Expendable Bathythermograph Observations from Ships of Opportunity
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EXPENDABLE BATHYTHERMOGRAPH OBSERVATIONS FROM SHIPS OF OPPORTUNITY J. F. T. Saur National Marine Fisheries Service, NOAA La Jolla, Calif. and Paul D. Stevens Fleet Numerical Weather Central, U. S. Navy Monterey, Calif. INTRODUCTION differences in a changing ocean. This procedure may be likened to the collection of weather observations ersonnel of merchant and fishing vessels, who that are used to determine storm patterns and fore- P have cooperated for many years in taking and re- castweather conditions. Such an oceanwide monitoring porting weather observations, are now beginning to system will require observations from many sources, make observations of water temperatures below the such as buoys, satellites, coastal and island stations, sea surface to depths of 760 m (2,500 ft). Such ob- oceanographic research ships, and especially ships servations from freighters, tankers, transports, and of opportunity. fishing vessels are made possible by the relatively Of the many variables to be observed it seems new expendable bathythermograph, commonly called logical to start with temperature, because it is easily the "XBT. Oceanographers call these cooperating measured and because it is of paramount importance nonoceanographic vessels "ships of opportunity. '' to the following marine biological and physical prob- lems: USES OF SUBSURFACE TEMPERATURE DATA 1. Marine organisms are affected both directly and indirectly by temperature. One well-known The primary impetus in the development of the XBT example of harmful temperature changes is the came from the U.S. Navy, whose advances in anti- El NiEo effect, which occasionally develops submarine warfare technology required a subsurface along the Pacific coast of South America in the system for recording temperatures that was more general region of Peru. Here a relaxation of accurate andeasier to use than the mechanical bathy- the upwelling of cold water from below the sur- thermograph (MBT). For a dependable observation face and the incursion from the north of much with the MBT, the ship must slow to 5 kt or less in warmer surface water, often accompanied by order to lower and raise the instrument to the re- the bloom of microscopic toxic organisms, quired depth (a maximum of 900 ft). This operation cause widespread destruction of coastal fauna. usually takes more than 20 min. With XBTs, obser- The commercially abundant anchovies (ancho- vations can be made in a few minutes at speeds as vetta) leave the surface layers to avoid the warm high as 30 kt without interfering with the ship's water, and the guano birds that normally feed operation. on them emigrate in search of other food or Although the military need made the development starve in large numbers. of the XBT feasible, other oceanographic studies of 2. Temperature is known to affect migration pat- the marine environment also awaited the development terns and group behavior of marine animals. of instruments that would economically provide the Many nekton (swimming marine organisms) pre- capability of oceanographic monitoring. By this, we fer certain temperature ranges. When the mean the collection of oceanographic observations albacore, a temperate-water tuna, migrate east- widely enough distributed in time and space to reveal ward from the central North Pacific Ocean the week-to-week, month-to-month, and year-to-year toward the coast of North America in early 1 summer, they generally appear in the fishing and the ultimate abundance of fish spawned that area where surface temperatures are between year. Further, the distribution of heat in the 15' and 20° C (59"-68' F). sea, noted next, is very dependent upon the An example of behavior affected by temperature ocean currents. can be found in tropical tunas, which are often 5. A major physical application of the knowledge fished by purse seine. Statistical studies have of temperature distribution and its changes is shown that when the thermocline (a layer of related to weather forecasting. Water has a water separating the warm surface layer from high heat capacity so that the ocean acts as a colder deeper water) becomes sharper and is giant heat reservoir. Not only does it store shallower than the bottom of the seine, there is vast quantities of heat during summer for re- less chance that the tuna will escape by diving lease in winter, but it may also store a fraction below the net before the bottom is drawn to- ofthe heat gainedoverperiodsof several years. gether, or "pursed." The tuna tend to avoid This abnormal storage, as recognized hy month- passing down through the thermocline. ly temperature anomalies (differences from The monitoring of large ocean current systems monthly averages), and the later release of the by direct measurement of currents is difficult heat may play a significant role in the modifi- and expensive, so oceanographers often turn to cation of large-scale weather patterns. Con- the temperature distribution from which many sequently, a knowledge of the changes in heat inferences about the circulation can be drawn. storage below the surfaceof theocean is neces- Circulation of waters in the sea, including not sary to research into long-range weather only horizontal ocean currents but also vertical forecasting. currents, is extremely important to the inor- 6. Finally, vertical temperature gradients in the ganic chemical nutrients essential to the growth ocean have a large effect on the propagation of of phytoplankton (microscopic plants, the first sound in the sea. Generally, cooling and deep- step in the food chain in the sea). In turn, the ening of the surface mixed layer increases the availability of microscopic food organisms to detection ranges of sonar equipment used by the larvae and juvenile fish, may affect the survival Navy and fishing vessels. Conversely, warming Launcher mor Shrn- I Sipnol Wire from Canitler Spool Wira from Probe SPOOl Breach Y pstorWire Spool Launchw/Recorder Coble LAUNCHER AC Power Coble \\Y\ Ground Strap Spool RECORDER Figure 1. --The expendable bathythermograph system and its operation. Three major components of the system are the recorder, launcher, and canister containing the expendable probe and signal wire. The dual-spooling system for the signal wire permits the probe to sink vertically from its point of entry and the observation to be recorded as the ship proceeds on course. n and a decrease in thc depth of the mixed layer The recorder operates automatically. When the reduces the detection ranges of hull-mounted release pin is pulled, the probe slides out of the sonar. launcher by gravity (fig. 3)--no explosive or propul- sion device is involved--and the recording cycle is triggered by entry of the probe into the water. The TIIE ESPENDABLE BATHYTHERhIOGRAPH temperature-depth trace is recorded as the probe SI'STEhl descends. When the signal wire is expended, it breaks and the probe is lost. After a fixed recording The expenrlahle bathythermograph system, which was period, the recorder automaticallg stops and returns engincered and developed by Sippican Corporation, * to a standby position, ready for the next observation. of llarion, Xlass., has three major components (fig. The system records over the normal range of sea I). Two components, the launcher and recorder, temperatures, -2" to 35" C (28"-95" F). Probes are small and light enough to be carried aboard ship by macle for two primary depth ranges: (a) to 4G0 m hand, are mounted semipermanently on the ship. The (1,500 ft) for ship's speed up to 30 kt and (b) to 760 ni third and expendable component, one of which must be (2,500 ft) Cor ship's speed up to 15 kt. The recording ~isctlfor each observation, is the canister (fig. 2), cycles last 90 sec and 180 sec respectively. which contains the cxpendable probe ancl signal wire. The temperature sensing element is a rapid- OBSERVATIONAL PROCEDURES response thermistor. It is mounted in the nosc of the Iiallistically shaped expendable probe (fig. l), ahich The work ahonrd ship xt each observational period is is ahout 22 cm (8.7 in.) long, G cm (2.4 in.) in di- neither difficult nor involved. ameter, and weighs about 700 g (under 2 lb). The I. The XBT svstcni is very simpletooperate. The dual-spooling svstem, see figures 1 and 2, for main- loading of the canister into the launcher, the re- taining nn electrical connection between the thermistor lease of the prohe, and the recording cycle take nnd shipboard recorder during the recoriling cycle is less than 4 min. basic to the expendable design. As the ship proceeds 2. A reference temperature observation is taken on course, the fine (3.5 mm) insulated two-conductor and recorded. This observation may be a sur- signal wire is paved out freely on the water from the face temperature taken with a bucket thermom- canister spool, which remains in the launcher. Dur- eter or a representative water intake tempera- ing the same period, the spool in the probe pays out wire freely from its finned tail and allows the probe to desccntl verticnllj. from its point of entry at the surface of the ocean. The probe, Lvhich is spin sta- bilized and carcfully weighed in manufacture, has a calibrated rate of fall, so that the depth is determined from the time interval from its entry into the water. *Ysc of company name does not imply endorsement of the product the r.S. Government. Figure 2. --Cutaway view of oanister (35 cm long and 6 cm in diameter) reveals the two spools which to- Figure 3. --A test drop of an XBT is made from the gether hold about 1.8 km (1.1 mi) of signal wire. National Marine Fisheries Service research ves- The thermistor for sensing temperature is mounted sel, DAVID STARR JORDAN. The probe canbe very near the nose of the probe (at right) in the tu- seen in midair at lower left, but the signal wire is bular opening extending along the axis of the probe.