Ocean Surface Wave Slopes and Wind-Wave Alignment Observed in Hurricane Idalia

Abstract

Drifting buoy observations in Hurricane Idalia (2023) are used to investigate the dependence of ocean surface wave mean square slope on wind, wave, and storm characteristics. Mean square slope has a primary dependence on wind speed that is linear at low-to-moderate wind speeds and approaches saturation at high wind speeds ($>$20 m $s^{-1}$). Inside Hurricane Idalia, buoy-measured mean square slopes have a secondary dependence on wind-wave alignment: at a given wind speed, slopes are higher where wind and waves are aligned compared to where wind and waves are crossing. At moderate wind speeds, differences in mean square slope between aligned and crossing conditions can vary 15%–20% relative to their mean. These changes in wave slopes may be related to the reported dependence of air-sea drag coefficients on wind-wave alignment.