Zonal Movement of Mascarene High in Austral Summer

Abstract:

Temporal variations in the Mascarene High (MH), defined by the sea level pressure (SLP) maximum within a region of [40°–120°E, 50°–10°S] in austral summer (November–January), were investigated using NCEP–NCAR reanalysis dataset. The MH intensity has interannual variations, which is associated with the Southern Annular Mode. The MH longitudinal position shows a longer timescale of 5–10 years compared to the MH intensity. It appears to be independent of the MH intensity variation because of no significant correlation between the MH intensity and longitudinal position. The MH longitudinal movement is caused by a combination of SLP variations in the eastern South Indian Ocean (ESIO) and western South Indian Ocean (WSIO). Pressure variations in the ESIO region are confined from the sea surface to the mid-troposphere, and are associated with the Matsuno­-Gill response due to El Niño-Southern Oscillation. The pressure variations in the WSIO region, characterized by a quasi-barotropic structure throughout the troposphere, are related to the meridional movement of storm track.

We examine the influence of MH variations on sea surface temperature (SST) field. The MH intensity variations form a northeast-southwest dipole pattern, which resembles the Indian Ocean Subtropical Dipole (IOSD) pattern. In addition, the MH longitudinal shifts also show the dipole pattern, but this is shifted 10° westward in longitude compared with that associated with the MH intensity. The correlation between the MH variations and IOSD index show significant values (0.42 for intensity and ­−0.58 for longitudinal position). Therefore, it is suggested that both the changes in the MH intensity and longitudinal position cause the IOSD.