chapter 11[1].pdf
TRANSCRIPT
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CHAPTER 11
CLIMATE
Photograph courtesy Richard Edwards
Photograph courtesy Satee Boodoo
Photograph courtesy Richard Edwards
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11 CLIMATE 11.1 Climate Trinidad and Tobago the two most southerly islands of the Eastern Caribbean have
two distinct and seasonal climatic types:
1. Tropical Maritime: - warm days and cool nights with rainfall mostly in the form of showers due to daytime convection. This typifies the dry
season months of January to May.
2. Modified Moist Equatorial: - low wind speeds with hot humid days and nights and a marked increase in rainfall, not always convective.
During this period, the islands come repeatedly under the influence of
equatorial weather systems.
The two climatic types result in two distinct seasons a dry season from January to
May and a wet season from June to December. Tobago, the more northerly of the
two islands, experiences drier seasons in comparison to Trinidad.
11.1.2 Climatic Determinants The main climatic determinants of Trinidad and Tobago are:
1. The latitudinal position and strength of the sub-tropical high pressure
(Bermuda-Azores High) and semi-permanent northern hemispheric
feature.
2. The Inter-Tropical Convergence Zone (ITCZ) a major rainfall
producing system, which is responsible for the rainy season.
3. The Mid-Atlantic Upper-level Trough an upper tropospheric feature
which assume prominence during the months of October to
December.
4. Orography, Land size and sea breeze effect
5. Tropical waves and cloud clusters
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11.2 Rainfall
The annual rainfall pattern in Trinidad is bimodal in distribution (see Figure 11.1). In
an average over the period 1971-2000, the first major peak occurred during June or
July and was associated with the northward movement of the ITCZ. The second
peak occurred in November. Tobago also has a bimodal rainfall distribution,
however, the major peak occurred in November and the secondary peak occurred in
June (see Figure 11.2)
FIGURE 11.1: COMPARISON OF THE LONG-TERM MONTHLY AVERAGES OF RAINFALL FOR THE STATIONS AT HOLLIS, NAVET AND PIARCO IN TRINIDAD, 1971- 2000
0
50
100
150
200
250
300
350
400
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rai
nfal
l (M
illim
etre
s)
PiarcoHollisNavet
Source: Meteorological Services Division
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FIGURE 11.2: COMPARISON OF THE LONG-TERM MONTHLY AVERAGES OF RAINFALL FOR THE STATIONS AT CROWN POINT AND HILLSBOROUGH DAM IN TOBAGO 1971 - 2000
0
50
100
150
200
250
300
350
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Rai
nfal
l (M
illim
etre
s)
Crown PointHillsborough
Source: Meteorological Services Division 11.2.1 Mean Annual Rainfall (Trinidad 1961 to 1990)
Map 11.1: Mean Annual Rainfall for Trinidad, 1961-1990 Climatologically, Trinidad had a
mean annual rainfall (1961 to
1990) maximum along the eastern
Northern Range, with the centre
on the eastern part of the range
between Sangre Grande and
Arima. The maximum also extends
southward on the eastern side of
the island through Rio Claro,
towards Moruga. The western side
of Trinidad is relatively dry in
comparison to the eastern side
(see Map 11.1), especially in the
southwestern peninsula. Source: Water Resources Agency/MET Office
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The mean annual rainfall ranges from in excess of 3500mm centred in the Northern
Range, to values below 1500mm along the western coast of the island and along the
south coast to the south east of Siparia.
11. 2. 2 Trinidad (1999) Map 11.2: Isohyetal Map for Trinidad, 1999 The rainfall pattern for 1999
differs slightly from the mean
annual rainfall pattern. The
major difference is a minimum,
which was located to the south
east of San Fernando and on
the eastern side of the island
to the north west of Point
Radix. (see Map 11.2). Further
an area of maximum rainfall
was located to the north east
of Point Fortin.
Source: Water Resources Agency/MET Office
Map 11.3: Isohyetal Map for Trinidad, 2000
11. 2. 3 Trinidad (2000)
Trinidad, during the year 2000,
experienced similar rainfall patterns to
that of 1999, albeit with higher values.
The location of the maximum shifted
to the Central Range, where the
precipitation exceeded 3500mm (see
Map 11.3).
Source: Water Resources
Agency/MET Office
289
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Map 11.4: Isohyetal Map for Trinidad, 2001
11. 2. 4 Trinidad (2001)
During the year 2001, the rainfall maximum
shifted to the eastern Northern Range as per the
mean annual pattern.However, due to the
extremely arid dry season, precipitation totals for
the year were lower than the mean especially on
the western side of the island (see Map 11.4).
Source: Water Resources Agency/MET Office
Map 11.5: Isohyetal Map for Trinidad, 2002 11. 2. 5 Trinidad (2002) Rainfall during 2002 was quite similar
to the mean in its major features. The
maximum was located along the
eastern Northern Ranges and its axis
was along the range. The western
side of the island continued to have
lower rainfall values in comparison to
eastern areas (see Map 11.5).
Source: Water Resources Agency/MET Office
290
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Map 11.6: Mean Annual Rainfall for Tobago, 1971- 2000 11.2.6 Mean Annual Rainfall
(Tobago 1971 to 2000) Tobago had much lower rainfall than Trinidad;
the mean annual maximum rainfall for the
island exceeds 2500mm compared to 3500mm
maximum in Trinidad.
The maximum was along the Main Ridge to the
north of Roxborough. Along the southwest it
was relatively dry in comparison to the
northeast (see Map11.6)
Source: Water Resources Agency/MET Office
Map 11.7: Annual Isohyetal Map for Tobago, 1999
11. 2.7 Tobago (1999) There was more precipitation during 1999 when
compared to the mean annual rainfall for the
island. During 1999, Tobago measured in
excess of 3250mm in areas along the Main
Ridge. The precipitation tapered in the
southwest but the values were higher, with
measurements in excess of 1300mm (see Map
11.7) Source: Water Resources Agency/MET Office
Map 11.8: Annual Isohyetal Map for Tobago, 2000 11.2.8 Tobago (2000)
The maximum during 2000 was lower than the
maximum of 1999 and the mean, which was
indicative of a drier regime in Tobago during that
year. The maximum precipitation recorded was in
excess of 2250mm, which was much lower than
the value above 3250mm recorded in the previous
year and the mean in excess of 2500mm (see Map
11.8) Source: Water Resources Agency/MET Office
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Map 11.9: Annual Isohyetal Map for Tobago, 2001 11. 2. 9 Tobago (2001)
During 2001, the location and amount of
maximum rainfall was almost the same as the
mean. However, in the south west of Tobago
precipitation was much less than the mean (see
Map11.9)
Source: Water Resources Agency/MET Office
Map 11.10: Annual Isohyetal Map for Tobago, 2002
11. 2. 10 Tobago (2002) There was an abundance of precipitation
during 2002, which enlarged the area of
higher than normal accumulation. This
maximum was spread over a larger area
when compared to the mean (1971 to 2000)
or in any of the previous three (3) years.
In the southwest of Tobago, the amount of
precipitation was much less than along the
Main Ridge, however, the areal extent was
smaller than the mean (see Map11.10).
Source: Water Resources Agency/MET Office
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11.3 Temperature 11.3.1 (Trinidad, 1999 to 2004) Climatologically, the long-term mean maximum temperature measured at Piarco, for
the period 1971 to 2000 is 31.3C, whereas the long-term mean minimum
temperature for the same period is 22.7C and the mean daily temperature for the
same period is 26.5C.
During the period 1999 to 2004, the mean annual maximum, minimum and average
daily temperatures were consistently above the long-term (1971 to 2000) average.
The average yearly maximum for the period 1999 to 2004 was 0.3C to 1.0C above
the long-term mean maximum. The yearly mean minimum temperature ranged
between 0.6C to 1.0C higher than the long-term mean minimum, the same was true
for the daily average temperature, but the range was smaller from 0.2C to 0.6C
higher than the long-term daily average temperature (see Figure 11.3)
FIGURE 11.3: BAR CHART OF THE MEAN MAXIMUM, MINIMUM AND MEAN TEMPERATURES FOR PIARCO, TRINIDAD, 1999-2004
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
1999 2000 2001 2002 2003 2004
Tem
pera
ture
Mean MinMean Temp.Mean Max.
Source: Meteorological Office
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11.3.2 Temperature (Tobago, 1999 to 2002) The long-term mean maximum temperature measured at Crown Point, Tobago, for
the period 1971 to 2000 was 30.5C, whereas the long-term mean minimum
temperature for the same period was 23.7C and the mean daily temperature for the
same period was 26.8C.
The mean maximum temperature for the period 1999 to 2002 exceeded the long-
term mean maximum (1971 to 2000) in every year, except in 2000, when the mean
was 0.1C lower. The mean minimum temperature for the period was higher than the
long-term mean minimum temperature (Figure 11.4), as was the mean temperature
except in 2000, when the mean yearly temperature equalled the long-term average.
FIGURE 11.4: BAR CHART OF THE MEAN MAXIMUM, MINIMUM AND MEAN TEMPERATURES FOR CROWN POINT, TOBAGO, 1999-2002
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Year
Tem
pera
ture
Mean Min.Mean TempMean Max.
Mean Min. 24.5 24.2 24.7 24.1
Mean Temp 27.2 26.8 27.2 27.2
Mean Max. 30.7 30.4 30.9 30.7
1999 2000 2001 2002
Source: Meteorological Office
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11.4 Relative Humidity and Sunshine Hours 11.4.1 Trinidad (1999 to 2004) Trinidad has an average (1971 to 2000) relative humidity of 82.2% and the average
number of sunshine hours per day is 7.2. The period 2001 to 2003 had more
sunshine hours per day than the average (see Figure 11.5), whereas the years of
2000, 2001, 2003 and 2004 were relatively drier than average.
Generally, relative humidity and sunshine hours have an inverse relationship; higher
numbers of sunshine hours leads to lower relative humidity values as shown in
Figure 11.5. The year with the most sunshine hours (2001) had the lowest relative
humidity. Concomitantly, the dry season of 2001 was extremely arid, with rainfall
stations reporting little or no rainfall in the period January to April.
FIGURE 11.5: AVERAGE SUNSHINE HOURS PER DAY AND AVERAGE RELATIVE HUMIDITY PER DAY FOR PIARCO, TRINIDAD, 1999-2004
7.0
7.2
7.4
7.6
7.8
8.0
Hou
rs
80.0
80.5
81.0
81.5
82.0
82.5
83.0P
rece
nt
Sunshine Hrs.Rel. Humidity
Sunshine Hrs. 7.2 7.1 7.9 7.5 7.8 7.2
Rel. Humidity 82.4 81.8 80.7 82.4 80.8 82
1999 2000 2001 2002 2003 2004
Source: Meteorological Office
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11.4.2 Tobago (1999 to 2002)
Sunset in North Trinidad
Tobago had an average (1971 to 2000) relative humidity of 79.5% while the average
number of sunshine hours was 7.7%. Compared to the period 1971 to 2000, the
period 1999 to 2001 had more sunshine hours per day than the average at Crown
Point, as well as a higher humidity than average (see Figure 11.6).
FIGURE 11.6: A COMPARISON OF AVERAGE SUNSHINE HOURS PER DAY TO AVERAGE RELATIVE HUMIDITY PER DAY FOR CROWN POINT, TOBAGO, 1999-2002
7.0
7.2
7.4
7.6
7.8
8.0
8.2
Year
Hou
rs
80.0
80.2
80.4
80.6
80.8
81.0
81.2
Perc
ent
Sunshine 7.8 7.8 8.1 7.4
Rel. Humidity 81.1 81.1 80.6 80.7
1999 2000 2001 2002
Source: Meteorological Office
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11.5 Wind Direction and Wind Speed Generally, the average wind speed
over Trinidad and Tobago shows a
pattern consistent with the
establishment of the sub-tropical high
pressure closer to tropical latitudes
during the northern hemisphere
winter/spring months and a northward
migration of the high-pressure system
as the northern hemisphere moves
into summer and autumn.
Figure 11.7 shows that during the
months of January to May, Piarco and
Crown Point experienced a steady
increase in the mean wind speed, with
the peak in May. Thereafter, there was
a steady decrease in the wind speed
until December when the mean wind
started to increase.
FIGURE 11.7: MEAN WIND SPEED FOR PIARCO (TRINIDAD) 1961-1990 AND CROWN POINT (TOBAGO) 1971-1990
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Month
Win
d Sp
eed
(kt)
Piarco 5.5 6.2 6.7 6.9 7.1 6.3 5.0 4.1 4.0 4.2 4.2 4.7
Crown Point 5.8 7.3 7.6 8.1 8.8 8.2 6.7 5.8 5.8 5.9 5.6 5.5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Source: Meteorological Office
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Trinidad and Tobago experiences a northward migration of the Equatorial Trough
during the month of August to November, which is characterized by weak easterly
wind speeds, high temperatures and localized torrential rainfall. During this period, a
sea breeze develops blowing from the Gulf of Paria inland. At times, the sea breeze
penetrates to the east of Arima. (see Appendix 11.1 for Wind Roses; Piarco -Trinidad
and Crown Point -Tobago)
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