Pre-Japan-ARGO: Experimental Observation of Upper and Middle Layers South of the Kuroshio Extension Region Using Profiling Floats

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1 Journal of Oceanography, Vol. 59, pp. 119 to 127, 2003 Short Contribution Pre-Japan-ARGO: Experimental Observation of Upper and Middle Layers South of the Kuroshio Extension Region Using Profiling Floats NAOTO IWASAKA 1,2 *, TOSHIO SUGA 1,3, KENSUKE TAKEUCHI 1, KEISUKE MIZUNO 4, YASUSHI TAKATSUKI 4, KENTARO ANDO 4, TAIYO KOBAYASHI 1, EITAROU OKA 1, YASUKO ICHIKAWA 1, MOTOKI MIYAZAKI 1, HIROSHI MATSUURA 1, KENJI IZAWA 4, CHAN-SU YANG 1, NOBUYUKI SHIKAMA 1 and MOMOKO AOSHIMA 2 1 Frontier Observational Research System for Global Change, Natsushima, Yokosuka , Japan 2 Tokyo University of Mercantile Marine, Etchujima, Koto-ku, Tokyo , Japan 3 Graduate School of Science, Tohoku University, Aoba-ku, Sendai , Japan 4 Japan Marine Science and Technology Center, Natsushima, Yokosuka , Japan (Received 18 February 2002; in revised form 9 May 2002; accepted 15 May 2002) We deployed two profiling floats in the region south of the Kuroshio Extension in March Temperature and salinity profiles from a depth of Pa to the surface are reported every two and four weeks, respectively. The floats performed very well for first four months after deployment. Later they failed in surfacing for a few months when the sea surface temperature in the region was high. The salinity sensors seemed to suffer from some damage during their failure-in-surfacing period. Despite this trouble, the results clearly demonstrate that the profiling float is a very useful and cost-effective tool for physical oceanographic observation in the open sea. Keywords: ARGO project, profiling float, salinity sensor. 1. Introduction There has been a strong demand from society for a substantial improvement in long-term weather forecast and short-term climate prediction in order to protect people s lives against meteorological disasters and to reduce the weather s impact on the economy and social systems. It is apparent that not only atmospheric observations but also oceanic data are necessary to improve long-term weather forecasts and climate change predictions. International meteorological and oceanographic organizations such as the World Meteorological Organization (WMO), the Intergovernmental Oceanographic Commission of UNESCO, and other related institutions have, therefore, started conducting an international project named ARGO. In the project about 3000 profiling floats will be deployed in the world ocean in a few years, so that we will be able to observe temperature and salinity profiles in the ocean s upper and middle layers in real * Corresponding author. iwasaka@ipc.tosho-u.ac.jp Present address: The Institute of Energy Economics, Japan, 13-1, Kachidoki 1, Chuo-ku, Tokyo , Japan. Copyright The Oceanographic Society of Japan. time, with spatial and temporal resolutions of about 300 km and ten days. The monitoring system will be the oceanic counterpart of the operational observation system of the global atmosphere (The Argo Science Team, 2000). The real-time network for monitoring the world ocean will help us to understand the Earth s climate system and improve the predictability of long-term weather forecasts and climate changes. In order to contribute the international effort to construct the monitoring system of the ocean, a five-year, inter-ministerial project, The Establishment of Advanced Ocean Observation System, has started under The Millennium Project of the Japanese Government. The research group of Oceanic Variation in the Subsurface Layer and in the Middle Layer, Frontier Observational Research System for Global Change (FORSGC) has been established to achieve the goals of the five-year project, in cooperation with the Ocean Observation and Research Department, Japan Marine Science and Technology Center (JAMSTEC). The project commenced in April 2000 and will be completed by the end of The project has been called Japan ARGO or J-ARGO. FORSGC and JAMSTEC deployed two profiling floats in the region south of the Kuroshio Extension in 119

2 March 2000, as a preliminary study for J-ARGO. The aim is to observe the performance of the floats since we had little current experience in using profiling floats equipped with temperature and salinity sensors for observing the upper and middle layers of the ocean. There were two major reasons that we chose the deployment region. One was that there were no strong currents there so the floats were expected to remain in the neighborhood of the deployment point. The other reason was that the region was scientifically very interesting since there were several important water masses such as the North Pacific Subtropical Mode Water (NPSTMW). NPSTMW, a thermostad in the upper layer south of the Kuroshio and Kuroshio Extension with temperature range of C (see reviews such as Hanawa and Suga, 1996; Suga, 1997; Hanawa and Talley, 2001), is likely to play an important role in the variation of the subtropical gyre and thus in climate variation. In the present paper, we describe the performance of the profiling floats as an oceanographic observation tool and discuss how useful the profiling floats are for oceanographic observations. 2. Profiling Float A profiling float is an instrument to measure temperature and salinity profiles in the upper and middle layers of the ocean and to send the data via satellites to the researchers on land periodically. A typical float that is used in the ARGO project consists of a conductivity-temperature-pressure (CTP) sensor, an ARGOS data transmitter, an engine, a control system, batteries, and a hard body. The engine usually consists of an oil pump and a bladder. The oil is pumped into or out of the bladder to change the volume of the bladder, thus changing the buoyancy of the float (Fig. 1). When the float is deployed in the sea, it sinks to its parking depth. The float drifts with the ocean current at that depth for a prescribed period, then it begins to rise. It measures temperature and conductivity of the layers through which it rises. When it reaches the sea surface, it transmits the data to orbiting satellites that are equipped with the ARGOS system. The float then begins another observation cycle (Fig. 2). A typical sampling depth of the ARGO float is Pa (2000 dbar, about 2000 m) and the sampling interval is 10 days. Parking depth, at which the float drifts, is usually Pa. Some floats may be programmed to drift at a shallower level in order to infer current speed at the layer shallower than the sampling depth. In such case the floats are programmed to come down to the sampling depth of Pa just before they rise up to the surface. The required accuracy of the temperature, salinity and pressure sensors on the typical ARGO float is better than degree, 0.01, and Pa, respectively (The Argo Sci- ARGOS system Sea surface Measuring and recording temperature and salinity as it rises Drifting Parking depth Sampling depth Fig. 1. Sketch of the Apex-type profiling float. A CTP sensor and an ARGOS antenna are on top of the float. Sampling interval Fig. 2. An observation cycle of the profiling float. 120 N. Iwasaka et al.

3 ence Team, 2000). The profiling floats used in the present study are Apex-type floats supplied by Webb Research Co., with CTP sensors from Sea Bird Electronics (SBE) Co. type 41. The parking depth of the floats is Pa, the maximum parking depth of this type of the float available at this time. The floats were programmed to measure temperature and salinity at 58 levels from Pa to the sea surface. The report from the float contains temperature and salinity values at the sampling depth, and the pressure of the depth. Some parameters indicating float condition are also reported. Sampling depth and an example of the report are shown in Table 1. A float s position at the sea surface is identified by the ARGOS system and reported to the user together with the data transmitted from the float. We could have more Fig. 3. (a) Trajectories of the two floats from their deployment through the end of October A large solid circle indicates the deployment position. Small open circles show the sinking positions and solid circles indicate surfacing positions. Broken lines indicate the periods when the floats failed in surfacing due to high SST during summer. Crosses indicate the CTD observation stations performed by the S/V Tenyo. (b) Same as (a), except for the period from March 21st through August 9th, Crosses indicate the CTD observation stations performed by the S/V Tenyo. Pre-Japan-ARGO: Experimental Observation South of the Kuroshio Extension Using Profiling Floats 121

4 Table 1. Sampling depth and an example of temperature and salinity data. The example is the data obtained from Float-28 on April 18, First two columns are for sampling level number and assigned depth (pressure) of the level. Third through fifth columns are for pressure, temperature and salinity values reported from Float-28 on April 18, Level No. Sampling level ( 10 4 Pa) Pressure ( 10 4 Pa) Temperature ( C) Salinity sampling layers if we had a high capacity, high-speed data transmission system, other than the ARGOS system. The batteries are expected to have sufficient capacity to repeat at least 100 observation cycles. One of the floats repeats its observation every two weeks and the other every four weeks. The specification of the floats is not the same as that for the ARGO project because the floats were deployed in a preliminary study for the J-ARGO project. The period of two weeks was chosen arbitrarily but is appropriate for monitoring low frequency variability south of the Kuroshio Extension. The four-week period was chosen for the second float in order to discover what would happen to the CTP sensor during long-term observation since it was expected to have lifetime of more than 7 years. Hereafter, we will call the float on the 2-week cycle (WMO ID 29033) Float-14 and that on the 4-week cycle (WMO ID 29032) Float-28, respectively. The two profiling floats were deployed on March 21st, 2000 at N, E, south of the Kuroshio Extension during the MR00-K02 cruise of R/V Mirai, JAMSTEC. The trajectories of the floats are shown in Fig. 3. They moved anti-clockwise from the deployment position until early August After that, however, both of them failed in surfacing because the sea surface temperature (SST) at that location was so high that the buoyancy of the floats was insufficient for them to reach the sea surface. Float-28 resumed surfacing in late October and Float-14 in December We cannot determine exactly at what level they drifted during the failure-in-surfacing period, but we have inferred that they drifted just beneath the sea surface until the SST reduced, according to the manufacture s explanation of the float control system. After they returned to observation, they have shown small but non-negligible biases on their salinity observations, as described in the following section. Since then, they have continued to observe, except 122 N. Iwasaka et al.

5 Level No. Sampling level ( 10 4 Pa) Table 1. (continued). Pressure ( 10 4 Pa) Temperature ( C) Salinity for a short period in summer Float-14 moved far to the east of its deployment position and drifted anti-clockwise. Float-28 surfaced to the east of its deployment point and moved towards the Mainland of Japan until late June After a short period of failure-in-surfacing, it appeared around 145 E and drifted along the Kuroshio Extension. 3. Performance of the Salinity Sensors on the Floats One of the major concerns about float observation of this kind is deterioration of the salinity sensor over long term periods of observation. Deterioration of temperature and pressure sensors of the CTD systems that are generally used in oceanographic observation is known to be very small and negligible (e.g., Millard and Yang, 1993). Matsumoto et al. (2001) confirmed this by examining the data obtained by TRITON Buoys (Kuroda and Amitani, 2001). On the other hand, salinity sensors of either electrode type or inductive conductivity cell type may suffer significant deterioration due to bio-fouling and other environmental causes (e.g., Matsumoto et al., 2001). In order to evaluate the accuracy of the salinity values obtained by the floats, we compared the float observations with hydrographic observation aboard the R/V Mirai, using a SBE-9 CTD, at the deployment point just before the floats were released into the sea. We also compared the float observations to a series of CTD observations, using a SBE-19 CTD, conducted aboard the Survey Vessel (S/V) Tenyo of the Japan Coast Guard in the region close to the points where the two floats surfaced on July 11th, The observation aboard the S/V Tenyo was designed to detect possible deterioration of the float sensors four months after their deployment. In the comparison here, we assume that the observed temperature data are accurate and do not need any corrections. Figure 4(a) shows profiles of the first observations of the temperature and salinity obtained by Float-14. The T-S diagram of the observation is shown in Fig. 4(b). Superimposed on the figures are those of the CTD observation aboard the R/V Mirai at the deployment position ( N, E). Temperature and salinity profiles and T-S diagram of the first observation by Float-28 Pre-Japan-ARGO: Experimental Observation South of the Kuroshio Extension Using Profiling Floats 123

6 (a) (b) In order to quantitatively evaluate the accuracy of the CTP sensors on the floats, we compared the observations obtained below about Pa (temperature below about 4.0 C). These levels were selected because seasonal variations of the temperature and salinity in the middle layer (below about 1000 m) were believed to be very small. Mean differences between the Float-14 observation and those of the CTD data for salinity are , less than the resolution of the salinity sensor of the float, and the root mean square of the difference is The average and root mean square differences for Float- 28 are and , respectively. On July 14th and 15th, researchers aboard the S/V Tenyo performed a series of CTD observations at 14 stations around the surfacing point of Float-28 on July 11th. The hydrographic observation stations and the trajectories of the floats are shown in Fig. 3(b). They used a SBE type 19 CTD. We compared the float observations obσ Fig. 4. (a) Temperature, salinity and density profiles observed by Float-14 on April 4th, 2000 at the point of N, E, the first observation of the float. Open circle ( ) denotes temperature, open square ( ) shows salinity and open triangle ( ) indicates sigma-t, respectively. Dots denote those of CTD observation aboard the R/V Mirai at the deployment point ( N, E) on March 21st, (b) T-S diagrams of the first observation by Float-14. Dots denote the T-S diagram of CTD observation made aboard the R/V Mirai at the deployment point. (figures not shown) are quite similar to those of Float- 14. Although the first observation of Float-14 was two weeks later and that of Float-28 was four weeks after deployment, they surfaced at positions very close to their deployment point, as shown in Fig. 3. That is, Float-14 surfaced on April 4th at N, E, 12.9 km away from the deployment point, and Float-28 surfaced at N, E on April 18th, 34.7 km from the point. The vertical profiles obtained by the floats are somewhat different from those of the CTD obtained aboard the R/V Mirai. However, the T-S diagrams show that the float observations and those of the CTD data obtained by the R/V Mirai coincide very well, except for surface and the salinity minimum layers where variability is generally large. Thus, the relatively large discrepancies in temperature and salinity profiles are presumably caused by excursion of isopycnals rather than changes in water masses. 124 N. Iwasaka et al.

7 (a) (b) σ Fig. 5. (a) Temperature, salinity and density profiles observed by Float-14 at the point of N, E on July 11th, Dotes denote those by CTD observation made aboard the S/V Tenyo at the station closest to the float surfacing point, N, E. (b) T-S diagrams for the observation by Float-14 on July 11th. Dots denote the T-S diagram of CTD observation made aboard the S/V Tenyo. tained during their rise on July 11th to the CTD observations conducted aboard the S/V Tenyo. Float-14 surfaced at N, E, and Float-28 at N, E, respectively. We chose the CTD station located at the western end of the stations for comparison with the data obtained by Float-14. This station is located at N, E, about 37.2 km from the float surfacing point. Figure 5(a) shows the profiles of temperature and salinity of the float observation and those of the chosen CTD observation made aboard the S/V Tenyo. The T-S diagram is shown in Fig. 5(b). We compared the salinity observations obtained below Pa levels again (temperature below about 3.7 C). The mean difference between the float observation and that of the S/V Tenyo is and the root mean square difference between them is For Float-28, we chose the CTD observation done at N, E on July 14th, the station closest (about 1.1 km) to the surfacing point of Float-28 on July 11th (figures not shown). The CTD station is located at the center of the observation stations visited by the S/V Tenyo. The average salinity difference between the CTD observation made by the S/V Tenyo staff and that of the float is and the root mean square difference is Based upon these comparisons, we can conclude that the salinity sensors on the floats had performed very well from March through July and had sufficient accuracy (better than 0.01) for most of analyses of upper and middle layer water masses and for the ARGO project. Thus, we do not apply any correction to the float observations obtained during the period. However, as mentioned before, the floats failed in surfacing during the high SST season because the SST was so high that the buoyancy of the floats was inadequate to surface in this region. This fact revealed that the floats were not properly designed for observation in the west- Pre-Japan-ARGO: Experimental Observation South of the Kuroshio Extension Using Profiling Floats 125

8 Fig. 6. Comparison of T-S diagrams of Float-14 between the first three observations after its failure-in-surfacing period (open circles, open triangles and open stars) and those obtained by last four observation before August 2000 (dots). ern North Pacific, where SST in the region reached its maximum of about 28 C. On the other hand, this type of profiling floats had worked well in the North Atlantic Ocean, where maximum SST is lower than that in the western North Pacific (e.g., Levitus, 1982). Float-14 surfaced again on November 19th, 2000 and Float-28 on October 19th 2000, respectively. If one inspects the T-S profiles before and after the summer, it can be seen that the observations made by Float-14 after the summer show significantly larger salinity values than previous values from below the 4 C level, as shown in Fig. 6. The differences are large, up to about Statistical examination based on Hydrobase, which is a high quality historical hydrographic database (Macdonald et al., 2001), did not clearly indicate that the shift in salinity values was due to an error on the salinity sensor, but we have concluded that the salinity sensor suffered some damage during their failure-in-surfacing period. It is not likely that a water mass difference caused the systematic shift of the salinity values because there is no observational evidence that the horizontal gradient of salinity in the middle layer in the region is large enough to show such a salinity shift, as shown in Fig. 7, which shows the T-S plots of the high quality historical data obtained in the regions around the float positions before and after the summer. As can be seen in the figure, there is no significant difference in salinity value between these two regions. We have no clue about what happened on the sensors exactly. But, as mentioned in Section 2, they might drift beneath the surface during the failure-in-surfacing period since the floats are designed to keep trying to rise until they actually come up to the sea surface when they are in the ascending mode. Fig. 7. T-S diagrams of historical observations obtained in the region between 28 N and 30 N, 141 E and 144 E, and that between 26 N and 29 N, 146 E and 149 E. The western region includes the last three observations by Float-14 before August and the eastern region includes first three observations after Float-14 resumed observation in November The upper panel shows the locations of the historical observations. Dots denote the observations obtained in the western region and crosses indicate those in the eastern region. Open circles indicate the float surfacing positions. The lower shows the T-S diagram. On the other hand, Float-28 worked well even after the summer until the end of Unfortunately, however, it seemed to show a small bias in the salinity values since January 2001 (figure not shown). The bias seems to be growing. Details of deterioration of the salinity sensors on the floats will be reported in the near future. 4. Discussion We deployed two profiling floats in the region south of the Kuroshio Extension in March They have been reporting vertical profiles of temperature and salinity from Pa to the surface every two and four weeks, respectively. The floats performed very well for at least the first four months after their deployment. Later they failed in surfacing for a few months when the SST in the region was high, after which they have shown slightly biased salinity values. Although they experienced some trouble, the results shown in the present study clearly demonstrate how useful and cost-effective profiling floats with temperature 126 N. Iwasaka et al.

9 and salinity sensor are in oceanographic observation of the upper and middle layers. The maximum range of depth observed by the floats is up to Pa, the accuracy of the salinity sensor is better than 0.01 and that of the temperature sensor is better than K. The depth range and sensor accuracy is much better than those of XCTD. The initial cost of the float observation, which includes the cost of the float itself, cost of deployment and so on, is estimated at about $20,000 25,000 or 2,500,000 3,000,000, so far about 50 times the cost of a XCTD probe. The cost of data transmission should also be factored in. But considering the quality of the observation and the lifetime, the total cost of the float observation is estimated equivalent to, or even lower than that of XCTD. A shortcoming of the float observation is that we cannot control the observation position after deployment. One major problem in the float observation is maintenance of the accuracy of the salinity sensor. But deterioration of the salinity sensors on the float seems very small compared to the CTD sensors installed on the surface mooring buoys such as TRITON (Kuroda and Amitani, 2001). We are trying to shorten the time during which the float stays at the sea surface for data transmission, making it as short as possible in order to reduce the risk of bio-fouling. An ideal data transmission time is a few minutes or less. The satellite data communication system for profiling floats should be improved. So far, the ARGOS system is the only practical satellite data transmission system from the float. But the ARGOS system has two major shortcomings. One is that the data transmission rate of the system is so low that we are forced to program the float to stay on the sea surface for at least 12 hours. We wish to shorten this time, as mentioned above, as much as possible. The other shortcoming is that because ARGOS is a one-way system we cannot control the float after deployment. If we were to use a two-way communication system between the floats and land stations, we could change sampling depth, period, layers, etc., even after deployment, so that we would have more a flexible observation system. We are very sure that in the near future profiling floats will be one of the major, important observation tools in physical oceanography, especially for monitoring purposes, despite the problems mentioned above. It is apparent, however, that a profiling float is not suitable for observation near shore and in strong current regions. XCTD and XBT are still very useful for such regions. For research purposes, very precise and accurate observations by research vessels are needed. Precise and accurate observations in the open sea are also desirable for calibrating the sensors on the profiling floats. Research vessels are also needed to sample water, deploy and retrieve moorings, observe currents and so on. Moorings will be used for deep sea observation and for monitoring at fixed positions. Since October 2000 we have deployed more than 30 floats in the western North Pacific and Indian Ocean as of the end of 2001 as J-ARGO floats. The J-ARGO floats have been improved significantly so that the engine is now designed to have sufficient buoyancy to rise up to the surface from Pa depth in almost all regions of the World Ocean. The results of the observations obtained by the newly deployed J-ARGO floats will be reported in the near future. Acknowledgements The authors are grateful to the captain and crew of the R/V Mirai, Dr. Kuroda of JAMSTEC, research engineers of Global Ocean Development Inc., and of Marine Work Japan for their help and cooperation in deploying the floats and performing related observations. They thanks to the Hydrographic Department of the Japan Coast Guard for providing a set of CTD observation data obtained aboard the S/V Tenyo. References Hanawa, K. and T. Suga (1996): A review on the Subtropical Mode Water of the North Pacific (NPSTMW). p In Biochemical Processes and Ocean Flux in the Western Pacific, ed. by H. Sakai and Y. Nozaki, Terra Sci. Pub. Co., Tokyo. Hanawa, K. and L. Talley (2001): Mode Waters. p In Ocean Circulation and Climate, ed. by G. Siedler, J. Church and J. Gould, Academic Press, New York. Kuroda, Y. and Y. Amitani (2001): TRITON: New ocean and atmosphere observing buoy network for monitoring ENSO. Umi no Kenkyu (Oceanography in Japan), 10, (in Japanese with English abstract and figure captions). Levitus, S. (1982): Climatological Atlas of the World Ocean. NOAA Professional Paper No. 13, U.S. Gov. Printing Office, Washington, D.C., 173 pp. Macdonald, A. M., T. Suga and G. Curry (2001): An isopycnally averaged North Pacific climatology. J. Atmos. Ocean Tech., 18, Matsumoto, T., T. Nagahama, K. Ando, I. Ueki, Y. Kuroda and Y. Takatsuki (2001): The time drift of temperature and conductivity sensors of TRITON buoy and the correction of conductivity data. JAMSTECR (Report of Japan Marine Science and Technology Center), 44, Millard, R. C. and K. Yang (1993): CTD calibration and processing methods used at Woods Hole Oceanographic Institution, Tech. Rep., WHOI 93-44, 104 pp. Suga, T. (1997): Climatology and variability of North Pacific Subtropical Mode Water as revealed by Hydrographic data analysis. Umi no Kenkyu (Oceanography in Japan), 6, (in Japanese with English abstract and figure captions). The Argo Science Team (2000): Report of the Argo Science Team 2nd Meeting (AST-2) March 7 9, 2000, Southampton Oceanography Centre, Southampton, U.K. Pre-Japan-ARGO: Experimental Observation South of the Kuroshio Extension Using Profiling Floats 127