CLIMATE VARIABILITY

refers to departures from the mean position and higher order statistics such as standard deviations, as well as changes in the occurrences of extremes. Moreover, it is used to indicate departures of the mean etc over a given time period (for instance a month, a season, a year) from the long term statistic for a matching time period. Climate variability is thus a measure of the departures which are usually called anomalies. Climate variability is caused by natural occurring internal processes which occur on all time and spatial scales outside of that of individual weather events, and involves many modes of variability involving components of the climate systems such as the atmosphere and the ocean. An example of a naturally occurring internal process which drives climate variability in Trinidad and Tobago is the coupled ocean/atmosphere El Nino-Southern Oscillation (ENSO- El Nino or La Nina) phenomenon.

CLIMATE CHANGE

refers to a change in the mean condition of the climate that can be identified by a change in the mean and or variability of its properties that is statistically significant and this change must continue for an extended period, typically decades or longer. It may be due to naturally occurring internal processes such as volcanic eruptions which can act to cool the atmosphere; or external forcing such as changes in solar radiation received at the earth surface which can be influenced for instance by change in the tilt of the earth axis away from its orbital plane which occurs naturally. Naturally occurring climate variability can also induce climate change. Climate change may also be due to persistent external influences which do not occur naturally such as the change in the composition of the atmosphere or change in land use generated by human activity such as the burning of fossil fuel and deforestation.

DISTINCTION

The major difference between climate variability and climate change is the persistent nature of the anomaly and the fact that the change cannot be explained by naturally occurring internal variability processes alone. For instance, rare events occur more often or less often e.g. the maximum temperatures increasingly breaking records each year or nights are consistently becoming warmer each year. Climate change detection is the process of demonstrating that the climate has changed in some defined statistical sense without providing a reason for the change. Climate change attribution is the process of establishing the most likely cause for the detected change with some defined level of confidence.

PROBLEM STATEMENT

This policy document aims to enhance the monitoring and assessment of dry spells in Trinidad and Tobago to minimize crisis responses. Dry spells impact water management, agriculture, health, and the economy, with extreme droughts posing significant risks, especially with climate change. Given population growth and increased agricultural demands, establishing a dry spell early warning system is crucial. Current procedures for identifying dry spells are inadequate, limiting science-based monitoring. This document addresses that gap by aligning Trinidad and Tobago’s meteorological service with international standards, ensuring reliable rainfall deficit and surplus monitoring for informed decision-making.

INTRODUCTION

Before implementing a dry spell and drought policy, it is crucial to understand their differences, the complexity of drought, and its various types. According to the WMO, a dry spell lasts at least 15 days without significant rainfall, whereas drought is an extreme climatic condition marked by prolonged low rainfall, insufficient to meet societal and environmental needs. Drought varies by sector: climatologists focus on rainfall deficits, agriculturalists on crop impact, hydrologists on groundwater levels, and sociologists on social expectations. Types include meteorological, agricultural, hydrological, and socio-economic droughts. This policy specifically addresses meteorological drought and dry spells, using the Standard Precipitation Index (SPI) as the primary monitoring tool, supplemented by other indices when necessary.

VARIABILITY DRIVERS

Most of the features which account for the variability of temperature and rainfall in Trinidad and Tobago originate in the tropics, but global features also impact the climatic variability. Phenomena such as El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are two of the major drivers of climate variability in Trinidad and Tobago. It has also been shown that the Madden-Julian Oscillation (MJO), the Saharan Air layer (SAL), tracks taken by tropical storms, migratory behaviour of the Inter-tropical Convergence Zone (ITCZ) and behaviour of the North Atlantic Sub-Tropical High (NASH) also influence the climate variability in Trinidad and Tobago.

Studies have shown that the El Nino phase of the ENSO modifies the annual cycle of rainfall over the Caribbean greatest, with varying results for different areas. The relationship between El Nino and the rainfall cycle starts with a drying trend during the latter half of the wet season which continues into the dry season and ends with a strong wet signal during the earliest period of the new wet season; this relationship was observed in Trinidad and Tobago during the 2009-2010 episode of the most recent El Nino event.

With regards to La Nina, it has been found that the Southern Caribbean including Trinidad and Tobago exhibits wetter than normal late wet season and dry season; while the early wet season at the end of a La Nina event is drier than normal. The link between the NAO and Caribbean rainfall is found in the relationship between pressure systems and rainfall and temperatures in the region.

A positive index is indicative of anomalously high pressures across the region resulting in a reduction in rainfall since higher pressure introduces stronger winds, more evaporation, cooler Sea Surface Temperatures (SST's), and less ascending motion. When the index is negative, rainfall in the early period of the wet season is enhanced since lower anomalous pressure introduces lighter winds, less evaporation, warmer SST's, and more ascending motion.

TRENDS

Several studies exist which have found recent changes in the climate in Trinidad and Tobago, especially with respect to temperature trends. Temperatures in Trinidad and Tobago have been found to have increased steadily from 1946 to 1995. More recently, it has been shown that over the last three decades there has been an upward trend in temperatures that is statistically significant and which has been induced by a steady increase in daily minimum temperatures ; this latter finding suggests that nights have become warmer.

Other studies are also consistent with the warmer temperatures. Mc Sweeney 3using a Global Climate Model (GCM) found significant increases in the mean annual temperature in Trinidad and Tobago with an increase around 0.6 oC since 1960; an average of 0.13 oC per decade. The Trinidad and Tobago Meteorological Service (TTMS) also found that at Trinidad's, as well as, at Tobago's reference climatological stations, the annual mean air temperature has also warmed over the period 1981-2010 by 0.8 and 0.5 oC relative to 1961-1990 and 1971-1990, for Trinidad and Tobago respectively. The observed anomalous warming of 0.27 C per decade at the station in Trinidad and 0.17 oC per decade in Tobago are consistent with the IPCC (2007) observed 0.2 oC per decade in the Caribbean region.

Rainfall in Trinidad and Tobago has not shown as distinct a change as has been observed with temperatures. It appears that changes in rainfall differ across various parts of the country and over different time periods. During the 1960's to 80's it has been shown that Trinidad and Tobago annual rainfall totals increased, but subsequently decreased onward to the 1990's. More recently, rainfall in Penal in South Trinidad has showed a decreasing trend while at Piarco in the north, an increasing trend was observed.
Another recent study conducted on behalf of the UNDP found fractional decreases in Trinidad and Tobago's rainfall over the last half century; however the trend was not statistically significant. The TTMS 8 also found that there has not been any significant change in the annual mean rainfall totals in Trinidad over the period 1981 to 2010 compared to 1961-1990; however, there were larger changes in the annual mean rainfall totals over the latter period.
These changes were driven by slight increases in the frequency of extreme dry and wet years, as well as, increases and decreases in extreme wet and dry years rainfall totals respectively. In Tobago however, the rainfall pattern has shown a different behaviour;Tobago's annual mean rainfall has decline by 36.4 mm per decade during the 1981-2010 period compared to the long term average of the 1969 to 1990 period, this was driven primarily by a decrease in the number of years with near normal rainfall.

Strong El Nino events in recent years have resulted in meteorological droughts in Trinidad and Tobago. It has also been shown that the El Nino of 1997/98 and associated drought conditions in Trinidad and Tobago were responsible for the decline in sugar production during that period. The Drought event of 2010 in Trinidad and Tobago resulted in hundreds of acres of natural forest burnt. Drought conditions have also been advanced as the reason for lost of near- shore habitat due to declines in freshwater. This was aided by saline intrusion in the Southern Caribbean and Trinidad and Tobago during 2009-2010. Increased sea surface temperatures during the summer of 2005 resulted in severe coral bleaching in Tobago during the summer period. Recent increases in intense rainfall events have been associated with increased flooding in the region including Trinidad and Tobago and this has no doubt impacted fresh water impoundments and water quality. The Caribbean region experienced an average sea level rise of about 10 cm over the twentieth century due to the warming; but this rise is not uniform throughout the region.

PROJECTIONS

Statistical downscaling of global climate model (GCM) HadCM3 using the A2 and B2 emission scenarios project a rise in temperature of between 1.5 - 2.0 Celsius in the Caribbean region including Trinidad and Tobago by the 2080's compared to the 1980 to 1999 period. This amounts to a projection of 0.29 Celsius per decade. If these projections are realized it is possible that an approximate 8% increase in the length of the dry season by 2050 and consequently a shorter wet season could result.
Projected trends in rainfall for Trinidad and Tobago remain unclear with large uncertainties. Model consensus among 21 GCM models project annual rainfall decrease in the range of 15% for Trinidad and Tobago by the 2080's, however 3-4 of these models projected increase in annual rainfall. If the over drying trend projections are realized then it is likely that there will be intensification of dry spells and droughts in the vicinity of Trinidad and Tobago towards the 2080's.

Results from HadCM3 model downscaled for the A2 and B2 emission scenarios using the Statistical Downscale Modeling (SDSM) also project increases in the early rainy season (June – August) rainfall in Trinidad and Tobago compared to decreases in other countries in the region apart from Barbados. The drying trend projected in annual rainfall towards the 2080's is linked to a consensus in the models showing a shift to a more positive phase of the North Atlantic Oscillation (NAO) and El Nino like pattern in the Pacific with higher temperatures.

Based on the projections local water resources can be expected to be impacted. Increase intense rainfall events over shorter periods will result in lower surface water quality, reduction in the recharge of ground water as run off would be at a maximum; while Increase in longer dry spells and drought events coupled with warmer temperatures would increase agricultural irrigation demands,affect crop scheduling, increase heat related health impacts, coral bleaching and saline intrusion.

Recent modelling of the current and future water resource availability on several small islands in the Caribbean including Trinidad and Tobago, using a macro-scale hydrological model and the SRES scenarios (Arnell, 2004), found that many of these islands would be exposed to severe water stress under all SRES scenarios, and especially so under A2 and B25. Also, recent variations in sea level on the western Trinidad coast indicate that sea level in the north is rising at a rate of about 1 mm/yr, while in the south the rate is about 4 mm/yr; the difference being a response to tectonic movements 6.
According to IPCC AR4 global Sea level is projected to rise between the present (1980-1999) and the end of this century (2090-2099) by 0.35 m(0.23 to 0.47 m) for the A1B scenario. Based on research evidence published over most recently, mean global sea level rise is projected to increase above that of IPCC AR4 projections by the end of the century7 8 9 10. Furthermore, recent studies also suggest that because of the close proximity of the Caribbean including Trinidad and Tobago to the equator, sea level rise may be more outstanding than some other regions.11
12 This suggests that, in the future, if projections materialize, coastal inundation, inland flooding, storm surge damage, and coastal erosion are likely to increase in the vicinity of Trinidad and Tobago, however uncertainty remains high.

MONTHLY RAINFALL

The Cumulative Rainfall graph shows the total amount of rainfall accumulated over a calendar year in comparison to the cumulated rainfall of the long term average, (the climatological standard normal).

Meteorologists and Hydrologists use such graphs to help visualize how rainfall accumulates over time, whether daily, weekly, or monthly.
It is useful in climate studies for comparing rainfall patterns across different seasons or years.
This data aids in predicting water availability, which is crucial for resource management, agriculture, and flood forecasting.

The TTMS observes and records the ongoing rainfall amounts for surplus or excess rainfall amounts to assess seasonal changes and adjust seasonal and climate forecasts.