Consider the following statements regarding the interplay between Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO) and their impact on the Indian Summer Monsoon:

Statement 1: A Positive IOD can counteract the negative effects of El Niño by increasing westerly winds over the Indian Ocean, enhancing monsoon rainfall over India.

Statement 2: A Negative IOD tends to suppress monsoon rainfall over India, but when combined with La Niña—which usually strengthens the monsoon—the net impact on Indian rainfall is uncertain and depends on which influence dominates.

Statement 3: The combined presence of a Positive IOD and La Niña weakens the monsoon, as both phenomena work together to reduce moisture flow into the Indian subcontinent.

Which one of the following is correct in respect of the above statements?

The correct answer is option 4

Key Points

• A Positive IOD (warmer Western Indian Ocean, cooler Eastern Indian Ocean) can partially offset the drying effects of El Niño by strengthening monsoon winds and increasing moisture transport to India. Hence, Statement 1 is correct.
• A Negative IOD (cooler Western Indian Ocean, warmer Eastern Indian Ocean) weakens the monsoon, but if La Niña is present, it enhances monsoon activity, rather than causing droughts. The incorrect part of the statement is the claim that Negative IOD + La Niña causes drought—it may lead to a mixed effect rather than severe drought. Hence, Statement 2 is correct.
• A Positive IOD and La Niña together actually strengthen the monsoon, not weaken it. Both phenomena increase moisture availability, leading to above-normal rainfall in India. Hence, Statement 3 is incorrect.

Important Points 

• Impact of Positive and Negative IOD with ENSO on Monsoon:
• Positive IOD + El Niño → Neutralizing Effect
• El Niño weakens monsoon, but Positive IOD enhances westerly winds and increases rainfall, partially offsetting El Niño's impact.
• Example: In 1997, despite a strong El Niño, a Positive IOD prevented a severe drought in India.
• Negative IOD + El Niño → Weak Monsoon & Drought
• Both suppress monsoon activity, leading to deficient rainfall.
• Example: In 2015, a strong El Niño and Negative IOD resulted in one of India's driest monsoons in decades.
• Positive IOD + La Niña → Strongest Monsoon
• Both enhance monsoon rainfall.
• Example: 2010 and 2021 had above-average rainfall due to La Niña + Positive IOD conditions.
• Negative IOD + La Niña → Mixed Effects
• La Niña strengthens monsoon, but Negative IOD reduces moisture supply from the Indian Ocean.
• The overall impact depends on which factor is stronger.

Additional Information 

• IOD and ENSO monitoring is crucial for predicting monsoon variability in India.
• The India Meteorological Department (IMD) closely tracks IOD and ENSO patterns to issue monsoon forecasts.
• ENSO and IOD can either amplify or counteract each other’s effects, influencing agriculture, water resources, and disaster preparedness in India