Bopha and Coriolis

   After weeks of next to no activity in the Northwest Pacific basin, there is finally some sign of life. Earlier today TD 26W became TS Bopha. It might not seem like much now, but this storm is actually very rare. What makes it special is the fact that officially became a tropical cyclone (TD 26W) while at a latitude of just 3.6 degrees north. Generally, storms form much further away from the equator due to its lack of a crucial force called the Coriolis Force.
   Technically, the Coriolis Force is not a real force, it is an apparent force resulting from the rotation of the Earth. If one were to observe the Earth from a fixed point in space, they would not be aware this kind of force. The result of the Coriolis Force is that objects in motion are deflected, to the right in the northern hemisphere and to the left in the southern hemisphere, from traveling in a straight path. This force is very important because it is the primary source of rotation in both the atmosphere and the ocean. However, at small scales it is insignificant as other forces become much more dominant. Therefore, something like a single storm cloud is going to be relatively unaffected by this force. It goes with out saying that this force will not cause you to mess up your ball game and it is not the reason for the direction your toilet flushes.
   The magnitude of the Coriolis Force is proportional to the sine of latitude [sin(lat)], so it is at its maximum at the poles (90 degrees) and zero on the equator (0 degrees). This is why tropical cyclones typically form some distance from the equator: to get some 'spin' from the Coriolis Force. Generally, I think of about eight degrees to be the closest a storm will likely get to the equator. This is why Bopha is so incredible, in the entire history of tracked storms (back to the 1840s) only a handful of storms have come so close to the equator. The map below shows the tracks of all Indian and Pacific storms ever recorded, the segments in yellow show where a tropical cyclone has tracked closer than 3.6 degrees from the equator (there were no cases in the other basins). As far as I can tell, first prize goes to Typhoon Anges (then a TD) in 1984, it was first recorded when at just 0.1 degrees north! If the Coriolis Force is so small, how did Bopha do it? The best I can figure is that it caught a bit of spin from a frontal zone in the South Pacific that included the invest area 97P.

 

 

UPDATE: 

This interesting statistic was posted on the blog of Japan's Digital Typhoon site:

"...since 1951 there are only 13 typhoons born to the south of 5 degree north. The most recent one is Typhoon 200206 [Chataan] ,so this typhoon [Bopha] is the first in these 10 years. It is known that areas around the equator is where the typhoon is formed infrequently due to weak force (Coriolis force) for spiral motion."

 

Note that this site refers to both typhoons and tropical storms as 'typhoons'.