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The Role of Observations in the Future of Oceanography

Raffaele Ferrari*


Galileo Galilee first pointed out that progress in physical sciences proceeds in three steps: observation of a new natural phenomenon, formulation of a hypothesis to explain the phenomenon, and an experiment to test the hypothesis. This implies that scientific progress is achieved only when observations of new phenomena become available. This has indeed been the case in oceanography since its early days. New observations have led progress in the field. An assessment of where the field might be fifteen years from now must therefore start from an evaluation of what observations are likely to become available in that time span. I will restrict my considerations to a few technologies that I believe are most likely to advance our field, and will leave it to others to attempt a comprehensive review of the future of physical oceanography.

Lacking a crystal ball or clairvoyance, the past is our best guide to predicting the future. I entered the field fifteen years ago, when satellite oceanography was taking its first steps. In those years, satellite measurements of SSH and sea surface temperature (SST) provided the first global view of the time dependent nature of the ocean circulation. The Argo float program complemented surface measurements with thousands of vertical profiles of temperature and salinity continuously repeated across the whole ocean. The old paradigm of an ocean circulation dominated by large scale steady gyres (~5000 km) was abandoned. A new view emerged, where the circulation is dominated by turbulent mesoscale

*

Massachusetts Institute of Technology



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The Role of Observations in the Future of Oceanography Raffaele Ferrari* Galileo Galilee first pointed out that progress in physical sciences proceeds in three steps: observation of a new natural phenomenon, for- mulation of a hypothesis to explain the phenomenon, and an experiment to test the hypothesis. This implies that scientific progress is achieved only when observations of new phenomena become available. This has indeed been the case in oceanography since its early days. New observations have led progress in the field. An assessment of where the field might be fifteen years from now must therefore start from an evaluation of what observations are likely to become available in that time span. I will restrict my considerations to a few technologies that I believe are most likely to advance our field, and will leave it to others to attempt a comprehensive review of the future of physical oceanography. Lacking a crystal ball or clairvoyance, the past is our best guide to predicting the future. I entered the field fifteen years ago, when satellite oceanography was taking its first steps. In those years, satellite measure- ments of SSH and sea surface temperature (SST) provided the first global view of the time dependent nature of the ocean circulation. The Argo float program complemented surface measurements with thousands of vertical profiles of temperature and salinity continuously repeated across the whole ocean. The old paradigm of an ocean circulation dominated by large scale steady gyres (~5000 km) was abandoned. A new view emerged, where the circulation is dominated by turbulent mesoscale * Massachusetts Institute of Technology 52

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53 RAFFAELE FERRARI eddies with scales of ~100 km, much like cyclones and anticyclones in the atmosphere. In parallel to these novel observations, ocean models also progressed from coarse and highly dissipative mesh grids to finer eddy- admitting grids. We are now able to produce simulations of the present state of the ocean that compare increasingly well to observations. How- ever, the skill of models in making long range predictions of the ocean is still very limited, because they lack a physically based representation of the physics at the submesoscale, i.e., scales of 1-100 km. Dissipation of momentum is achieved through enhanced viscosities and drag laws with little physical validation. Turbulent transport of tracers like heat, salt, carbon and nutrients is represented with unphysical constant eddy diffusivities. The first observational breakthrough is likely to come from an upcom- ing generation of radar altimeters. Conventional nadir-looking radar altimeters have an alongtrack resolution of ~100 km, barely sufficient to resolve the largest mesoscale eddies. The technique of radar interferome- try, recently demonstrated by NASA’s Shuttle Radar Topography Mission, offers an approach to mapping SSH at 10 km resolution over a wide swath of 100 km. NASA and the French space agency CNES (Centre National d’Études Spatiales) plan to use this technology in the Surface Water Ocean Topography (SWOT) mission to be launched sometime after 2016. SWOT will provide the first global observations of the surface mesoscale field and a large fraction of the submesoscale field. This kind of resolution is crucial to assess whether the turbulence generated by high resolution ocean models is realistic—mesoscale eddies contain 90% of the ocean’s kinetic energy and submesoscale eddies and fronts dominate the vertical velocities. Biogeochemically, these eddies set the physical and chemical environment of ocean ecosystems on space scales of kilometers and time scales of days, through their stirring of tracers and nutrient supply control by vertical motion across the euphotic zone. Indeed, one can think of the mesoscale as an ocean life “evolutionary hot-spot” in time and space. Just as it is not a coincidence that elemental ratios in seawater are the same as those in life, so the lifecycle of phytoplankton is in synchrony with finescale physics. These turbulent flows may well be key determinants of the structure and function of the entire marine food web. Moreover, the average structure of marine ecosystems will reflect the integrated, and rectified, effects of the finescale processes, modulating primary produc- tion and community structure and hence the export of organic carbon to the interior ocean with obvious implications for climate. Seismic reflection profiling, a technique that has been used for decades to image the solid earth beneath the ocean, could also become a revolutionary tool for oceanographic studies. Boundaries between bodies of water have a very faint sonic signature, which the oil industry used

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5 OCEANOGRAPHY IN 2025 to treat as noise. But in 2003, a team led by W. Steven Holbrook of the University of Wyoming adopted the technique and created unexpect- edly clear acoustic images of density boundaries in the ocean with an outstanding resolution of 10 × 10 meters. The information in these images is not quantitative (it is difficult to invert for density from sound), but it provides detailed pictures of turbulent structures in the ocean on scales from hundreds of kilometers down to a few meters. Much like schlieren images used to study turbulence in the laboratory, the 2D seismic sections might shed light on how energy is transferred from geostrophic eddies to 3D turbulence. This is an essential question because the pathway of energy from the large to the dissipation scales sets the equilibration of the ocean circulation. Numerical models must accurately represent this energy transfer if they are to be believed in their forecasts. Last, but not least, in the next fifteen years the oceanographic climate record will be substantially extended. Record lengths of SSH and scat- terometer winds will be doubled. Monitoring of ice sheets from space will extend to a couple of decades. The Argo observing system, which has just become operational, will provide the first glimpse of ocean vari- ability below the sea surface. The combination of these measurements is not only crucial to monitor the anthropogenic effect on Earth’s climate, but it is also essential to test the skill of ocean models in reproducing natural climate variability. Ocean models are often run for centuries to study climate change, but it is not clear whether they have any skill on those time scales. Fifteen years from now, it will finally be possible to test model skill vs. data. The list of observational techniques presented is not meant to be comprehensive. Rather, I emphasize techniques that will sample tempo- ral and spatial scales that are not yet observed but are crucial to improve our understanding of the role of the oceans in climate. Climate change has been called the defining issue of our era. The future trajectory of the Earth and its management in the coming century can only be informed by numerical models that couple atmosphere, ocean and cryosphere, as well as the complex interaction and co-evolution of its physics, chemistry and biology. The chemical and physical environment of life in the ocean is controlled by oceanic turbulence, which draws heat from the surface into its interior and, through its interaction with life, draws carbon to depth. The observations I described will move us toward a full understanding of ocean turbulence and its impact on life. This will be a milestone achieve- ment towards our goal of predicting climate and climate change.