Phenomenal seas*

Several storms with hurricane-force winds cross the North Atlantic Ocean every year. And, indeed we saw that the highest waves were recorded there (Image of the Month, June 2011). However, observations of winds and waves during these storms is a particular challenge because there are not many in situ observations in the middle of the ocean, so there are few opportunities to validate remote sensing techniques. In fact, significant wave height altimeter measurements over 15 m are considered to have a lower level of confidence due to the lack of validation data. Moreover, models are reputed to underestimate the severity of extreme events, partly due to low spatial resolution but also due to underestimation of these values by the wind fields used to force the models.

The Quirin storm occured in February 2011, and extreme values of significant wave heights were observed in the North Atlantic Ocean. To study this particular storm, observations from satellite altimeters, in situ buoys, and ground-based seismometers (which record the seismic noise from storms as it propagates far from the source) were used, as well as a numerical wave model. This study showed consistency between these different sources, both for significant wave heights in the open ocean generated in the storm area and for observations of the swell field it produced as it reached European shores a day or two later.

 
(* Following the World Meteorological Organization, a "phenomenal sea" is a sea with wave heights of more than 14 m.)