On yesterday's episode of WB, we talked about Cyclone Emma, a strong midlatitude cyclone that moved across Central Europe last week. Winds of up to 136 MPH were reported with the storm. Emma was unusually strong, but fundamentally it was cyclone just like the ones that influence the weather of NE and SD every few days.
Somewhat confusingly, another “cyclone” has been in the news lately, and that's Cyclone Ivan. Cyclone Ivan was a hurricane in the southern Indian Ocean. Ivan made landfall on the island of Madagascar on February 17, 2008. Shortly before landfall, Ivan was a category 4 hurricane, with winds of up to 135 MPH. The storm weakened slightly as it came ashore, but it was still as strong or stronger than Hurricane Katrina was when that storm hit New Orleans. Over 80 people died in Cyclone Ivan, and nearly 200,000 people were left homeless. Almost a million acres of crops were destroyed.
Normally, we don't hear much about hurricanes at this time of year. Hurricanes primarily occur in the autumn, so in the US we tend to be more aware of hurricanes in August and September. There are several reasons why autumn is the preferred time of year for hurricane formation or, technically, “tropical cyclogenesis”, but the main reason is the warm sea surface temperatures that are common at that time of year. Hurricanes get their energy from the ocean, so the time of the year with the warmest waters will generally be the time of year with the most hurricanes. Right now, the waters of the Atlantic are just too cold to support a hurricane, and no Atlantic hurricane has ever formed in the month of March. Well, no NORTH Atlantic hurricane, anyway—later this month we'll mark the anniversary of a SOUTH Atlantic hurricane, but that's a story for another day.
It's easy to forget that while March is late winter or early spring in the US, it's late summer or early fall in the Southern Hemisphere, which, of course, is where Madagascar is. Therefore, this is the time of year when islands like Madagascar or Australia would EXPECT hurricanes. The Southern Indian Ocean has seen 17 named tropical storms since November, so that is a busy part of the world for extreme weather.
Hurricanes in the Southern Hemisphere work pretty much the same way as hurricanes in the Northern Hemisphere—they are also areas of low pressure of tropical origin, they also have a ring of strong thunderstorms surroundings an “eye” of relatively undisturbed weather, and they also have their strongest winds at the surface. The main difference is the way that the winds blow—in the Northern Hemisphere, winds around a hurricane rotate counterclockwise at the surface, whereas in the Southern Hemisphere the winds rotate clockwise. The reason for the difference is the reversal of the so-call Coriolis Force at the Equator. In the Northern Hemisphere, the Coriolis Force deflects all motions to the right, whereas in the Southern Hemisphere, the Coriolis Force deflects all motions to the left. You don't notice the Coriolis Force in your day to day life, but this force becomes a critical factor in atmospheric motions.
So why did they name the storm “CYCLONE Ivan” and not “HURRICANE Ivan”? Hurricanes are actually called different things in different parts of the world. For example, in the Western Pacific, hurricanes are known as “typhoons”, even though a “typhoon” is exactly the same kind of storm that we would call a “hurricane” in the Atlantic. In the same way, in the Indian Ocean, hurricanes are called “cyclones”. There have been several important “cyclones” in the Indian Ocean in the last year, including both Cyclone Sidr and Cyclone Gonu. The terminology is just a bit confusing because in the US and in Europe the term “cyclone” means something else entirely. Here, a cyclone is an area of low pressure with fronts and a trough in the jet stream. “Cyclones” in the US have very little in common with hurricanes or “cyclones” in the Indian Ocean, other than they are both low pressure systems.
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