Cyclone formation in Arabian Sea afternoon update

The mechanism behind the current monsoon onset intensifying into a cyclone in the Arabian Sea

Figure 1: Zonal vertical wind shear over the north Indian Ocean. The pink line denotes the shear value this year and the black line denotes the climatology of shear. Source: Monsoon Online, IITM

This year the vertical wind shear is running below normal, below normal vertical wind shear during monsoon onset favors the monsoon onset vortex to intensify into a cyclone. The reason for the weak vertical wind shear this year is the following: Due to frequent western disturbances this summer, above-normal rainfall occurred in north India as shown in Figure 2 below. This led to below-normal temperatures in north India, as shown in Figure 3. Below normal temperatures in north India lead to a weak north-south temperature gradient and weak meridional pressure gradient. This has resulted in weak monsoon circulation (weak low-level winds, as shown in Figure 4 and weak upper-level winds also called as TEJ). Due to weak monsoon circulation in the first week of June, the vertical wind shear is below normal favoring the monsoon onset vortex to intensify into a cyclone.

Figure 2: Observed rainfall (leftmost plot) over India in May 2023 and rainfall anomaly (rightmost plot). Blue shades in the rightmost plot denote above-normal rain.

Figure 3: Maximum, minimum and mean temperature (top row) and their anomalies (bottom row) in May 2023. Green shades in the bottom row denote below-normal temperatures. Source: IMD

Figure 4: Somali jet index. The blue line indicates the current Somali jet wind speed, black line indicates the climatology. Source: Monsoon Online, IITM