Exploring Rogue Waves from Observations in South Indian Ocean by Paul C. Liu (NOAA/GLERL, U.S.A.) and Keith R. MacHutchon (Liebenberg & Stander International, South Africa)

What is a rogue wave? H max /H s > 2 How high can this ratio be?

Western Cape Coast Line of South Africa showing FA Platform location Atlantic Ocean Indian Ocean FA Platform at 38S 22E

Wave Measurement from a Marex Wave Recorder Significant wave height (H s ) Maximum wave height (H max ) Average zero crossing wave period (T z )

FA Platform Wind and wave data Wave data Wind data Significant wave height Maximum wave heights Zero crossing periods Hm/Hs Ratios Hm/Hs < 2 Hm/Hs > 2 2< Hm/Hs < 3 3< Hm/Hs < 4 Hm/Hs > 4 Hourly ave 1/3 highest Wind Direction Hourly maximums Hourly averages FA PLATFORM: Wind & Wave Data Structure Temporary Record Time series wave elevations Hourly maximums Hourly averages PRIMARY PROCESSED DATA BY WAVE RECORDER SECONDARY PROCESSED DATA Wind Speeds

But that’s not the whole story!

What happen when H max /H s is much larger than 2?

Conventional Rayleigh conditions

H max /H s = N = N = 6.5x10 7 (65 million) 7.9x10 13 (79 trillion) 5.2x10 21 (5.2 sextillion) 1.9x10 31 (19 nonillion) Estimated time needed for occurrence: 14.5 years 17 million years 1 quadrillion years (10 15 ) 4 septillion years (10 24 ) Conventional Rayleigh conditions

Comparing the results with some laboratory experiments by Chin Wu of University of Wisconsin Normalize the wave heights with their corresponding zero-crossing wave period, H/gT 2

Concluding remarks No. 1

Agulhas Retroflection Short and medium period waves, with lengths from <200m to 500m, refracted by the Agulhas Current Agulhas Current Medium and long period waves, with lengths from 200m to 1500m, refracted by the Agulhas Bank Composite wave fronts arising from the deep sea over a long fetch Short waves, with lengths <200m, passing over the Agulhas bank Schematic Diagram showing a possible mechanism for the combination of wave spectra, leading to the presence of Freak Waves, on the Agulhas Bank

Concluding remarks No. 2

Proposing of a new classification: Typical rogue waves 2.0 < H max /H s < 4.0 Uncommon rogue waves 4.0 < H max /H s

Concluding remarks No. 3

With respect to typical rogue waves, Rayleigh distribution needs revision. With respect to uncommon rogue waves, Rayleigh distribution needs overhaul!

Concluding remarks No. 4

We need more, actual, continuous wave time series measurements in order to substantially verify or refute the existence of these uncommon rogue waves!

Progress in science is sometimes made by great discoveries. But science also advances when we learn that something we believed to be true isn’t. When solving a jigsaw puzzle, the solution can sometimes be stymied by the fact that a wrong piece has been wedged in a key place. -- Richard Muller