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Inter Tropical Convergence Zone

What it is and its role in Trinidad and Tobago Rainfall

25/06/2018

Pictured above: a hyperactive and prolonged ITCZ in October 2017

The Inter Tropical Convergence Zone or ITCZ as it is commonly called is one of the main climatological features of the global atmosphere.

It is a zone (belt) of low atmospheric pressure that forms where the northeast trade winds meet the southeast trade winds (zone of convergence) near the earth's real equator.

When the two trade wind regimes meet, moist warm air is forced upward. This causes water vapour to condense as the air cools and rises, resulting in the formation of a band of clouds with heavy rainfall.

Photo credit: http://principiojoanne2196.blogspot.com/2012/06/intertropical-conversion-zone-itcz.html

The location of the ITCZ varies throughout the year and dramatically affects rainfall in areas where it is located.

It is also called the thermal or climate equator because the zone of low pressure associated with the ITCZ migrates with the changing position of the thermal equator.

The thermal equator is that zone that receives the most intense heat from the sun and hence moves with the movement of the sun. The ITCZ, therefore, appears to move north and south with the overhead sun, because where the sun is overhead is likely to be the hottest area.

As such, the movement of the thermal equator shifts the belts of planetary winds and low pressure zone to the north and to the south annually.

When it moves north over Trinidad and Tobago it brings wet weather and significant rainfall, with thunderstorms regularly occurring directly within the associated band of cloud mass. Land areas heat up a lot more quickly than the sea so you will find that the ITCZ is particularly active over land areas during the day and more active over coastal and sea areas during the night.

Trinidad received around 1643 mm of rainfall per wet season.

Trinidad has a twin-peak rainfall regime, with a major peak in June-July-August and a smaller peak in November as the graph above shows for Piarco, Trinidad.

This twin peak in rainfall occurs because the ITCZ moves northward in the early part of the wet season, which brings much rainfall with it, As it moves southward-- as the sun migrates later in the season-- it brings another peak in the rainfall totals.

Tobago has average rainfall totals per wet season of around 1250 mm and, as the graph below shows, Crown Point in Tobago receives an early peak in rainfall in July when the ITCZ is furthest north and its main peak in November when the ITCZ is on its way southward.

It is important to note that the ITCZ is not the only feature affecting rainfall during the wet season.

Usually, the wet season accounts for 75 to 80% of the annual rainfall total in Trinidad and Tobago.

While the ITCZ is not the only main rainfall-producing feature of our local wet season, it does account for a significant part of the annual rainfall. This is because when it’s in the area, rainfall activity associated with it, although not always continuous, can span one to three days or sometime more.

#itcz #intertropical convergence zone

Thick Dust Haze Over Trinidad and Tobago Once Again

Heavy amounts of dust have been in the air over Trinidad and Tobago for the last three days, and are expected to continue for a few more days.

June 06th, 2018. Pictured above: Piarco

Thick dust haze is in the air over Trinidad and Tobago once again.

This has resulted in the atmosphere appearing dull and hazy and making it difficult to discern parts of the landscape that are just four to five kilometres away.

The dust haze which has been linked to suppression of hurricane activity in particular, and to rainfall occurrence in general, has provided the hurricane season with dusty start.

Coming all the way across the Atlantic Ocean (the extent of the current dust intrusion can be seen in the image below): the dust would have begun as dust storms in the Sahara desert that were pushed towards the Caribbean by low- to mid-level winds.

Source: http://forecast.uoa.gr/dustindx.php

Whenever this type of dust-filled air engulfs the area, it is known to be a hazard for people with respiratory conditions.

The Real Story Behind The Dust Haze

There is a layer of air called the Saharan Air Layer (SAL) which is an extremely hot, dry and dust-laden layer of air that originates over the Sahara Desert of North Africa, where it extends from the surface upwards to several kilometres. It is fed by strong low to mid-level easterly winds over the desert which pulls sand and dust particles into the atmosphere. This is where the dust haze really emanates from.

This layer of dusty, very dry and warm air is pushed westward by the easterly winds and, on reaching the west African coast or eastern Atlantic Ocean, it rides over the cooler, more moist surface air of the Atlantic Ocean, forming what is called an atmospheric inversion layer or boundary: with warm, dry air aloft and cooler, moist air below.

How is dust haze linked to the suppression of rainfall and hurricane activity?

In an inversion, the temperature increases with height. The inversion boundary between the SAL and the cooler, more humid surface air prevents deep clouds from forming easily due to convection and hence rainfall is suppressed.

Convection is the upward movement of warm air or warm air rising. So the inversion boundary acts as a cap and suppresses deep or thick low level cloud formation or what is referred to as convective clouds.

The SAL has also been observed to suppress the formation and intensification of tropical cyclones and can make tropical waves appear ragged or even disintegrate them due to the mixing of the SAL’s dry air into these weather systems. Why is that? The Saharan air layer typically contain lots of dry air, aerosol dust particles, and strong winds, all of which act against the development of cloud and tropical cyclone formation.

Once the SAL reaches the Atlantic, the easterly trade winds continue to carry this dust across the Atlantic at the lower and mid-levels, where eventually some of it, oftentimes large pools of it, lands over Trinidad and Tobago and the southern Caribbean. As the SAL moves westward, it maintains the characteristics of warm stable air at its base, and dryness and dustiness throughout its vertical profiles.

Records show a strong cycle that suggests that dust haze is most prevalent in Trinidad and Tobago during June to August and least during December to February.

The SAL is therefore set to bring occasional dust haze over Trinidad and Tobago over the next three months.

#sahara dust #dust haze

Erratic Rainy Season Ahead

The rainy season is upon us again and, this year, we should prepare for an erratic one.

We don’t expect this year to be abruptly ambushed at the start of the rainy season by intense and highly impactful rainfall events; instead, we expect a slow onset to this rainy season. More than likely, heavy and prolonged rainfall events will gradually keep increasing as the season progresses.

June to August is expected to be drier than usual.

However, September to November favoured to be wetter than usual with above normal rainfall totals having the largest chance of occurring.

The map below shows the average amounts of rainfall that various areas in the country are predicted to receive over the rainy season:

Temperature

Prepare for hotter than usual days this season, as we expect maximum temperatures to exceed the average, producing warmer than usual days.

Prepare for nights to also be warmer than usual, as minimum temperatures at night will likely exceed the average.

Key points:

· Sunny skies in the wet season can turn cloudy with intense rainfall in less than half an hour. So they needed to stay informed of conditions that could change rapidly.

· Heavy and prolonged rainfall events are expected to increase in frequency as the season progress and this poses serious risks of flooding and landslides.

· The wet season brings increased potential for lightning strikes and floods so be prepared for power outages.

· If you live in a flood-prone area, have a plan in case flood waters quickly threaten your home.

· Citizens are urged to use the seasonal rainfall outlooks to prepare so that unpreventable flooding do not turn into unmanageable disasters. Be sand bag ready.

· Expect rainy weather impacts: such as increased road travel times as this can reduce productivity.

· Expect the unexpected. Have a plan and have the needful to survive for about 3 days to 7 day during the season.

· Don’t be vulnerable. Be sensible! Act now and prepare for heavy rainfall, flood, high wind, hot spells and spikes in dengue cases.

#rainy season #rainy season 2018 #wet season #rainy season outlook

Implementation of a new colour-coded Early Warning System

The Trinidad and Tobago Meteorological Service implemented a new, risk-based system, effective Monday, April 16th, 2018.

The new Early Warning System (EWS) complies with the World Meteorological Organization’s international standards for effective early warnings. The structure of the warnings conforms to the format of the Common Alerting Protocol (CAP), an international standard for emergency alerting and public warning. The TTMS will no longer be issuing Bulletins.

Early Warning Messages will now be issued as ‘Public Warning Messages’, which will take the form of ‘Alerts’, ‘Watches’ or ‘Warnings’ based on the amount of time between the current time and the expected time of impact. Alerts will have the longest lead time, while Warnings will have the shortest lead time.

Early Warning Messages will be issued for hydro-meteorological hazards such as Severe Weather, Thundershowers, Flooding, Dry Spells/Droughts, Extreme [High] Temperatures, and Hazardous Seas, in addition to already established notifications for Tropical Cyclones (Depressions, Storms, and Hurricanes). These hazards are already continuously monitored by the TTMS.

Public Warning Messages will contain indicators for the urgency (time available to prepare), certainty (probability of occurrence) and severity (intensity of impact). The risk posed to Trinidad and Tobago is determined by combining the certainty and severity of the hazard’s impacts. The risk level will be communicated using a colour scheme of Yellow-Orange-Red to indicate Moderate-High-Very High risk respectively.

Example of a Yellow-level Early Warning Message issued earlier this year for Hazardous Seas

Other features of the CAP compliant messages include: validity time, response type, description of the hazard event, instructions for appropriate response by message recipients and area description.

This new system aims to markedly improve the country’s Early Warning System in order to strengthen resilience, reduce vulnerability and minimize loss and damages associated with adverse weather and climate events.

#cap #common alerting protocol #early warning system

If you’d like to see more of our El Nino / La Nina updates as we track their progress throughout the year, you can find them on our website.

#enso #la nina #enso neutral

Winners of Art and Poetry Competition

On Friday, March 23rd, the day known as World Meteorological Day, we celebrated and awarded the winners of our poetry and art competition.

The poetry competition was open to primary school students, while the art competition was open to secondary school students. The theme of the competitions was “Weather-Smart, Climate-Ready”, in keeping with this year’s World Met Day theme!

Pictured: The Director of the Trinidad and Tobago Meteorological Service, Mr. Marlon Noel, the Minister of Public Utilities, Senator the Honourable Robert Le Hunte, and some of the winners of our art and poetry contests.