Hazardous Seas

Throughout the year, Trinidad and Tobago experience rough sea conditions for a multitude of reasons. These rough seas not only impact the lives of mariners but can translate into hazardous conditions nearshore along bays and beaches and can even contribute to coastal flooding and erosion.

Content
Causes & Frequency
Long Period Swells
Strong Low-Level Wind-Generated Waves
Thunderstorms & Tropical Cyclone Generated Waves
Monitoring & Forecasting
Effects
Major Events
Safety

Hazardous Seas Causes & Frequency

Most waves across Trinidad and Tobago are wind-driven. This means that waves are generated by the local prevailing wind. These waves would vary in size according to the length of time a particular wind has been blowing, the fetch (the distance the wind has blown over the sea) and the water depth.

Hazardous seas across Trinidad and Tobago’s coastal waters mainly come from three sources; long-period swells from distant weather systems, strong low-level winds agitating sea surface conditions and wind-driven waves from local weather systems such as strong thunderstorms or tropical cyclones.

Long Period Swells

Large weather systems (extra-tropical or tropical low-pressure systems) with sustained winds can blow across long fetches and travel thousands of kilometers from their point of origin.

Swells are also different from regular waves as it has a longer period. The period of a wave is the between the crests or peaks of a wave, usually measured in seconds. For a swell to be considered a long period, the wave period is typically greater than 11 seconds.

Water-Wave diagram showing characteristics of a wave. Credit: Wikipedia
Water-Wave diagram showing characteristics of a wave. Credit: Wikipedia

In deep, open water, swells present less of a hazard to most mariners than wind waves. Swells become more dangerous as it moves into shallow waters because of a process called shoaling. This allows swells to become steeper and taller. These swells can double in height at it moves into shallow water and can break in open waters.

Diagram showing swell generated waves due to shoaling. Credit: University of Wisconsin-Madison
Diagram showing swell-generated waves due to shoaling. Credit: University of Wisconsin-Madison

Long period swells during December through May tend to originate from strong extra-tropical low-pressure systems or winter storms moving off the Eastern Coast of the United States. During an El Niño year, when winter storms take a more southern track across the United States, Trinidad and Tobago typically experience increased swell events. From May through November however, long period swells in the Atlantic Ocean tend to originate from passing tropical cyclones across the Atlantic Basin.

A swell event generated by Tropical Storm Leslie in the North-Central Atlantic Ocean. Large swells affected coastal Trinidad and Tobago during September 28th through October 5th 2018.
A swell event generated by Tropical Storm Leslie in the North-Central Atlantic Ocean. Large swells affected coastal Trinidad and Tobago from September 28th through October 5th, 2018.

Strong Low-Level Wind-Generated Waves

Predominantly during the dry season, between the end of December through mid-May, Central Atlantic high-pressure systems dominate across the Atlantic Basin. This weather feature is responsible for suppressed rainfall across the Eastern Caribbean, as well as breezy conditions with prevailing winds generally from the northeast to east.

Association between wind speed and distance between isobars (lines of equal pressure). In the illustration above thicker arrows represent relatively faster winds. Credit: Physical Geography

Association between wind speed and distance between isobars (lines of equal pressure). In the illustration above thicker arrows represent relatively faster winds. Credit: Physical Geography

At times, the pressure gradient tightens, meaning areas of low pressure and high pressure is close to each other. This tight pressure gradient can generate strong low-level winds that blow across the Atlantic Ocean, bringing gusty winds on-land and at sea, but with accompanying rough seas (waves ≥ 2.5 meters).

Thunderstorms & Tropical Cyclone Generated Waves

A tropical cyclone is a strong, closed low-pressure system, with strong winds and associated showers and thunderstorms. (you can read more about tropical cyclones here.) Tropical cyclones in the Atlantic Basin usually occur between June 1st and November 30th, but out of season storms have happened in the past.

The wind associated with a tropical cyclone blows over a large fetch and can cause waves of significant height to impact coastlines due to water piling up along coastlines. This can cause coastal flooding of tsunami-like proportions.

 Storm Surge vs. Storm Tide. Credit: NOAA/The COMET Program
Storm Surge vs. Storm Tide. Credit: NOAA/The COMET Program

The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm’s center.

Wind and Pressure Components of Hurricane Storm Surge. Credit: NOAA/The COMET Program
Wind and Pressure Components of Hurricane Storm Surge. Credit: NOAA/The COMET Program

Thunderstorms, particularly severe thunderstorms can produce locally rough seas as strong downdrafts can agitate the sea surface and cause damaging waves out at sea. In rare cases, in fast-moving severe thunderstorms, meteotsunamis can also occur.

Hazardous Seas Monitoring & Forecasting

Hazardous seas are fairly predictable events in this modern era as forecasters have a wide array of satellite data (RapidSCAT, ASCAT, WindSat, Oceansat-2, SMAP, SSM/I, GCOMW1-AMSR2 Radiometer) and a network of buoys and ship reports relaying sea conditions in a near real-time basis.

This collection of data is then fed into several waves and swell models, most notably the ECMWF Wave Model (WAM) and the GFS Wavewatch III Model. Forecasters can use a combination of all of these pieces of information and issue forewarning.

In Trinidad and Tobago, the Trinidad and Tobago Meteorological Service issues Hazardous Seas Alerts, Watches and Warnings at different levels (yellow, orange and red) depending on the severity of the conditions.

Most countries in the Caribbean and internationally, follow a rigid marine warning system, issuing small craft cautions, advisories, and warnings based on expected wind speeds and sea wave heights for mariners as well as high surf advisories and warnings based on wave heights for beachgoers and coastal communities.

There are several wave heights that can be used in wave forecasts. However, most meteorological offices use total or significant wave heights.

The most frequent wave height is 0.64 of the significant wave height. The significant wave height is the average height of the top one-third of all wave heights. About 14% of waves (1 in 7) will be higher than the significant wave height.

Maximum waves are twice the significant wave height. It is normal to expect a wave of twice the height of the significant wave about 3 times in 24 hours. This means that you need to be prepared for a wave of this height before heading out to sea!

For example, if the forecast calls for waves of 2.0 meters in open waters, this is the significant wave height. This would then mean that the most frequent wave height you would experience at sea would be approximately 1.28 meters. This also means that the maximum wave height you can experience out at sea would be 4.0 meters. You need to be prepared for a wave of this height before heading out to sea!

The sea forecast, i.e. the significant wave height, takes into account the combined wind-generated waves and swells using the following table:

Significant wave height, taking into account wind wave height and swell wave height.
Significant wave height, taking into account wind wave height and swell wave height.

In sea forecasts, seas can be qualitatively described as calm, slight, moderate, rough and in rare instances, very rough to high. What do these terms actually mean? See the below table for more information.

Sea (Wind Wave)
Description
Height
(Meter)
Effect WMO Sea
State Code
Calm (Glassy) 0 No waves breaking on the beach 0
Calm (Rippled) 0 – 0.1 No waves breaking on the beach 1
Smooth (Wavelets) 0.1 – 0.5 Slight waves breaking on the beach 2
Slight 0.5 – 1.25 Waves rock buoys and small craft 3
Moderate 1.25 – 2.5 Sea becoming furrowed (corrugated) 4
Rough 2.5 – 4.0 Sea deeply furrowed 5
Very Rough 4.0 – 6.0 Sea much disturbed with rollers having steep fronts 6
High 6.0 – 9.0 Sea much disturbed with rollers having steep fronts (damage to foreshore) 7
Very High 9.0 – 14 Towering seas 8
Phenomenal ≥ 14.0 Precipitous seas (experienced only in cyclones) 9

Sea description, typically used to describe the sea state in forecasts. Source: Australian Government Bureau of Meteorology

Hazardous Seas Effects

Injuries or Loss of Life. For beachgoers and coastal communities, hazardous waves and swells near or onshore produce dangerous rip currents that can carry even the strongest swimmers out to sea. High surf can also knock spectators off exposed rocks and jetties. Because of these hazardous conditions, beaches can be closed to the public. For mariners, small craft may be difficult to navigate and waves can be hazardous to small craft. Inexperienced mariners, especially those operating smaller vessels, should avoid navigating in these conditions.

Beach Erosion. Swells can produce large breaking waves in nearshore and onshore areas. These waves can cause severe erosion along with coastal communities. This has been the case in Trinidad and Tobago along Tobago’s Atlantic Coastlines, Mayaro and Manzanilla in Eastern Trinidad and Cedros and Icacos in Southwestern Trinidad.

Coastal Flooding. Particularly during high tides, large waves can produce localized coastal flooding. Seawater can also splash onto low-lying roadways such as Mosquito Creek and the Manzanilla-Mayaro Road.

Economic Losses. Hazardous seas can close beaches and shut down most marine activities such as fishing and marine recreation. This has further consequences for businesses and economies that rely on tourism, fishing, etc. In severe conditions, coastal flooding and/or large battering waves can cause significant damage to property and boats in coastal communities.

Environmental Impacts. In turbulent and rough seas, sensitive coral reefs can be damaged. Coastal floods can cause saltwater intrusion into farmland and the water table, disrupting potable water from desalination.

Major Hazardous Seas Events

Hazardous Seas Safety

Rip currents are powerful channels of water flowing quickly away from the shore, which occur most often at low spots or breaks in the sandbar and near structures such as groins, jetties, and piers. If caught in a rip current, relax and float. Don`t swim against the current. If able, swim in a direction following the shoreline. If unable to escape, face the shore and call or wave for help.

Secure property. Hazardous sea events are fairly well forecast. If hazardous seas are forecast for your location, secure small crafts if you own a marine vessel. If you live on a waterfront property, sandbagging is useful in the event coastal flooding materializes.

Stay out of the ocean. If you are a mariner that owns a small craft, it will be dangerous for you to venture into the ocean, particularly if you are inexperienced. Remember, even if the seas forecast calls for waves of 2.5 meters in open waters, the maximum wave height is double that number, i.e. 5 meters. You need to be prepared for a wave of this height before heading out to sea! Waves of this height are capable of capsizing larger vessels. Recreational boaters should remain in port, or take shelter until waves subside.  Commercial vessels should prepare for rough seas and consider remaining in port or taking shelter in port until hazardous seas subside.

Stay away from beaches or entering the water. If you are considering visiting the beach during a period of hazardous seas, don’t! Beaches can be closed. Large breaking waves can knock you down and cause serious injuries. Rip currents can be life-threatening. Your safest option is to wait until seas return to normalcy.

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