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OVERVIEW AND PRINCIPAL RECOMMENDATIONS 5 1 Overview and Principal Recommendations TERMS OF REFERENCE The Panel on Near-Tenn Development of Operational Ocean Observa- tions was given a charge to: 1. Develop a design for a .global coupled ocean-atmosphere observa- tional system required for climate prediction in the post-Tropical Ocean and Global Atmosphere (TOGA) program period. The system is to be built on the observational programs utilized in support of TOGA. Appropriate space and time scales should be taken into account as well as the development of data sets required for the use of current and anticipated prediction models. 2. Provide advice on methods and facilities for implementation of this observational system. Implicit in this language is that the panel's focus should be on oceanic and atmospheric observations of utility for short-range climate prediction on TOGA-like scales in the immediate future, following the formal 1994 end of TOGA. This is only a modest piece of the overall climate observa- tion problem, but it is an important one. Much thought, effort, and funding have already been invested in bringing the TOGA observing system and TOGA predictions that rely on them to their current status and in develop- ing TOGA models that will make increasing use of the observations in the future. The progress of both observing power and predictive ability during 5
6 OCEAN-ATMOSPHERE OBSERVATIONS the relatively short course of TOGA to date has been impressive. The formal program end should not be the signal to decimate either aspect of this progress. TOGA PLANS AND REQUIREMENTS FOR OBSERVATIONS The TOGA program has been clear and consistent over the years about its requirements for observations. Table 1, from the fourth edition of the International TOGA Implementation Plan (ITPO, 1992), has appeared in essentially the same form in other TOGA reports [e.g., TOGA Scientific Plan of 1985 (World Climate Research Programme (WCRP), 1985)] and in reports of other organizations. That report and its predecessors also outline the degree to which the required observations have been implemented. Associated with the sampling description of Table 1 has been an equally steady articulation of the need for a data set of at least 10 years' duration. As the 1985 Scientific Plan states, "The TOGA Programme will require an internationally coordinated measurement programme to provide a consistent ten year record of the basic geophysical variables describing the variability of the coupled tropical ocean and global atmosphere system." The underly- ing rationale for this duration has been straightforward: the most signifi- cant short-term variations of the physical climate [El Nino/Southern Oscil- lation (ENSO) events] are interannual, so a 10-year record is minimal for describing such events and studying their predictability. It can be argued that the sampling designs of Table 1 are simplistic. They were initiated in large part from educated guesses based on then- available information and on the most basic kinds of considerations of typi- cal amplitudes and scales of important seasonal to interannual variations, like ENSO. But this is not an unreasonable basis on which to have begun, and adjustments to such plans should be made cautiously so as not to break up ongoing time series without having thought through the consequences. Methods and platforms that can, in principle, produce the required ob- servations are well known. Table 2, taken from ITPO (1992), summarizes these techniques. While identification of these methods has not changed greatly during TOGA to date, the relative emphasis in some cases has. Earlier in the program there was anticipation of a larger role for remote sensing in the determination of surface wind. Delays of satellite launches and technical difficulties in converting from what is remotely sensed to the geophysical parameter desired have led to a greatly increased emphasis on direct wind observations in the crucially important tropical Pacific-the TOGA Tropical Atmosphere Ocean (TAO) array (Hayes et al., 1991). Be- cause of its late move to center stage in this respect, the full array of approximately 70 moorings is only due to reach full strength in 1993. Simi- larly, a recently expanded role for surface drifting buoys in the determina-
OVERVIEW AND PRINCIPAL RECOMMENDATIONS 7 TABLE 1 TOGA Data Requirements Horizontal (Vertical) Time Parameter Resolution Resoluttion Accuracy 1. Upper-air winds SOO km (two levels: 1 day 3 m/s 900 and 200 mb) 2. Tropical wind profiles 2SOO km (100 mb) 1 day 3 m/s 3. Surface pressure 1200 km 1 day 1mb 4. Total-column precipitable water SOO km 1 day O.S g/cm 2 s. Area-averaged total precipitation 2Â° lat x 10Â° long S days 1cm 6. Global sea-surface temperature 2Â° lat x 2Â° long 30 days O K .S 7. Tropical sea-surface temperature 1Â° lat x 1Â° long 1S days 0.3-0.S K 8. Tropical surface winda 2Â° lat x 100 long 30 days O.S mls 9. Tropical surface wind stress a 2Â° lat x 10Â° long 30 days 0.01 Pa 10. Surface net radiation 2Â° lat x 100 long 30 days 10 W/m 2 11. Surface humidity 2Â° lat x 10Â° long 30 days o.s glkg 12. Surface air temperature 2Â° lat x 10Â° long 30 days O K .S 13. Tropical sea level b 1 day 2cm 14. Tropical ocean subsurface temperature and salinity c c c 1S. Tropical ocean-surface salinity 2Â° lat x 100 long 30 days 0.03 PSU 16. Tropical ocean-surface circulation 2Â° lat x 100 long 30 days O.ot m/s 17. Subsurface equatorial currents 300 long As 0.01 m/s (five levels) recorded awhile the accuracy requirements given are for 30-day averages, daily values are required for resolution of 30- to 60-day oscillations. bAs permitted by the existence of suitable sites and satellite altimetry. csee discussion in Chapter 3. tion of global sea-surface temperature (SST) is now under way under the auspices of the World Ocean Circulation Experiment and TOGA. This recognizes the fact that other sources of SST data (VOSs, satellites) have some attractive sampling and cost attributes but also have unavoidable er- rors of serious magnitude (hull and intake plumbing near-field effects for VOSs, volcano dust interferences and internal radiometer calibration diffi- culties for satellites). A network of reliable, accurate surface observations is required to reduce such errors in the VOS and satellite data sets to ac-
TABLE 2a Observing Systems for Atmospheric Parameters Oo Parameter Observing Systems Phenomenon Observed Upper-air winds and Rawinsonde, Doppler wind profilers, Rotational wind field, divergent tropical wind profile aircraft, geostationary satellites motion in the tropics, monsoon variability, El Nino, wind anomalies for diagnosing the Southern Oscillation and the 30- to 60-day oscillation Humidity (total precipitable water) Radiosonde, rawinsonde, satellite Convective heating remote sensing Surface pressure World Weather Watch (WWW) surface Southern Oscillation and 30- to 60- stations, volunteer observing ships day oscillation, El Nino (VOSs), drifting and moored buoys Surface winds/wind stress WWW island and coastal stations, Surface fluxes of momentum, monsoon c VOSs, moored buoys, satellite remote variability, El Nino, wind ~ sensing anomalies for diagnosing the Southern Oscillation and the 30- ~ to 60-day oscillation ~ Precipitation CLIMAT rain gauge network, satellite Rainfall pattern ~ remote sensing ~ Global sea-surface VOSs, drifting and moored buoys, Surface temperature pattern ~ temperature satellite remote sensing ~ ~ Source: ITPO (1992). :::! ~
c ~ :otl :s ~ ;... TABLE 2b Observing Systems for Oceanic Parameters ~ '"a Parameter Observing Systems Phenomenon Observed 2!! ~ Q Tropical sea level Tide gauges, satellite altimeters, Variability of thermocline and ~ expendable bathythermographs (XBTs), ocean-surface topography, surface l"'o ~ drifting thermistor chains, moored geostrophic currents, response to temperature sensors wind stress and barometric pressure Tropical ocean subsurface XBTs, drifting thermistor chains, Variability of ocean heat storage ~ temperature moored temperature sensors ~ ~ :::! Tropical ocean salinity Conductivity-temperature-depth probes, Variability of ocean density, c surface sampling, expendable stratification, and upwelling ~ conductivity-temperature-depth probes Subsurface and near- Drifters, moored current meters, Surface and subsurface ocean surface current velocity current profilers currents Tropical sea-surface Drifting and moored buoys, VOSs, Anomalous ocean circulation events, temperature satellite remote sensing upwelling Source: ITPO (1992). IC
10 OCEAN-ATMOSPHERE OBSERVATIONS ceptable levels and will be required for the foreseeable future. Drifters can fill this role. Technical advances affect any such scheme as Table 2. Already meth- ods exist to improve some of the measurements noted and to add new measurements to the same platforms in cost-effective ways, and further advances are in progress. Some comments on the most important of these developments are given below. Neither the complete sampling scheme envisioned in Table 1 nor the decadal duration of that sampling will have been realized by the end of TOGA in 1994. The specifications exist, but the execution has fallen short. This is perhaps not surprising in retrospect given the extraordinary levels of technical, fiscal, political, and international effort and coordination needed to realize such ambitious plans. The wonder is not that there are now significant discrepancies between the situation in place and the specifica- tions of Table 1 but rather that TOGA has moved as far toward the scheme of Table 1 as it has. BEYOND TOGA: ONGOING RESEARCH ON SHORT-TERM CLIMATE VARIABILITY AND PREDICTION It is clear that scientific effort to study and predict seasonal to interannual variability of the physical climate system will not become a closed book at the end of TOGA. Social and economic interest in useful predictions, especially among tropical countries most affected by ENSO events, will only increase. TOGA prediction schemes have achieved noteworthy suc- cess in predicting the major signal (ENSO) within the tropical zone and particularly in the Pacific. But nobody supposes that the present situation is the best that can be done. Nobody supposes that the increase of predictive skill for ENSO or the extension of skillful predictions to the rest of the tropics or to midlatitude regions will be easy. Progress on these fronts will be the logical extended aims of TOGA-like research in the post-TOGA era. A TOGA Program on Seasonal to Interannual Predictions (Provisional Working Group, 1991) is only a year old and will, one hopes, continue beyond 1994. Based on the predictive skill demonstrated to date, and in response to the obvious social and economic needs for useful predictions of climate fluc- tuations, the International TOGA Scientific Steering Group (SSG) has en- dorsed the development of an International Research Institute for Climate Prediction. A new program called the Global Ocean-Atmosphere-Land System (GOALS) is in its formative stages now, in conjunction with the emerging WCRP's Climate Variability effort. The draft objectives of GOALS are "to describe and model the variability of the coupled global ocean-atmosphere- land system on seasonal and interannual time scales and to understand the mechanisms and processes underlying this variability and its predictability,
OVERVIEW AND PRINCIPAL RECOMMENDATIONS 11 and "to investigate the feasibility of predicting short term climate variabil- ity using coupled models of the global ocean-atmosphere-land system." These sound very similar to the TOGA objectives and for good reason. There is still a great deal about the fluctuations of the physical climate system on these time scales that is not well observed, not well modeled, not well predicted, and not well understood. PRINCIPAL CONCLUSIONS Four main conclusions flow from the fact that the full TOGA observing network will not have had a complete I 0-year run by 1994, while TOGA- like prediction efforts and research to improve them will continue and ex- pand into the indefinite future: I. Present TOGA observations should be continued. The single most critical effort to be sustained, because of its late establishment and because of the central importance in TOGA predictions of the fields it measures (tropical wind stress, SST, upper-ocean thermal structure), is the full TOGA TAO array of approximately 70 moorings. In recommending continuation the panel finds itself in close agreement with the international TOGA SSG (ITPO, 1992). 2. TOGA atmospheric observations stem largely from operational me- teorological stations, a huge international enterprise in which the needs of weather forecasting are paramount and tend to drown out climate concerns. Serious degradation of this system has been noted in previous TOGA re- ports (National Research Council, 1990), but it continues today. Govern- ment officials with responsibilities for climate questions should redouble efforts to arrest and, where possible, reverse the decline of this essential observing system. 3. Regarding the gap between the observing coverage set forth in Table I and the existing system in place, it appears that the most important av- enue for future expansion of the network is to establish in the other tropical oceans, first arid foremost the Indian Ocean, a system of ongoing observa- tions similar in quality and sampling patterns to the system now nearly in place in the Pacific. This implies beginning to construct an Indian Ocean TAO array, starting with an exploratory array and sampling studies like those done earlier in the Pacific, and expanding VOS and drifter coverage of this ocean. Further discussion of important expansions, both in coverage and in techniques/fields measured, is given below. 4. While it might be ideal to achieve the sampling of Table I and then to keep it intact for I 0 years, the reality is that observing systems evolve, circumstances and logistical constraints change, and financial support fluc- tuates on shorter time scales. The history of TOGA observations to date is
12 OCEAN-ATMOSPHERE OBSERVATIONS testimony to these facts. Continuous reassessment, adaptation, and modifi- cation of individual measurement series are required-an evolutionary pro- cess. There is, however, an objective standard by which to accomplish natural selection in this evolution: prediction. Observations now in plllce to support prediction (items 1 and 2) or added lllter for this reason (item 3) should thereafter be sustained until such time as a serious study of their impact on predictions reveals them to be of marginal value or until a new and more cost-effective technique is demonstrably ready to replllce them without disrupting or biasing the geophysical time series. Observations used to initialize, validate, or update predictions "support prediction," in our usage. Any such demonstration of marginal impact on predictions ought to be taken as a necessary but not a sufficient condition for curtailment of an observation series. It is quite possible that a data series could become marginal for use in the short-term climate predictions of interest here but still retain high value for climate research on other time scales or in other areas (e.g., the deep ocean, which is generally of second-order importance for TOGA-like climate questions). All scientific constituencies for a par- ticular series should be consulted before curtailment actions are taken. Two further main conclusions have an organizational flavor: 5. Because the observing system is evolutionary, whether we wish it to be or not, and because many individual measurements within it are simulta- neously important to more than a single climate research/prediction con- stituency, guidance for its future development must come from ongoing mecha- nisms involving an appropriate range of concerned scientists, program managers, and users. It is unrealistic to expect a comprehensive and durable system design from a panel such as this, which is "short term" not only with regard to the climate variations on which it has focused but also in its mandated lifetime. 6. Particularly for ocean observations and particularly in the U.S. structure, measurements tend to begin in the government and academic research com- munities and later are proposed for "transition" to an operational govern- ment agency in order to have long-term support. Where these transitions of data collection efforts make sense, they must be prepared carefully and methodically, so that commitment, resources, and technical capability are in place for the handover. Thereafter, continued strong involvement of the research community, through advisory mechanisms and hands-on work with the data, is vital for the effective maintenance, assessment, modernization, and advocacy of the operational effort. We were asked to "develop a (more or less static) design" and have
OVERVIEW AND PRINCIPAL RECOMMENDATIONS 13 ended up with a view of a system always changing and some recommenda- tions on how to guide that change. This should not be construed as defeat; the evolutionary model has much to recommend it: "TOGA has proved itself to be a wonderfully cooperative and successful research endeavor, rich in results and thought-provoking scientific papers. Its mix of system- atic observations of all kinds, of theoretical dynamical ideas and numerical modeling, and even of trial predictions really sets an excellent example of how a scientific study and evolving monitoring effort can interplay to pro- duce guidelines for a rational design of a monitoring effort of the kind that the National Oceanic and Atmospheric Administration can sustain" (H. M. Stommel, personal communication toR. A. Knox, 1991). Moving forward in this spirit to enhance, to extend, and, if deadwood be convincingly identi- fied, to prune the present TOGA network in close connection with the future development of modeling and prediction research results will be our best avenue toward increased understanding and practical predictions of earth's short-term climate variations.