16.2.2 Climate and weather

16.2.2.1 General features

The climate regimes of small islands are quite variable, generally characterised by large seasonal variability in precipitation and by small seasonal temperature differences in low-latitude islands and large seasonal temperature differences in high-latitude islands. In the tropics, cyclones and other extreme climate and weather events cause considerable losses to life and property.

The climates of small islands in the central Pacific are influenced by several contributing factors such as trade wind regimes, the paired Hadley cells and Walker circulation, seasonally varying convergence zones such as the South Pacific Convergence Zone (SPCZ), semi-permanent sub-tropical high-pressure belts, and zonal westerlies to the south, with ENSO as the dominant mode of year-to-year variability (Fitzharris, 2001; Folland et al., 2002; Griffiths et al., 2003). The Madden-Julian Oscillation (MJO) is a major mode of variability of the tropical atmosphere-ocean system of the Pacific on time-scales of 30 to 70 days (Revell, 2004), while the leading mode of variability with decadal time-scale is the Interdecadal Pacific Oscillation (IPO) (Salinger et al., 2001). A number of studies suggest that the influence of global warming could be a major factor in accentuating the current climate regimes and the changes from the normal that come with ENSO events (Folland et al., 2003; Hay et al., 2003).

The climate of the Caribbean islands is broadly characterised by distinct dry and wet seasons with orography and elevation being significant modifiers on the sub-regional scale. The dominant influences are the North Atlantic Sub-tropical High (NAH) and ENSO. During the Northern Hemisphere winter, the NAH lies further south, with strong easterly trades on its equatorial flank modulating the climate and weather of the region. Coupled with a strong inversion, a cool ocean, and reduced atmospheric humidity, the region is generally at its driest during the Northern Hemisphere winter. With the onset of the Northern Hemisphere spring, the NAH moves northwards, the trade wind intensity decreases, and the region then comes under the influence of the equatorial trough.

In the Indian Ocean, the climate regimes of small islands in tropical regions are predominantly influenced by the Asian monsoon; the seasonal alternation of atmospheric flow patterns which results in two distinct climatic regimes: the south-west or summer monsoon and the north-east or winter monsoon, with a clear association with ENSO events.

The climates of small islands in the Mediterranean are dominated by influences from bordering lands. Commonly the islands receive most of their rainfall during the Northern Hemisphere winter months and experience a prolonged summer drought of 4 to 5 months. Temperatures are generally moderate with a comparatively small range of temperature between the winter low and summer high.