Final Report Summary - IOWAGA (Interdisciplinary Ocean Wave for Geophysical and other applications)
The project “Integrated Ocean WAves for Geophysical and other Application” has led to novel observation techniques, based on stereo-video imagery, satellite remote sensing and seismic waves, to better understand the physical processes responsible for the evolution of wave energy. This energy balance, with energy coming from the wind and being lost by breaking or friction at the air-sea interface is very different for waves of different scales. This knowledge has been integrated into “parameterizations”, which are rules used to compute the gains and losses of energy in numerical wave models. Our new parameterizations have been demonstrated in wave forecasts and hindcasts performed at Ifremer, leading to their widespread use now by many weather forecasting agencies (Météo-France, the U.S. NOAA/NCEP, Environnement Canada … ). These are all available in the latest version of the wave forecasting model WAVEWATCH III, which is developed jointly with the NOAA/NCEP and the U.K. Met. Office, among others. This computer code was published in March 2014.
Among the new capabilities, we have demonstrated that we could predict new parameters relevant for a wide range of applications. Among these, some statistics of ocean surface slopes are used by NASA and European researchers to better understand the measurements from satellite missions Aquarius and SMOS, designed to measure ocean salinity. Another new result is the prediction of the sources of the seismic noise. Indeed, the background oscillations of the Earth are mainly due to ocean waves, and after establishing a more complete theory for how waves make noise, we have produced the first maps of where different types of seismic waves are generated in the ocean. As you are reading these lines, unless you are on an airplane, you are moving up and down every 5 seconds or so, with an amplitude of a few micrometers. For reference a human hair is about 50 microns across, so it is a small motion, but this so far above the detection threshold of today's seismometers, that it actually obscures the signal from small earthquakes. Seismologists are now using this noise to make tomographic maps of the inside of the Earth, and we have also shown that ocean sediment properties could be inferred from that noise. We even predict the “seismic weather” for the coming days, which may have application for delicate problems like fundamental physics experiments trying to measure gravitational waves, or helping the detection of explosions.
Working on this “noise” has helped us understand the limitations of our wave model parameterizations, in particular it appears that short waves, with wavelength around 1 meter, have directions much further away from the wind, when the wind is strong, than what we expected. This was confirmed by our analysis of the stereo-video data. We do not know yet how this happens, but the surface current and effect of large breaking waves are our main suspects. Correcting the wave models for such errors should allow a better understanding of radar echoes from the sea surface, which can be used to measure winds and surface currents. Our understanding of the spatial distribution of seismic noise sources also means that we can now analyse seismic records to estimate wave properties, like wave heights and wave periods. We have demonstrated this for locations in the East Pacific and Scotland. Seismic data is thus a great complement to buoys, which are not common in the Southern hemisphere, and satellites which do not measure all wave properties. Besides, there are no satellite measurements of waves before 1985, and the first buoys were installed in the 1980s. Seismic records can go as far back as 1880.
Understanding better waves and their relation to coastal currents and water levels is also very important for the management of water quality and coastal hazards with submersion during big storms. We have particularly worked on weaving together numerical models for waves and ocean currents. One demonstration project is already used by a major water company to better predict the fate of bacterial contamination in a popular small bay in the South West of France, where closing the beach to tourist has a big impact on the local economy. This required a novel combination of models for currents with different layers capable to represent fresh water from local rivers, with the impact of waves breaking in shallow water and pushing the water around. When waves reach the shore, they come in groups which drive oscillations of the sea level with periods of a few minutes. We have demonstrated that these oscillations are highest on steep slopes, with amplitudes reaching a record 4 m, where they contribute to the overwash of cliffs and deposition of big rocks very high above the level of the tides. We have also explained how these same oscillations, when the propagate to the open ocean, can interfere with satellite measurements of sea level, and also how they generate seismic noise... which was an enigma since the discovery of these long period seismic oscillations in 1998. This shows the power of interdisciplinary integration around the topic of ocean waves.
Among the new capabilities, we have demonstrated that we could predict new parameters relevant for a wide range of applications. Among these, some statistics of ocean surface slopes are used by NASA and European researchers to better understand the measurements from satellite missions Aquarius and SMOS, designed to measure ocean salinity. Another new result is the prediction of the sources of the seismic noise. Indeed, the background oscillations of the Earth are mainly due to ocean waves, and after establishing a more complete theory for how waves make noise, we have produced the first maps of where different types of seismic waves are generated in the ocean. As you are reading these lines, unless you are on an airplane, you are moving up and down every 5 seconds or so, with an amplitude of a few micrometers. For reference a human hair is about 50 microns across, so it is a small motion, but this so far above the detection threshold of today's seismometers, that it actually obscures the signal from small earthquakes. Seismologists are now using this noise to make tomographic maps of the inside of the Earth, and we have also shown that ocean sediment properties could be inferred from that noise. We even predict the “seismic weather” for the coming days, which may have application for delicate problems like fundamental physics experiments trying to measure gravitational waves, or helping the detection of explosions.
Working on this “noise” has helped us understand the limitations of our wave model parameterizations, in particular it appears that short waves, with wavelength around 1 meter, have directions much further away from the wind, when the wind is strong, than what we expected. This was confirmed by our analysis of the stereo-video data. We do not know yet how this happens, but the surface current and effect of large breaking waves are our main suspects. Correcting the wave models for such errors should allow a better understanding of radar echoes from the sea surface, which can be used to measure winds and surface currents. Our understanding of the spatial distribution of seismic noise sources also means that we can now analyse seismic records to estimate wave properties, like wave heights and wave periods. We have demonstrated this for locations in the East Pacific and Scotland. Seismic data is thus a great complement to buoys, which are not common in the Southern hemisphere, and satellites which do not measure all wave properties. Besides, there are no satellite measurements of waves before 1985, and the first buoys were installed in the 1980s. Seismic records can go as far back as 1880.
Understanding better waves and their relation to coastal currents and water levels is also very important for the management of water quality and coastal hazards with submersion during big storms. We have particularly worked on weaving together numerical models for waves and ocean currents. One demonstration project is already used by a major water company to better predict the fate of bacterial contamination in a popular small bay in the South West of France, where closing the beach to tourist has a big impact on the local economy. This required a novel combination of models for currents with different layers capable to represent fresh water from local rivers, with the impact of waves breaking in shallow water and pushing the water around. When waves reach the shore, they come in groups which drive oscillations of the sea level with periods of a few minutes. We have demonstrated that these oscillations are highest on steep slopes, with amplitudes reaching a record 4 m, where they contribute to the overwash of cliffs and deposition of big rocks very high above the level of the tides. We have also explained how these same oscillations, when the propagate to the open ocean, can interfere with satellite measurements of sea level, and also how they generate seismic noise... which was an enigma since the discovery of these long period seismic oscillations in 1998. This shows the power of interdisciplinary integration around the topic of ocean waves.