Cyclone secrets beneath the surface of the ocean

As we approach the cyclone season across the world, we have to prepare for the worst and hope for the best.

- Raghu Murtugudde

Tropical cyclone predictions are getting better, but there is always room for further improvement considering the havoc cyclones wreak on lives, livelihoods, and socioeconomics.

Researchers try relentlessly to understand the processes that lead to the birth of cyclones. While there are many indicators for cyclone genesis, i.e., conditions favorable for the seeds of cyclones, we do not have a complete understanding of how these seeds germinate into full-blown cyclones.

A new study, of which this writer is a co-author, reports that the subsurface ocean holds some secrets to whether a cyclone seed will germinate and grow into a cyclone. This is a critical insight since it points to the data needed to further improve cyclone predictions.

Cyclone genesis is the process where seeds for cyclones are thrown onto the ocean surface. These seeds are borne out of a confluence of factors. The most important factors are warm sea surface temperatures (SSTs), the circulatory tendency in the horizontal variations of wind fields called vorticity, moisture in the middle atmosphere since condensing moisture releases energy for cyclones, and weak variations in winds from the surface into the atmosphere.

Strong wind variations with altitude, called vertical shear, can chop off the cyclones and prevent them from growing. Vertical shear is precisely what prevents cyclone formation during the monsoon, thus relegating the cyclone seasons to the pre- and post-monsoon months when the vertical shear is weak. Or so it has been assumed thus far. Does the dark ocean have something to say about that?