The climate of Aldabra Atoll

Meteorological observations made by members of the expedition since August 1967 are the first to appear from Aldabra which cover a sufficient time span to merit analysis. Data for the period November 1967 to October 1968 are compared with marine derived parameters available from climatological atlases. Aldabra experiences a 5- or 6-month wet season in most years and its climate may be classified as V 2 /V 3 . During midsummer 1967/8 a marked positive pressure anomaly was observed: this coincided with a 3-month failure of the monsoon. For 7 months of the year the southeast trade winds blow, with a constancy of more than 90%. Summer maximum temperatures average 32 °C (90 °F) at the Settlement synoptic station, but this station is atypical for much of the atoll where maximum temperatures are about 3 °C lower. Winter minimum temperatures average 22 °C (72 °F). The annual pattern of temperature fluctuation for Picard compares most closely with the Comoro Islands. Summer maxima are identical with Diégo Suarez values; winter minima with the Seychelles. The highest and lowest shade temperatures recorded in the year were 36.3 °C (97.4 °F) and 19.5 °C (67.1 °F). The diurnal temperature range varies little throughout the year, averaging 6.5 °C, though extremes of 11 °C are occasionally recorded. Lagoon water temperatures are tidally controlled. At springs mean values may exceed the local air shade temperature by more than 3 °C. This could lead to very high temperatures in the lagoon in summer, but the exact magnitude must be ascertained from future observations. Sunshine measurements have not yet been made on the atoll. Calculations of probable monthly sunshine totals based on cloud observations show that October is clearly the sunniest month with a computed average of 8.6 h/day; January and April are least sunny, with 5.4 h/day. The annual total calculated for Aldabra (2400 h average; 2100 h 1967/8) is considerably at variance with regional values shown on world maps of climatology (indicating 1800 h). This can be resolved only by obtaining instrumental records from the island. The mean annual rainfall of 670 mm (26.5 in) derived from 7 years records is strikingly lower than atlas predictions. It places Aldabra in the most arid sector of the western Indian Ocean. From 1950 to 1960 a sequence of pairs of wet and dry years can be discerned, with extreme yearly totals of 1200 and 350 mm (47 and 14 in). The monsoon may occasionally be as short as 2 months, and droughts of 3 months or more are recorded during the trades. In 1967/8 summer rainfall showed a continental-type bimodal distribution, though on average it shows a broad plateau tending towards a late summer maximum (cf. Lindi, Tanzania). In 1967/8, 42% of the yearly rainfall fell on 5 days as violent showers with more than 40 mm (1.6 in). Local climatic effects were investigated from an auxiliary meteorological station set up at Middle Camp, Passe Houareau. In October 1968 the mean trade-wind speed exceeded 9 m s -1 (19 knots) from 08h00 to 13h00; which indicates a tendency to underestimate its strength considerably when observing at Settlement. Appreciable east—west variation in rainfall totals occurred. It is this essential to evaluate any precipitation gradient which may exist during the monsoon, for rainfall totals from Picard may bear little relation to the southeastern region. Analysis of autographic charts indicates that relative humidity varies from stable night values of around 93% to minima of 70% during early afternoon. In late September/October, values as low as 50% have been recorded. A strongly semi-diurnal distribution of rainfall is shown by Dines tilting syphon raingauge charts, which correlates with the S 2 atmospheric tide. During the monsoon frequency of rain shows a pre-dawn maximum: during the trades a pre-dusk maximum, which is not typical of a truly oceanic atoll.