RECENT IMPROVEMENTS TO DEEP-SEA RESEARCH THROUGH
USE OF SUBMERSIBLES, ACOUSTIC TOMOGRAPHY AND IN-
SITU LONG TERM OBSERVATIONS (IAPSO)
Location: Arts Building, 125 LR1
Friday 30 July AM
Presiding Chairs: A. Flosadottir (Univ. of Washington, Seattle, USA),
R. Iwase (Deep-Sea Res., JAMSTEC, Yokosuka, Japan)
P16/W/05-B5 0830
LONG-TERM MONITORING OF OCEANIC SEISMICITY USING UNDERWATER ACOUSTIC TECHNIQUES
Christopher G. FOX, Robert P. Dziak (both at NOAA, Pacific Marine Environmental Laboratory, 2115 S.E. OSU Drive, Newport, OR 97365 USA email: fox@pmel.noaa.gov)
Monitoring oceanic seismicity using underwater acoustic techniques offers several advantages over land-based seismic networks, including a lower detection threshold, improved location accuracy, and the ability to monitor large areas of the ocean with relatively few sensors. The availability of the U.S. Navy's SOund SUrveillance System (SOSUS) to the scientific community has allowed long-term monitoring of seismicity in the North Pacific Ocean to be accomplished at relatively low cost. Detailed maps of overall seismicity are now available for structural interpretation of plate tectonic activity. Episodes of volcanic seismicity associated with seafloor spreading, generally with magnitudes less than 3.0, have been successfully detected by SOSUS three times and verified in the field. Patterns of intraplate deformation have also been discovered, including both isolated intraplate earthquakes and patterns of intraplate activity relating to large-scale stresses on the plate. Based on the success of its SOSUS efforts, NOAA developed and deployed an array of autonomous hydrophones in the equatorial Pacific in May, 1996. Seismicity from the equatorial region of the Pacific shows a similar pattern to that in the northeast Pacific, with most activity confined to subduction zones and transform fault zones, with some indications of intraplate seismicity and volcanic seismicity along the ridge crest. The inferred volcanic episodes along the fast-spreading-rate East Pacific Rise (EPR) are of short duration relative to confirmed activity along the medium-spreading-rate Juan deFuca/ Gorda Ridges (hours-days versus days-weeks). These shorter volcanic episodes may simply represent the nature of the activity or perhaps reflect lower seismic magnitudes due to a thin brittle layer. In February 1999, a second array of hydrophones was deployed in the North Atlantic between 10N and 40N in a two-year experiment to monitor the Mid-Atlantic Ridge (D. Smith, WHOI; M Tolstoy, LDEO; C. Fox, NOAA, PIs). Analogy to the Pacific examples would predict fewer, longer duration volcanic episodes and larger magnitude fault plane earthquakes. Additional monitoring efforts, based on either routine ship access or the establishment of permanent ocean stations, will depend on partnerships with the international research community and other U.S. agencies, but could eventually lead to a global acoustic monitoring system.
P16/L/01-B5 0910
FOUR-YEARS' CHANGE OF THE TAG HYDROTHERMAL MOUND OBSERVED BY SUBMERSIBLESHINKAI 6500
Kantaro FUJIOKA (Deep Sea Research Department, Japan Marine Science & Technology Center, Yokosuka, 237-0061 JAPAN, email fujiokak@jamstec.go.jp), Hitoshi Chiba (Institute for study of the Earth’s Interior, Okayama University, 827 Yamada, Misasa, Tottori, 682-0193 JAPAN, email hchiba@misasa.okayama-u.ac.jp), Harue Masuda (Department of Geophysics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585 JAPAN, harue@sci.osaka-cu.ac.jp), MEGATRAIN scientific party
The TAG hydrothermal mound is one of the largest hydrothermal mounds known in the world. Since the first finding of the mound its whole edifice, stratigraphy and structure, chemistry and long-term change were made clear by the previous studies. Here we present new data sets on the location of active vents, type of smokers, and chemistry of the hydrothermal fluids. The TAG hydrothermal mound is migrating with NNE-SSW trending fault movement since 1994 to form the two new black smoker mounds east of the Central Blacksmoker Complex previously existed. The chemical compositions of the vent fluids did not change al all even 17 ODP drill holes were dug up on and around the TAG mound. We detected big change of the Eh, pH, CTDT and gamma ray intensity along the vents and faults on the mound. We also found the existence of the plume structure by the measurements during the descent and ascent the submersible. The TAG hydrothermal mound is now migration together with fault system suggesting that the pass way of the hydrothermal fluid changes without any reaction with the mound forming materials since 1994.
P16/E/03-B5 0930
DEEP-SEA SUBMERSIBLE MAGNETICS ACROSS A MID-OCEAN RIDGE
J. DYMENT, M. Ravilly (both at CNRS, IUEM, Univ. Bretagne Occidentale, Plouzané, France; jerome@univ-brest.fr); C. Honsho (Ocean Research Institute, Univ. Tokyo, Japan); M. Perrin (CNRS, ISTEEM, Univ. Montpellier II, Montpellier, France); H. Horen (Ecole Normale Supérieure, Paris, France); P. Gente (CNRS, IUEM, Univ. Bretagne Occidentale, Plouzané, France)
A unique set of magnetic data has been acquired on the Mid-Atlantic Ridge spreading segment at about 21°40'N during the Tammar cruise of R/V Nadir and deep sea submersible Nautile. The data include surface scalar magnetic field collected by the ship, sea-bottom vector magnetic field continuously measured along the dive track by a three component magnetometer attached to Nautile, and measurements of rock magnetic properties and absolute paleointensities on samples collected by the submersible. Fifteen dives make three almost-continuous traverses of the spreading center, and four additional dives at the axis allow examination of magnetic variations along the axis. The surface magnetic anomaly and the long-wavelength part of the sea-bottom magnetic anomaly were inverted to equivalent magnetization. Because they were measured close to the seafloor, the observed sea bottom anomalies contains short-wavelength variations produced by the topography and the up-and-down motion of the submersible. We estimate the magnetisation of the seafloor along the dive tracks by comparing the amplitude of these short-wavelengths with synthetics calculated for different uniform magnetisation of the seafloor. About 130 direct measurements of the natural remanent magnetisation and magnetic susceptibility have been made on the samples. In addition, successful determinations of the geomagnetic field paleointensity by the Thellier method have been obtained for 34 samples of the two traverses located in the segment center. These data offer a unique opportunity to study the structure and magnetic properties of the oceanic crust and the source of marine magnetic anomalies.
P16/E/13-B5 0950
LONG TERM DEEP SEAFLOOR MONITORING AT COLD SEEPAGE SITE OFF HATSUSHIMA ISAND IN SAGAMI BAY
Ryoichi IWASE, Hiroyasu Momma, Kyohiko Mitsuzawa and Katsuyoshi Kawaguchi (Deep Sea Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan, email: iwaser@jamstec.go.jp)
Real time deep seafloor monitoring has been carried out for more than 5 years by the cable-connected observatory which was deployed at the cold seep community site at a depth of 1174m off Hatsushima Island in Sagami Bay, central Japan in 1993. It is equipped with video cameras, subbottom temperature probes, a current meter, a CTD, a seismometer and a hydrophone. Through 5 year monitoring, seasonal variations of subbottom temperatures which increased in spring and had a peak in April or May were observed, although the seasonal variations of water temperature were not so obvious. The increase of the amount of suspended particles which are considered to be produced by the spring bloom in the shallow water were also observed in spring by the video camera. The sedimentationn of these particles seems to affect the subbottom temperatures. The daily variations of subbottom temperatures which correlated with the variation of hydraulic pressure i.e. ocean tide were also observed. These are considered to have some relation with the behaviour of cold seepage. When the earthquake swarms occurred at about 7 km southwest of the observatory in March, 1997 and in April and May, 1998, mudflows were observed by the observatory. Considering from video images, bottom current profiles and the results of deep-tow survey around the observatory after the earthquake in 1997, mudflows seem to be caused by the landslides of the unstable surface sediment on the slope in the west of the observatory. The subbottom temperatures increased more than 1 degree C in only several days just after the occurrence of mudflows. These seem to be caused mainly by rapid sedimentation associated with mudflows.
P16/E/10-B5 1010
COMPREHENSIVE DEEP SEAFLOOR MONITORING SYSTEM OFF CAPE MUROTO, WESTERN JAPAN
Hiroyasu MOMMA (Ocean Engineering Department, JAMSTEC, 2-15 Natsushimacho Yokosuka 237-0061, Japan, e-mail: mommah@jamstec.go.jp)
Great submarine earthquakes at a scale of magnitude 8 (M8) occurred successively in 1944 and 1946 in the Nankai Trough off Cape Muroto and Kii Peninsula , the western region of Japan. Since similar great earthquakes have repeated at intervals between 100 and 150 years, there has been no seafloor seismic network in this area. As a step to increase the seafloor network, a comprehen sive deep seafloor monitoring system was developed by the JAMSTEC to monitor the seismic activity and deep-sea environment in the Nankai Trough off Cape Muroto. The system consists of observatories with a cable and without cable. The cabled observatory comprises of two accelerometer type seismometers, two Tsunami pressure gauges, a multi-sensor cable-end station with video camera, CTD, current meter, etc., 125 km long optical submarine cable, and a land station at Muroto. All the data are sent and stored in real time to the land s tation, and they are also transmitted to JAMSTEC and Meteorological Agency of Japan. It was deployed at water depths between 1,290 meters and 3,572 meters in March 1997. The observatory without cable comprises of a multi-sensor mother station and four satellite stations. The mother station is equipped with a velocity type seismometer, a Tsunami pressure gauge, a digital camera with flash, a CTD, an electro-magnetic current meter, two heat flow temperature probes, a hydrophone and 16 pop-up buoys. The data in mother station could be monitored monthly through satellites by releasing pop-up buoys. After one year long observation, all the data in 8 GB hard disc is retrieved by recovering the station. Each satellite station comprises of a long-term velocity type digital ocean bottom seismometer (OBS) which could be recorded three -component seismic data continuously for two months. The data in 2 GB Exabyte tape could be retrieved by recovering the station. The observatory without cable finished preliminary observation in 1998 for four months at a depth around 1,400 meters. In total, five systems, similar to that off Cape Muroto, will be deployed around Japan.
P16/E/04-B5 1050
MONITORING THE MID-ATLANTIC RIDGE: THE MOMAR PROJECT
Pascal TARITS and the Momar Working Group (IUEM/UBO, UMR CNRS 'Domaines Océaniques', Place Nicolas Copernic, F-2980, Plouzané, France, tarits@univ-brest.fr)
Over the coming years, the European and international community of multidisciplinary ridge researchers is seeking to initiate a comprehensive long-term program to monitor the Mid Atlantic Ridge (MAR) near the Azores (MOMAR). This program is designed to conduct in-depth inquiries into the issues of:
How does this mid-oceanic ridge environment change with time (in terms of seismicity, volcanism, hydrothermal venting and the distribution and characteristics of biological populations)? How do these changes affect heat and chemical transfer to the overlying ocean, ecosystem development, faunal succession and biological productivity? What are the components and space/time extent of the subseafloor biosphere? What are the dominant controls on volcanism, hydrothermalism and faulting at this ridge axis, and how are these processes connected?
This program is being initiated by InterRidge, an international program for the coordination of ridge studies that has a strong European component (7 out of the 9 member countries are European). It is planned to be the most ambitious and comprehensive effort to date for the multidisciplinary monitoring of a mid-ocean ridge system. It will follow up on previous experiments carried out by American researchers in the Pacific (mainly on the Juan de Fuca ridge) and by Japanese researchers on the East Pacific Rise. The program will extend the scope of these experiments beyond the monitoring of individual volcanoes or vent sites, to integrated monitoring of all axial processes in the studied region.
The MOMAR project will include the deployment in the Azores region of a variety of monitoring tools, some of which have been successfully deployed in the past, while some will need to be specifically designed. Efforts will be made to involve engineering specialists from industry backgrounds in the development of these tools, and to publicise their potential for monitoring other active underwater environments. The use of submarine cables and moored buoys to transmit data and provide the energy necessary to work these tools are also envisioned as part of the project and will require co-ordinated technological efforts on the part of the participating countries.
P16/W/03-B5 1110
NEPTUNE: A FIBER OPTIC TELESCOPE TO INNER SPACE
A.D. CHAVE (Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA; e-mail: alan@whoi.edu), J.R. Delaney (University of Washington, Seattle, WA 98195, USA)
Investigation of the Earth as a dynamic system will require new intellectual approaches in the ocean sciences. We submit that it will also require a co-ordinated investment in a new mode of conducting oceanographic investigations through the establishment of long-term (i.e., "permanent") observatories on the seafloor. To effectively model the complex interactions involved in ocean systems, a submarine presence must be used which returns four-dimensional (three spatial dimensions plus time) data sets from multiple arrays of physical, chemical, and biological sensors deployed specifically to characterise the covariant nonlinear behaviour of the major planetary systems. North East Pacific Time-integrated Undersea Networked Experiments (NEPTUNE) is a concept for a series of interactive seafloor observatories to investigate highly active plate tectonic and oceanographic systems off the Washington-Oregon coast. This can be accomplished by establishing a series of strategically-located, state-of-the-art seafloor observatories capable of 1) real-time transmission of data and images from many hundreds of instruments, 2) routine robotic responses controlled from shore, and 3) distribution of power to the system. The single-most important technology required to achieve this capability is a fibre-optic cable system, designed to link a series of seafloor communication nodes to shore, enabling and controlling the data flow and power transmission required for comprehensive long-term observations and routine remote interventions. In fact, with such a facility, individual seafloor instruments or arrays can become virtual Internet sites from which data retrieval, instrument control, or robotic intervention may be accomplished remotely by designated shore-side users.
P16/W/04-B5 1130
GLOBAL MULTIDISCIPLINARY OBSERVATIONS FROM SEAFLOOR OCEAN OBSERVATORIES
Dr. John A. ORCUTT IGPP (0225) (Scripps Institution of Oceanography, La Jolla, CA 92093, USA, email: jorcutt@igpp.ucsd.edu), Dr. Adam Schultz (Institute of Theoretical Geophysics, Department of Earth Sciences, Downing Street, Cambridge, CB2 3EQ, UK)
Oceanography, in contrast to many other Earth Sciences, has advanced almost solely through expeditions which examine phenomena in a limited area over a very limited time. Technology can now support, and scientific problems demand, continuous observations on a broad range of scales in understanding the dynamics involved in global processes in addition to the traditional approach of exploration. For example, the observation of change can only be accomplished by such continuous observations, whether a change in a climate system or the eruption of sections of a mid-ocean ridge. It is essential that oceanographers pursue the development, deployment, and operation of an integrated, permanent ocean observing system for seismology, oceanography, geomagnetism, geodesy, climate and other critical observations. Such observations will lead directly to fundamental discoveries about the physical, chemical, and biological processes that, on all time scales, determine the structure, evolution, and climate of our planet. We envision a coherent network of observatories in the oceans with data broadly available to scientists, students, commercial interests, and other individuals and organisations. While the emphasis is on long-term observations at fixed, globally-distributed locations, it is essential that shorter-term, smaller-scale studies be promoted to minimise aliasing of important phenomena. The network will include 20-30 observatories distributed globally to provide measurements of geophysical phenomena. Most of these sites are in isolated regions and near-real-time delivery of data and the supply of power require specially-designed, long-life buoys which will be serviced annually (SeaStation) or less frequently (SeaBase). The buoys designed for global observations must be used to support shorter term (2 - 4 years) regional observations with plug-in access for individual experiments. These will be complemented by autonomous stations to ensure that the variety of spatial scales which characterise all geophysical fields can be sampled. 5 - 10 additional observatories will be required for this more temporary component of the global network.
P16/L/02-B5 1150
A BENTHOPELAGIC CONTACT ZONE IN THE OCEAN PATTERNS OF THE PLANKTONDISTRIBUTION IN THE NEAR-BOTTOM LAYER
Alexander L. VERESHCHAKA (Institute of Oceanology, Moscow, Russian Academy of Sciences.
E-mail: alv@ecosys.sio.rssi.ru)
During 12 biological dives in the North Atlantic (hydrothermal fields Rainbow, Broken Spur, Logachev, Norwegian Sea, off Newfoundland, depths range from 1700 m to 3800 m), methods of the planktonic visual observations have been developed. This has became possible due to the huge energetic store (1.5-2 times more than in any other submersible) which characterises "Mir" submersibles and allows illumination of the water column during many hours of observations. Emphasis has been made on the detailed observations of the plankton distribution and biology within few hundreds of meter above bottom. All planktonic animals were shown to be divided into 2 principal groups: pelagic (not related to the bottom) and benthopelagic (associated with the sea- floor, this group may be subdivided further). Each group is characterised by certain patterns of distribution, behaviour, and feeding. Near the bottom there is an area called benthopelagic contact zone, where benthopelagic animals dominate and biological processes related to the bottom define the structure and function of the near bottom communities. This zone seems to be ubiquitous in the deep ocean, and was found an all studied depths both above hydrothermal fields and background areas. The thickness of this zone depends upon various biotic and abiotic parameters (examples given), the structure of this zone may be very complex (exemplified by the Broken Spur contact zone). The concept of a benthopelagic contact zone allows more perfect understanding of the biological structure of the ocean.
Friday 30 July PM
Presiding Chairs: C. Fox (Pacific Marine Environmental Lab., Newport, USA);
I. Nakano (Affiliation Ocean Obs.-Res. Div., JAMSTEC, Yokosuka, Japan)
P16/E/07-B5 1400
UNDERWATER CABLE OBSERVATIONS OF OCEAN VARIABILITY IN THE NORTH PACIFIC
Agusta FLOSADOTTIR (Joint Institute for Studies of the Atmosphere and Ocean, JISAO/PMEL, Box 357941, University of Washington, Seattle, WA98115, U.S.A., email: agusta@pmel.noaa.gov)
Ocean current variability is a major contributor to the large-scale voltage differences observed between points on the sea floor. An example of successful interpretation of a voltage time series in terms of ocean transport variability has been given by J.C. Larsen's ongoing time series of cross-stream voltages in the Straits of Florida on time scales from days and now approaching the inter-decadal. Until recently, the Florida Current work was the only long underwater cable voltage time series in existence. Over the last few years, however, a network of basin-scale voltage measurements has been established in the North Pacific by the groups of L.J. Lanzerotti at Lucent Technologies' Bell Laboratories and A.D. Chave at WHOI in collaboration with the group of H. Utada at Tokyo University. The two longest time series are Hawaii-California, since 1990, and Guam-Japan, since 1992. While these data have been used in applications ranging from the Earth's geophysical structure to space weather, a large part of the frequency range has been shown to be dominated by signals of oceanic origin. Oceanographic analysis, however, has so far been based on what is by now less than half of the time series. An overview will be given of the cable voltage observations in the North Pacific, and their place among other large-scale ocean observations. The full Hawaii-California time series will be presented and discussed in terms of ocean variability.
P16/E/11-B5 1440
CENTRAL EQUATORIAL PACIFIC TOMOGRAPHY EXPERIMENT: PLANS AND PRELIMINARY RESULT
Iwao NAKANO, Toshiaki Nakamura, Hidetoshi Fujimori (Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Japan Email: nakanoi@jamstec.go.jp) and Bruce Howe (Applied Physics Laboratory, University of Washington, Seattle, USA, email: howe@apl.washington.edu )
JAMSTEC and APL made and started a joint program to set the acoustic tomography system to measure a shallow overturning which is thought to be related to interdecadal climate variability (Gu and Philander, Science 1997) as well as to measure the heat storage variation due to El Nino for two years (1998-2000). The acoustic tomography system was successfully deployed by the JAMSTEC tomography team in the Central Equatorial Pacific Region (0.5N to 13.6N, 177.7E to 172.1E) in early January, 1999. This system has a capability to transfer measured data for the experiment sites to the Land Station in real-time in addition to store it in the internal HDD. These transferred data is to be analyzed to reconstruct temperature and current fields in the region using the tomographic inversion. Topics at the early stage of the experiment will be presented and mentioned to the relation of El Nino.
P16/E/06-B5 1500
SIMULTANEOUS TRANSMISSION OF FIVE TRANSCEIVERS BY THE USE OF MULTIPLE M-SEQUENCES IN THE CENTRAL EQUATORIAL PACIFIC TOMOGRAPHY EXPERIMENTS
Toshiaki NAKAMURA, Tomoyuki Kanaizumi, Hidetoshi Fujimori, Iwao Nakano, (Japan Marine Science and Technology Center, Japan); Kurt Metzger (Michigan University, USA); and Bruce Howe (Applied Physics Laboratories, University of Washington, USA)
A tomography array which consists of five 200 Hz transceivers were deployed in the Central Equatorial Pacific in December 1998 and are under measurements for two years. The objective of this experiment which is an international collaborative work between JAMSTEC and APL-UW is to Monitor the variability of water temperature and current in relation to the El Nino. In our previous experiments, each transceiver transmitted in order every 30 minutes, and the total turn-around time was 2.5 hours. In this experiment, we intend to shorten a measurement time by a simultaneous transmission of five transceivers, which improves an accuracy of measurement for water current velocity due to reduce the effects of internal waves. In this case, however, each transceiver receives an overlapped signal of other transceivers. Multiple M-sequence signals are needed to discriminate mutual signals among transceivers. Five sets of M-sequences were chosen carefully and a simulation for discrimination of the overlapped signals were conducted under the condition of the practical geometry of the five transcievers. As the result, it is expected that the signal of each transciever can be received with the S/N ratio of over 20 dB.
P16/E/01-B5 1520
THE APPLICATION OF HIGH DATA RATE TRANSMISSION ON OCEANOGRAPHIC RESEARCH
XU Tianzeng (Department of Oceanography, Xiamen University, Xiamen, 361005, China, email: xmxu@jingxian.xmu.edu.cn)
Over the last decade, more attentions have been paid to underwater acoustic data transmission techniques. The partial reason is the increasing requirements of developing underwater vehicle, such as manned research submersibles, ROV and AUV. But the mission of high data rate transmission through underwater acoustical channel have to face a variety of serious and difficult problems arising from the channel: multipath spread, noise, fluctuation and so forth.
With experiments, we have studied on some statistical characteristics of signal, multipath and noise as the theoretical foundation of signal processing in underwater acoustic channels for many years. Theoretical analyses and experimental results show that the techniques based on MFSK, together with corresponding anti-multipath, anti-fluctuation and anti-noise is an effective signal processing method suitable for high data rate transmission in the channels. Combined with the respective characters of acoustic data telemetry, digital speech communication and video image transmission, the corresponding experimental prototypes have been developed. These prototypes were tested in Xiamen harbor and got good results, for example, the video image can be transmitted omni-directionally to about 10km at the data rate of 8kbits/s by occupying 5 kHz frequency bandwidth, the speech communication can be transmitted to the same distance at 6kbits/s by occupying 4kHz bandwith. We also developed the deep-sea transduce. For the acoustical conditions in the deep-sea are much better than in the shallow-sea, the prototypes would be used to transmit the relative signals in the deep-sea research.
P16/E/02-B5 1600
ARCTIC CLIMATE OBSERVATIONS USING UNDERWATER SOUND (ACOUS)
Peter N. MIKHALEVSKY (Science Applications International Corporation, Ocean Sciences Division, 1710 Goodridge Dr., McLean, VA 22101, USA)
Since the early 1990's it appears that inflow of warmer Atlantic Water into the Arctic Ocean increased resulting in temperature increases in the Atlantic Layer that are continuing until today. Point measurements from icebreakers in 1991 and 1993 showed temperature increases of several tenths of a degree C over historical climatologies. In 1994 acoustic transmissions were made from a site north of the Svalbard Archipelago across the entire Arctic Ocean to receiver arrays located in the Lincoln Sea and the Beaufort Sea. The travel time measurements revealed an average .4 C increase in the Atlantic Layer. This was the first basin scale measurement of this large scale warming. Since 1994 there have been annual trans-Arctic submarine cruises and one trans-Arctic icebreaker cruise that have also observed these changes. The 1999 submarine SCICEX cruise in the Arctic is likely to be the last such cruise to be undertaken for some time. A network of sources and receive arrays arrays (the latter cabled to shore) would be able to provide year-round real-time masurements. Acoustic energy traverses the Arctic basin in approximately 30 mins. providing an integrated measurement of the Arctic Ocean temperature. Observations can be made on many acoustic paths over the Arctic on time scales unachievable by submarine, icebreaker or ice camps. Coupled atmosphere-ice-ocean modelling and analysis of recent data appear to support a decadal scale oscillation in the atmospheric and ocean circulation in the Arctic that may explain the recent warming and portend a return to cooler temperatures. The need for year-round real-time data from the Arctic is evident. A program called Arctic Climate Observations using Underwater Sound (ACOUS, from the Greek word "akous" meaning "listen!") is underway to establish an acoustic monitoring network in the Arctic. The first source and receive array were installed in Oct. 1998. Funding constraints limited the array to be autonomous, but a cabled array is being planned for installation in the spring of 2000. In addition to using the travel time measurements to obtain ocean temperature, research is underway to use the acoustic attenuation changes to measure changes in average sea ice roughness and thickness, and the use of multiple frequencies to measure the depth of the thermocline and thus the thickness of the upper mixed layer in the Arctic Ocean.
P16/W/01-B5 1620
COHERENCE OF BOTTOM AND SUB-SURFACE PRESSURES AND SEA LEVELS AROUND ANTARCTICA
P.L.WOODWORTH, C.W.Hughes, M.Smithson and J.M.Vassie (Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside L43 7RA, UK, email plw@pol.ac.uk) T.Whitworth (Department of Oceanography, Texas A&M University, College Station, TX 77843, USA, email twhitworth@tamu.edu) and other US Collaborators R.M.V.Summerson (National Resource Information Centre, Canberra ACT 2600, Australia, email rupert.summerson@brs.gov.au)
Since the late 1980s, the Proudman Oceanographic Laboratory (POL) and Texas A&M University have deployed bottom pressure recorders (BPRs) either side of, and in the middle of, the Antarctic Circumpolar Current (ACC) 'choke points' at the Drake Passage and south of South Africa and Australia. The object has been to provide information for the World Ocean Circulation Experiment (WOCE) on ACC transports for comparison to those obtained from coastal tide gauges, satellite altimetry and numerical models. This presentation shows some of the findings from the BP deployments, tide gauges and models. One significant feature observed is that large-scale coherence at the several mbar (cm) level does appear to exist between BPR sites and gauges many 1000s of kilometres apart from low frequencies (e.g. annual) to timescales of order 10 days. The data analysis is continuing, but so far the signals appear to be larger than those suggested by numerical models such as FRAM and OCCAM. Ongoing BP deployments will continue to be required beyond the WOCE obervational phase, in order to acquire an extended time series of ACC transport, aided by the constant improvement in technology (e.g. via the use of air-launched 'expendable' BPRs). In addition, the availability and suitability of data from the Antarctic coastal sites (e.g. Faraday, Syowa, Mawson, Dumont d'Urville and several other sites) suggest that they might be appropriate data sources of ACC variability, although perhaps containing a greater degree of near-coastal local 'noise'. The continued development of the GLOSS tide gauge network in this part of the world is essential.
P16/W/02-B5 1640
DEPARTURES FROM THE INVERSE BAROMETER MODEL OBSERVED IN TIDE GAUGE AND ALTIMETER DATA AND IN A GLOBAL BAROTROPIC NUMERICAL MODEL
Lucy MATHERS, Philip Woodworth (both at Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead L43 7RA, UK, email lucy@pol.ac.uk)
The 'local inverse barometer' (LIB) model provides a simple description of the response of sea level to forcing from air pressure changes. It is widely used in oceanography as a correction to sea level data prior to ocean circulation studies. At high frequencies (periods typically less than 2 days), at very low frequencies (periods typically seasonal and longer), and in certain areas at particular timescales (e.g. in tropical areas at periods of around 5 days), the LIB model is known not to work well. In most other parts of the temporal spectrum, it is assumed to work reasonably well. In this presentation, results will be given of a worldwide search for departures from the simple LIB model using tide gauge data from about 100 sites in the global network and from the TOPEX/POSEIDON altimeter satellite. In addition, results will presented from a coarse-grid global barotropic model (a development of the coastal tide-surge model of Flather) forced by ECMWF air pressure and wind fields. Apparent dynamical features evident in all three data sets will be discussed.