"Phenomenal sea states and swell from a North Atlantic Storm in February 2011: a comprehensive analysis"

An extra-tropical cyclone in the North Atlantic in February 2011 provided the opportunity to study extreme wind and wave conditions in the open ocean and the subsequent swell field generated by a storm with extensive hurricane-force winds. In a paper soon to be published in the Bulletin of the American Meteorological Society (Hanafin et al, 2012, in print), with many of the OceanFlux-GHG researchers as co-authors, remarkable consistency was found between many different satellite and land-based observations and numerical models, considering the extreme conditions produced by this storm.

Annual average frequency of the low pressure centers with hurricane-force winds based on the NOAA OPC 6-hourly surface pressure analyses and QuikSCAT winds. The average was calculated based on data from September though May 2002-2009. The track of Quirin at 6 hourly intervals from 00Z on 13th February till 18Z on 14th February is overplotted. The size of the circle symbol at each time step reflects the surface area of winds ≥ 24.5 m/s and the colour represents the maximum wind speed.

 The spatial extent of the areas experiencing storm force and hurricane force winds compared very well between satellite-borne scatterometers and NCEP (National Center for Environmental Prediction) numerical model analysis fields. Wave heights up to 20.1m were observed by altimeters during the storm, which was the highest single measurement in a nine-year record, as was the along-track average of 16.2m over 533km. A series of hindcasts were run using the numerical wave model, WAVEWATCH-III ®, forced with NCEP analysis wind fields and comparisons with altimeters show that the model is capable of reproducing the extreme wave heights observed in the open ocean.

Top: altimeter H_s measured by 4 altimeters (Jason-1, Jason-2, ERS-2 and Envisat) on February 13^th (left panel) and February 14^th 2011 (right panel). The black square in the left (right) panel indicates the location of the most extreme sea states measured during these two days by the Envisat (Jason-2) altimeter, respectively.

Middle: Focus on the altimeter (black) H_s values estimated along the Envisat (left) and Jason-2 (right) tracks shown in figure 3 and indicated by the squares above, and computed from the WW3 model forced by ECMWF (red), NCEP (green) and NCEP+10% (blue) winds. A running average has been applied to the altimeter data (~5km resolution) to better match the resolution of the WW3 model (0.5°).

Bottom: Wind speed from different sources interpolated on the same Envisat (left) and Jason-2 (right) altimeter tracks. For both panels, black (green) lines give the altimeter (NCEP) wind speed. For the left (right) panel, the dashed red line gives the ASCAT scatterometer (Jason-2 radiometer) wind speed. On the left panel, the blue line gives the Oceansat-2 wind speed. All estimates have been computed at the spatial resolution of the NCEP fields. The dashed blue lines show the storm force (V ≥ 24.5 m/s) and hurricane-force (V ≥ 32.7 m/s) wind thresholds. A running average was again applied to the altimeter data to better match the resolution of the other data sources (~25km).

The extreme conditions observed during the storm generated swell of periods up to 25 seconds along the coasts of western Europe. The model hindcasts show that the model can also reproduce the swell field generated by the storm very well, as the model results were compared with ocean buoy data and seismic station observations around the Atlantic basin in the days following the storm as the swell made landfall. The time of arrival, the peak periods and the wave heights of the modelled wave fields were in very good agreement with the buoy and seismic station observations.

Top: Peak periods of the swell field: as calculated by WW3; from SAR observations; from wave buoy data; and from seismic buoy data. The background shows the output from the model at 12Z on the 15^th, as the longest swells were encroaching on the west coast of Scotland. The square symbols represent the wave buoy data, the size of the symbol signifying the H_s at the time of the maximum peak period observed and the color signifying the value of the peak period at this time. Beside each symbol is printed the time of arrival of the maximum peak period at each buoy. The circle gives the location of the SAR observations and diamond symbols represent the seismic stations, also colored according to the peak periods observed.

Bottom: Time series of the 3 hour median of the vertical ground displacement variance averaged over 20 minutes, from several stations around the North Atlantic, from February 14^th to 17^th. A timeseries of H_s from a buoy (OLERON) located off the west coast of France is also shown.

The Atlantic extra-tropical storm Quirin produced, on 14^th February 2011, wave heights that are expected to occur only about once a year over the globe, according to our hindcast results. Over a 12-year hindcast period, this storm ranked 3rd largest in terms of significant wave height in the North Atlantic. Waves from the center of the storm radiated as swell with very long periods, from 20 to 25 s, and were recorded around the northern and eastern Atlantic basin. Although the maximum values for wind and wave estimates are difficult to validate, the evidence presented in this study gives credence to the observed scales over which hurricane-force winds and sea state developed. Once the forcing wind field was adjusted to better match the satellite observations, a numerical wave model performed very well in reproducing the local sea state and swell field around the basin, given the extreme input conditions. We are encouraged by these results to report that our ability to both model and observe extreme wave events has improved greatly in recent years, while a novel look at century-old seismic records will help refine the climatology of such rare events.