Typhoon Bopha’s Southern Track

Posted in Non-US Weather, Tropical Weather, Weather Education, Weather News at 5:09 pm by Rebekah

Typhoon Bopha narrowly missed Palau last night on its westward track through the west central Pacific. The typhoon has weakened slightly from the equivalent of a Category 4 on the Saffir-Simpson Scale to a Category 3 as it makes its way towards the southern Philippines.

It is unusual for a strong typhoon to exist so close to the equator; a recent article from The Weather Channel points out that Bopha is the strongest typhoon to track through the southern Philippines in 22 years.

Bopha’s history (above) and 5-day forecast (below); Kwajalein Atoll (where I live) is circled in red. All images courtesy of Weather Underground.

But why is this unusual? Put simply, tropical cyclones require a certain amount of rotation to be able to strengthen. You probably know that wind in the Northern Hemisphere is deflected one way, while wind in the Southern Hemisphere is deflected the other way–this is known as the Coriolis force.

The closer you are to the poles, the stronger the deflection; which means the closer you are to the equator, the less the wind favors deflecting one way or the other. Even if you have all the other necessary “ingredients” for a tropical cyclone to form, if the trough of low pressure does not start to rotate, or does not rotate very much, the central pressure will fail to deepen and the wind speeds will not increase very much (remember, the stronger the pressure gradient, the stronger the winds as the atmosphere tries to balance out the pressure differences — see Meteorology 101: Elements of Weather – Wind).

Generally you will not see tropical cyclones forming between about 5°N and 5°S, let alone strengthening to Category 4 storms. This is why here at Kwajalein, at 8°N, we don’t expect we could get much more than the equivalent of a Category 2. Many tropical cyclones form near the Marshall Islands, but in order to get a stronger storm tracking through, it would need to form further upstream and closer to the equator, which is possible but not very common.

See also one of my previous posts on how tropical cyclones form: Tropical Cyclones: How They Form, Move, and Strengthen (aka, What Danielle, Earl, and Frank Are Up To) – Part 1

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