9.2. Regional Climate
9.2.1. Some Common Influences
Although there is much climatic variation between localities, some factors
and characteristics are common to most small islands-mainly as a result of their
insular natures and tropical locations. For instance, it is generally true that:
- The ocean exerts a strong influence on the climate of islands.
- Temperatures usually are high, with mean annual values of 20°C and above.
- Diurnal and seasonal variations in temperature are low, with values around
5°C and below.
- Many small island states are influenced by tropical storms and cyclones
(i.e., hurricanes and typhoons).
- In the tropical Pacific, most islands are strongly influenced by the ENSO
phenomenon and associated high interannual variations in rainfall and sea
level (Hay et al., 1993a). The ENSO phenomenon also has an influence on the
weather and climate of islands in the Caribbean Sea (Centella et al., 1996),
as well as in the Indian Ocean.
9.2.2. Observed Trends
126.96.36.199. Temperature and Precipitation
Global and regional temperature and precipitation trends are presented in Annex
A of this report. Figure 9-2a shows that, for the
Caribbean islands, average annual temperatures have increased by more than 0.5°C
over the period 1900-1995; the seasonal data are consistent with this overall
trend. In the specific case of Cuba, for which a study of observed temperature
trends has been undertaken, Centella et al. (1996) found that mean air temperature
has risen by 0.6°C during the past 45 years. Rainfall data for the same period
show much greater seasonal, interannual, and decadal-scale variability, although
a declining trend in average annual rainfall-on the order of 250 mm-is evident
(see Figure 9-2b). Average annual temperature also
has increased since 1900 in the Pacific islands; the magnitude of the increase
in this area, however, is less than 0.5°C, and seasonal trends are not coherent,
nor do they track the annual average. Similarly, the rainfall data show considerable
decadal-scale fluctuations-on the order of 200 mm for annual average rainfall
and 50-100 mm for seasonal rainfall (Figure 9-2d,e).
|Figure 9-2: Time series of climate observations in the Caribbean
islands (a-c) and Pacific islands (d-f). Observed annual temperature anomalies
(a,d) and observed annual precipitation anomalies (b,e) both relative to
1961-90 means. Longer term variation of annual anomalies are emphasized
by the smooth curve using a nine-point binomial filter. Number of Atlantic
hurricanes including those in the Caribbean region (c), and annual numbers
of tropical cyclones in the southWest Pacific region (f). Note that while
there was an increase in the number of tropical cyclones detected since
the advent of weather satellites in 1969, there is no clear evidence that
tropical cyclones have been changing in number or intensity over the years.
A (a,b,d,e); Environment Division,
Ministry of Health and Environment, Barbados (c); Climate Research and Information
Services, National Institute of Water and Atmospheric Research, Wellington,
New Zealand (f).
Further details of climate trends for the southWest Pacific region have been
reported by Hay et al. (1993a,b) and Salinger et al. (1995); several subregions
with coherent trends and variability were identified. These studies indicate
that the region as a whole has warmed at a rate of about 0.2°C per decade and
that the 1981-1990 decade was the warmest on record. Within the region, there
has been a steady increase in temperature to the south of the South Pacific
Convergence Zone (SPCZ)-an area that includes Fiji and Tonga-since the 1880s.
A rapid increase has occurred north of the SPCZ since the 1970s, after an earlier
cooling trend from the 1940s; this region includes Tuvalu, Kiribati, Western
Samoa, and the northern Cook Islands.
Rainfall in the southWest Pacific has been more variable, both temporally and
spatially, over the past 100 years, and long-term trends are difficult to ascertain.
In Kiribati, Tuvalu, and the northern Cook Islands, wetter-than-average conditions
have prevailed since 1975; conditions have been drier in Fiji, Tonga, and Western
Samoa. Rainfall patterns in the region clearly are associated with the ENSO
phenomenon (Salinger et al., 1995), although the precise nature of the relationship
is not clearly understood.