AWT Weather Routing/Routeing: Applied Weather Technology

Tropical Cyclone Effect on the Monsoon Winds Over the South China Sea

By George Schlinkert

Yearly, questions arise from our clients regarding the Northeast monsoons that affect the South and East China Seas during the autumn and winter seasons. Confusion starts in October and November when the speeds of vessels that are not particularly close to the stated wind fields of a tropical cyclone are greatly reduced. Charterers believe the ship must be underperforming and the profits from the voyage are being greatly diminished or negated.

In one such instance, the cyclone bulletin from the Japanese Meteorological Agency stated that at 05/1200Z Severe Tropical Cyclone Peipah was at 17.8N 118.8E moving west slowly. 50 knot winds prevailed within 40NM, while 30 knot winds prevailed within 150NM (2.5 degrees) in the north semicircle.

The following image shows an example of such a situation.

Was the forecast agency in error? The answer is “no,” but the user of this information needs to understand what the numbers mean. When discussing the radius of cyclonic winds, the bulletins are discussing the winds that are in direct circulation around the cyclone. During the monsoons, large bands of northeast winds prevail between the cold air masses of Asia and the cyclone itself. These winds are not rotating about the cyclone itself.

Compounding the issue is the storm’s speed of movement. This storm was only moving slowly westward. In this situation, the strong winds blow over the same waters for a long period of time. The fetch of Northeast winds becomes large and the waves increase dramatically. BF 8 conditions may generate 5 meter waves within 6 hours which is common during the passage of a transitory low. However, if blowing for 24 hours over the same waters, the significant wave height may reach around 8 meters, with maximum wave heights even greater.

This in turn can lead to the extensive band of very heavy northeasterly waves which may make northbound transit west of the cyclone prohibitive in some situations and certainly add significant time to voyages in other situations.

The wind and wave forecast is equally as important as the actual cyclone forecast, and we recommend to our clients utilizing AWT services to always access these wind analyses and forecasts through the AWT Fleet and Weather Portal.