The amazing century-long rainfall data over India has provided many insights into the vagaries of the monsoon. Weeks of rain followed by weeks of dry periods, known as the active and break events, are critical for agriculture. Year-to-year variabilities come with pre-monsoon anxiety about whether the monsoon will be normal or not.
El Nino forecasts raise flags about potential deficits and yet only about 60 per cent of the drought years are explained by them. Seasonal rainfall documents a distinct decade-to-decade variability, but a clear scientific explanation for its cause has yet to be discovered. This timescale of decades is called multi-decadal variability.
Multi-decadal ups and downs in monsoon
The clear evidence of multi-decadal variability in the monsoon raises a critical question — wouldn’t it be really useful to know if the coming decade or two will see above or below the long period average rainfall?
This is especially important since the monsoon projections from international body Inter-governmental Panel on Climate Change (IPCC) are not accurate enough for making plans for water resources, agriculture, energy and health. Some studies have interpreted the multi-decadal monsoon changes as downward or upward trends. This can be misleading.
A variability differs from a trend in that it will experience downtime before returning to bountiful times. This underscores the critical need to improve understanding and establish a focused effort on decadal predictions in India.
Multi-year to decadal timescales are also known as societally relevant timescales, because predictions at these timescales can inform socioeconomic decisions and policies for water, agriculture, energy and health.
What drives multi-decadal variability of monsoon?
While data indicates a robust multi-decadal monsoon signal, the science of this timescale remains incomplete. There are correlations between the decadal variability in the sea surface temperatures over the Indian Ocean and the decadal variability over the core monsoon zone over central India. The Indian Ocean temperatures themselves are related to decadal temperature changes in the Pacific Ocean.
El Nino and La Nina are changes in Pacific Ocean temperatures at year-to-year or interannual timescales and they clearly affect the monsoon rainfall in certain years. It is this same relation that seems to translate to a relation from the Pacific Ocean temperatures to the multi-decadal variability of the monsoon.
Decadal clustering of El Ninos and La Ninas
A new study, co-authored by this writer, reported on the process that determines the transition of an El Nino or La Nina from its peak warming or cooling in the Pacific Ocean in December-January-February to the following summer, i.e., temperatures from June to September. El Nino and La Nina together are referred to as the El Nino Southern Oscillation (ENSO).
A climate signal called the ENSO transition mode (ETM)has been identified in the southern Pacific Ocean. This mode with a sea level pressure and wind co-variation, perturbs the winds in the tropical Pacific Ocean.
These tropical winds then influence whether an El Nino in one year will continue to be warm in the following summer or transition to neutral or cold conditions. Similarly, it also determines the transition of a La Nina in one year into a continued La Nina state into the following summer.
The crucial insight is that the ETM has a multi-decadal variability as well. This means that some decades tend to have El Ninos that will continue to be warm from one year into the next summer and decades where La Ninas will continue to be La Ninas from one year into the following summer.
A multi-decadal variability in the ETM is a key indicator that it may be able to explain the multi-decadal variability in the monsoon.
ETM and monsoon multi-decadal variability
The newly found ETM offers new science for understanding monsoon muti-decadal variability. Even if only 60 per cent of the deficit monsoon years are related to El Nino and only 60 per cent of the surplus years are related to La Nina years, the decadal preference for El Ninos to remain warm or La Ninas to remain cold, open up avenues to explain the multi-decadal variability in the monsoon using the ETM mode.
Furthermore, this can also lead to successful predictions of the multi-decadal surplus and deficit monsoon years.
Combining reliable decadal monsoon predictions with the current skillful short (1-3 days), medium (3-10 days) and extended (weeks 2-4) range forecasts and seasonal to interannual predictions, will provide India a robust scientific basis to manage water, agriculture, energy and health effectively and efficiently.
The great news is that the personnel and computing facilities to pursue this critical timescale are already in place at the various centres, such as Indian Institute of Tropical Meteorology in Pune.
Raghu Murtugudde is Professor of Climate Studies at IIT Bombay and Emeritus Professor at UMD.
Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth
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