Tropical Cyclones
The previous chapter discussed about the most desirable and sought-after sailing conditions - the trade winds. This chapter talks about tropical cyclones - the least desirable and most avoidable sailing conditions.
Tropical cyclone (TC) is a general term for rotating organized storms that form over the warm tropical oceans. When tropical cyclones reach a certain strength, they are called Hurricanes in the Atlantic and the Eastern/Central Pacific Ocean (east of the dateline). In the Northwest Pacific, west of the dateline, TCs are called Typhoons. In the southwest Pacific and in the Indian Ocean they are called Cyclones.
Tropical cyclone structure
Figure 1. Tropical cyclone structure. The eye that is relatively clear is surrounded by eyewall that has the strongest winds. Outside the eyewall are the rainbands that spiral around the cyclone. (Image courtesy of UCAR/COMET.)
The innermost part of a tropical cyclone is the eye that is typically 20-40 km wide. Inside the eye winds are calm and skies are clear or covered with some cirrus clouds. Tropical cyclones are warm-core systems and therefore the minimum pressure in a tropical cyclone can be found in the eye. In other words, the air temperature throughout the eye is higher than its surroundings and because warm air is lighter than cold air, the air pressure is the lowest in the eye. The eye is surrounded by eyewall that is the most vigorous part of a tropical cyclone. The eyewall has the maximum wind speed in a tropical cyclone. The eyewall has typically strong updrafts, heavy rain, and often lightning. The eyewall is usually not vertical but it tilts outward. The eye and eyewall are called the core of a hurricane.
Outside the core are the rainbands that are sometimes divided into inner and outer rainbands. The inner rainbands are located 50-150 km radius from the eye and the outer bands 150-300 km from the eye. They consist of bands of cumulonimbus clouds that spiral around the core of a tropical cyclone. These cloud bands can produce heavy rain and lightning.
The size of tropical cyclones vary greatly. Some small TCs have tropical storm-force winds only some tens of kilometers in diameter, whereas in some large hurricanes the tropical storm winds can extend out to 2200 km, such as in super typhoon Tip in 1979. Tip also has the record for lowest surface pressure (870 mb).
It should be noted that the low surface pressure area in TCs is usually very small and is located in and near the core. The pressure of even a strong hurricane can rise above 1000 mb just some tens of kilometers away from the core. Therefore the boat's barometer is obviously a poor hurricane forecaster, because once the barometer shows any significant pressure change, it is probably too late to make any routing decisions!
Tropical cyclone strength
Tropical cyclones are typically categorized according to their strength to tropical depressions, tropical storms (when they are named), and hurricanes/typhoons/cyclones.
- Tropical depression: A weak tropical cyclone with a closed surface circulation. They have closed surface isobars and their highest sustained wind speed is less than 34 kt.
- Tropical storm: A tropical cyclone with closed isobars and highest sustained wind speed of 34-63 kt.
- Hurricane/Typhoon/Cyclone: A tropical cyclone with highest sustained wind speed more than 64 kt.
Below is the famous Saffir-Simpson hurricane scale that divides hurricanes into five different categories. The categories are based on wind speed, so the pressure and storm surge numbers are only informational and can vary from storm to storm.
Table 1. Saffir-Simpson hurricane scale. Pressure values and storm surge heights are estimates.
| Saffir-Simpson Hurricane Scale | ||||||
|---|---|---|---|---|---|---|
| Category | Wind (kt) | Wind (m/s) | Wind (km/h) | Wind (mph) | Pressure (mb) | Storm surge |
| 1 | 64-82 kt | 33-42 m/s | 119-153 km/h | 74-95 mph | >980 | 1.0-2.0 m |
| 2 | 83-95 kt | 43-49 m/s | 154-177 km/h | 96-110 mph | 965-979 | 2.0-2.5 m |
| 3 | 96-112 kt | 50-58 m/s | 178-208 km/h | 111-129 mph | 945-964 | 2.5-4.0 m |
| 4 | 113-136 kt | 58-70 m/s | 209-251 km/h | 130-156 mph | 920-944 | 4.0-5.5 m |
| 5 | > 137 kt | > 70 m/s | > 252 km/h | > 157 mph | <920 | > 5.5 m |
Tropical cyclone formation
Tropical cyclones form over tropical warm waters where the humidity is high. Over most of the world, the conditions for TC formation are the most favorable during the late summer. The majority of the Atlantic TCs are formed from African easterly waves. The waves have their origin over North Africa and they propagate westward over the Atlantic in the trade wind flow. Tropical cyclones can form if the environmental conditions are favorable. The following necessary ingredients are needed for tropical cyclone formation. However, even if all the conditions are apparently met, not all the disturbances develop into TCs.
Figure 2. African easterly waves form near the African easterly jet and propagate westward. Most of the Atlantic TCs form from these waves. (Image courtesy of UCAR/COMET.)
- Sea surface temperature over 26.5°C (80°F). The warm ocean layer should be at least 30-50 m deep so that mixing and upwelling don't cool down the surface water rapidly.
- Instability through the troposphere is required at the early stages of the formation. This ensures that the convective clouds can grow high.
- High humidity through the lower and middle troposphere. Dry layers can suppress the convection and prevent the clouds from growing high.
- Pre-existing disturbance with sufficient low-level horizontal vorticity. In other words, there should be some rotation near the surface so that the system can start spinning. For a cluster of thunderstorms to organize into a tropical cyclone, there has to be some mechanism that triggers the formation. Such a mechanism can be surface winds converging along the ITCZ, a tropical easterly wave, or a pre-existing system such as an old mid-latitude front.
- Sufficient Coriolis force. That requirement does not fulfill near the equator, between about 3 °N and 3 °S because Coriolis force is zero at the equator. (The impact of this requirement can be clearly seen in Figure 4 that has a clear zone near the equator.)
- Small vertical wind shear. The difference in wind speed between the lower and upper parts of the storm should be less than about 20 kt. That prevents the environmental wind from tearing the storm apart.
Tropical Cyclone Basins
There are seven ocean basins where tropical cyclones form regularly (Figure 3). Below is a summary of TC activity for each basin.
Figure 3. Seven ocean basins where tropical cyclones form. (Image courtesy of NOAA.)
Atlantic Ocean
- Term: Hurricane
- Season: 1 June - 30 November
- Peak: September
- Average number of TCs per year: 12
The Atlantic tropical cyclone basin covers the north Atlantic Ocean between the Americas and Africa/Europe, including the Caribbean and Gulf of Mexico. Most of the TCs form off the coast of Africa from African easterly waves and travel westward or northwestward (Fig. 4).
There is usually no TC formation in the south Atlantic Ocean because of the unfavorable environmental conditions, such as cooler sea-surface temperature and strong vertical wind shear. However, in March 2004, an extratropical storm transitioned to a subtropical storm off the coast of Brazil and later gained tropical cyclone characteristics. The storm reached category 1 hurricane force winds and made landfall in Brazil. The storm was the first hurricane-force TC to form and make landfall in the south Atlantic. The storm was unofficially named Catarina. A few other cases of subtropical/tropical cyclone formation have also been reported in the south Atlantic, but it is not considered an active TC basin.
Figure 4. Tracks and intensity of all known tropical cyclones through 2006. Tropical depression (TD) and tropical storm (TS) tracks are colored in blue shades and category 1-5 hurricanes/typhoons/cyclones have yellow-red colors. (Image courtesy of NASA).
Northeast Pacific Ocean
- Term: Hurricane
- Season: 15 May - 30 November (1 June - 30 November in Central Pacific)
- Peak: August - September
- Average number of TCs per year: 15
This basin covers the northern Pacific Ocean from the west coast of Central America to dateline. Majority of the TCs form over the eastern part of the region and travel westward or northwestward sometimes making landfall in Central America. Most of the TCs reported in the Central Pacific (140-180°W) form in the eastern Pacific, but sometimes TCs also form in the Central Pacific.
Northwest Pacific Ocean
- Term: Typhoon
- Season: year round
- Peak: September
- Average number of TCs per year: 26
Northwest Pacific Ocean is the most active TC basin in the world. It can be active all year round although there is a distinct minimum in February-March. The main season is between July and November and the peak months are August-September.
North Indian Ocean
- Term: Cyclone
- Season: 1 April - 31 December
- Peak: May and November
- Average number of TCs per year: 5
This basin is impacted by the Asian southwest monsoon and has a double peak in TC activity. The first peak is in May before the monsoon onset and the other peak occurs in October-November, after the monsoon. During the monsoon, the surface winds are southwesterly whereas the upper level winds are easterly. Together they create strong vertical wind shear that is unfavorable for TC formation. The basin covers the Bay of Bengal and the Arabian Sea. The Bay of Bengal is more active area with an average of 4 TCs per year while the Arabian Sea has only 1 TC per year.
Southwest Indian Ocean
- Term: Cyclone
- Season: 1 November - 30 April
- Peak: January - February
- Average number of TCs per year: 9
The Southwest Indian Ocean basin covers the waters south of the equator from the African coastline to 90°E. The main season is between November and April although TC formation can occur at any time of the year.
Australia
- Term: Cyclone
- Season: 1 November - 30 April
- Peak: January - February
- Average number of TCs per year: 11
The Australian TC basin extends from 90°E to 160°E and therefore covers the Southeast Indian Ocean, northern Australia up to the equator, and the southwesternmost part of the Pacific Ocean. The season runs from November to April with peaks in January and February.
Southwest Pacific Ocean
- Term: Cyclone
- Season: 1 November - 30 April
- Peak: February - March
- Average number of TCs per year: 9
The southwest Pacific basin extends from 160°E to 120°W thus crossing the dateline. The season starts in November and continues through April with a peak in late February and early March.
Table 2. Summary of tropical cyclone activity in different basins. The number of TCs per year is the average number of TCs in one season with at least tropical storm force winds (>34 kt). That number may vary slightly depending on the source and as new data are analyzed.
| Basin | Term | Season | Peak | TCs/year | Monitoring Center |
|---|---|---|---|---|---|
| North Atlantic | Hurricane | 1 June - 30 November | September | 12 | National Hurricane Center |
| Northeast Pacific | Hurricane | 15 May - 30 November | August-September | 15 | National Hurricane Center / Central Pacific Hurricane Center |
| Northwest Pacific | Typhoon | Year round | August-September | 26 | Japan Meteorological Agency |
| North Indian | Cyclone | 1 April - 31 December | May and November | 5 | India Meteorological Department |
| Southwest Indian | Cyclone | 1 November - 30 April | January-February | 9 | Meteo France, La Reunion |
| Australia | Cyclone | 1 November - 30 April | January-February | 11 | Bureau of Meteorology |
| Southwest Pacific | Cyclone | 1 November - 30 April | February-March | 9 | Fiji Meteorological Service |
Tropical cyclone advise for sailors
The best advise for mariners is obviously to avoid sailing in the ocean basins where and when tropical cyclones can occur. Although modern Numerical Weather Prediction (NWP) models can sometimes predict the formation and track of TCs surprisingly well, TCs can often form from small cloud clusters and develop rapidly without an NWP model catching them. Once a TC has formed, the models can usually predict the track better than the intensity. As discussed earlier, the boat's barometer is a poor forecaster because the pressure changes on the outskirts of a TC are relatively small. If an encounter with a TC is unavoidable and there is some information available of the TC track, the following facts should be taken into account: For example, if a TC with sustained winds of 80 kt is traveling westward at 20 kt, the winds on the poleward-side of the storm add up to 100 kt whereas the winds on the equator-side of the storm are only 60 kt. Tropical cyclones come in many sizes and shapes and the tropical storm-force winds can extend anywhere from a few tens of kilometers to a thousand kilometers from the eye. However, the strongest winds are typically seen in the eyewall and that region should be avoided at any cost.
