The Indian monsoon is a complex weather phenomenon crucial to the country's agriculture and water resources. It is primarily influenced by the differential heating of land and sea, but two significant global factors — jet streams and El Niño — also play a critical role in determining the onset, intensity, and distribution of the monsoon. Understanding their roles provides insight into the variability and unpredictability of the Indian monsoon system.
Jet streams are narrow bands of high-speed winds found in the upper levels of the atmosphere, typically in the tropopause. These winds flow from west to east and have a substantial influence on weather patterns across the globe, including the Indian subcontinent.
Subtropical Westerly Jet Stream (SWJ):
Present during winter months (October to March).
Located approximately at 25°N–35°N latitude over India.
Responsible for bringing western disturbances that cause winter rainfall in North India.
Its retreat northward is essential for the onset of the summer monsoon.
Tropical Easterly Jet Stream (TEJ):
Develops during summer (June to September).
Flows from east to west around 15°N latitude.
Strong TEJ is associated with a good monsoon season.
During summer, the SWJ shifts northward, allowing the moist monsoon winds to enter India.
The establishment of the TEJ is crucial for the low-pressure systems over the Indian landmass to intensify and attract monsoon winds from the Indian Ocean.
Any disturbance or weakening in the jet stream system can delay or weaken the monsoon.
El Niño is a periodic warming of the central and eastern Pacific Ocean that significantly affects global weather patterns, including the Indian monsoon.
In normal conditions, trade winds push warm surface waters toward the western Pacific, allowing cold waters to rise along the South American coast.
During El Niño, these trade winds weaken, and the warm water spreads eastward across the Pacific, disrupting the typical atmospheric circulation (Walker Circulation).
Weakened Monsoon Winds:
The weakening of trade winds reduces the pressure gradient between the Indian Ocean and the Indian landmass, resulting in weaker monsoon winds.
Drought Conditions:
El Niño years often correspond with below-normal rainfall and even droughts in India. For instance, the droughts of 2002 and 2009 coincided with strong El Niño events.
Reduced Low-Pressure Systems:
The formation of low-pressure areas over the Bay of Bengal becomes less frequent, further weakening the monsoon.
While jet streams and El Niño individually influence the monsoon, their combined interaction can amplify or moderate the effects.
Strong El Niño + Weak TEJ:
Leads to severely deficient rainfall and weak monsoon currents.
Neutral El Niño + Strong TEJ and Proper Jet Stream Shift:
Often results in normal or above-normal monsoon.
Jet Stream Anomalies + El Niño:
If both the jet stream movement is abnormal and El Niño is active, monsoon onset and withdrawal can become highly erratic.
2015 Monsoon:
Influenced by a strong El Niño, leading to 14% rainfall deficit in India.
2020 Monsoon:
Despite a weak La Niña, favorable jet stream patterns led to above-normal rainfall.
These examples show that while El Niño is a powerful influencer, jet stream behavior often determines the final outcome.
The Indian monsoon is governed by a delicate balance of various atmospheric and oceanic phenomena. Jet streams play a direct role in steering and strengthening monsoon currents, while El Niño acts as a remote influencer by altering global pressure systems. Accurate prediction and understanding of these two factors are essential for monsoon forecasting, which in turn has deep implications for agriculture, economy, and disaster management in India. As climate variability increases, studying these phenomena becomes even more critical.