Tropical cyclone seasons

Learning when the tropical cyclone season is and the general tracks of these systems can help avoid a potentially dangerous situation, whether in port or out at sea. Let’s evaluate the tropical cyclone seasons for the Pacific and Indian Ocean basins. The general lifecycle of a tropical cyclone begins as a broad circulation or tropical low.

As this system becomes better organised, it becomes a tropical depression, when sustained winds can reach 33 knots or less around an enclosed circulation. When the sustained winds reach 34 knots, it becomes a tropical storm and is given a name. When the tropical storm has sustained winds of 64 knots, the system becomes classified as a hurricane in the north-eastern Pacific.

North-east/central Pacific Ocean tropics

The north-east Pacific tropics cover from north of the equator westward to approximately 140°W, and commences on 15 May and lasts through 30 November. In the central North Pacific, from 140°W to the International Dateline, the tropical cyclone season is from 1 June through 30 November (see Figure 1).

In the north-east Pacific, tropical cyclones usually form offshore of the western coast of Central America or Mexico, and will track westward or turn more north-west towards the Mexican Coast.

In the central Pacific, tropical cyclones generally track west to west-north-west until a strong cold front passes to the north of the tropical cyclone. This could turn a tropical cyclone more to the north-west and eventually north-east ahead of the cold front. Once the tropical cyclone reaches cooler waters, it will transition into an extratropical cyclone, or become a low or gale that is found over the mid-latitudes.

North-west Pacific Ocean tropics

The north-west Pacific tropical cyclone area covers north of the equator, and from the South China Sea and eastern China eastward to the International Dateline. This is the most active basin for tropical cyclones.

The tropical cyclone season commences 1 April and ends 31 January, although tropical cyclones can form year-round.

The life cycle of a tropical cyclone is similar to that of the north-east Pacific, except that hurricanes are called typhoons.

Tropical cyclones generally develop between 05°N and 15°N and often track westward over the Philippines and into the South China Sea, before moving inland over South-East Asia. At other times, tropical cyclones will track north-west towards the East China Sea, or turn to the north-east ahead of a cold front and can impact North-East Asia before accelerating north-east over the north-west Pacific (see Figure 2).

Northern Indian Ocean tropics

In the northern Indian Ocean (north of the equator), the tropical cyclone season is from 1 April to 31 December. However, the peak of this tropical season occurs twice: once in April through early June, and again in September through December during breaks in the monsoon.

These systems are called tropical depressions when they first form with sustained winds of 17 knots or more and become tropical cyclones when the winds reach 34 knots or greater. Typically, tropical systems form in the Southern Bay of Bengal or Southern Arabian Sea and track to the west or north-west.

Southern Indian and Southwest Pacific Ocean tropics

South of the equator, tropical cyclones rotate clockwise instead of rotating counter-clockwise as they do in the northern hemisphere.

In the South Pacific Ocean, tropical cyclones do not develop east of 140°W. However, from 140°W to the Indian Ocean, tropical cyclones generally develop from 1 November through 30 April. These systems are referred to as tropical cyclones when they develop and typically form south of the equator, before tracking to the west towards eastern Australia or towards Madagascar.

At other times, these systems will turn out ahead of cold fronts and track to the south-west and, eventually, to the south-east before weakening over cooler waters.

El Niño and La Niña impacts

El Niño and La Niña can impact the tropical cyclone seasons of the Pacific Ocean. El Niño is when sea surface temperatures are warmer than normal over the eastern and central equatorial Pacific Ocean, while La Niña is when sea surface temperatures are cooler than normal.

During El Niño, tropical cyclone frequency increases over the north-eastern Pacific due to the warmer sea surface temperatures. However, during La Niña, the tropical cyclone frequency in this area is usually below normal due to strong winds aloft that are unfavourable for tropical development.

Over the south-west Pacific, tropical cyclone frequency lowers near Australia due to lower-than-normal sea surface temperatures. However, the tropical cyclone frequency increases and the season extends into May from New Caledonia eastward to French Polynesia. During La Niña, tropical cyclones become more frequent around northern and eastern Australia.

Monsoon seasons

A monsoon is a seasonal reversal of the winds which impact the amount of precipitation over an area. Areas that experience a monsoon are over the northern Indian Ocean, South-East Asia, northern Australia and Indonesia (see Figure 3).

There are two different types of monsoons in the northern hemisphere: the south-west monsoon and north-east monsoon. During the south-west monsoon, weak thermal lows develop inland due to warmer land temperatures in the summer. This, in contrast with cooler sea surface temperatures, allows winds to blow from south-west to north-east. Abundant moisture from the Indian Ocean and South China Sea flows inland and generates the rainy season over southern Asia.

During autumn, the south-west monsoon gradually transitions to the north-east monsoon. Stronger highs develop over Asia and colder air migrates to the south over warmer sea surface temperatures. Rainfall diminishes and the dry season commences during the north-east monsoon. By winter the transition is complete and the north-east monsoon lasts until spring before transitioning again.

The Indo-Australian monsoon is similar, where weak lows develop along a trough inland over northern Australia and generate westerly winds across Indonesia and northern Australia.

These winds transport abundant rain over these regions from the southern Indian Ocean during the summer. During the winter, the winds become easterly as the trough weakens and shifts to the north, while strong highs build over southern Australia. Dry air moves over northern Australia and Indonesia as a result.

Trade winds, doldrums and the horse latitudes

Trade winds are mainly easterly winds that can be found between approximately 5° to 20° in both the northern and southern hemispheres. The strength of the trade winds is affected by two weather features: the equatorial trough and the semi-permanent ridge of high pressure.

The equatorial trough is found along the equator between 5°N and 5°S. The semi-permanent ridge of high pressure is found between latitudes 25° and 30° during the winter and between latitudes 30° and 35° in the summer in both hemispheres.

During the winter, the trade winds are strongest due to a stronger interaction between the equatorial trough and the ridge, and during the summer the trade winds are weakest since the ridge has shifted farthest from the equatorial trough.

There are two regions of light winds that can be found in the northern and southern hemispheres. ‘The doldrums’ refers to the region under the equatorial trough where winds are generally light and variable, but numerous showers/squalls can be found moving east to west. This region is where the trade winds converge and moist air rises and condenses into squalls. This rising air minimises wind at the surface.

Light westerly winds can be found around latitude 30° in both hemispheres and this is referred to as the horse latitudes. This is typically found along the periphery of the semi-permanent ridge of high pressure.

Thermal troughs

Areas of elongated low pressure, or troughs, can be found along western coasts of places such as Baja California, Australia, India and Madagascar. These troughs typically develop due to the interaction between the warm land temperatures and cooler water temperatures. The troughs can interact with strong highs nearby to produce enhanced winds along and offshore the coast (northerly winds in the northern hemisphere and southerly winds in the southern hemisphere) before diminishing when the high weakens.

Gale/storm tracks

The gale/storm track is most active in winter across the North Pacific, where most of the gales/storms track over the East China Sea and across Japan, to the North Pacific between 35°N and 50°N. The storms strengthen and track northeast over the Gulf of Alaska.

Secondary gales will develop along cold fronts near 35°N over the Gulf of Alaska and will track to the east towards the US West Coast. Cold fronts will extend as far south as the northern South China Sea before tracking to the east between 25°N and 30°N across the North Pacific. At times, large storms that develop over the North Pacific will generate large north to north-west swell that can propagate south of the equator.

The gale/storm track shifts to the north while weakening and, by summer, the gales are located from northern Japan north-east over the Bering Sea or track north-east over the Gulf of Alaska before moving over British Columbia. These weather features reverse to the south and strengthen through autumn.

During spring and autumn, cut off lows/gales can develop over the eastern Pacific between 30°N and 35°N and from 130°W to 170°W. These systems generally develop along a cold front but are left behind and can meander for a few days before turning north-east ahead of another cold front (see Figure 4).

Over the northern Indian Ocean, the weather is mainly dictated by the monsoons. However, the weather over the Red Sea is affected by highs and cold fronts. During winter, cold fronts will track east across the northern Red Sea every two to three days. Behind these cold fronts, a strong ridge will build over the northern Red Sea which will generate enhanced northerly winds through the Red Sea.

During spring and autumn, the cold fronts shift farther north over the eastern Mediterranean Sea and remain over Eastern Europe in summer. High-pressure ridges build over the northern Red Sea during spring through autumn every few days. Farther south, a trough of low pressure generally develops over the central Red Sea during the spring through autumn. This, interacting with a broad high over the Arabian Sea, will produce enhanced southerly winds through Bab El Mandeb and the southern Red Sea. These conditions typically last approximately two to three days.

Over the South Pacific Ocean and southern Indian Ocean, the gale/storm track is usually located between 35°S and 50°S and cold fronts extend as far north as 25°S to 28°S during winter. These fronts move through South Africa and south of Madagascar, as well as through Australia and New Zealand every two to three days. These storms can also produce large south to south-west swell propagating north of the equator.

During spring the gale/storm track weakens and shifts to the south, and by summer the gale track is mainly south of 45°S. Cold fronts only extend as far north as 35°S. These fronts move through every four to five days to the south of Africa and Australia. Cut off gales can develop offshore of eastern Australia and east of New Zealand during autumn through spring, and typically form between 30°S and 40°S. These systems act similarly to the cut off lows/gales in the North Pacific (see Figure 5).

Additional weather information

Having a general knowledge of what weather conditions to expect in certain regions will assist with your planning. Knowing what your vessel can handle in different weather conditions will also help in determining when there is a window to transit. It is best to be flexible in your plans. Always have the option to stop somewhere or divert your route to avoid weather systems if necessary, and be prepared for delays to allow the weather to improve.

It is also best to have the information at hand to monitor the weather before you leave port and while underway. There are plenty of sources that can be found online, and government agencies produce weather reports that are routinely updated. Weather programs can be downloaded through software, as well as grab files prior to departing port and while underway.

You can request assistance from a professional marine meteorologist to monitor your weather conditions and provide routing advice. These meteorologists can monitor your progress 24 hours a day and contact you if anything were to change. Sometimes these meteorologists can provide additional information such as passing along tsunami warnings.

Regardless, making the appropriate preparations and staying on top of the latest weather reports will provide the best chance at a memorable trip to your dream destinations.