Indian Monsoon – Beyond Weather

Have you ever wondered how Rajendra Chola I conquered places as far as Indonesia, Malaysia & Thailand in Southeast Asia? Or for that matter have you wondered how Rome and China were connected through an ancient maritime trading empire.  Read on.

Early Seafarers from the various cultures along the ring of Indian Ocean used to run an active maritime trading operations all the way from Arabia to China.  So in an era which was few centuries before Industrial Age how did these merchants travel all the around the world over the seas. The answer to both of the questions asked in the early part of the post lie in a weather phenomenon called “Monsoon”.  The word Monsoon owes its origin to the Arabin Word Mausim which signified the seasonal shift in wind pattern.

It was probably the early Arabic Traders who identified the seasonal pattern of winds and possibly started the Maritime trading across the Arabian Sea towards the Indian Peninsula.  They used the Southwest Monsoon period to make their journey from Arabia towards India for their trading requirements, waited for the winds to change back to Easterlies to make their journey back home.

So what is Southwest Monsoon? During Summer the land over the Indian Sub Continent heats up getting the air to rise up creating a Low Pressure Area.  In order to fill this Vaccum winds start flowing in from the Southern Hemisphere aided by the High Pressure Area over Southern Indian Ocean that gets created during the Southern Winter Season Between Australia & African Coasts. Called “Mascarene High” the outlflow form this  high pressure zone gets bounced across by the African Highlands over the equator to be called the Somali Jet / Findlater Jet which pick moisture while travelling over Arabian Sea thereby providing rainfall over West Coast during the period June to September.  During this period the winds are predominantly Westerlies.  One arm of the Somali Jet moves around Sri Lanka to blow over towards the Southeast Asia region and parts of East India & Northeast India.

Not only did Southwest Monsoon aid the Arabian Traders to reach the Indian Peninsula but it also aided Rajendra Chola I who possibly built the biggest maritime army among all South Indian dynasties to go across the Bay of Bengal to capture far away kingdoms like Srivijaya Empire (Modern day Sumatra / Indonesia) and Kedah (modern day Malaysia). If one could observe the arc of Overseas Chola Empire it would not be easy to miss its close link with the Monsoon Trade Winds.  Not only did Rajendra Chola I used the Monsoon trade winds to reach out to Southeast Asia, the Ancient Indian Traders also used the same technique to build their trade relationship with their Malacca counterparts providing them access to the Chinese as well through the Malayans.

So how did the return journey happen for these traders & for the naval fleet of Rajendra Chola I?

Towards late September a High Pressure Area starts to build in Siberia near the Lake Baikal region which starts to influence the weather pattern across Northern Hemisphere.  The Siberian High starts to push cold dry air from Central Asia which brings about a shift in the wind pattern over Southeast Asia & Indian Subcontinent from Westerly to Easterly.  This is called Northeast Monsoon or in other words the retreating monsoon.  Just like how the Somali Jet picks up moisture over Indian Ocean / Arabian Sea, these easterly winds pick up moisture while traveling over Bay of Bengal bring in Rains to extreme Southern Peninsula with the state of Tamil Nadu getting almost 50% of its annual rainfall during the October – December period.

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With the easterly winds behind them the Indian traders made their journey back home from Southeast Asia while the Arabian traders were on their way back home to the Arabian Peninsula from the Indian Subcontinent using the same Northeast Monsoon Trade winds. The entire cycle took them almost a year to complete.

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So the next time when you think of Northeast Monsoon do not forget to think of the importance of a High Pressure Area which develops over Siberia that is a good 10000 kms away from us.

In other Weather Updates, Rains are expected to return to Southern Peninsula form today with heavy spells of rains expected over Kerala / South Interior Karnataka and parts of Western Tamil Nadu.  By Friday the rains are expected to return to more parts of Rayalseema & South AP along with parts of North Coastal TN.