Falling monsoon depression frequency: a Gray-Sikka conditions perspective.

Area of Science:

• Meteorology
• Climatology
• Atmospheric Science

Background:

• Monsoon depressions (MDs) are crucial weather systems impacting India.
• A decreasing trend in MD frequency has been observed along the east coast of India.
• This decline is more pronounced in the southern Bay of Bengal.

Purpose of the Study:

• To investigate the reasons behind the weakening frequency of MDs in the southern Bay of Bengal.
• To analyze atmospheric parameters influencing tropical cyclogenesis.
• To validate findings using observational and reanalysis datasets.

Main Methods:

• Analysis of statistically significant trends in MD frequency (1979-2010).
• Examination of seasonal average in-situ atmospheric parameters.
• Utilizing observational data from the India Meteorological Department (IMD) and three reanalysis datasets.
• Conducting numerical sensitivity experiments.

Main Results:

• A statistically significant decreasing trend in annual monsoon depression frequency making landfall on India's east coast from 1979-2010.
• Approximately 80% of this decrease is concentrated south of 20°N.
• The weakening of MD frequency south of 20°N since the 1950s is strongly correlated with a declining trend in mid-tropospheric relative humidity over the Indian region.

Conclusions:

• Declining mid-tropospheric relative humidity is a likely cause for the reduced frequency of monsoon depressions in the southern Bay of Bengal.
• Numerical experiments support the link between reduced humidity and weakened MD activity.
• This finding has implications for understanding regional climate variability and monsoon dynamics.