THE WOCE SYNTHESIS PHASE
TRANSITION TO FUTURE PROGRAMS
- Results from the Subduction Experiment (Jenkins)
- Comparing TOPEX/POSEIDON Data with XBT/XCTD Data (Roemmich)
- Is U.S. WOCE meeting its objectives? - Objective 5 (Jenkins)
- Pacific He/Tr Data (Jenkins)
U.S. WOCE Science Steering Committee - R. Fine, G. Johnson, C. Koblinsky, J. Marotzke, J. McWilliams, W. Nowlin (Chairman), B. Owens, S. Riser, D. Roemmich, L. Rothstein, P. Schlosser.
Other Attendees - P. Chapman (USWO), D. Goodrich (NOAA), J. Gould (IPO), C. Hill (USWO), W. Jenkins (WHOI), R. Lambert (NSF), E. Lindstrom (USWO), R. Lukas (U. Hawaii), S. Walters (USWO).
Nowlin welcomed members and visitors, particularly the invited speakers (W. Jenkins and R. Lukas).
Agenda - The agenda distributed prior to the meeting was adopted without changes (Appendix 1). There were no outstanding action items and no additional questions.
Chapman reviewed the status of the Indian Ocean field program. Clearance has been obtained for work within the Indian EEZ during cruise I1, but Indonesian clearance for work south west of Java remains uncertain. This may affect cruises I10 and I2. The U.S. WOCE Office (USWO) and representatives of the U.S. State Department continue to work on resolving this problem.
McWilliams asked about the status of XBT coverage. Roemmich and Lindstrom pointed out that with the end of the Tropical Ocean and Global Atmosphere (TOGA) Program, much of the funding in other countries has disappeared. It is probable that the present XBT coverage is as large as it will be for the foreseeable future.
Nowlin reviewed the background to the Atlantic Circulation and Climate Experiment (ACCE) proposal preparation and submission of August 1995. A joint Science Steering Committee (SSC)/Atlantic Climate Change Program Science Working Group review panel met September 11, 1995 to review all ACCE proposals submitted for funding. Nowlin reviewed problems with the proposals' construction; these problems were partly due to time constraints. (Still needed is better integration of the field programs with the modeling proposals.) He also stated the ACCE could be considered a first step in a transition toward the Climate Variability and Predictability Programme (CLIVAR).
The meeting attendees discussed the process leading to the submission of ACCE proposals. There was general agreement that clear, firm guidance had not been adequately provided to potential proposers-there was no strawman plan prior to the initial community meeting in Atlanta nor were budget limits or priorities assigned by the Atlantic Ocean Science Steering Committee. The result was considerable hostility and proposals out of proportion to the funds available. There was discussion of the ACCE review and funding process relative to the current budgetary restrictions.
Lindstrom reviewed the status of the WOCE data system and gave a quick tour of the World-Wide Web (WWW) pages of several of the WOCE Data Assembly Centers (DACs) and Special Analysis Centers (SACs). The "Tour of the WOCE Data System" can be found at http://www.acess.digex.net/~woce/tour.html
The SSC discussed several other data-related problems. The recent resignation of Chuck Corry has left a vacancy at the WOCE Hydrographic Program Office (WHPO). Possible options for accomplishing the work needed from the WHPO were discussed; the funding agencies are to follow up on options with Terry Joyce (WHPO Director).
The National Oceanic and Atmospheric Administration (NOAA) is suffering budget cutbacks. As a result, the Oregon State University current meter DAC and the Scripps Institution of Oceanography upper ocean thermal DAC have lost funding. Funding of the sea level DAC in Hawaii is anticipated to be an issue in 1996. Changes also are expected at the National Oceanographic Data Center (NODC). Goodrich stated that it is not NOAA's intention to permanently close these centers, and funding should be restored at some level in the 1996 budget.
Another issue is the apparent cessation in supply of delayed-mode data from the Voluntary Observing Ships (VOS) XBT program. Almost no delayed-mode data have been forwarded to the DAC in Brest since 1992. Members of the Data Products Committee and the WOCE/CLIVAR XBT panel have been requested to follow up on national submissions to the archive. The WOCE Data Information Unit (DIU) also has been actively pursuing missing data.
Finally, there remains the problem of the intended abandonment of the carbon dioxide survey by the Department of Energy (DOE). The latest information indicates that DOE managers intend to stop funding for the program by the end of 1995. The SSC and WOCE International Project Office (IPO) were unclear if this would allow continued sampling during the remaining Indian Ocean WOCE cruises. Many scientists have written to DOE deploring this move.
The need for the continued existence of the DIU was raised. Because principal investigators are not to releasing data to the DACs on the originally agreed upon time schedule, the SSC agreed that the DIU remains an essential part of the data system. It is the only central source of information regarding data for the WOCE program. Additionally, the SSC agreed that although the WOCE DACs and SACs have limited life spans, the methodology behind them should remain in place for future programs as part of the International Oceanographic Data Exchange archive system.
Gould reminded SSC members that an international WOCE bibliography is now available via the WWW from the DIU. Also, P. Saunders has started an electronic abstract service for WOCE publications. (Action Items 1-2)
Koblinsky discussed the status selected ocean scientific satellite programs. He reported that TOPEX/POSEIDON continues to be very stable and that the data are widely used. The program is assured funding through 1998. Disagreements in sea surface height of ~10-20 cm remain between TOPEX/POSEIDON altimetry and hydrography in the latitude band 5°S-20°N; this is probably because of errors in the geoid. In contrast, data from 30°S agree within 5 cm.
Because of improved sea surface height measurements, Nerem has calculated an annual global rise in sea level of 5.8 mm/yr, which agrees well with changes in sea surface temperature (SST) from the National Meteorological Center (NMC) analyses.
Other satellite programs are faring less well. The NSCATT launch has been rescheduled for fall 1996. A gravity mission that would improve knowledge of the geoid is still not regarded as a top priority by space agencies, but various designs are being investigated in the U.S. and Europe. Also, the TOPEX/POSEIDON Follow On remains in limbo; there are no funds presently budgeted for a U.S. follow-on mission. Nevertheless, NASA plans to release a research announcement for both the TOPEX/POSEIDON extended mission (1996-1998) science team and the EOS Radar Altimeter science team in early 1996.
Koblinsky reported that Bruce Douglas has been appointed the new Physical Oceanography Program Director within the National Aeronautics and Space Administration (NASA). Donna Wells Blake will remain with the program until early 1996. However, the Ocean Data Assimilation Program is currently unfunded, and it appears that ocean research is being downgraded within NASA. (Action Item 3)
Discussion of the WOCE synthesis phase began with Marotzke presenting his thoughts on possible strategies for the modeling component. He cited from "The Status, Achievement and Prospects for WOCE" (IPO, 1995) that ". . .the assembly of the individual [observational] contributions into basin-wide, and ultimately global, dynamically consistent, data sets is the major challenge for WOCE." In this context, "dynamically consistent" means providing an estimated state of the ocean compatible with the conservation equations for mass, salt, heat, momentum, and the equation of state. Additionally, he posed several questions that must be integral parts of every synthesis effort. For example: What model should be used for each estimate? Are current models realistic enough? If the model does not fit the observations, which do you reject?
He suggested that a General Circulation Model (GCM) be used to synthesize all data types. It should contain an inversion to allow proper dynamic interpolation between the data points, otherwise there will be a tendency for the model to disagree with known physics. However, the present resolution of GCM inversions is too low (in the order of 2°) to provide good estimates. Additionally, hydrographic data are particularly difficult to accommodate because of their non-linear dynamics. For example, it is difficult to relate observed changes in a model to incorrect surface fluxes applied (say) 20 years earlier and 4000 km upstream. At present, there are few ongoing efforts in this field.
Incorporating altimetry into GCMs is apparently an easier task, and several groups are working on the problem. Several different approaches (e.g., optimal interpolation, Green's functions, reduced Kalman filter) are being used with considerable success. Differences between model data and observations are about 5-6 cm, except where lack of knowledge of the geoid is a problem.
There seems to be relatively little study of how to incorporate other data types, except in the tropics. Scientists at NMC are using XBT data, but their estimation technique is less formal than the GCM with inversion proposed above.
Marotzke ended with some suggestions for future work. These included:
Scientifically, this means building better models that can incorporate hydrography through adjoints and provide proper accounts of both model and data errors.
Nowlin gave an overview of the letters sent to federal funding agencies regarding the need for some institutional support for synthesis and the responses received. It was suggested that the next SSC meeting would be a good time to present high-ranking members of the agencies with a review of WOCE accomplishments and discuss the matter of an assimilation center. (Action Item 4)
Nowlin reviewed the latest version of the draft U.S. WOCE Synthesis document. The meeting attendees considered discussion topics to be addressed in the synthesis planing meeting scheduled for October 3-5, 1995. It was agreed that topics should include the additional needs for synthetic data products and synthesis through models (Bennett to lead discussion) and the development and comparison of better forward-looking models (McWilliams to lead discussion).
It also was agreed that the final version of the synthesis document should make it possible to identify what is needed to complete the WOCE synthesis phase in no more than five years. Examples include combining all tracer sets from different laboratories, producing basic data sets for individual ocean basins, etc.
As the WOCE field phase winds down, scientific interest (and funding) in many countries is turning toward follow-on activities. One important global program is CLIVAR, a study of climate variability and predictability at scales of seasonal and longer. Given the obvious link between WOCE and CLIVAR, the SSC requested updates on the progress of the CLIVAR GOALS (seasonal-to-interannual climate prediction of the Global Ocean Atmosphere Land System) and CLIVAR DecCen (decadal-to-centennial climate variability and predictability) components..
The objectives of GOALS are:
Lukas reviewed the status of the GOALS program. He said that a CLIVAR Science Steering Group has been established along with a Project Office in Hamburg. The CLIVAR Science Plan has been published, and a detailed Implementation Plan is being developed. Additionally, panels have been set up to cover modeling on a seasonal-to-interannual basis (NEG-1), modeling on a decadal-to-centennial basis (NEG-2), the synthesis phase of the TOGA-COARE (Coupled Ocean Atmosphere Response Experiment) program, the Asian-Australian monsoon, and the upper ocean (i.e., CLIVAR Upper Ocean Panel). Lukas provided the SSC with the terms of reference for the CLIVAR Upper Ocean Panel. Lukas is a member of the monsoon panel, which will be developing a strategy for long-term monitoring, process studies, and modeling.
Within the U.S., there is a National Research Council (NRC) GOALS Advisory Panel and a published document ( i.e., Global Ocean-Atmosphere-Land System for Prediction of Seasonal-to-Interannual Climate, NRC 1994) that addresses long-term monitoring, process studies, and modeling activities supporting the prediction of seasonal-to-interannual climate variability. Additionally, the document "Ocean-Atmosphere Observations Supporting Short-Term Climate Predictions" (NRC, 1994) addresses how to proceed with observations following TOGA. The main items of concern in this document are the need to maintain the present TOGA observing system and its capability for El Nino/Southern Oscillation (ENSO) prediction; to expand coverage from the Pacific to the global tropics, including land; and to expand coverage to mid-latitudes.
While GOALS concentrates on shorter-term predictions, there is a continuum between GOALS and DecCen. There is already some intention to coordinate the two programs (e.g., studies of ENSO decadal modulations, the Atlantic SST dipole, sea ice). The need remains for continued efforts in calibration and continuity to ensure that measurements taken for GOALS also will be useful for DecCen research. Where possible and as justified in the context of GOALS objectives, GOALS will maintain ongoing time series with the ultimate aim of linking them to remote sensing capabilities to obtain global coverage. There also will be process studies (e.g., concerning the Asian-Australian monsoon and Pan-American climate studies).
Some gaps still remain in both the international and U.S. GOALS efforts. These include the need for implementation plans and sufficient resources. In the U.S., the GOALS program will not have a detailed implementation plan; however, it will have a documented implementation strategy including a set of guiding principles that will allow reviewers and program managers to prioritize among proposals directed at GOALS objectives.
Additionally, needs for an Intergovernmental CLIVAR Board and a U.S. Interagency GOALS Project Office remain to be considered. There are also issues regarding observational priorities given restrictive budgets. For example, the XBT program begun under TOGA and continued by WOCE is coming under some pressure.
At the November 1994 SSC meeting a sub-committee (chaired by Roemmich) was established to consider the transition from WOCE to CLIVAR. The sub-committee met at the IAPSO meeting in Hawaii in August 1995 and produced a set of recommendations for determining WOCE measurements likely to be proposed for future CLIVAR DecCen funding. These are based loosely on the recommendations used in the ACCE program. These were agreed to in principle by the SSC. A copy of the recommendations is given in Appendix 2. Lambert stated that the National Science Foundation (NSF) is already able to consider certain proposals for CLIVAR projects.
Gould said that he constructed a list of WOCE observations that could possibly be handed over to CLIVAR. It was agreed that he would merge his list with the Roemmich panel recommendations. (Action Items 5-6)
Fine reviewed progress toward the DecCen component of CLIVAR. The scientific objectives of DecCen are:
Two meetings of the U.S. NRC DecCen Panel have taken place, and Fine and McCartney provided oceanographic input. The key science issue was the need to "design a comprehensive system to predict changes to climate mean and variability on decade-to century time scales."
Internationally, three workshops will be conducted within the next 15 months to consider the effects on decadal-to-centennial variability of surface forcing, ocean circulation, and water mass transformation. Convenors will be Sarachik, Gordon, and Schott respectively. It was stressed that individuals are welcome to submit recommendations to the DecCen panel on topics to be covered during these workshops.
Nowlin outlined the structure of the Global Ocean Observing System (GOOS) and the Global Climate Observing System (GCOS) and their interrelationships.
Within the U.S., the federal funding agencies have established an Ad Hoc Interagency GOOS Working Group (chaired by Mel Briscoe) to coordinate and report the U.S. contribution to GOOS. Additionally, the NRC is setting up a GOOS committee under the Ocean Studies Board with input from the Board on Sustainable Development, the Board on Atmospheric Sciences and Climate, and the Marine Board. Despite this activity, few new funds have yet been allocated to ocean work. By contrast, new money is being made available through the World Meteorological Organization coordination for atmospheric observations to support GCOS.
Nowlin stated that certain ongoing programs (e.g., Navy/NOAA ice center products, TAO array) could be considered operational GOOS programs. However, many of these are now funded with research rather than operational funds. Despite slow progress within the U.S., however, he remained optimistic about future support for GOOS. Other countries are working to determine what is needed from GOOS by various industries (e.g., shipping industry, offshore industries, marine insurers). Such activities are beginning in the U.S. and should force the U.S. government to provide some support.
Lambert reported that the National Science Foundation (NSF) funding in FY96 may be level or have a minor (few percent) decrease. Goodrich reported that given the present uncertainty regarding the future of NOAA, no firm statement could be made regarding future support. It is unlikely that the federal budget process for FY96 will be decided before Thanksgiving.
Hill reported that U.S. WOCE was featured in May 1995 on a program titled "Prime Time Texas." This program was produced by the ABC affiliate in Dallas and aired in eight Texas cities. The WOCE clip focused on how improved knowledge of oceanography may assist in better climate and weather prediction. Hill is now working with a producer out of Washington to include WOCE in a Discovery Channel feature titled "Understanding Oceans." The proposed production date is early 1996.
Gould reported that IPO has been approached by the Canadians for help with an educational video on WOCE. He stated that the World Climate Research Programme (WCRP) can provide some funding if it is for international release.
Four science talks were presented during the meeting. Abstracts are given below.
Jenkins began by reviewing the goals of the Subduction Experiment, emphasizing his particular involvement. The goals were to:
Methods/observations used in the experiment follow:
Initial attempts to compare ECMWF gridded data, climatological data and observations from the moorings showed good agreement between the first two. The subtropical maximum in wind stress curl was more widespread and complex than in the climatology. Comparison of near surface current meter data with observed meteorological forcing was consistent with Ekman dynamics, but simple one-dimensional modeling was unable to replicate the observed evolution of the mixed-layer, indicating the importance of lateral advective processes.
Subduction was observed in situ on several cruises. This was shown both from the CTD isotherm development and from changes in the cycling depths of the bobber floats. The bobber floats were set to cycle between two isotherms. As the depth of the isotherms deepened during subduction, so the bobbers cycled to deeper depths. A comparison of measured vertical velocities at 150 and 280 m agreed well with calculations using the average CTD climatology of the area. Potential vorticity apparently increased downstream for the bobber floats, but this may have resulted from the bobbers tracking isotherms, not isopycnal surfaces. Since there are significant thermohaline gradients along isopycnal surfaces, the two types of surfaces do not coincide. Simple calculations suggest that the bulk of observed potential vorticity changes observed may be explained by this, but further analysis is required.
Jenkins' talk concentrated on results from the tracer studies. He pointed out that ^3H/^3He were good tracers for this experiment because they provided both direct visualization of subduction and linkage to average rates of water mass turnover. Data from the Subduction Experiment showed no direct ventilation on the 26.6 sigma-theta surface, although ventilation reached the 26.4 surface. The apparent ventilation age on the shallower surface increased and deepened from northeast to southwest across the area; this was different on the deeper surface. Because the bobber floats follow isotherms rather than isopycnals, they all ended up in water "older" than predicted. (For example, if the floats were released onto the two-year-old isochronic surface, two years later they were on a surface downstream more than four years old.) The deviation generally was equivalent to about 0.25° in temperature and six to nine months in apparent age. When this difference is accounted for, the two methods (bobber floats vs. tritium-helium age gradients) agree within uncertainties. Using the tritium-helium age, we can measure the horizontal velocity field.
Another use for the ^3H/^3He tracer couple is to determine directly the vertical velocity. This comes from the age gradient with respect to depth on an isopycnal. Comparison of the vertical velocity as a function of depth shows that it varies in a way consistent with conservation of potential vorticity (beta v = f dw/dz). One also can estimate isopycnal subduction rates from the vertical age gradient. Subduction rates for the isopycnals that outcrop locally in the winter are about 35 m/y. This is somewhat lower than older climatological estimates but consistent with both the ECMWF estimates and the age-estimated vertical velocity curve extrapolated to the surface. Thus, the tracer and ECMWF data point to slightly lower rates of Ekman pumping than previously thought. Although this may be a result of secular changes in Ekman pumping rates, the fact that the tracers integrate over many years suggests a long-term variation.
Data from near the Azores front showed sharp age gradients. These data give values of the cross-stream horizontal divergence of about 7000 m^2/s. This assumes that the only important process is isopycnal movement (although diapycnal mixing is undoubtedly important also). Local rates of subduction were about 35 m/yr, based on a multi-year average with approximately a decadal time scale, although rates deeper in the water column tend to increase. This rate, combined with the age patterns observed on the deeper isopycnals, suggests that frontal mixing processes are important for ventilation (apparent subduction?) of surfaces deeper than sigma-theta 26.6.
Roemmich posed the question "How well can you predict sub-surface temperature structure and hence geostrophic velocity, transport, and heat flux from TOPEX/POSEIDON measurements of sea surface height?" Most of the change in sea surface height in the tropical and subtropical Pacific is due to vertically coherent changes in the temperature of the upper 800 m of the water column. While this depth, the maximum reached by XBTs, is not an ideal level of zero motion, most of the stratification and shear occurs above it.
Using the combined XBT/XCTD lines PX10, PX37, and PX44 across the North Pacific at about 24°N between Taiwan and San Francisco, he showed that the warmest layers move northward (dominated by Ekman transport) while the cooler layers (below about 24°C) move southward in accordance with geostrophy. The heat transport estimated from eight high-resolution transects summed in 1° bins was about 0.75 PW.
The same eight transects showed very good agreement between the 0-800 m dynamic height estimates from geostrophy and TOPEX/POSEIDON data. The rms difference was about 5 cm (greater near the western boundary and less in the interior), and fluctuations in the two data sets were highly coherent. The TOPEX/POSEIDON and XBT/XCTD data sets indicated large changes in the basin-integrated surface transport of up to 8 Sv per 100 m depth over the same two-year period (November 1992 to November 1994). Vertically integrated transport (0-800 m) varied by more than 10 Sv, and estimated variability in meridional heat flux was large (about 0.5 PW).
In conclusion, the time-varying fields in the ocean have large effects on calculated fluxes. Therefore, relying on one-time survey data may not lead to well-constrained models (as Roemmich showed previously in the South Pacific). Variability occurred in the Kuroshio, in its tight recirculation, and in circulation components on large spatial scales. The powerful combination of high resolution XBT/XCTD data and TOPEX/POSEIDON altimetry provides basin-wide snapshots revealing the detailed evolution of geostrophic velocity and transport fields.
U.S. WOCE Scientific Objective 5: "To obtain quantitative estimates of the large-scale exchange of buoyancy and chemical constituents between the upper boundary layer and the ocean interior, by adequately describing the properties of the surface layer, including its horizontal mass transport and divergence."
Jenkins pointed out that the main mechanisms for this objective are the wind-driven and thermohaline circulation and subduction, water mass production, modification, and destruction. To understand these, we must quantify Ekman transport/divergence, determine surface heat/buoyancy fluxes, and estimate ventilation rates and mechanisms. In many respects, the issue of determining surface flux and flux divergence has been focused on by other groups. Therefore, Jenkins concentrated his remarks to the role of tracers in elucidating these mechanisms. Tracers can provide information on ventilation, circulation, and mixing as well as integrate over characteristic time scales. However, tracer data are colored and limited by boundary conditions, time scales, and data quality and quantity. Also, the models used with tracer data provide ambiguous answers. However, tracer data ultimately will be used in large-scale models constrained by other data (e.g., float trajectories).
Using multiple tracer date sets can improve matters since different tracers behave differently in the ocean. They also provide information on a variety of boundary conditions and time histories. Four parallel streams of work are being conducted in WOCE:
|Stream 1||Defining the large-scale tracer distributions.|
|Stream 2||Using tracers to improve parameterization of unresolved processes in large-scale models.|
|Stream 3||Refining knowledge of their boundary conditions and time histories.|
|Stream 4||Improving measurement procedures and data quality.|
Jenkins illustrated the improvements presently occurring with reference to three specific examples: improvements to ^3H source functions, ventilation "fluxes" and mixing, and a novel use of tracers.
1. Improving ^3H Source Functions
^3H is a dye tracer that was injected in a pulse around 1965 via precipitation and vapor exchange, largely in the northern hemisphere. It is radioactive, decaying to the stable isotope ^3He. ^3He is lost to the atmosphere; therefore, its concentration increases down the water column from almost zero levels at the surface. There is also a second (volcanic) source of ^3He in the deep ocean, and the two components can be separated. Tritiugenic ^3He can be used to determine the time since a water mass was last at the surface, with useful time scales from a few months to about 20 years.
In the North Atlantic ^3H inputs can be broken down into vapor exchange (mainly in 1965), precipitation, runoff, and inflow from the southern hemisphere and Arctic. Since 1970, the dominant input has been from the Arctic, where the tritium "spike" was held up in the fresh water component. This can be seen in the tritium rich East and West Greenland and Labrador Currents. There is strong inverse correlation between ^3H and salinity in the surface waters of the Atlantic subarctic.
Refinements in our ability to quantitatively describe the delivery of tritium to the oceans should significantly improve our ability to use this tracer in large-scale ocean models as a diagnostic of large-scale thermohaline circulation and ventilation.
2. Ventilation Fluxes and Mixing
Because ^3H distributions differ from chlorofluorocarbon distributions, we can use the data qualitatively to show how much ventilation has taken place in the North Atlantic since 1965. The southward penetration of tritium in the Atlantic basin gives a picture of the flux of ventilated water into the global thermohaline circulation.
Data from a time series of ^3H and ^3He at Bermuda (from the late 1960s to the late 1980s) show the staggered arrival of the "tritium front" at intermediate depths and deeper in the water column. Also, one sees a downward movement of the bomb-tritium maximum into the main thermocline at a rate of 18 m/yr. This is a direct measure of vertical motion in the thermocline. Summing the two tracers yields a stable, conservative dye-like tracer that also appears to move downward into the thermocline at 18 m/yr. More importantly, the amplitude of this tracer maximum is attenuated by more than a factor of two in 15 years. This is not simply vertical mixing, as the vertically integrated inventory appears comparably diminished.
Such time-dependent variability is not restricted to Bermuda. Examination of the entire Transient Tracers in the Ocean/North Atlantic Tracer ^3H/^3He data set for 1981 to 1983 shows that no water within the subtropical gyre main thermocline has remained unmixed between 1964 (bomb-tritium transient) and 1981. The ^3H/^3He relationships indicate that there has been at least a three-fold dilution of the tritium transient since that time. Therefore, mixing is an important and perhaps dominant process in determining the thermocline inventories of tracers on those time scales.
One can compute a "ventilation flux" by dividing the volume of a water mass by its tracer age. Conceptually, this is some measure of the flux of surface (zero age) water required to ventilate this water mass. This flux is not advective but rather the sum of the effects of convection, advection, subduction, and mixing processes. Doing such a calculation for the North Atlantic reveals a ventilation flux as a function of density that, when integrated, yields an effective ventilation flux of order 100 to 200 Sv. Since this is much larger than the meridional thermohaline cell, it suggests that water mass transformation and mixing processes within the North Atlantic basin are certainly vigorous and need accounting for if we are to model the oceans' role in climate.
3. Novel Use of Tracers
It is important to study the "return side" of the oceanic overturning cell. An example is the tropics, where wind driven upwelling brings carbon dioxide and nutrient laden water to the surface. ^3He is a useful tool for quantifying this process. In this study the tracer's boundary conditions characterize its utility in studying oceanic phenomena. Unlike (for example) freons, which enter the ocean from the surface, ^3He is produced within the water column and hence highlights the return path of waters from the thermocline.
In the tropical Pacific, there is a midwater maximum in ^3H (produced by in situ tritium decay), which is upwelled toward the equator across isopycnals. It is clearly visible in the WOCE meridional sections that cross the equator (e.g., P16C and P17C) and a zonal section at 10°N. Examining the WOCE Pacific ^3He data, one sees a signature of this upwelling as a small but significant excess of ^3He in the mixed layer. The excess is larger in the tropics and increases to greatest values in the east. It implies a loss of ^3He from the mixed layer to the atmosphere by gas exchange, which must be balanced by upward transport of ^3He from below. From known gas exchange coefficients we can calculate the upward flux. When taken with subsurface values of ^3He, the flux leads to an upwelling rate of order 65-80 Sv for the tropics between 10°N and 10°S. As with the ventilation flux calculations discussed for the Atlantic, this ^3He flux is the result of both mixing and advection in the tropics. Therefore, we must consider the "net effective upwelling rate" when assessing nutrient budgets (and hence biological productivity) and carbon dioxide fluxes for the tropics.
Summary - WOCE Objective 5 is aimed at assessing the exchange fluxes between the ocean surface layer and the subsurface interior. Tracer measurements are uniquely positioned to do this, although colored by their boundary conditions and nature. The point to note is that processes other than direct advection may play an important, if not dominant, role in the transport of material by the oceans. The information obtained from tracers likely will be best used in the context of sophisticated numerical models that are only now being developed. In the meantime, the simple discussions presented above indicate what may be possible in the future.
Jenkins gave an overview of the helium/tritium results from WOCE sections in the Pacific. All data are being made available on the WWW (http://kopernik.whoi.edu/wpac/wpac. html) as analyses are finished. It is anticipated that all WOCE Pacific helium/tritium data will be completed and made available by mid-to-late 1996. Jenkins showed data from several vertical sections and across several sigma-theta levels.
Sections of both tracers show a strong front at the equator because the source function is situated mainly in the northern hemisphere. The different production rates of ^3He from tritium decay and hydrothermal input (50 atoms/cm^2/s compared with 5 atoms/cm^2/s respectively), combined with the relatively weak interaction between abyssal and thermocline waters, suggest that it should be possible to separate the two components.The deep, primordial ^3He plumes are large compared to the shallow tritiugenic ^3He tongues because the abyssal water has integrated the primordial flux for many centuries. Separating the two maxima is a layer of low ^3He (low tritium intermediate water). Jenkins presented a simple scheme using dissolved silica as an indicator of upward mixing of deep waters. The scheme is predicated on the similarity of deep ^3He and silica distributions (between 1500 and 2500 m the ratio of 3He/silica varies by only +/-25 percent), and assumes in situ remineralization is not dominant.
Using the corrected ^3He distributions, it is possible to calculate the helium/tritium age, which can then be used to estimate advection and mixing rates as well as rates of in situ oxygen consumption.
Schlosser described the status of the upcoming Ewing Symposium on Tracers in Oceanography scheduled at the Biosphere II site in Arizona October 16-20, 1995. He stated that the meeting, which will concentrate on the past, present and future use of tracers in oceanography, has attracted widespread interest with more than 70 attendees. Included in the meeting will be sessions on global and regional circulation, integrating tracers into models, and the production of integrated data sets. The proceedings will be published.
Gould discussed the proposal from the IPO for a series of WOCE-sponsored workshops. The idea is to bring data originators and their data together with modelers to facilitate the creation of combined data sets and prepare the ground for joint publications. These meetings should be different from usual conferences since much of the work will be performed using computer terminals.
The following workshops have been proposed:
|October 1995||Meeting on Air-sea Fluxes and Oceanic Fluxes (at ECMWF) sponsored by WCRP.|
|June 1996||South Atlantic workshop (Brest). [Subsequent to the meeting, the French announced that 1997 would be better for this meeting.]|
|August 1996||Pacific workshop to be conducted in the western U.S. Talley will chair the international workshop steering committee and has passed a list of potential committee members to the SSG for approval. USWO will assist as needed with site selection, arrangements and logistics.|
|Late 1997||Southern Ocean workshop (Hobart). [This may be moved to 1998.]|
|Undecided||Indian Ocean workshop. U.S. to take the lead in planning and logistics.|
|Undecided||North Atlantic workshop.|
The SSC endorsed the idea of a series of such workshops and stressed the importance of inviting modelers as well as observationalists. U.S. action will focus on the Pacific and Indian Ocean workshops.
Additionally, Gould reported that the Canadians have offered to host a WOCE Conference in Halifax in May 1998. The SSC remains unclear as to the need for such a conference, given the plethora of basin meetings planned and the need to continue WOCE momentum through the synthesis phase. (Action Items 7-8)
It was agreed to have the next SSC meeting in Washington, D.C. at the new NSF building in April 1996. The USWO will invite high-level attendees from the federal funding agencies to attend (Action Item 9).
Tuesday - September 12, 1995
|8:30||Welcome (W. Nowlin)
Local Arrangements (S. Riser)
|8:45||Update on Action Items (Document 1 - P. Chapman)
Update on Indian Ocean Progress (Document 2 - P. Chapman)
|9:00||Science Talk: Results from the Subduction Experiment (W. Jenkins)|
|10:15||Transition to Future CLIVAR
Progress with CLIVAR DecCen (R. Fine)
Progress with CLIVAR GOALS (R. Lukas)
Progress/Plans of SSC WOCE-CLIVAR Transition Panel (Document 3 - D. Roemmich)
|12:00||Lunch (Served at APL)|
|1:00||Update on Atlantic Ocean Planning and Proposals (P. Schlosser, J. McWilliams) Likely Funding (R. Lambert, D. Goodrich)|
|3:15||Update on U.S. WOCE Attainment of Objective 5 (W. Jenkins)|
Wednesday - September 13, 1995
|8:30||Science Talk: Pacific He/Tr Data (W. Jenkins)|
|9:00||Data Management (Document 4)
|10:30||Transition to Future Programs - GCOS/GOOS (W. Nowlin)|
|12:15||Lunch (On Your Own)|
|1:45||TOPEX and High Resolution XBT/XCTD Transport Estimates (D. Roemmich)|
|2:05||Return to former topics as required|
|2:30||Satellite Update (Document 5 - C. Koblinsky)|
|3:00||Synthesis (Documents 6 & 7)
|4:30||Agency Matters (Agency Representatives)|
Thursday - September 14, 1995
|8:30||Future of U.S. WOCE (Document 8 & 9)
|10:00||Return to former topics as required
Date and Place of Next Meeting
D. Roemmich, R. Fine, L. Rothstein, E. Itsweire (representing NSF), and M. McCartney (representing CLIVAR DecCen)
The field phase of WOCE is scheduled to end in 1997. Data analysis, synthesis and modeling activities will continue for several additional years. Although a Draft Science Plan for CLIVAR has been circulated, it may be years before an Implementation Plan is produced and put into effect. Hence, unless the CLIVAR startup is accelerated, a critical gap will occur between the end of the WOCE field program and the availability of support for CLIVAR field activities. It is that gap which the Transition Panel regards as the most critical and immediate issue on its agenda. We are not excluding modeling activities during the transition period. But we note that with the synthesis phase of WOCE continuing beyond the field phase, the need to define the hand-off of modeling problems is less urgent. We also do not consider infrastructure here. There may be need on a case-by-case basis for support of data infrastructure if justified by early initiation of CLIVAR field work.
A variety of WOCE measurements will be continued under CLIVAR and/or the operational climate programs GOOS and GCOS. The boundary between these programs is vague, and to avoid making that an issue here, we will consider all measurements which are critical to the basic research objectives of CLIVAR to be part of CLIVAR. The possible hiatus in sampling between the end of WOCE and the beginning of CLIVAR poses serious problems. Not only would this create scientifically damaging gaps in time series measurements, but it also would result in the loss of technical and logistical capabilities needed to restart the programs and to carry out the measurements in support of CLIVAR. Technical capabilities that have been painstakingly ramped up during WOCE would be lost. It is vital to bridge the gap by carrying on with measurements which are strong candidates to be part of CLIVAR.
The aim of this report is to urge the accelerated startup of CLIVAR to allow a seamless WOCE-to-CLIVAR transition and to outline a rationale for identifying specific WOCE measurements which should receive highest priority for continuation during the interim period.
Measurements during the WOCE-to-CLIVAR transition may consist of:
Measurements receiving high priority should be those which both fit one of the categories above and in addition are directed toward the scientific objectives of CLIVAR. Among CLIVAR four overall objectives (CLIVAR Science Plan, p. 7), the one which most closely follows WOCE science is the first:
To describe and understand the physical processes responsible for climate variability and predictability on seasonal, interannual, decadal, and centennial time scales through the collection and analysis of observations and the development and application of models of the coupled climate system, in cooperation with other relevant climate research and observing programs.
We view a part of the Atlantic Circulation and Climate Experiment (ACCE) as an appropriate model to be generalized for the WOCE-to-CLIVAR transition. ACCE is a merged program combining WOCE Atlantic and NOAA Atlantic Climate Change Program elements. While WOCE in the Pacific and Indian Oceans focused on the basin-scale one-time survey, the focus in the ACCE is on observing variability in the meridional overturning circulation, and linking this variability to decadal-scale changes in observed property distributions at the sea surface and at depth, and to air-sea fluxes and feedback mechanisms. In generalizing the ACCE to global scale, it may also be desirable to recognize explicitly that there are climatically critical elements of ocean circulation that cannot be identified with meridional overturning circulations (MOCs). The following objectives, essentially a generalization of ACCE goals, are suggested as defining the WOCE follow-on in CLIVAR:
WOCE measurements, as listed above, should receive high priority for continuation if they address these objectives in a direct and practical fashion. The CLIVAR Science Plan may be of further assistance in the identification process, in its listing of CLIVAR DecCen measurements under the headings of Process Studies (p 123-124) and Elements of a Monitoring Program (p 125-127). Implementation of the latter is thought to rest mainly with GOOS and GCOS. A synopsis of the (WOCE-related) observations listed in the CLIVAR Science Plan follows. A few of these have been reworded for clarity. It may be possible to rewrite this list with fewer items by eliminating some redundancy; here we have kept with the listing in the Plan.
To General Index
To WOCE Home Page (email@example.com)