Found this interesting article to share, an
insight abstract in relating why it was truly an Epic Day at ManaMana on Christmas Eve 2001 and one of the best sailing sessions. Oh Com'on babe, bring on another hard one .....!!!!
Due to the diminishing Coriolis effect , the belt 300 km either side of the equator has been considered tropical cyclone-free. Typhoon Vamei, which developed near Singapore on 27 December 2001, was the first recorded tropical cyclone formation within 1.5 degrees of the equator. The development was the result of two interacting systems, a weak Borneo vortex that drifted into the southern tip of the South China Sea and remained there for four days, and a strong and persistent cold surge that created the large background cyclonic vorticity at the equator. The probability of a similar equatorial development is estimated to be once every 100-400 years.
Historically, the equatorial zone has been considered by sailors to be free from tropical storms. This is because tropical cyclones have rarely been observed to form equatorward of 5° latitude where the diminishing Coriolis effect prevents effective generation of relative vorticity by horizontal convergence. Typhoon Vamei formed near Singapore on 0600 UTC 27 December 2001. With its circulation center at 1.5°N and a radius of convective cloud area of near 200 km, the storm circulation was on both sides of the equator. Naval ships reported maximum sustained surface wind of 39 m s-1 and gust wind of up to 54 m s-1 (Joint Typhoon Warning Center, 2002), which caused damages to the carrier USS Carl Vinson and an accompanying ship. This was the first-ever recorded tropical cyclone formation at such equatorial latitude. Typhoon Vamei was such a rare event.
During the northern hemisphere winter monsoon, northeasterly cold surges sporadically spread equatorward from the East Asia continent on the southeast edge of surface highs The strongest cold surges occur over the South China Sea, where the air is no longer cold but the freshening of the northeast winds progresses southward rapidly as the sea narrows towards the equator. The cold surge air can reach the equator in about two days. Conservation of potential vorticity causes the air to turn eastward after it crosses the equator. These Southern Hemisphere equatorial westerlies may enhance the Australian monsoon trough farther south between 10°S-20°S where tropical cyclogenesis occurs frequently.
Southeast of the main surge wind belt in the South China Sea is the island of Borneo, where a quasi-stationary low-level cyclonic circulation centered near its northwest coast is a persistent feature of the boreal winter climatology. This circulation will be called the Borneo vortex even though its circulation may not be completely closed on the east side over the island. It is maintained by the shear vorticity and convergence that result from the interaction between the northeast monsoon winds and the mountainous Borneo terrain. The Borneo vortex is often associated with deep convection and intense latent heat release, and upper-level divergence is often present. However, because most of the time a significant part of the vortex circulation is over land, even when a vortex drifts to northern Borneo between 5°N-7°N, which are latitudes considered more favorable for tropical cyclone development, it is very difficult for the vortex to develop into a tropical cyclone.
Starting from 19 December 2001, a cold surge developed rapidly over the South China Sea while the Borneo vortex center was located near 3°N on the northwest coast depict the southwestward movement of the vortex from along the Borneo coast toward the equator. By 21 December, the center of the vortex had moved off the coast to be over water, where the open sea region in the southern end of the South China Sea narrows to about 500 km with Borneo to the east and the Malay Peninsula and Sumatra to the west. This over-water location continued for several days. While the vortex center remained in the narrow equatorial sea region, the strong northeasterly surge persisted, and was slightly deflected to the northwest of the vortex. Consequently, the cross-equatorial flow wrapped around the vortex and the net result was a spinning up of a rapid counter-clockwise circulation that is similar to the spinning of a top played by a child, and this led to the development of Typhoon Vamei. While cold surges are frequent events in the South China Sea during boreal winter, the surge preceding the development of Typhoon Vamei was especially intense and long lasting.
The development of Typhoon Vamei was a result of several factors. Prior to its formation, a strong cold surge persisted for nearly one week over the narrowing South China Sea provided a source for background cyclonic vorticity as the surge wind crossed the equator. The anomalous strength and persistence of this surge was related to the anomalously strong meridional gradient of sea-level pressure in the equatorial South China Sea during December 2001. The narrowing of the South China Sea at the equator appears to be an important factor for the rare occurrence of the typhoon formation. On the one hand, the channeling and strengthening of the cross-equatorial surge winds helps to produce the background cyclonic vorticity at the equator. On the other hand, the open water region of approximately 5° longitude is just sufficient to accommodate the diameter of a small tropical cyclone. However, it is too small for most synoptic-sized disturbances to remain over the water for more than a day or so. In the unusual case of Typhoon Vamei, the Borneo vortex migrated to this small region and stayed for 96 hours – long enough to become the early stages of the typhoon.
Thus, of all the conditions that led to the formation of Vamei, we estimate the probability for a typhoon to develop in the equatorial South China Sea to be about 0.12-0.49% a year, or an expectation of about once every 300-400 years.
Ref - C.-P. Chang, Ching-Hwang Liu1 , Hung-Chi Kuo2 
