Preprints
https://doi.org/10.5194/wcd-2022-31
https://doi.org/10.5194/wcd-2022-31
 
14 Jun 2022
14 Jun 2022
Status: a revised version of this preprint is currently under review for the journal WCD.

Nonlinear intensification of monsoon low pressure systems by the BSISO

Kieran Mark Rainwater Hunt1,2 and Andrew George Turner1,2 Kieran Mark Rainwater Hunt and Andrew George Turner
  • 1Department of Meteorology, University of Reading, UK
  • 2National Centre for Atmospheric Sciences, University of Reading, UK

Abstract. More than half of the rainfall brought to the Indian subcontinent by the summer monsoon is associated with low-pressure systems (LPSs). Yet their relationship with the Boreal Summer Intraseasonal Oscillation (BSISO) – the dominant intraseasonal forcing on the monsoon – is only superficially understood. Using reanalysis data, we explore the relationship between the BSISO and LPS intensity, propagation, and precipitation, and associated underlying mechanisms.

The BSISO has a large impact on mean monsoon vorticity and rainfall as it moves northward – maximising both in phases 2–3 over southern India and phases 5–6 over northern India – but a much weaker relationship with total column water vapour. We present evidence that LPS genesis also preferentially follows these phases of the BSISO. We identify significant relationships between BSISO phase and LPS precipitation and propagation: for example, during BSISO phase 5, LPSs over north India produce 51 % heavier rainfall and propagate northwestward 20 % more quickly. Using a combination of moisture flux linearisation and quasigeostrophic theory, we show that these relationships are driven by changes to the underlying dynamics, rather than the moisture content or thermodynamic structure, of the monsoon.

Using the example of LPSs over northern India during BSISO phase 5, we show that the vertical structure of anomalous vorticity can be split into contributions from the BSISO and the nonlinear response of the LPS to anomalous BSISO circulation. Complementary hypotheses emerge about the source of this nonlinear vorticity response: nonlinear frictional convergence and secondary barotropic growth. We show that both are important. The BSISO imparts greater meridional shear on the background state, supporting LPS intensification. The BSISO background and nonlinear LPS response both contribute significantly to anomalous boundary layer convergence, and we show through vortex budget arguments that the former supports additional LPS intensification in boundary layer while the latter supports faster westward propagation.

This work therefore yields important insights into the scale interactions controlling one of the dominant synoptic systems contributing to rainfall during the monsoon.

Kieran Mark Rainwater Hunt and Andrew George Turner

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wcd-2022-31', Anonymous Referee #1, 14 Jul 2022
  • RC2: 'Comment on wcd-2022-31', Anonymous Referee #2, 14 Sep 2022
  • RC3: 'Comment on wcd-2022-31', Anonymous Referee #3, 14 Sep 2022

Kieran Mark Rainwater Hunt and Andrew George Turner

Data sets

Monsoon low-pressure-system tracks over South Asia (1979-2019) Kieran Mark Rainwater Hunt https://doi.org/10.5281/zenodo.5575336

Kieran Mark Rainwater Hunt and Andrew George Turner

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Short summary
More than half of India’s summer monsoon rainfall arises from low-pressure systems: storms originating over the Bay of Bengal. In observation-based data, we examine how the generation and pathway of these storms are changed by the “Boreal summer intraseasonal oscillation” – the chief means of large-scale control on the monsoon at time scales of a few weeks. Our study offers new insights for useful prediction of these storms, important for both water resources planning and disaster early warning.