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

The Indian monsoon is critical since it supplies most of the water for drinking, sanitation, industry and agriculture for a billion people.  The onset of monsoon 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.  The timing of monsoon onset is already known to be influenced by tropical variability such as the Madden-Julian Oscillation.  New research has shown that the mid-latitudes also exert a powerful control, but the full extent of this extratropical role in monsoon onset progression and in the timing of active and break periods is poorly quantified and understood.

The team behind the new MiLCMOP project earlier led the INCOMPASS field campaign to India, taking new measurements and generating new hypotheses 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.

This poster describes some of the preliminary results on which the project is designed and explains the approach that MiLCMOP will use, including established techniques and development of new metrics to quantify the interactions between monsoon progression and extratropical forcing.  These methods will include use of vorticity budgets and Lagrangian feature tracking, applied to reanalysis and model data in case study years of fast and slow onset behaviour, to determine the dominant mechanisms controlling monsoon progression.  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.

The MiLCMOP project will eventually answer the following key questions:  (1) How are the pace and steadiness of Indian monsoon progression affected by interactions with the extratropics?  (2) What are the mechanisms of extratropical control on monsoon 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?