Turbulent characteristics of momentum flux in the marine atmospheric boundary layer of North Bay of Bengal

Abstract

Abstract We use a 16-month-long, 20 Hz wind data from a mooring deployed in the Bay of Bengal (BoB) to study the characteristics of turbulent wind stress ( $${u}{\prime}{w}{\prime})$$ u ′ w ′ ) events in the marine atmospheric boundary layer (MABL). Quadrant analysis of the motion-corrected $${u}{\prime}$$ u ′ and $${w}{\prime}$$ w ′ suggests that sweep and ejections, representing downward stress transfer into the ocean, dominate the $${u}{\prime}{w}{\prime}$$ u ′ w ′ (~ 140%). In comparison, outward and inward interactions representing an upward stress transfer into the atmosphere provide the counter-contribution (~ 40%). We found a wind speed (ws) dependency on stress transfer for ws > 3 m/s, while for low ws, the swell-dominated ocean state modulates the $${u}{\prime}{w}{\prime}$$ u ′ w ′ with a significant reverse stress transfer into the atmosphere, especially during intermonsoon periods. It is found that for weak winds ( $$ws$$ ws < 3 m/s), the number of turbulent events (N) is less, but they frequently repeat with more considerable flux per event ( $$\widehat{f})$$ f ^ ) , with outward and inward interactions (sweeps and ejections) dominating during intermonsoon periods (monsoon periods). For medium to strong winds, sweeps and ejections dominate $${u}{\prime}{w}{\prime}.$$ u ′ w ′ . Ejections are found to be the most efficient method of stress transfer in the BoB, contributing 80% of $${u}{\prime}{w}{\prime}$$ u ′ w ′ , compared to sweeps contributing ~ 60% and interaction processes contributing ~ − 20% each to the $${u}{\prime}{w}{\prime}$$ u ′ w ′ . Though the duration of sweep events is larger than ejections and with comparable flux energy per event ( $$\widehat{f}$$ f ^ ), the larger number N of ejection events makes it the dominant stress transfer mechanism in the Bay in all seasons.