The Myth That Tropical Cyclones Cannot Form Near the Equator

Historically it has been stated that tropical cyclones cannot form close to the Equator because the Coriolis force is too small there. On December 27, 2001 a tropical cyclone named Vamei reached typhoon intensity at latitude 1.5˚N and longitude 105˚E. Thus, it is clearly possible for tropical cyclones to form and intensify near the Equator. Early simplifications of the equations of motion were based on scale factors appropriate for the mid-latitude synoptic scales of motion. Those simplifications often eliminate a term related to the cosine of the latitude in the term for motion in the x-direction. When the simplified equations of motion are used to create a vorticity theorem, they also neglect that term. However, if there is a sufficiently large horizontal gradient of vertical motion, then it is possible for this term to generate sufficient vorticity to spin up a tropical cyclone near the Equator.