The Effects of a Small Island on Marine Boundary Layers

The presence of an island is known to affect the marine environment in multiple ways depending on the size, shape, and particularly the topography of the island. The dominant effect is seen in the atmospheric boundary layer through the significant change in surface roughness and land surface heating, while steep topography on a small island may affect large scale systems such as tropical cyclones. In this study, we investigate the role of a small island, Diego Garcia, a coral atoll in the middle of the Indian Ocean (7o18.7''S, 72o25"E) in modifying the marine environment. The objective is to access the measurements from Diego Garcia in representing the marine environment. Diego Garcia has a land rim that varies in width from a few hundred metres to 2.4 km, enclosing a lagoon 21 km long and up to 11 km wide, with a 4 miles (6 km) pass opening at the north. The maximum elevation is 9 m above mean low water. During the Dynamics of Madden-Julian Oscillation (DYNAMO) experiment, extensive rawinsonde measurements were made from Diego Garcia, which is an important component of the DYNAMO sounding array. Although it was anticipated that the measurements from the island may be affected by the island itself, the specifics of such influences are not known. These include the horizontal and vertical extent and the magnitude of the effect, the variables that are most affected by the island, and the general weather conditions that may result in significant boundary layer modifications. These subjects will be the focus of discussion in this presentation using aircraft flight level, dropsonde, as well as ground based measurements from Diego Garcia. Efforts in high-resolution numerical modeling is ongoing to help extend the measurements into a 3-D context and explain the physical processes involved.