Intraseasonal Variability of Monsoon Systems

Intraseasonal variability in monsoon systems refers to coherent fluctuations in rainfall, winds and convection occurring on timescales of roughly 10–60 days. These fluctuations manifest as alternating ‘active’ and ‘break’ phases that modulate agricultural productivity, water resources and flood risk across Asia, Africa and Australia. The boreal summer intraseasonal oscillation (BSISO) is a dominant mode over the Indian Ocean and western Pacific, characterised by a quasi-periodic envelope of enhanced convection propagating northward and eastward from the equator. In South Asia the monsoon intraseasonal oscillation (MISO) governs the arrival and retreat of active spells through a northward-propagating band of rainfall. Intraseasonal modes interact with sea surface temperatures, land–atmosphere feedbacks and large-scale circulations to shape regional extremes and subseasonal predictability. Progress in understanding these processes has grown through advances in high-resolution modelling, machine learning classification of propagation modes and targeted field campaigns. Improved characterisation of propagating envelopes and their driving mechanisms offers the prospect of extending reliable forecasts into the crucial 10–30-day ‘predictability gap’, thereby enhancing early warning of monsoon extremes and informing water-management strategies.

Research from Nature Portfolio

Recent studies have used unsupervised machine-learning to identify three distinct propagation modes of the boreal summer intraseasonal oscillation across the Indo-Pacific: a north-eastward track, an eastward-blocked pattern and a quasi-stationary mode. Analysis shows that Pacific sea surface temperatures modulate these pathways by altering the background moist static energy through local overturning circulations, offering potential for rainfall-extreme warnings up to four weeks in advance. Complementary aircraft observations during a tropical supercluster campaign over the Bay of Bengal revealed the vertical structure of convectively coupled Kelvin waves embedded within large-scale clusters. Dropsonde measurements documented zonal wind reversals, cold-pool characteristics and convergence zones that uplift unstable air parcels, supporting the notion of self-similar dynamics among mesoscale convective systems and large-scale intraseasonal waves.

Intraseasonal Variability of Monsoon Systems publication trend

The graph below shows the total number of articles in intraseasonal variability of monsoon systems across all publications each year (not limited to Nature Index journals).

Technical terms

Intraseasonal oscillation: Climate variability on 10–60-day timescales characterised by organised bands of convection and precipitation.

Boreal Summer Intraseasonal Oscillation (BSISO): A northward and eastward propagating convective envelope active during the Northern Hemisphere summer, modulating monsoon rainfall.

Monsoon Intraseasonal Oscillation (MISO): A northward-propagating oscillation in South Asian monsoon rainfall responsible for alternating active and break spells.

Convectively Coupled Kelvin Wave (CCKW): An eastward-propagating equatorial wave that is strongly linked to organised deep convection in the tropics.

Moist Static Energy (MSE): The sum of an air parcel’s sensible, latent and potential energy used as a proxy for convective potential.

References

1. Propagation pathways of Indo-Pacific rainfall extremes are modulated by Pacific sea surface temperatures. Nature Communications (2023).
2. Aircraft observations in a tropical supercluster over the equatorial Indian Ocean during MISO-BOB field campaign. Scientific Reports (2024).
3. Improved subseasonal prediction of South Asian monsoon rainfall using data-driven forecasts of oscillatory modes. Proceedings of the National Academy of Sciences of the United States of America (2024).
4. The influence of boreal summer intraseasonal oscillations on precipitation extremes and their characteristics in Southeast Asia. npj Climate and Atmospheric Science (2024).
5. Contrasting controls on convection at latitude zones near and away from the equator for the Indian summer monsoon. Environmental Research Letters (2023).