The Late Retreat of the Indian Monsoon

  • A satellite image from NASA showing monsoon activity.

  • The Late Retreat of the Indian Monsoon
    17.09.2011 14:22

    The Intertropical Convergence Zone (ITCZ) is an area close to the equator where winds from the northern and southern hemisphere converge together. The location of the ITCZ over the land is largely to do with the zenith sun point and moves north and south as the earth rotates through each season. The northward shift of the ITCZ in the northern hemisphere commences the south-east Asian monsoon season.

    The Monsoon of south-east Asia occurs every year from approximately June through to September, yet it is always difficult to predict its distribution, strength and variability. Over 80% of India’s rainfall occurs during this season, impacting directly agriculture and flora and fauna which in turn impacts the economy and social standards.

    In September the southern hemisphere is entering into Spring and the northern hemisphere is entering into Autumn. The monsoon usually retreats fast as the zenith angle retreats and the Northeast Monsoon begins.

    The Northeast monsoon occurs when air cools and air pressure rises. Winds will become north-easterly bringing cold air from the Himalayas and the Indo - Gangetic Plain in Pakistan. As this air travels over the ocean, it picks up moisture bringing more rain to Sri Lanka and the peninsular part of India. Rainfall totals are usually lower here during the monsoon season from June to September.

    This September the monsoon is highlighting its variability while hitting Pakistan. On Monday, Hyderbad recorded 151mm of rainfall in just 12 hours. The cause of this was an upper trough combined with a moist south-west flow at the surface. On Wednesday and Thursday a further 230mm of rain feel in Islamabad. The average total for September in the city is normally 92.2mm of rain.

    The heavy rain in Pakistan has led to a growing humanitarian crisis with over two million people fleeing their homes and vital crops have been washed away. Infrastructure has been destroyed and many people at risk from disease. The death total is thought to be in the order of 300. In 2010 Pakistan was also impacted by major floods where over 2000 people were killed and approximately 11 million people were made homeless.

    By: Aisling Creevey