The Southeast Trade Winds

Most of the western region of the tropical South Pacific benefits from the Southeast Trade Winds. These are produced by the effect of Coriolis deflection on surface air drawn towards the low pressure region at the Equator, called the Equatorial Trough. The Coriolis deflection of winds, which is to the left in the Southern Hemisphere, is caused by the Earth's rotation. The Southeast Trade Winds are persistent for most of the year, although they tend to be weaker in the summer season (from November to April), and stronger in winter (from May to October).

As the Southeast Trade Winds blow across vast stretches of open ocean, they collect large amounts of moisture derived from evaporation at the sea surface. Vertical mixing of this moist air may give rise to some condensation and clouds. In general though, trade wind weather over the atolls and the other low islands of southwest Pacific is clear and fresh. This is because the lack of significant relief on low islands means that no clouds form by the process of orographic lifting, so rainfall is mainly convectional and frontal. This situation

1 For Fiji: http://www.met.gov.fj; for French Polynesia: http://www.meteo.pf; for New Caledonia http://www.meteo.nc; for New Zealand: http://www.niwascience. co.nz; for Samoa: http://www.meteorology.gov.ws; for Tonga: http://www.mca.gov. to/met; for Vanuatu: http://www.meteo.vu.

is in marked contrast to high volcanic islands, where the orographic effect is the most important rainfall-generating mechanism on the windward southeast sides of islands facing into the Trade Winds. This can result in big geographical variation in annual rainfall totals across many high islands, and local people often refer to the 'wet' and 'dry' sides of their islands.

The interaction of volcanic relief and the Southeast Trade Winds on rainfall distribution is illustrated in Table 1.1 for Viti Levu island in Fiji. Suva city on the southeast coast of Viti Levu lies on the wet side of the island, and receives almost 3,000 mm of rain annually during 240 rain days. Lautoka city on the northwest coast enjoys a drier location in the lee of the volcanic highlands in the interior of the island. Lautoka therefore receives 1,903 mm of rain a year in less than half the number of rain days experienced by Suva. Relative humidity is correspondingly higher in Suva than in Lautoka, and as might be expected, this has an inverse effect on daily sunshine hours for the two cities. Isohyets extrapolated across entire Viti Levu are seen in Fig. 1.3, showing that the wettest part of the island is the central volcanic highlands.

Table 1.1. Long-term climatic averages for two coastal sites in Fiji. Suva and Lautoka cities are located on the southeast windward and northwest leeward coasts of Viti Levu Island, respectively. Suva City

Station location: Laucala Bay; Latitude 18°09S Longitude 178°27E; Elevation: 6 m

Jan Feb Mar Apr May Jun

Rainfall in millimetres (1942-2002) 343 293 373 366 245 168

Number of rain days (1942-2002) 23 22 24 22 20 18 Relative humidity at 9 a.m. in percent (1942-2002) 81.3 82.4 83.5 83.2 81.6 82.3 80.4

Sunshine hours per day (1926-2002) 6.1 6.0 5.5 5.1 4.7 4.5

Jul Aug Sep

142 148 191

Nov Dec Year

254 269 2998

Lautoka City

Station location: Lautoka Sugar Mill; Latitude 17°37S Longitude 177°27E; Elevation: 19 m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Rainfall in millimeters (1910-2002)

302 326 338 181 99 65 52 68 73 89 124 186 1903 Number of rain days (1910-2002)

Relative humidity at 9 a.m. in percent (1957-2002) 74.3 76.5 77.0 75.5 74.1 74.2 71.7 69.2 68.7

Sunshine hours per day (1957-2002)

They are separated by volcanic mountains that rise in the centre of the island to over 1,300 m above the sea level. Data Source: Fiji Meteorological Service.

Fiji Meteorological Service Rainfall

Fig. 1.3. Isohyets of annual precipitation on Viti Levu, the main island in Fiji, illustrating the difference in rainfall between the wet windward and drier leeward sides of the Island. The pattern is produced by the orographic effect of the high interior volcanic relief on the dominant Southeast Trade Winds. Source: Fiji Meteorological Service.

Fig. 1.3. Isohyets of annual precipitation on Viti Levu, the main island in Fiji, illustrating the difference in rainfall between the wet windward and drier leeward sides of the Island. The pattern is produced by the orographic effect of the high interior volcanic relief on the dominant Southeast Trade Winds. Source: Fiji Meteorological Service.

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Responses

  • maxima
    Why is the humidity high in Suva than in Lautoka?
    7 years ago
  • KISANET ABAALOM
    Which part of the year is southeast trade winds rain experienced?
    1 year ago
  • will bolger
    What is the other name for southeast trade wind?
    4 months ago

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