Observations of rogue seas in the Southern Ocean

TL;DR

This study provides direct observational evidence of rogue seas in the Southern Ocean, highlighting the role of wind forcing and nonlinear interactions in generating extreme waves during wave growth phases.

Contribution

It offers the first direct measurements of rogue seas in the Southern Ocean, demonstrating the link between wind conditions, wave growth, and heavy-tailed wave statistics.

Findings

Rogue seas occur during wave growth with strong wind forcing.

Heavy-tailed wave statistics are observed beyond bound mode theory.

Wave statistics normalize after wind forcing ceases.

Abstract

We report direct observations of surface waves from a stereo camera system along with concurrent measurements of wind speed during an expedition across the Southern Ocean in the austral winter aboard South African icebreaker S.A.~Agulhas~II. Records include water surface elevation across a range of wave conditions, spanning from early stages of wave growth to full development. We give experimental evidence of rogue seas, i.e., sea states characterizided by heavy tails of the probability density function well beyond the expectation based on bound mode theory. These conditions emerge during wave growth, where strong wind forcing and high nonlinearity drive wave dynamics. Quasi-resonance wave-wave interactions, which are known to sustain the generation of large amplitude rogue waves, capture this behaviour. Wave statistics return to normality as the wind forcing ceases and waves switch to a full developed condition.