Monsoon of South Asia
The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.
The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic, and geographical factors, influence the behavior of the monsoon. Because of its effect on agriculture, on flora and fauna, and on the climates of nations such as Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka – among other economic, social, and environmental effects – the monsoon is one of the most anticipated, tracked,[3] and studied weather phenomena in the region. It has a significant effect on the overall well-being of residents and has even been dubbed the "real finance minister of India".[4][5]
The word monsoon (derived from the Arabic "mausim", meaning "seasonal reversal of winds"), although generally defined as a system of winds characterized by a seasonal reversal of direction,[6] lacks a consistent, detailed definition. Some examples are:
Observed initially by sailors in the Arabian Sea[10] traveling between Africa, India, and Southeast Asia, the monsoon can be categorized into two branches based on their spread over the subcontinent:
Alternatively, it can be categorized into two segments based on the direction of rain-bearing winds:
Based on the time of year that these winds bring rain to India, the monsoon can also be categorized into two periods:
The complexity of the monsoon of South Asia is not completely understood, making it difficult to accurately predict the quantity, timing, and geographic distribution of the accompanying precipitation. These are the most monitored components of the monsoon, and they determine the water availability in India for any given year.[11]
Pink arrow: Southwest monsoon wind direction; green arrow: northeast monsoon wind direction.
Orographic precipitation.
Geographical features of the Indian subcontinent.
Daytime scene showing the pre-monsoon air over India.
Recent trends in average annual precipitation from monsoons.
Theories for monsoon variability[edit]
The jet stream effect[edit]
The jet stream theory also explains the variability in timing and strength of the monsoon.
Timing: A timely northward shift of the subtropical westerly jet at the beginning of summer is critical to the onset of the southwest monsoon over India. If the shift is delayed, so is the southwest monsoon. An early shift results in an early monsoon.
Strength: The strength of the southwest monsoon is determined by the strength of the easterly tropical jet over central India. A strong easterly tropical jet results in a strong southwest monsoon over central India, and a weak jet results in a weak monsoon.
Impact of climate change[edit]
Since 1950s, the South Asian summer monsoon has been exhibiting large changes, especially in terms of droughts and floods.[32] The observed monsoon rainfall indicates a gradual decline over central India, with a reduction of up to 10%.[33] This is primarily due to a weakening monsoon circulation as a result of the rapid warming in the Indian Ocean,[34][35] and changes in land use and land cover,[36] while the role of aerosols remains elusive. Since the strength of the monsoon is partially dependent on the temperature difference between the ocean and the land, higher ocean temperatures in the Indian Ocean have weakened the moisture bearing winds from the ocean to the land. The reduction in the summer monsoon rainfall has grave consequences over central India because at least 60% of the agriculture in this region is still largely rain-fed.
A recent assessment of the monsoonal changes indicate that the land warming has increased during 2002–2014, possibly reviving the strength of the monsoon circulation and rainfall.[37] Future changes in the monsoon will depend on a competition between land and ocean—on which is warming faster than the other.
Meanwhile, there has been a three-fold rise in widespread extreme rainfall events during the years 1950 to 2015, over the entire central belt of India, leading to a steady rise in the number of flash floods with significant socioeconomic losses.[38][39] Widespread extreme rainfall events are those rainfall events which are larger than 150 mm/day and spread over a region large enough to cause floods.