The Future of Physical Oceanography1
William R. Young
Scripps Institution of Oceanography, University of California, San Diego
The National Science Foundation (NSF) tasked the U.S. physical oceanographic community in 1997 to evaluate the current status of research in physical oceanography and to identify future opportunities and infrastructure needs. A workshop was held in Monterey, California from December 15-17, 1997 and was attended by 46 scientists representing the community of NSF-supported investigators. A subtheme of the meeting was the role and effectiveness of the NSF's core program in physical oceanography. Input via electronic mail from the wider scientific community was sought both before and after the meeting.
The community was asked to consider advances in physical oceanography over the last twenty years. The following items were widely hailed as significant recent achievements: a revolutionary understanding of the coupling of the tropical ocean and atmosphere and the development of predictive El Niño models; estimation of the global distribution of mesoscale variability in the word ocean and theories and models of this geostrophic turbulence; completion of the World Ocean Circulation Experiment and improved estimates of the pathways and timescales of the circulation; and quantitative measurements of the strength of small-scale ocean mixing and the dependence of this mixing on the strength of the internal wave field and other environmental conditions.
The community was also asked to look into the future and forecast advances for the next twenty years. Great excitement was expressed at the prospect of new tools that might solve the problem of observing the global ocean. Already the TOPEX/POSEIDON satellite mission has measured the topography of the sea surface to 3 cm accuracy at 7 km spacing for 5 years. Future developments in satellite oceanography promise global measurements of sea surface salinity and precipitation. These measurements are crucial if we are to understand the climate system and the hydrologic cycle. Yet sea-truth is essential and in situ water-column observations made by an unprecedented class of autonomous instruments are anticipated. Integrating measurements, such as tomography, and the installation of cheap and easy-to-use probes on ships-of-opportunity, hold great promise.
Even with present technology, a description and an understanding of the spatial distribution of turbulent processes in the global ocean is achievable in the next decade. Our present conception of ocean dynamics is largely ignorant of processes with relatively short horizontal length scales (say 100 m to 50 km). Yet biological variability is concentrated on these short scales. It is the dynamics on these same scales that is parameterized by eddy-resolving circulation models. Further, in the coastal zones, cross-shelf exchanges are likely mediated by instabilities and topographic influences whose horizontal scales are much less than those of the well-studied alongshore flows. Exploring these largely unvisited scales is a new frontier for physical oceanography.
Several problems facing physical oceanography were identified at the meeting. These are: (1) large sea-going groups are retrenching and there is a consequent loss of technicians, engineers, and the hardware that these people maintain; (2) sustaining the funding of long time series observations is difficult; (3) physical oceanography is not visible to undergraduate mathematics, physics, and engineering majors, and so does not attract many graduate applicants from that population; (4) the organization of NSF physical ocean
Excerpted from The Future of Physical Oceanography: Report of the APROPOS Workshop. http://www.joss.ucar.edu/joss-psg/project/oce-workshop/apropos/report.html, 9/11/99. The APROPOS committee was co-chaired by William Young (Scripps Institution of Oceanography) and Thomas Royer (Old Dominion University). Other members of the steering committee included John Barth (Oregon State University), Eric Chassignet (University of Miami), James Ledwell (Woods Hole Oceanographic Institution), Susan Lozier (Duke University), Stephen Monismith (Stanford University), Peter Rhines (University of Washington), and Peter Schlosser (Lamont-Doherty Earth Observatory). William Young presented a summary of the APROPOS activity at the symposium.
ography makes it difficult to fund projects of intermediate size and this difficulty is compounded if the project is interdisciplinary.
Despite these problems, there was consensus that the National Science Foundation's core program is an invaluable asset of the field. The peer-review system maintains a balance between scientific rigor and responsiveness and ensures continuing support for innovative and fundamental science.
Challenging as it may be to make progress on any scientific problem, it is even more difficult to predict the future course of scientific progress. One might say that every important discovery in science is, almost by definition, unpredictable and so it is futile to guess at future triumphs. Indeed, it is worse than futile if these guesses are used to ''manage" the direction and content of science. It is our belief that basic research, independent of any practical concerns, is critical to the advance of science and the development of technology. Science is the most serendipitous of human enterprises and the ability of physical oceanography to solve problems of social concern depends on a healthy commitment of resources to basic research on fundamental scientific issues.
The economic benefits of understanding the role of the ocean in the climate system are enormous. And accumulating evidence of man-made climate change has brought these issues to the attention of the public. These concerns coincide with recent successes in long-term weather forecasting associated with El Niño, and with advances that enable detailed measurement of climate variables. (For instance, in the last ten years, the errors in surface heat fluxes obtained from moorings have been reduced by a factor of forty so that the present uncertainty is 5 Watts per square meter.) These factors imply that climate studies will be a significant path for future research in oceanography.
The development of long-term forecasting skill raises challenging scientific problems. These include: understanding and quantifying turbulent mixing, convection, and water-mass formation and destruction; the thermohaline circulation and its coupling to the wind-driven circulation; the generation, maintenance, and destruction of climatic anomalies; climatic oscillations and the extratropical coupling of the ocean and atmosphere on seasonal, decadal, and inter-decadal timescales; and the physics of exchange processes between the ocean and the atmosphere. All these problems are of fundamental scientific and practical importance.
Will there be substantial progress on these issues during the next decade? Many physical oceanographers have already begun an enthusiastic frontal assault under the banner of CLIVAR. It is likely that the economic issues that surround global change and climate prediction will motivate continued financial support from society. If people and money are what counts, then we have every reason to be optimistic.
The problem of global climate prediction is the most difficult that our field has encountered. Unlike equatorial oceanography and El Niño, there is not going to be a theory based on linear waveguide dynamics that decisively identifies timescales and cohesively binds oceanography and meteorology. Further, the decadal timescale of extratropical dynamics means that scientists see only a few realizations of the system within their own lifetime. This is bad for morale, but even worse, we cannot wait to gather enough data to reliably verify the different predictions of climate models. Could meteorologists have developed daily weather prediction models if these scientists saw only three or four independent realizations of the system in a lifetime? The only way around this statistical problem is to expand our data base and frame hypotheses about past climate change and ocean circulation using paleoceanographic studies. An important challenge is to test the dynamical consistency of these hypotheses.
The Hydrologic Cycle
An emerging theme, which is strongly related to climate, is the ocean's role in the hydrologic cycle. New satellite technologies promise to measure sea surface salinity and precipitation. These, coupled with improvements in the computation of evaporation via indirect methods, will improve our picture of the freshwater flux in the oceans. The freshwater sphere is an encompassing topic that spans oceanography, the atmospheric sciences, polar ice dynamics, and hydrology. Our knowledge of the oceanic freshwater source-sink distribution is far poorer than our knowledge of the source-sink distribution of heat. Yet salinity and temperature contend in their joint effect on the density of seawater and in their influence on the ocean circulation, and the climate system. Knowledge of freshwater input from continents, precipitation, and sea-ice is poor. Observational techniques addressing these issues (for example, the use of oxygen isotopes, and tritium/helium to diagnose freshwater sources) herald progress.
Coupled with improved estimates of the freshwater sources at the surface, will be an increased understanding of water-mass dynamics and transformations. We can look for advancement on such fundamental issues as the causes of the temperature-salinity relationship, thermocline maintenance, and interhemispheric water-mass exchanges.
Observing the Ocean
We will see explosive development of new observational tools, such as those used by the TOPEX/POSEIDON
satellite mission. Future developments in satellite oceanography promise more of the same at ever-increasing accuracy, coupled with the deployment of new satellite-borne instruments. Yet sea-truth is essential and we envisage in situ observations that will be made by an unprecedented class of autonomous instruments and probes. The ability to manipulate these tools in mid-mission is developing. While we are making enormous strides in sampling the global ocean better, we still have far to go for truly adequate spatial and temporal sampling, though the era of grossly undersampling the global ocean is dead.
A national effort to support sustained high-quality global observations over decades is needed. Measurements of air-sea fluxes of heat, fresh water, and gases, of surface and sub-surface temperature, salinity, and velocity, are all necessary to meet new scientific challenges and practical needs. Looking beyond the equatorial TOGA-TAO array, long-term subsurface measurements spanning the global ocean are required.
Given the rapid increase in Lagrangian measurements by drifting and profiling floats, and the parallel increase in geochemical tracer data, an intense approach to Lagrangian analysis of advection and diffusion is warranted; our existing base of theoretical tools and concepts is not worthy of the observations that we are about to receive.
Global and Regional Connections
Many emerging physical oceanographic issues concern connections between large-scale and small-scale motions; for example, the relation between small-scale turbulent mixing and the large-scale meridional overturning circulation. Analogous connections and interactions between scales are arising in issues of societal concern, often centered around the increasing recognition that many issues previously regarded as regional now require a global perspective. Anthropogenic pollutants have reached the open ocean and are known to be transported far from their sources. A better understanding is needed of small-scale processes and small-scale aqueous systems (estuaries, wetlands, coral reefs) and their impacts on global issues. For example, the growth of plankton populations, which affect carbon dioxide levels and thus may be important in global warming scenarios, is dependent on details of circulation at fronts, sea-ice, and mixed-layer boundaries.
In most coastal regions, the strongest persistent gradients in properties (for example, salinity, temperature, nutrients or suspended materials) are found in the cross-shelf direction. This is because cross-shelf flow is often inhibited by topography and because the coastal ocean is the contact zone between terrestrial influences, such as river runoffs, and oceanic influences characterized by nonlinear physical dynamics and oligotrophic biological conditions. Progress has certainly been made on some aspects of the flows that determine cross-shelf transports, especially those related to surface and bottom boundary layer processes. A good deal more has yet to be learned about exchanges that occur in the interior of the water column. The problem is difficult because it often appears that the processes that are relevant for the dominant alongshore flows do not apply to cross-shelf flows. For example, it is likely that instabilities and topographic influences may dominate the exchange process. The exchange itself needs to be understood if we are to address issues such as the control of biological productivity in the coastal ocean, or the removal of contaminants from the near-shore zone.
In addition to cross-shelf exchange processes themselves, there is the question of how the coastal ocean couples to its surroundings on both the landward and seaward sides. Estuarine processes are important for determining the quantity and quality of terrestrial materials that reach the open shelves. The oceanic setting, including eddies, filaments, and boundary currents, in turn determines how effectively coastal influences can spread offshore, or how the oceanic reservoir will affect shelf conditions. Consequently, the study of the continental shelf demands consideration of both offshore and near-shore (estuarine and surf zone) dynamics.
Inland Waters and Environmental Fluid Dynamics
Our understanding of inland waters, such as estuaries, wetlands, tide flats, and lakes, will be aided by the same observational and computational technologies that promise progress on the general circulation problem. This work will afford exciting opportunities for interdisciplinary research blending physical oceanography with biology, geochemistry, and ecology. Examples are tidal flushing through the root system of a wetland, and the physical oceanography of coral reefs.
Lakes can be useful analogs of the ocean, with wind and thermally driven circulations, developing coastal fronts, and topographically steered currents. Lakes are important as model ecosystems that are simpler and more accessible than ocean ecosystems. Significant progress can be foreseen in the coming decades in limnology, helped by the tools and ideas developed for the ocean.
The expertise of the physical oceanography community should make possible substantial advances in the understanding of all these shallow systems. Because of the major roles played by turbulence and complex topography, these systems pose impressive and fascinating challenges to physical oceanography.
Turbulent Mixing and Unexplored Scales
Past achievements in quantifying small-scale turbulent mixing in the main thermocline, coupled with exciting re
cent measurements in the deep ocean, suggest that a description and an understanding of the spatial distribution of turbulent mixing in the global ocean is achievable in the next decade. Unraveling the possible connections between the spatial and temporal distribution of mixing, the large-scale meridional overturning circulation, and climate variability are important aspects of this research.
Knowledge of the horizontal structure of the ocean on scales between the mesoscale (roughly 50 km) and the microscale (roughly less than 10 m) will be radically advanced and altered. The growing use of towed and autonomous vehicles, in combination with acoustic Doppler current profilers, will revolutionize our view of the ocean by exploring and mapping these almost unvisited scales throughout the global ocean. While this research is driven by interdisciplinary forces (biological processes and variability are active on these relatively small horizontal scales) it is also a new frontier for physical oceanography, and one in which even present technology enables ocean observers to obtain impressive data sets.
Numerical Modeling as an Integrative Tool
Large-scale numerical models of the ocean, and of the coupled ocean-atmosphere, are becoming the centerpiece of our science. This is not to say that numerical models dominate our science, but rather that results of theory and observational data are often cast into the form of numerical models. This happens either through data assimilation or through process-model explorations of theoretical ideas. Yet the fundamental difficulty of computer modeling remains: the ocean has, in its balanced circulation, energy-containing eddies of such small scale (less than 100 km) that explicit resolution of these dominant elements is marginally possible. Compounding this difficulty are the unbalanced, three-dimensional turbulent motions that are known to be important in select areas, such as the sites of open ocean convection.
We now have a well-acknowledged list of subregions of general circulation models that are greatly in need of improvement. These include: deep convection; boundary currents and benthic boundary layers; the representation of the dynamics and thermohaline variability of the upper mixed layer; fluxes across the air-sea interface; diapycnal mixing; and topographic effects. Progress in all of these areas is likely as our capacity for modeling smaller scale features increases, and as physically-based parameterizations are developed.