Structure and maintenance mechanisms of the Mascarene High in austral winter

The Mascarene High (MH), is a key component of the Asian-Africa-Australia monsoon system in austral winter (JJA), spanning over the South Indian Ocean (15 degrees-35 degrees S, 15 degrees-110 degrees E). Its three-dimensional structures and maintenance mechanisms are examined in this study. It is a low-level subtropical high dominating the southern Africa and South Indian Ocean, characterized by a north-westward tilt with height, which is attributed to its spatially inhomogeneous thermal structure. Large-scale subsidence characterizes the main body of the MH, with the stronger subsidence to the east than to the west. Diagnosis using the complete form of the vertical vorticity tendency equation shows that the anticyclonic structure of the MH, which can be described by the distribution of meridional wind, is maintained mainly by the vertical gradient of diabatic heating, change in static stability, and friction dissipation. In particular, a combination of sensible heating and longwave radiative cooling results in a vertical decreasing gradient of diabatic heating in the lower troposphere. It generates the stronger southerlies over the subtropical South Indian Ocean than over the southern Africa. Meanwhile, over the South Indian Ocean, the increasing static stability as a result of the downward transport of a more stable atmosphere partly offsets the effect of the vertical gradient of diabatic heating, and southerlies still prevail there. Over the southern Africa, topographic friction dissipation induces northerlies, balancing the effect of the vertical gradient of diabatic heating with a stronger magnitude, and northerlies prevail.