Tropical Revolving Storms

(Pictured above is a satellite picture of Cyclone Pam which hit the American Samoa and New Zealand in March 2015)

A Tropical Revolving Storm (TRS) is a weather system that forms in the tropical latitudes. The cause very high winds and heavy seas and provide the most dangerous weather a seafarer can encounter. They can cause massive devastation to areas that they hit. For example Hurricane Katrina devastated New Orleans in May 2005 and more recently Hurricane Pam which devastated Vanuatu in the Pacific in March 2015. It is essential as a seafarer to be fully aware of the dangers of a TRS and to successfully detect and avoid them.

TRSs form in both the northern and southern hemispheres. In the Northern Hemisphere in the early and late season a TRS will likely form between latitudes 5-15 degrees north. At the height of the season a TRS will likely form between latitudes 10-25 degrees north. In the North Atlantic Ocean a TRS, throughout the season, will commonly form between latitudes 25-30 degrees north. In the Southern Hemisphere, a TRS commonly forms between latitudes 5-18 degrees South. For a TRS to form, aside from being in the appropriate location on the Globe, high air temperatures, high sea temperatures (the minimums sea temperature required from research is 27 degrees Celsius) and high humidity are all required.

The seasons of TRSs and their regional names around the world are:

The data above referenced from ‘Meteorology For Seafarers, authored by Lieutenant-Commander R.M. Frampton and P.A. Uttridge.

If within any of the areas mentioned above in the seasons in which TRSs are expected to form, the OOW must be fully aware of the signs of a TRS being in his or her vicinity. The following changes in weather indicate the presence of a TRS:

• An appreciable change in the direction and strength of the wind
• A long low swell originating from the centre of the TRS
• As the TRS approaches large amounts of Cirrus clouds, followed by Alto Stratus and then broken Cumulus
• If, once corrected, the barometer reads 3hpa below the mean for the time of year (found in the Climatic Atlas or the appropriate volume of Admiralty Sailing Directions for the location) this should arouse suspicion that a TRS is in the vicinity
• If, once corrected, the barometer reads 5hpa below the mean for the time of year then avoiding action must be taken as a TRS is almost certainly present
• Coriolis force, weakest at the equator and strongest at the Poles, is also required for a TRS to form. This is the apparent force caused by the rotation of the Earth and causes the TRS to rotate. Without Coriolis force a TRS can’t form and this is why a Tropical Revolving Storm will not form below 5° N/S.

To monitor for the signs of changes in wind and pressure readings, it is good practice to log the wind direction and force along with the pressure readings every hour whilst suspicious that a TRS may be in the vicinity. This allows for patterns to be spotted within the hourly observations that may match with the above warning signs that a TRS may be approaching.

Figure 1 (referenced from ‘Navigation, Advanced, Mates/Masters authored by Capt. Nadeem Anwar)

With reference to ‘Figure 1’, after a TRS forms they usually travel Westerly then North Westerly in the northern hemisphere and South Westerly in the southern hemisphere. They then usually recurve (known as ‘recurvature’) within a latitude of around 20 degrees. After this they usually head to the North East in the Northern Hemisphere and to the South East in the Southern Hemisphere. It must be noted though that recurvature may not take place. Previous TRSs have looped back onto their original track and if two TRSs meet, they sometimes interact and start rotating around each other. The tracks that a TRS takes, shown in figure one, should only be used as a guide as all TRSs are different and can behave in different and potentially unexpected ways.

There are two sides to a TRS. The dangerous semi circle and the navigable semi circle. The dangerous semi circle is so named as the wind in the dangerous Semi Circle moves in the same direction in which the TRS is travelling. Therefore in this half of the TRS very strong winds will be experienced as the combination of the movement of the storm plus the already fast moving winds cause stronger winds than elsewhere in the TRS. The navigable semi circle is the safer half of a TRS. In the navigable semi circle, if the TRS was to recurve, the TRS would recurve in a direction away from the vessel. The navigable semi circle will still be dangerous due to strong winds and heavy seas, but the wind and sea state will be less severe than if within the dangerous semi circle. The winds in the navigable semi circle will also push the vessel out of the path of the storm whereas the winds on the dangerous semi circle will push the vessel into the path of the TRS.

The dangerous quadrant is the most dangerous area in the TRS. This is because if the TRS was to recurve and the vessel was in this quadrant, the vessel would have limited options as to how to escape the TRS as the TRS would be recurving around the vessel.

Action to avoid a TRS

With reference to figure one the following actions are to be taken to avoid a TRS:

Observations –

In the Northern Hemisphere:

• If the wind is veering (changing direction in a clockwise direction), the vessel is located in the dangerous semi-circle
• If the wind is backing (changing direction in an anticlockwise direction), the vessel is located in the navigable semi-circle
• If the wind is steady, the vessel is located in the path of the TRS

In the Southern Hemisphere:

• If the wind is veering, the vessel is located in the navigable semi-circle
• If the wind is backing the vessel is located in the dangerous semi circle
• If the wind is steady, the vessel is located in the path of the TRS

Once aware of the vessels location in relation to the TRS appropriate actions must now be taken to avoid the TRS –

In the Northern Hemisphere:

• If located in the dangerous semi circle, put the wind on the starboard bow and alter course to starboard as the wind veers
• If located in the navigable semi circle or in the path of the TRS, put the wind on the starboard quarter and alter course to port as the wind backs

In the Southern Hemisphere:

• If located in the dangerous semi circle, put the wind on the port bow and alter course to port as the wind backs
• If located in the navigable semi circle or the path of the TRS, put the wind on the port quarter and alter course to starboard as the wind veers

Ships Weather Routing Services

Ships weather routing services are based at shore side locations and are made up of teams of experienced seafarers with access to the latest analysis and forecast charts, ice information, weather warnings, bulletins and satellite pictures and they are also briefed by experienced meteorologists. With all of this expertise and information they aim provide the safest and most efficient routes for ships to follow that use their services. This is so ships can avoid the worst weather situations whilst at sea.

Using the positions provided by the warnings received through the EGC messages the TRS can be successfully plotted. Buy Ballots Law can also be used to plot the position of the TRS. This can be done by facing the wind and the centre of the low pressure system, in this case the TRS, will be from 90 degrees to 135 degrees on your right hand side in the northern hemisphere and on your left in the southern hemisphere.

TRS forecast information

Weather satellites are now predominantly used to detect and track a TRS from its very early stages. Meteorological reconnaissance aircraft may sometimes be sent into the circulations of the storm to receive readings. This information is then provided to seafarers through various means. Specifically to Aurora this information comes in on Enhanced Group Calling (EGC) messages received via the Inmarsat C system onboard MV Aurora, the NAVTEX System as well as WRI (Weather Routing Inc.), the Bon Voyage system (a present and future weather forecasting software for Auroras position and intended tracks) and any other information required about the TRS can be found online via the appropriate Met Office or forecasting service for the vessels location.

If there is suspicion of a TRS or if the vessel is in the vicinity of a TRS and no reports of one have been received then the Master, so as to comply with ‘SOLAS (Safety Of Life At Sea) Chapter 5, Regulation 31, Danger Messages’ is obliged to send all information he or she can about the TRS to all ships in the vicinity along with the nearest maritime radio station or signal station which they can communicate with. If winds of force 10 and above are encountered without any prior reports then this should also be reported in the same manner. For the duration of the time that the vessel was within the TRS reports should be made, if possible, every three hours.

This report should include the following:

• Approximate position of the storm as well as the GMT time and date that it was encountered
• Position, true course and speed of the vessel at the time of observation
• The Barometric pressure and any changes in the previous 3 hours (not corrected for Diurnal variation). In temperate latitudes the Barometric pressure changes quickly and irregularly on a day to day basis due to the movement and development of weather systems. In the tropical latitudes in normal weather conditions the Barometric pressure readings are small and follow a regular pattern. This ‘Diurnal Variation’ in the tropical latitudes is usually corrected using a correction table but whenever Barometric pressure readings are included in a weather report or entered into the ships log the corrections are not to be applied.
• True direction and the force of the wind
• The sea state
• The direction, height, period and length of the swell

How MV Aurora took action to avoid Hurricane Pam in March 2015 (A vessel I sailed on in 2015)

Aurora can be seen in the pictures that follow (the red vessel icon) passing Cyclone Pam and how she avoided it. The screenshots are taken from the ‘Bon Voyage’ weather forecast program used onboard MV Aurora. The track that was to be followed originally is to the right of the pictures and this would have sent Aurora into the path of the storm and would have ran the risk of her getting caught in the dangerous semi circle of the TRS where the highest winds and highest seas are. Efforts were made to avoid passing any closer to the TRS than required by following the track to the left using advice provided by ‘WRI’ (the weather routing service detailed previously) of the safest route to take. This is as a TRS should not be passed by any less than 80 Nautical Miles. If possible the TRS should be passed by at a distance of at least 250 Nautical Miles, which is the distance Aurora endeavored to adhere to.

Picture 1 – Aurora can be seen passing the TRS at a safe distance and staying to the side in which the navigable semi circle is located. This is so as to avoid the more adverse weather conditions found within and around the dangerous semi circle.

(Height of significant waves key at the bottom right of the image below)

In the weather detailed above (linked to picture 1) it can be seen that there is a high humidity of 82%, a high sea surface temperature of 24.1 degrees Celsius and a high air temperature of 23.9 degrees Celsius. All of these conditions (high humidity, air and sea water temperatures) are required for a TRS to form and continue to develop. Winds were from the South at Force 8 at a speed of 35 knots which would help to further confirm that a TRS was present as TRS wind speeds are usually force 6 and above.

Picture 2 – Aurora at this point was still passing the TRS at a safe distance whilst still remaining inside the navigable semi circle. Around this time was when the worst weather encountered occurred due to the fact that, even though she was still in the navigable semi circle, Aurora was skirting the outer limits of the dangerous semi circle (seen on the East side of the TRS).

The wind direction in comparison to picture one has moved from coming from the South to now coming from the West South West. Therefore the wind has veered proving that Aurora was in the ‘Navigable Semi Circle’ throughout this period in time. Additionally, to escape the TRS in the southern Hemisphere if located in the ‘Navigable Semi Circle’ you must, as previously mentioned, put the wind on the port quarter and alter course to starboard as the wind veers. By comparing the two pictures above, along with the changes in wind direction, it can be seen that this is exactly what Aurora was doing. The pressure has also dropped from 987mb to 984mb, indicating that Aurora had become closer to the TRS than she was previously. Humidity, air and sea temperatures have all remained generally similar. The significant wave height has increased to around 8 meters in comparison to around 4 meters in picture one which provides a further indication that Aurora had become closer to the TRS.

Picture 3 – Aurora has now, after taking avoiding action, continued on a set heading as the TRS has now continued to head to the South East and is now off the coast of the North Island of New Zealand.

Winds are now coming from the West at Beaufort force 5 at a speed of 21 knots. Previously in picture 2 they were from the West South West at Beaufort force 8 at a speed of 35 Knots, so the wind has continued to veer and has decreased in strength (this references back to the appropriate avoiding action to be taken). This is a good sign that Aurora had taken appropriate action to avoid the TRS. The pressure has increased from 984mb to 999mb as well as the significant wave height dropping to around 5 meters in comparison to 8 meters in picture 2. These are both good indications that the TRS was moving away from Aurora and that she was successfully moving away from it.

Picture 4 – Aurora had now passed clear of the TRS and it was now starting to cross the coast of the North Island of New Zealand. At this point in time the TRS was starting to dissipate due to a lack of moisture supply from the land.

The pressure had increased to 1003mb from 999mb in picture 3. The wind had now dropped to Beaufort Force 4 at a speed of 15 knots from the West in comparison to it blowing from the west at Beaufort force 5 at a speed of 21 knots. The significant wave height was at around 3 meters, down from around 5 meters in picture 3. The humidity had also dropped to 80% when it had been previously 84%. All of these weather changes are very clear indications that show that the TRS was now passing clear of Aurora.

Picture 5 – This picture is included to show when the TRS had finally died out off the East Coast of the North Island of New Zealand and had now become a large low pressure system. This, as previously mentioned, is due to a lack of moisture being supplied to the TRS as it passed over land.

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