| Dr. Steve Lyons, Tropical Weather Expert Each fall usually in November and early December a major change in Earth's atmosphere takes place in the Eastern Hemisphere! The summer monsoon across Southeast Asia and India with its life giving rains within southwest winds very suddenly stops and is quickly replaced by northeast winds and drier weather we call the winter monsoon. Yes, typically in less than 45 days a complete reverse in the direction winds blow across a huge area of the tropics occurs. At the same time hurricanes begin to form south of the equator in the Indian Ocean (we call them tropical cyclones down there)! Well if you are at all curious like I am, you have to ask the question "why?" How does the atmosphere know its time to start making "tropical cyclones" in the Southern Hemisphere Indian Ocean? Well in detail this is a long and rigorous dissertation that involves the fundamentals of why there is a "monsoon" in the first place. And, by the way, in case you did not already know the definition of a "monsoon" is; a seasonal reversal in wind direction. That's it, but that shift in wind is typically associated with very different weather/rain as well! So, in a nut shell, how does the atmosphere know how to turn on queue tropical cyclones in November in the South Indian Ocean? Well it turns out that when shifts in solar energy are the fundamental cause, the devil is in the details. As fall begins we see a net cooling of the Northern Hemisphere and warming of the Southern Hemisphere as the sun's energy maximum begins to shift into the Southern Hemisphere. It reaches a maximum in northern extent and amount around the 3rd week in June and a maximum southern extent and amount around the 3rd week of December (summer and winter solstice). This annual shift in sun's energy maximum is caused by the fact the Earth is tilted approximately 23.5 degrees latitude from the axes it rotates around (tropic of Cancer and tropic of Capricorn). Sun heating (cooling) of land happens pretty fast while sun heating (cooling) of ocean is delayed by a few months from these dates. The combination of land heating and cooling and ocean heating and cooling in BOTH hemispheres drives the entire summer and winter monsoons of Asia, Australia and Africa. Obviously you don't have to live within the Asian monsoon to know we have seasons, in most locations across the U.S. the seasons have been grouped into four (winter, spring, summer and fall) while winds can remain generally from the same direction in many areas. In the monsoon region seasons are primarily grouped into two big ones (summer and winter monsoon) with a nearly 180 degree reversal in wind direction from one to the other. In November the "monsoon trough" of low pressure in the north Indian Ocean caused by warm water and the sun's energy can no longer be maintained by rapidly decreasing solar energy, and it begins to dissipate. This monsoon trough acts as the focus of tropical cyclone development in this area. At the same time increases in sun's energy in the Southern Hemisphere warms land and ocean and forms a new monsoon trough that typically lines up along about 8-10 degrees latitude south of the equator (south of that over land areas of Australia). So as one tropical cyclone generating monsoon trough dissipates in the Northern Hemisphere a unique one forms in the Southern Hemisphere. The monsoon trough does NOT migrate from one hemisphere to the other. So our atmosphere in many ways is simply subservient to sun's incoming energy that shifts back and forth across hemispheres causing our seasons. Although we can predict the angle Sun's energy strikes Earth with great accuracy years in advance. The atmosphere is "not perfectly subservient" to the sun because we have variations in snow/ice cover, soil moisture, rain and amount of reflected or absorbed sun energy from surface vegetation changes that can all act to shift (slightly) the change of seasons most commonly locally but occasionally globally. So the answer to my proposed question, how does our atmosphere know, is simple. Earth gets its queue from sun energy changes which differ very little in space and time from one year to the next. Hence we often see, right on time, tropical cyclones form in the Southern Hemisphere in November. 2009 is no exception as we have or had (depending on when you read this) category 3 tropical cyclone "Anja" recently formed south of Diego Garcia in the Southern Hemisphere! |