30 July 2018

Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century

South Asian monsoon depressions in the 21st century.

Science

1) Developed a new index that explains the sources of errors in the simulation of MDs in the current generation of GCMs. 2) No significant impact on the frequency and trajectory of MDs in the future period simulations of GCMs. 3) Changes in the extreme wet events are dominated by non-depression day occurrences, which may render the monsoon extremes less predictable in the future.

Impact

MDs are one of the most important sources of moisture transport over the South Asian. They contribute significantly to the total precipitation and often times cause severe flooding and loss of life. Despite the lack of significant trends in MDs, there are important changes in the monsoon behavior at seasonal to daily time-scales that may heighten the unpredictable nature of monsoon extremes

Summary

Monsoon depressions (MDs) are one of the most important synoptic scale transient weather systems that transport a large amount of moisture over the South Asian landmass and contribute significantly to the total precipitation. In this study, we investigate the representation of MDs in the Coupled Model Intercomparison Project Phase 5 general circulation models, their simulated responses to increase in radiative forcing during the 21st century under Representative Concentration Pathway 8.5, and the resulting changes in precipitation characteristics at intraseasonal timescales. We show that 21st-century increase in radiative forcing does not significantly impact the frequency of South Asian summer monsoon depressions or their trajectories in the general circulation models. Our results indicate a significant relationship between the climatological occurrences of MDs and the background upper (lower) tropospheric meridional (zonal) winds and tropospheric moisture in the core genesis region of MDs. While monsoon dynamics progressively weaken in the future, atmospheric moisture exhibits a strong increase, limiting the impact of changes in dynamics on the frequency of MDs. Our results also indicate that future increase in the extreme wet events is dominated by nondepression day extremes, which may render the monsoon extremes less predictable in the future.

Contact
Moetasim Ashfaq
Oak Ridge National Laboratory (ORNL)
Publications
Rastogi, D, M Ashfaq, L Leung, S Ghosh, A Saha, K Hodges, and K Evans.  2018.  "Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century."  doi:10.1029/2018GL078756.