Monsoon

Monsoon (pronounced mon-soon)

(1) The seasonal wind of the Indian Ocean and southern Asia, blowing from the southwest in summer (associated with heavy rain) and from the northeast in winter.

(2) On the Indian sub-continent and in nearby countries, the season during which the southwest monsoon blows, commonly marked by heavy rains; the rainy season (known as the Asiatic monsoon).

(3) Any wind that changes directions with the seasons (rare) or any persistent wind established between water and adjoining land.

(4) In colloquial use, sudden, hard rain.

(5) Entire meteorological systems with such characteristics.

1547: From the Raj-era English monsoon (alternating trade wind of the Indian Ocean), from the now obsolete Dutch monssoen, from the Portuguese monção, from the earlier moução, from the Arabic موسم (mawsim) (time of year, appropriate season (for a voyage, pilgrimage etc.)), from وَسَمَ‎ (wasama) (to mark, to brand; he marked).  Monsoon has a specific technical meaning in meteorology but in casual use it’s sometimes used as a synonym for (especially sudden) hard rain as an alternative to terms like deluge, rainstorm, storm & squall.  Monsoon is a noun and monsoonal & monsoonish are adjectives; the noun plural is monsoons.

Lindsay Lohan caught in a monsoon in Confessions of a Teenage Drama Queen (2004).

The Arabic word came into use among Portuguese sailors crewing ships which plied the Indian Ocean trade routes.  In the Arabic, mawsim could be used to describe anything recurrent, especially annual events such as festivals and, confusingly to the Portuguese, it could reference difference seasons (spring, summer etc) because each could be associated with the appropriate time for some activity (a pilgrimage, a harvest et al).  Under the Raj, in the sub-continent and adjacent lands, it came to be applied specifically to the seasonal (April through October) south-westerly winds which both brought the rains and were best suited to the sailing ships making voyages to the East Indies (modern-day Indonesia).  Technically, the winter’s north-easterly winds were also a monsoon but because the summer monsoon generated much heavier rain, it came emphatically to be spoken of as "the monsoon".  Because of the similarity of the conditions, use of the word (as a technical term) has extended from the original (Asian-Australian) to describe the rain patterns in West Africa and the Americas associated with seasonal changes in the direction of prevailing winds but, because the change is not as dramatic (especially in North & South America), some meteorologists prefer other terms.

To a meteorologist a monsoon is not just the summer rains but a system of winds which influences the climate of a large area which stretches as far south as northern Australia, the prevailing direction reversing with the change in seasons.  Although affected by ocean temperatures, monsoons were long thought primarily caused by the much greater annual variation in temperature over large land masses but the influence of oceanic temperatures is now becoming clear.  This variation induces higher atmospheric pressure over the continents in the winter and much lower levels in summer, the disparity causing the strong winds to blow between the ocean and the land, accounting for the heavy seasonal rainfall.

Monsoon storm event over Tuscon, Arizona.

That climate change is caused by the increased levels of atmospheric CO₂ is now accepted by just about everybody except some right-wing fanatics and those who get their medical and scientific advice from their hairdressers or personal trainer.  In the last decade, enough data has been accumulated to build models which predict the changes the Asian-Australian monsoon is expected to undergo and although there are variations between them, all seem to suggest a net increase in monsoon rainfall on a seasonal mean, area-average basis, the causes essentially two-fold: The rise in the land-sea thermal contrast and, of greater significance, warming over the Indian Ocean which means the monsoon winds will carry more moisture to the sub-continent.  There are variations in estimates but typically most models suggest the increase in total rainfall over India will be around 5-10%.  That figure is often misunderstood because it refers to a long-term average number and given that in some years rainfall will actually be below average, in some years it will be much above and climate simulations also show different patterns of geographic distribution which means it’s difficult to predict specific outcomes except to say the trend-lines are upward.  The effect on the Asian-Australian monsoon of anthropogenic climate change is thus certain in direction (and to a degree in extent) but unpredictable at the margins.  The mechanism is well known:  A warming climate allows more moisture to be held in the atmosphere which means rainfall when it does occur will be heavier.  Carbon is a form of energy so more of it in the atmosphere means a more energetic atmosphere and thus climate events, when they occur, will probably tend to the extreme in frequency and severity.