Tides & The Supermoon

Tides affect Trinidad and Tobago year round, with spring tides and king tides sometimes causing nuisance coastal flooding to parts of Mosquito Creek. During hazardous sea events, either through wind generated waves, long period swells or storm surges, tides can produce more severe flooding to coastal communities.

Some key phrases for this post:

  • Syzygy – Linear alignment of the Sun, Moon, and Earth, which occurs during the new and full Moon, causes the respective gravitational pulls of the Sun and Moon to reinforce each other.
  • Spring Tide – The effect of syzygy causes an increase in the tidal range.
  • Perigee – Occurs when the Moon reaches the closest point to the Earth during its elliptical orbit.
  • Perigean Spring Tide – Occurs when the Moon is closest to the Earth during a Spring tide.

Moon Phases

As the Moon orbits the Earth, it goes through 8 lunar phases. The four that affect tides on Earth are:

A New Moon occurs when the Moon is aligned between the Sun and the Earth. This event occurs approximately every 29.5 days. The hemisphere that is viewable from earth is completely in the Sun’s shadow and mostly invisible. It is still possible to see the Moon faintly during a New Moon due to Sunlight reflecting off Earth, a phenomenon called Earthshine.

Spring Tides resulting from a new moon

During the New Moon phase, syzygy occurs, and this results in a Spring tide. Spring tides, as you will soon see, are not uncommon and occur twice a lunar month. During Spring tides, the tidal forces due to the Sun and the Moon reinforces one another and increases the tidal range – i.e. we observe higher high tides and lower low tides.

First Quarter & Third Quarter

Neap Tides resulting from 1st and 3rd Quarter Moons

This occurs when 50% of the viewable hemisphere of the Moon is illuminated by direct Sunlight. The first quarter occurs after the new Moon while the Third Quarter occurs after the Full Moon. Neap tides occur during this period, which typically occur 7 days after the Spring tide associated with the full Moon or the new Moon. During a Neap tide, the Sun and Moon are separated by 90° (when viewed from Earth). This results in the tidal force from the Sun partially canceling the Moon’s tidal force. A Neap tide results in lower than usual high tide but a higher than usual low tide. It is less extreme than Spring tides.

Full Moon

Spring Tides associated with a Full Moon

A Full Moon occurs when the Earth is directly aligned between the Sun and the Moon and is completely illuminated as seen from Earth. (In actuality, only the hemisphere of the Moon that faces the Earth is illuminated.) This phenomenon also occurs every 29.5 days. During a Full Moon, similar to a New Moon, a Spring Tide also occurs.

Colors of the Full Moon (Blood Moons)

A “Blood” Moon

The Moon may sometimes appear reddish and is colloquially called a “Blood Moon.” This is not a scientific term and is actually the result of the Moon travelling through the Earth’s shadow. This is known as a total lunar eclipse. This results in the blockage all of the direct sunlight that reaches the Moon and illuminates the hemisphere that we see on Earth. However, the Moon does not turn completely dark – we see a red Moon.

A Total Lunar Eclipse

The Sun’s rays pass through Earth’s atmosphere and a phenomenon called Rayleigh Scattering occurs. Our atmosphere essentially acts as a “filter” for light, “filtering” (or scattering) a number of wavelengths but affecting red wavelengths the least. Since so many colors are affected, the Moon may also appear yellow, orange or even brown during a total lunar eclipse. This phenomenon is the same mechanism that causes our colorful sunrises and sunsets. The next lunar eclipse that will be fully visible from Trinidad and Tobago will occur on January 20th-21st 2019.

Supermoons & Micromoons
(Perigees & Apogees)

Perigee versus Apogee

The definition of a supermoon was arbitrarily defined by astrologer Richard Nolle in 1979. Nolle defined it as a new or full moon that occurs when the natural satellite (the moon) is at or near (within 90 percent of) its closest approach to Earth in a given orbit (perigee). In other words, any full moon or new moon that comes to within 361,740 km or 224,775 miles (or less) of our planet, as measured from the centers of the moon and Earth, is a supermoon. At this distance, the moon appears approximately 7% brighter than an average full moon.

Similarly to the elliptical orbit of the Earth around the Sun, the Moon also has an elliptical orbit around the Earth. This results in a point in its orbit where the Moon will be closest to (perigee) and furthest away (apogee) from earth.

During a Supermoon, which is not a scientific term, a perigee syzygy occurs – meaning that the Earth, Sun, and Moon are all in alignment. Generally, a Supermoon occurs approximately every 13.9443 synodic months (New Moon, 1st Quarter, Full Moon, 3rd Quarter cycles). However, halfway through the cycle the Full Moon will be close to apogee, and the New Moons immediately before and after can be Supermoons.

On the opposite spectrum, a Micromoon or an apogee syzygy occurs where the Earth, Sun and Moon are still aligned but the Moon is now farther than 405,000 Kilometers away from the center of the Earth.

Tides

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted from the moon, the sun and the Earth’s rotation. While the Sun and the Earth’s rotation do affect tidal levels, the main contributor to extending the tidal range is the effects of the Moon’s orbit.

Regular Tides (Diurnal, Semi-diurnal, Mixed Semi-diurnal)

This map shows the geographic distribution of different tidal cycles along the Earth’s coastlines. Regions experiencing diurnal tides are marked in yellow, regions experiencing semidiurnal tides are drawn in red and regions with mixed semidiurnal tides are outlined in blue. Trinidad and Tobago experience mixed semi-diurnal tides. Image from NOAA Ocean Service Education.

There are generally three types of tides: diurnal – one high and low tide each day, semi-diurnal – two high and low tides each day, and mixed – two high and low tides each day of different heights. The different types of tides occur due to the arrangement of the land on earth, the shape of the local coastline, and the depth of the water near the coastline.

Types of tides: diurnal, semidiurnal, and mixed. Figure from NOAA Co-OPS Education.

In Trinidad and Tobago, as well as the Caribbean, we experience mixed semi-diurnal tides.

Spring Tides

Tides of a Lunar Cycle

Every lunar month (29.5 days), there are two Spring tides and two Neap tides. Spring tides correspond to the occurrence of a New or Full Moon (as detailed above). Neap tides correspond to the occurrence of the 1st and 3rd Quarter Moons (also as detailed above). These tides are also completely normal as they tend to occur twice each lunar month all year long, without regard for the season. During spring tides, the tidal range is larger than normal i.e. higher than average high tides and lower than average low tides occur. During neap tides, the tidal range is smaller.

Perigean Spring Tides (King Tides)

Perigean Spring Tides

As explained above, Spring tides occur twice every lunar month during either a full or new Moon. However, approximately 3-4 times a year, the New or Full Moon coincides closely in time with the perigee of the Moon – the point where the Moon is closest to the planet. These Perigean Spring Tides are sometimes called King Tides.

If a Supermoon occurs during a perigee and a Perigean Spring Tide also occurs during a perigee, does this mean that every time a Perigean Spring Tide occurs, is there a Supermoon? This is correct. The only reason you may hear or read about some Supermoons versus others is that some Supermoons occur during a Full Moon, where it’s visible, versus a New Moon, where it is not.

An important note, Supermoons are typically closer to the Earth in the Northern Hemisphere Winter months compared to other Supermoons during the year. During the Winter Months, the Sun is closer to the Earth (as a result of the Earth’s elliptical orbit around the Sun). This results in the Sun’s gravity pulling the Moon closer to the Earth. This also means higher Spring Tides or Perigean Spring Tides during November through March yearly.

King Tides & Peak Spring Tide Calendar 2019

2019 King Tides & Peak Spring Tides Calendar. Credit: North Carolina King Tide Project

King Tides & Coastal Flooding in T&T

During a Perigean Spring Tide, Trinidad and Tobago observes higher than average high tides and low tides. For very flood prone areas such as the Manzanilla-Mayaro Road and the South Trunk Road at Mosquito Creek, Perigean Spring Tides pose a significant issue.

A Look at Mosquito Creek, South Trinidad

Several times throughout the year, high tides near the height of the Mosquito Creek sea wall. The height of this wall is no taller than 4 feet, meaning when tides near this level, chaos ensues on the roadway.

Due to a lack of maintanence on the wall, several cracks have developed. While some mitigating measures have been put in place to reduce water pressure on the wall, usually when the water level exceeds 3.5 feet, salt water begins to trickle through the cracks and cause ponding on the roadway.

At 3.8 feet, this is where there is cause for concern. Due to prevailing winds, wind-driven waves can add an additional 0.2 feet to the height of the water column along the sea wall. When the water level is at the sea wall’s height, sea water can overtop the wall and begin to splash onto vehicles and the roadway.

The risk of flooding increases during periods of heavy rainfall, were drains are already at capacity along the creek or long period swells make its way into the Gulf of Paria, causing larger than normal waves along the sea wall.

Sea Level Rise & Trinidad and Tobago

Characteristically, Trinidadians know that particularly November through February, the Manzanilla-Mayaro Road experiences flooding – either through rough seas, a heavy rainfall event or a combination of both. During prolonged, heavy showers, Southerners know that Mosquito Creek floods – especially during high tide. Residents and commuters alike treat the frequent flooding in Port of Spain as the norm. What Trinbagonians now need to come to grips with is sea level rise – which impacts all the aforementioned circumstances in some way or shape.

A Flooded Manzanilla-Mayaro Road on October 25th 2017. (Credit: Richard Ramrattan)

While it has been difficult to acquire the data on average sea level rise around Trinidad and Tobago over the last few years, it is a known fact that as the planet warms, glaciers, and polar ice caps melts – resulting in global sea level rise. This global rise in ocean levels will not only begin to flood low-lying areas across the world, including parts of Trinidad and Tobago, but will also make the effects of Perigean Spring Tide events more apparent. This means an increase in frequency of flooding along Mosquito Creek if the wall isn’t raised or new infrastructure isn’t built. It also means that new coastal areas on both islands will begin to experience unprecedented coastal flooding.

Looking even more locally, ocean temperatures surrounding Trinidad and Tobago can contribute more than you’d expect both to local weather and sea level rise. Heat causes expansion, even in ocean waters. This expansion results in high than normal ocean temperatures, which then causes higher than average tides even during a regular tide cycle. This fact aside, warmer ocean waters locally affects our climate by providing more thermal energy for convective showers to build and strengthen as clusters near the islands – fueling heavier rainfall – which will cause coastal flooding in addition to inland freshwater flooding.

All signs are pointing to an increased potential for flooding events across Trinidad and Tobago and its time that the proper plans are implemented to mitigate its impacts.

A complete watercourse assessment across both islands in needed and appropriate action has to be taken.

Critical infrastructure such as the South Trunk Road at Mosquito Creek and the Manzanilla-Mayaro Road needs to be reinforced or rebuilt to mitigate the effects of both salt and freshwater flooding.

A comprehensive assessment needs to be done across Port of Spain’s drainage network to mitigate flooding in the capital, even after the lightest shower. On a citizen level, Trinbagonians need to come to terms and understand that our islands are highly susceptible to flooding, regardless of how much action is taken by officials.

These “unprecedented” amounts of rainfall in short periods are not going to remain one-offs. These events will undoubtedly become more and more frequent as our planet and ocean warms and unfortunately, we are in the line of fire for its effects.

Sources

Supermoon: https://www.timeanddate.com/astronomy/moon
NOAA: https://oceanservice.noaa.gov/facts/perigean-spring-tide.html
NOAA: https://oceanservice.noaa.gov/facts/springtide.html
Wikipedia: https://en.wikipedia.org/wiki/Perigean_spring_tide
Wikipedia: https://en.wikipedia.org/wiki/King_tide
Ocean Analysis: https://www.tropicaltidbits.com/analysis/ocean/

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