Enhanced freak wave occurrence with narrow directional spectrum in the North Sea

Wind and wave records obtained from the Kvitebjorn platform (2.3 degrees E, 61.0 degrees N, 190 m deep) in the northern North Sea from 2003 to 2005 were analyzed. Among the 2723 20-min records taken during storm conditions, 57 cases included freak waves exceeding twice the significant wave height. Comparisons between various wave parameters and the freak wave occurrence index did not show any significant correlation. Thus, the in situ wave record was used to select the days when relatively more or less freak waves were observed. The days were classified into freakish and non-freakish days, respectively. On freakish days, the Icelandic low was enhanced. Hindcasts performed by a third-generation wave model suggest that this synoptic atmospheric pressure difference produces approximately 7.6 degrees narrower directional spreading of the wave spectra during freakish days at the Kvitebjorn platform. This result is consistent with the physical mechanism of freak wave generation through nonlinear self-focusing in random wave fields. Citation: Waseda, T., M. Hallerstig, K. Ozaki, and H. Tomita (2011), Enhanced freak wave occurrence with narrow directional spectrum in the North Sea, Geophys. Res. Lett., 38, L13605, doi: 10.1029/2011GL047779.