Trinidad and Tobago's close proximity to the equator enables the country to have two climate types producing two opposing seasons. These seasons are differentiated by distinct dry and wet season regimes. The dry season which occurs during January to May is symbolized by a tropical maritime climate that is characterized by moderate to strong low level winds, warm days and cool nights, with rainfall mostly in the form of showers due to daytime convection. A modified moist equatorial climate characterized by low wind speeds, hot humid days and nights, a marked increase in rainfall which results mostly from migrating and latitudinal shifting equatorial weather systems, symbolizes the wet season during June to December. The periods late May and December are considered as transitional periods to the wet and dry seasons respectively. Variations in these two climatic seasons between the islands of Trinidad and Tobago are primarily as a result of difference in land size, orography, elevation, orientation in terms of the trade winds and geographical location. Within the wet season is the hurricane season which runs from June to November, peaking between August and October. Trinidad’s geographical location puts it on the southern periphery of the North Atlantic hurricane basin. As such, Trinidad is not affected directly by storms as frequent as Tobago; however, peripheral weather associated with the passage of tropical storm systems impacts Trinidad and Tobago similarly.
Trinidad and Tobago's daily temperature cycle is more pronounced than its seasonal cycle. The long term mean (1971 - 2000) annual maximum and minimum temperatures are 31.3 Celsius and 22.7 Celsius respectively with a mean daily temperature of 26.5 Celsius. Generally, the wet season temperatures are warmer than the dry season temperatures with September being the warmest wet season month and March the warmest in the dry season. Annually and seasonally Trinidad is wetter than Tobago; however, the rainfall pattern in both islands display a distinct bi-modal behaviour with early (June) and late(November) rainfall season maxima occurring. Trinidad's primary rainfall mode occurs in June while Tobago's primary mode occurs in November. The annual rainfall totals are largely driven by multiple competing weather features, chief of which are the latitudinal position and strength of the North Atlantic Sub-Tropical High (NASH) pressure cell, meridional shift of the Inter-Tropical Convergence Zone (ITCZ), westward propagating Tropical Waves and cyclones (depression, storms, hurricanes), Mid-Atlantic upper level trough system, localized sea-breeze effects, cloud clusters driven by large scale low level convergence and orography. Generally, during the dry season the NASH pressure cell center migrates more southerly than during the wet season and expands equator-ward resulting in generally subsiding air, low level moisture evacuation and strong trade winds over Trinidad and Tobago, which result in a drier state of the atmosphere. Usually during May, the slow but sure shifting of the NASH pole-ward with winds on its southern flank converging more, allows rain bearing systems such as the ITCZ and Tropical waves to penetrate northward and eastward; resulting in the transition to and eventual onset of the wet season regime.
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.
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.
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