Delhi-NCR’s June deluge a result of rapid, strong monsoon onset in the presence of active Western Disturbance: Experts

Sub: Geography

Sec: Climatology

Extreme Rainfall and Flooding in Delhi and NCR:

The extreme rainfall and flooding in Delhi and the National Capital Region (NCR) on the night of June 27-28 was due to the rapid and strong onset of the southwest monsoon, combined with an active western disturbance (WD).
Interactions between the southwest monsoon and WDs have become more frequent due to changes in the characteristics of WDs.
The Safdarjung observatory recorded 228 millimetres of rainfall by 8:30 am on June 28, marking the second-highest June rainfall on record, following the 235.5 mm recorded on June 28, 1936.

Cause of extreme rainfall:

The primary cause of the extreme rainfall was the rapid and intense onset of the southwest monsoon over the region on June 28, following a long hiatus from early to mid-June.
The presence of a WD and an associated cyclonic circulation over the region contributed to the heavy rainfall.
WDs, which are more common in winter, have been occurring more frequently in the summer months in recent years, due to a strengthening subtropical jet stream and its delayed northward retreat, leading to interactions with the monsoon trough and causing catastrophic floods.
- The Inter-Tropical Convergence Zone (ITCZ) changes to 20°-25° N latitude in July and is positioned in the Indo-Gangetic Plain, while the southwest monsoons blow from the Arabian Sea and the Bay of Bengal. The ITCZ at this position is commonly referred to as the Monsoon Trough.
Excess moisture from the warm Arabian Sea also contributed to the torrential rains. This system allowed the jet stream to pump large amounts of moisture into northwest and central India.

What are Western Disturbances?

Western disturbances are storms that originate in the Caspian or Mediterranean Sea and bring non-monsoonal rainfall to northwest India.
They are labelled as an extra-tropical storm originating in the Mediterranean, an area of low pressure that brings sudden showers, snow and fog in northwest India.
The disturbance travels from the “western” to the eastern direction.
These travel eastwards on high-altitude westerly jet streams – massive ribbons of fast winds traversing the earth from west to east.
Disturbance means an area of “disturbed” or reduced air pressure.
Equilibrium exists in nature due to which the air in a region tries to normalise its pressure.

Sub-tropical jet stream:

A subtropical jet stream is a belt of strong upper-level winds lying above regions of subtropical high pressure.
Unlike the polar front jet stream, it travels in lower latitudes and at slightly higher elevations, owing to the increase in height of the tropopause at lower latitudes.
The associated horizontal temperature gradients of this jet stream do not extend to the surface, so a surface front is not evident.

Source: DTE