Observed Ocean Surface Winds and Mixed Layer Currents Under Tropical Cyclones: Asymmetric Characteristics

Tropical cyclones (TC) transfer kinetic energy to the upper ocean and thus accelerate the ocean mixed layer (OML) currents. However, the quantitative link between near-surface currents and high wind speeds, under extreme weather conditions, remains poorly understood. In this study, we use multi-mission satellites and drifting-buoy observations to investigate the connections between TC surface winds and currents, including their spatial distribution characteristics. Observed ageostrophic current speeds in the OML increase linearly with wind speeds (for the range 20-50 m/s). The ratios of the ageostrophic current speeds to the wind speeds are found to vary with TC quadrants. In particular, the mean ratio is around 2% in the left-front and left-rear quadrants with relatively small variability, compared to between 2% and 4% in the right-front and right-rear quadrants, with much higher variations. Surface winds and currents both exhibit strong asymmetric features, with the largest wind speeds and currents on the TC right side. In the eyewall region of Hurricane Igor, high winds (e.g., about 47 m/s) induce strong currents (about 2 m/s). The directional rotations of surface winds and currents are resonant and dependent on the location within the storm. Wind directions are approximately aligned with current directions in the right-front quadrant; a difference of about 90 degrees occurs in the left-front and left-rear quadrants. The directional discrepancy between winds and currents in the right-rear quadrant is smaller. Reliable observations of the wind-current relation, including asymmetric features, support published theories developed in idealized numerical experiments to explain the upper ocean response to TCs. Plain Language Summary Observations of surface winds and currents under extreme weather conditions are essential to characterize upper ocean responses to tropical cyclones. To better understand air-sea coupled processes, it is necessary to quantify the connections between surface winds and upper ocean mixed layer currents, and their spatial distribution characteristics. Using an analysis of satellite-derived winds and drifter-observed currents, we find that in severe storm conditions, the near-surface current speeds increase linearly with wind speeds. Moreover, the ratios of ageostrophic current speeds to wind speeds are found to be significantly different on the two different sides of a tropical cyclone track, indicating that the surface winds and currents have strong asymmetry. High-resolution wind fields from spaceborne synthetic aperture radar and currents from collocated drifters further demonstrate the asymmetric features. Stronger winds and currents occur on the right side. Additionally, rotations of surface winds and currents are resonant in the right-front quadrant.