IMD: Monsoon to withdraw from October 10

The Indian Meteorological Department said that the retreat of monsoon that was delayed by more than a month is to happen by October 10, 2019. The monsoon this year was 110% of Long Period Average (LPA) with above average precipitation. The LPA between 1961 and 2010 was 88 centimeters.

The retreat of monsoon is set to be the most delayed in 60 years. The delay ravaged Bihar raising death toll to 100. UP and Bihar were the worst affected states due to the delay. Hospitals, schools and health care facilities were hampered in the states due to heavy rains. Failure of desilting of lakes, ponds and rivers in the states were the main reason behind floods.

The unusual events that caused the shift

Monsoon usually starts retreating from west Rajasthan from September 1. However, this time the retrieval began on September 16. The reasons behind the late retrieval include

  • Easterly winds – This year they were still blowing in upper latitudes.

The Easterly winds originate from sub tropical high-pressure belt of northern hemisphere. They blow towards south Equatorial Low 0 Pressure area. They shift to lower latitudes causing the retreat of monsoon.

By the end of September, the low-pressure trough of Ganga plains moves southwards in response to the southward march of the sun. (The sun in the months of August – September shifts from Summer Solstice to Autumnal Equinox). This causes the shift of easterly winds.

  • Monsoon Trough remained south of its usual position because of successive low pressure in the Bay of Bengal. The number of such low pressure were more than usual this year.

The axis of monsoon trough during the retreating period usually extends between Punjab in the west and West Bengal in the East. However, this year the axis shifted over northern plains causing rains in Bihar and Uttar Pradesh. Monsoon trough is the confluence of winds.

  • The break in period of the monsoon began late during August. It usually breaks in July.

During the break in period the monsoon trough shifts towards north bringing rainfall to the north eastern states and Himalayan states. During this period the rest of the country remains dry. This failed to happen this year.

The above factors led to the unusual and untimely rainfall in central India and in southern states.