When Two Similar El Niño Events Produced Different Indian Summer Monsoons: Insights from 1997 and 2015

 Background

One of the most intriguing questions in climate science is why the Indian summer monsoon behaves so differently even when large-scale climate conditions appear remarkably similar.

Consider the years 1997 and 2015, for example. Both years witnessed one of the strongest El Niño events on record, accompanied by a positive Indian Ocean Dipole (IOD). Based on these two climate signals alone, one might expect the Indian summer monsoon to respond in a similar way.

Yet the outcomes were strikingly different.

  • 1997: India received normal monsoon rainfall.
  • 2015: India experienced a below-normal monsoon, with widespread rainfall deficits.

This intriguing contrast motivated our recent study, published in Atmospheric Research, where we investigated why two seemingly similar climate years produced such different monsoon outcomes.

Figure 1. Seasonal rainfall anomalies over India during the monsoon seasons of 1997 and 2015, along with their difference (1997–2015). Although both years experienced Super El Niño and positive IOD conditions, the spatial distribution of rainfall differed markedly across India, highlighting the role of atmospheric circulation and the Madden–Julian Oscillation (MJO) in shaping the seasonal monsoon.

Why are these climate phenomena important?

The Indian Summer Monsoon Rainfall (ISMR) supplies nearly 80% of India's annual rainfall and supports agriculture, water resources, hydropower generation, and the livelihoods of millions of people. Among the many factors that influence the monsoon, three major climate drivers are particularly important:

  • El Niño, which generally weakens the Indian monsoon.
  • Indian Ocean Dipole (IOD), whose positive phase often favours enhanced rainfall over India.
  • Madden–Julian Oscillation (MJO), an eastward-moving tropical weather disturbance that modulates rainfall on timescales of 30–60 days.

While each of these climate modes has been extensively studied individually, much less is known about how they interact when they occur together. Our study focused precisely on this question.

The years 1997 and 2015 offered a unique opportunity. Both years experienced:

  • Super El Niño conditions over the tropical Pacific,
  • Positive IOD conditions over the Indian Ocean,
  • Similar seasonal evolution of sea surface temperatures.

Despite these similarities, India's monsoon behaved very differently. This made the two years an ideal "natural experiment" for understanding the combined influence of ENSO, IOD, and MJO.

What did we discover?

1. Not all El Niño events are identical

Although both years experienced very strong (Super) El Niño conditions, the spatial pattern of sea surface temperature anomalies differed between 1997 and 2015. These differences altered the large-scale atmospheric circulation over the Indo-Pacific region, ultimately affecting the strength of the Indian monsoon.

2. Atmospheric circulation played a crucial role

Compared with 1997, the 2015 monsoon season exhibited:

  • weaker southwesterly monsoon winds,
  • stronger atmospheric subsidence over India,
  • reduced moisture transport towards the Indian subcontinent.

These conditions suppressed cloud formation and rainfall over much of the country.

3. Rainfall characteristics were markedly different

One interesting finding was that the seasonal rainfall deficit in 2015 was not simply due to fewer heavy rainfall events. Instead, 2015 experienced:

  • many more dry days,
  • fewer rainy days,
  • more frequent breaks in the monsoon.

In contrast, 1997 had more rainy days and stronger rainfall during the peak monsoon months (July and August), helping the country achieve near-normal seasonal rainfall. These differences are clearly reflected in the contrasting rainfall anomaly patterns shown in Figure 1.

4. The Madden–Julian Oscillation made the difference

Perhaps the most interesting finding of our study concerns the role of the Madden–Julian Oscillation (MJO). Although both years experienced Super El Niño and positive IOD conditions, the MJO behaved very differently. During 1997, the MJO spent more time over the Indian Ocean, corresponding to phases that generally favour convection and rainfall over India. During 2015, however, the MJO remained active for longer over the western Pacific, corresponding to phases that suppress rainfall over the Indian region.

This difference provides one of the key explanations for why two years with nearly identical El Niño and positive IOD conditions produced such contrasting monsoon outcomes.

Why does this matter?

Seasonal monsoon prediction cannot rely on a single climate indicator. Our study demonstrates that the Indian summer monsoon is governed by the combined influence of multiple climate drivers operating on different timescales:

  • ENSO influences interannual variability.
  • IOD modulates ocean–atmosphere interactions over the Indian Ocean.
  • MJO regulates rainfall on intraseasonal timescales.

The Indian summer monsoon is often described as one of the most complex components of the Earth's climate system. Even when two years appear remarkably similar from a global perspective, subtle differences in ocean temperatures, atmospheric circulation, and intraseasonal variability can produce dramatically different monsoon outcomes over India. Understanding these interactions is essential not only for advancing monsoon science but also for improving seasonal prediction and strengthening operational climate services provided by agencies such as the India Meteorological Department (IMD).

This study also points to an important direction for future research. Improving our understanding of how ENSO, IOD, and MJO interact—and how these interactions are represented in climate models—will contribute to more reliable seasonal monsoon forecasts and more effective climate services for agriculture, water resources, and disaster risk reduction.

Published in:

Atmospheric Research (2024)

Combined influence of El Niño, IOD and MJO on the Indian Summer Monsoon Rainfall: Case study for the years 1997 and 2015. 

DOI: https://doi.org/10.1016/j.atmosres.2023.107214




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