Zonal Wind Shear Index (Webster and Yang, 1992)
Time-series representation of the zonal wind shear index between 200 hPa and 850 hPa averaged over the North Indian Ocean and southern Asia (shown by the rectangle on the bottom: equator – 20°N, 40°E – 110°E). The zonal wind shear index is calculated after Webster and Yang (1992). The thick and thin pink lines indicate seven-day running mean and daily mean values, respectively. The black line denotes the normal (i.e., the 1991 – 2020 average), and the gray shading shows the range of the standard deviation calculated for the time period of the normal.
Somali Jet Index (Boos and Emanuel, 2009)
The Somali jet index is a measure of the speed of the Somali jet: the square root of twice the domain mean kinetic energy of the 850 hPa horizontal wind in the region (50 E – 70 E, 5 S – 20 N). Data is from NCEP operational analyses for the current year, and NCEP reanalysis data for the climatology.