooling of the Asian continent and keeps the oceanic air out of the continent. In the summer season, the subtropical jet weakens and jumps to the north of these southern Asia highland areas and the oceanic air is allowed to incur into the continent.