Wind and waves in extreme hurricanes

TLDR

Ocean‑surface wave breaking shapes air‑sea dynamics, chemistry, and biology, and the prevailing theory predicts that white‑capping and aerodynamic roughness rise with wind speed until a plateau—a relationship that has not yet been tested under extreme hurricane winds. Observations show that beyond ~40 m/s wind, white caps give way to foam and spray streaks, the surface roughness falls to near zero by ~80 m/s, and a high‑velocity surface jet develops, rendering the interface extremely smooth in the most intense hurricanes, though cross‑swell can temporarily raise roughness.

Abstract

Waves breaking at the ocean surface are important to the dynamical, chemical and biological processes at the air‐sea interface. The traditional view is that the white capping and aero‐dynamical surface roughness increase with wind speed up to a limiting value. This view is fundamental to hurricane forecasting and climate research but it has never been verified at extreme winds. Here we show with observations that at high wind speeds white caps remain constant and at still higher wind speeds are joined, and increasingly dominated, by streaks of foam and spray. At surface wind speeds of ∼40 m/s the streaks merge into a white out, the roughness begins to decrease and a high‐velocity surface jet begins to develop. The roughness reduces to virtually zero by ∼80 m/s wind speed, rendering the surface aero‐dynamically extremely smooth in the most intense part of extreme (or major) hurricanes (wind speed > 50 m/s). A preliminary assessment shows that cross swell, dominant in large regions of hurricanes, allows the roughness under high wind conditions to increase considerably before it reduces to the same low values.