By Ray Bell
In winter-time when the temperature gradient between the pole and the equator sharpens, the North Atlantic becomes a breeding ground for intense storms (extratropical cyclones). Occasionally associated with intense winter storms is a ‘phenomenal sea state’ (WMO define a phenomenal sea state as having a significant wave height larger than 14 m; see Hanafin et al. 2012). These extreme ocean waves pose life safety risks at sea and can damage coastal communities (see the ferocity of the ocean observed at Dawlish, South Devon on 11 November 2014). They also pose risks to offshore structures such as oil and gas platforms as well as wind turbines.
The size of ocean waves depends not only upon wind speed but is also related to wind duration (how long the wind has been blowing) and fetch (the distance in which the wind transfers energy to the waves). Figure 1 is a nomogram showing the relationship of these characteristics to significant wave height (a measure of the average wave height of a series of individual waves).
Figure 1. Nomogram showing how fetch, surface wind speed and minimum wind duration combine to influence wave height (Met Office Forecasters’ Reference Book, 1997).
Ocean wave measurements are sparse (link 1, link 2) and therefore we have limited understanding of the behaviour of extreme ocean waves. Oil and gas platforms in the Central North Sea provided a unique insight into the behaviour of extreme ocean waves during a storm on 15 December 2012 (Figure 2). The storm generated waves well in excess of the 100 year wave height from the west. The significant wave height (average wave height) was 10-11 m, however, 11 waves were measured at twice the height of the significant wave height at around 20 m (one definition of a rogue wave – an individual wave much larger than surrounding waves in space and/or time). That’s a wall of water the size of a seven-storey building.
Figure 2. Met Office North Atlantic Analysis chart showing the low-pressure system which gave rise to rogue waves in the Central North Sea (source: http://www1.wetter3.de/Archiv/archiv_ukmet.html). Animation of the synoptic evolution – Animations of the associated wind speed and significant wave height given in ERA-Interim
During this extreme event, the low pressure system was located to the east of Scotland and winds were westerly. Although this is not the ‘worst-case’ synoptic forcing for extreme waves in the North Sea (a low pressure centre over Scandinavia driving northerly winds has caused larger waves in the North Sea), the sea-state had interesting characteristics. Due to the short fetch and high wind speed the waves were very steep – large wave heights and a relatively short wave period (the distance between wave peaks). A steep sea-state is often conducive for rogue waves. More details of this storm can be found in Gibson et al. (2014).
These measurements can be used to calibrate and validate ocean wave models to reproduce extreme ocean waves and therefore improve their predictability for life safety purposes.
Jennifer A. Hanafin, Yves Quilfen, Fabrice Ardhuin, Joseph Sienkiewicz, Pierre Queffeulou, Mathias Obrebski, Bertrand Chapron, Nicolas Reul, Fabrice Collard, David Corman, Eduardo B. de Azevedo, Doug Vandemark, and Eleonore Stutzmann, 2012: Phenomenal Sea States and Swell from a North Atlantic Storm in February 2011: A Comprehensive Analysis. Bull. Amer. Meteor. Soc., 93, 1825–1832. doi: 10.1175/BAMS-D-11-00128.1
Gibson, R., M. Christou, and G. Feld, 2014: The statistics of wave height and crest elevation during the December 2012 storm in the North Sea. Ocean Dyn, 64 (9), 1305–1317, doi: 10.1007/s10236-014-0750-5 , 13th wave special issue