Mid-Latitude Controls on Monsoon Onset and Progression (MiLCMOP)

The Indian summer monsoon is critical to the well-being of a billion people since it supplies most of the water for drinking, sanitation, industry and agriculture. Society is so finely tuned to the rains that any variation in their timing, intensity and duration has profound impacts. The onset of monsoon rains typically starts in southern India by 1 June, taking up to 6 weeks to cover the country. Meanwhile, during the monsoon, variations on time scales of a week or more give rise to periods of excess and reduced rainfall, known as active and break events.

Being able to better predict the onset of the rains, their progression, and of active and break events in the monsoon would be of great benefit particularly in agriculture, for improved decision-making for planting and use of irrigation and fertilizer. Such advances need improved understanding of how the monsoon progresses across the country and of its behaviour in the computer models used for forecasting and climate prediction.

The timing of the monsoon onset is already known to be influenced by variability in the tropics known as the Madden-Julian Oscillation. New research has shown that the mid-latitudes also exert a powerful control. But compared to tropical drivers, the full extent of the extratropical role in monsoon onset progression and in the timing of active and break periods is poorly quantified and understood.

The MiLCMOP team earlier led the INCOMPASS field campaign to India, taking new measurements and putting forward new ideas on how the monsoon is controlled, including the concept that monsoon progression can be described as a "tug-of-war" between tropical and extratropical airmasses. This "tug-of-war" is an unsteady process, with a back and forth of the two airmasses before the moist tropical flow takes over for the rest of the season.

The MiLCMOP team brings together expertise in monsoon systems and extratropical processes, state-of-the-art modelling at global and convective scales, and in the latest machine learning and causal inference techniques. The time is ripe for MiLCMOP to establish the role of extratropical dynamics on Indian monsoon onset and progression through a detailed investigation of the mid-latitude processes at play.

MiLCMOP will use established techniques and develop new metrics to quantify the interactions between monsoon progression and extratropical forcing. These methods will include use of vorticity budgets (examining the sources of changes in circulation) and Lagrangian feature tracking (determining the origin and properties of the different airmasses involved). The vorticity budget and Lagrangian feature tracking tools will be applied to observation-based "reanalysis" and model data in case study years of fast and slow onset behaviour, to determine the dominant mechanisms controlling monsoon progression. The latest global climate models contributing to the IPCC 6th Assessment Report will be tested to see if observed onset processes are represented, while initialised subseasonal and seasonal forecasts (including from project partners at the UK Met Office and India's NCMRWF) will be interrogated to test whether skilful forecasts can be made. New model experiments will be designed and performed to isolate the mechanisms by which extratropical drivers affect monsoon onset and its progression. Finally, novel causal inference techniques will be used to disentangle the effects of extratropical drivers from those in the tropics.

MiLCMOP will answer the following key questions: (1) How are the pace and steadiness of Indian monsoon progression affected by interactions with the extratropics in observations and models? (2) What are the mechanisms of extratropical control on ISM progression and variability? (3) In what way do the causal extratropical and tropical drivers of ISM progression offset or reinforce each other and can the competing roles of tropical and extratropical processes be generalised to other monsoons?