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Some Thoughts on Physical Oceanography in 2025

Ken Melville*


Projecting forward to 2025 is not so large a leap: 16 years, perhaps 40% of a typical post-doctoral working career, three Ph.D.s, a few El Niño-Southern Oscillation (ENSO) cycles and a few Intergovernmental Panel on Climate Change (IPCC) assessment reports. Looking back to 1993, could we have guessed where we would be today?

For physical oceanography, some of the technological advances that have revolutionized the field in the intervening period have turned out to be: radar altimetry from the TOPEX/Poseidon mission launched in 1992; profiling floats that became operational in the early 1990s and now constitute the 3000-float global Argo system; gliders that became operational almost a decade ago and now are about to be mass produced; computational power that has permitted ever more realistic global physical models while also permitting ever higher resolution for local process studies. All of the elements of these technological advances, and others one could cite, were in place in the early 1990s.

By and large it has been the physical oceanography (PO) community that has led the way in developing autonomous platforms for measuring the traditional variables in the ocean: currents, temperature and salinity. It is to be expected that extreme weather (e.g., hurricanes, Southern Ocean), climate variability, marine ecosystems and Navy needs will remain strong drivers of oceanography over the coming decades. As detailed comparisons of numerical models and observations move from the mesoscale

*

Scripps Institution of Oceanography, University of California, San Diego



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Some Thoughts on Physical Oceanography in 2025 Ken Melille* Projecting forward to 2025 is not so large a leap: 16 years, perhaps 40% of a typical post-doctoral working career, three Ph.D.s, a few El Niño- Southern Oscillation (ENSO) cycles and a few Intergovernmental Panel on Climate Change (IPCC) assessment reports. Looking back to 1993, could we have guessed where we would be today? For physical oceanography, some of the technological advances that have revolutionized the field in the intervening period have turned out to be: radar altimetry from the TOPEX/Poseidon mission launched in 1992; profiling floats that became operational in the early 1990s and now con- stitute the 3000-float global Argo system; gliders that became operational almost a decade ago and now are about to be mass produced; computa- tional power that has permitted ever more realistic global physical models while also permitting ever higher resolution for local process studies. All of the elements of these technological advances, and others one could cite, were in place in the early 1990s. By and large it has been the physical oceanography (PO) community that has led the way in developing autonomous platforms for measuring the traditional variables in the ocean: currents, temperature and salinity. It is to be expected that extreme weather (e.g., hurricanes, Southern Ocean), climate variability, marine ecosystems and Navy needs will remain strong drivers of oceanography over the coming decades. As detailed compari- sons of numerical models and observations move from the mesoscale * Scripps Institution of Oceanography, University of California, San Diego 22

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23 KEN MELVILLE to submesoscale to microscale, it is inevitable that the need for a better understanding of coupled physical, biological and chemical processes will be needed. Interpretation of chemical and biological variability in the ocean will require a foundation in physical oceanography. For example, ocean acidification and its impact on marine organisms is not just an issue for chemical and biological oceanographers, but also for physical oceanographers, since the air-sea fluxes of CO2, which are poorly understood, depend on physical processes in the lower atmo- sphere, the upper ocean and at the air-sea interface. Implicit in current parameterizations of gas transfer velocities based on just the wind speed is the assumption that the turbulence that supports the fluxes depends only on the wind speed. This assumption is untenable in non-stationary wave, current and turbulence fields. Since bubble-mediated gas trans- fer may be significant, acoustical oceanographers may also be expected to play a role in developing improved air-sea flux measurements and models. The inference to be drawn from this example, and there are others, is that the capability of platforms and instrumentation will have to be expanded to include small, low power, low maintenance chemical, bio- logical and acoustic sensors along with the payload and power to support them. Recent attempts have been made in this direction with bio-optical and carbon chemistry sensors on profiling floats or gliders, but much more needs to be done so that the Argo array and perhaps glider arrays can expand their global capability into these areas. With the success of the TOPEX/Poseidon mission, plans are now underway at NASA to develop higher resolution radar altimetry that will resolve sea surface height (SSH), ocean winds and wave measurements down to the submesoscale. While this would permit snapshots of subme- soscale processes, satellite altimetry is still limited by the O(10)-day repeat cycle, which may be much longer that the time over which these processes evolve. This and other aspects of orbital remote sensing highlight the need to supplement the global coverage of satellite remote sensing with sub- orbital or airborne remote sensing capabilities for submesoscale process studies over shorter timescales. Access by the oceanographic community to research aircraft is very limited, with few aircraft and funding a more explicit consideration than it is with getting access to UNOLS (University-National Oceanographic Laboratory System) vessels. For process studies, radar or Light Detec- tion and Ranging (LIDAR) altimetry and related measurements can be made from aircraft. For air-sea flux studies in the lowest layers of the atmosphere, manned flight at O(10) m above the surface is risky in all but the most benign conditions and this points to the need for Unmanned Aircraft Systems (UASs) to undertake such measurements. For example,

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2 OCEANOGRAPHY IN 2025 with the technical difficulties of deploying and maintaining air-sea flux moorings in the Southern Ocean, and with the need to provide areal as well as temporal coverage, UASs could play an important role there, as they are beginning to do in hurricane research. Ship-launched and recovered UASs would significantly enhance the capabilities of research vessels. Ship-based UASs have already been used for marine geomagnetic measurements. Another potentially important role for aircraft, whether manned or unmanned, is in the air-deployment of instruments. Traditionally, air- borne expendable bathythermographs (AXBTs) or Global Positioning System (GPS) dropsondes have been deployed, but smaller shorter-lived air-deployable profiling floats or gliders with physical, biological and chemical sensors would significantly complement the on-board remote sensing capabilities of the aircraft. In the field of air-sea interaction, we do not have a good understand- ing of surface-wave processes. However, we know that the sources of turbulence and mixing on both sides of the surface are dependent on the wave field. Increasingly, and not surprisingly, it is being found that air-sea fluxes of mass, momentum and energy depend on the wave field, effects that were hidden in the large scatter of earlier measurements. However, with our increasing capability to undertake field measurements that approach laboratory quality, and with the constraints that the basic conservation laws impose, there is reason for optimism about the progress to be made by 2025. It is also the case that the synergy between Large Eddy Simulation (LES) numerical models and measurements will propel the field forward at an accelerating pace. The results of this research will ultimately lead to an improved understanding of air-sea fluxes with this understanding finding its way into improved subgrid-scale closures for coupled atmosphere-ocean models, and climate models. Last year marked the 50th anniversary of the CO2 record (The Keel- ing Curve) at Moana Loa, and this year will mark the 60th anniversary of the California Cooperative Oceanic Fisheries Investigations (CalCOFI). The value of long continuous time series in climate research is no longer in dispute, but their maintenance over 50 years or more has from time to time required great tenacity and fortitude on the part of individuals and institutions to maintain the flow of funding in the face of competing priorities and budget shortfalls. If we are to understand climate variabil- ity, the time scale over which these observations need to be maintained stretches indefinitely into the future, and we need to find a better way of making sure it happens. We need planning and funding on the decadal and longer time scales to match those of the climate itself. While the capability of numerical models has expanded signifi- cantly in recent years, we must avoid the mistake of assuming that those

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25 KEN MELVILLE advances can be maintained without new and improved data to better understand the processes being simulated, and to test and improve the models. In a time of increasing pressure on research budgets, the tempta- tion to fund more PIs to do modeling rather than fewer PIs to do more expensive instrument development and field work will be great. This issue is also reflected in our educational programs, where more expen- sive lab classes have tended to decline as educational budgets have come under pressure. There is a larger educational question, and that is how do we attract and educate the coming generations of oceanographers so they have the motivation and skills to make the discoveries needed for us to better understand the ocean and its impact on society? The fresh postdoc in 2025 is about 11 years old now. How do we convince a middle schooler that our science is fun? How do we ensure that the high schooler and undergraduate see oceanography as a career option and ultimately get the science background needed for graduate school? And how in graduate school do we provide breadth across the subdisciplines of oceanography, while providing depth in specific areas? I do not think we will solve these problems in two days, but I do think we should begin to address them, both at the institutional level and as a community.