Structure and development of the atmospheric boundary layer over a small island (Mahe Island, Seychelles) in the equatorial Indian Ocean

The island of Mahe, Seychelles (4.68(degrees)S, 55.53 degrees E) is located close to the region responsible for the initiation and eastward propagation of the Madden Julian Oscillation in addition to being part of the Seychelles dome (an oceanic thermal dome) and the Seychelles-Chagos Thermocline Ridge each of which influences the atmosphere at multiple time scales. The representativeness of the island of the oceanic region surrounding it is limited by its atmospheric boundary layer (ABL). This tiny island (of area similar to 150 km(-2)) can be considered as a large ship permanently anchored at the equatorial Indian Ocean capable of making long-term atmospheric observations representing its surroundings provided its ABL structure and development is known. This study presents, for the first time, in situ observations of day-night variability of ABL over Mahe, Seychelles using high vertical resolution radiosonde ascents from November 2011 to January 2012. The ABL height (ABLH) was observed to vary in a range of 200-1300 m in the course of a day with a daytime mean of 855 +/- 300 m and a nighttime mean of 165 +/- 85 m. For an unstable boundary layer, a statistically significant positive (negative) correlation was observed between ABLH and near-surface temperature (near-surface relative humidity) with correlation coefficient R = 0.77 (R = - 0.45) indicating that a warmer island produces a deeper boundary layer subject to limits imposed by the marine influence. It also indicates the significant contribution of sensible heat flux to ABLH at the site. Observations indicate near-surface wind speeds greater than 5 m/s either disrupt stable air by mechanical wind shear or advect them out of the island.