The dipole factor in summer monsoon rainfall

• The Indian summer monsoon rainfall is influenced by a system of oscillating sea surface temperatures known as the Indian Ocean Dipole (IOD) in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.
• The IOD is commonly measured by an index that is the difference between sea surface temperature (SST) anomalies in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E and 10°S to 0°S) equatorial Indian Ocean. The index is called the Dipole Mode Index (DMI).

There are three type of Dipole existed:
a) Positive dipole

• A positive IOD occurs when the sea surface temperatures are greater than normal in the Arabian Sea and less than normal in the tropical eastern Indian Ocean.
• A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days

b) Negative dipole

• When the reverse of positive dipole happened it is termed as a negative IOD.
• Negative IOD leads to less rainfall and more monsoon break days (no rainfall).

c) Early IOD

• Early IOD, which peaks in the mid-monsoon months (July and August), plays a significant role in enhancing monsoon rainfall even though its intensity is medium compared to other IODs.
• Early IOD peaks in the peak monsoon months (July and August) and thus plays a vital role in the monsoon rainfall over the Indian subcontinent.
• The normal IOD and prolonged IOD peak in September, October, November and hence play a lesser role in the monsoon rainfall 

By: Vishal