Abstract
The island of Mahé, Seychelles (4.68oS, 55.53oE) 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 ~ 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 Mahé, 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.
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Data availability
The radiosonde dataset used in the study is available in a public repository https://data.eol.ucar.edu/dataset/347.046 and is provided by NCAR/EOL under the sponsorship of the National Science Foundation. The boundary layer height dataset from ERA5 reanalysis is available at https://doi.org/10.24381/cds.adbb2d47.
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Acknowledgements
The author wishes to thank the organizers, scientific and technical staff that made possible the valuable radiosonde data from Seychelles as part of the DYNAMO 2011-12 field campaign. The radiosonde data is provided by NCAR/EOL under the sponsorship of the National Science Foundation. The author would like to thank the ECWMF for ERA-5 data. The author wishes to thank the Indian Space Research Organisation (ISRO) for supporting the work. The author also thanks Dr.V.Sathiyamoorthy (SPL, Thiruvananthapuram) for encouraging this work. The author thanks the anonymous reviewers for their constructive comments and suggestions, which improved the content of this paper.
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Santosh, M. Structure and development of the atmospheric boundary layer over a small island (Mahé Island, Seychelles) in the equatorial Indian Ocean. Meteorol Atmos Phys 134, 91 (2022). https://doi.org/10.1007/s00703-022-00924-3
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DOI: https://doi.org/10.1007/s00703-022-00924-3