TAIYO KOBAYASHI and Shinya Minato

1. What observation scheme should we use for profiling floats to achieve the Argo goal on the accuracy of salinity measurement? • Outline • What change occurs on the quality of delayed-mode QCed salinity data as profile depth of measurement is changed? • Does new sampling scheme developed by Prof. S. Riser work well from the viewpoint of quality control? TAIYO KOBAYASHI and Shinya Minato

2. Introduction • For observations by profiling floats, we can freely determine several parameters such as profile depth, observation cycle, etc. • Among them, profile depth of measurement largely affects the data quality, especially salinity. • In the Argo Project, T/S measurements to the depth of 2000dbar were agreed to evaluate data quality by comparison with the climatology in deep layer. => Delayed-Mode Quality Control The goal on accuracy for salinity measurement: 0.01psu • However, many floats measuring only shallower profiles are deployed in the tropical regions due to their insufficient buoyancy to rise up to the sea surface from 2000dbar. • In this study, we evaluate the accuracy of quality-controlled salinity data, and its changes as profile depth is changed.For DMQC, we use the method of Wong et al. (2003), and historical salinity dataset of SeHyD version 1.0.

3. Bladder volume needed for surfacing From 1500dbar From 2000dbar Have a risk to measure 2000dbar profile Possible to use for surfacing Case of APEX (in October, climatology)

4. Examples of salinity correction by Wong’s method Salinity correction on 2C isotherms Measured salinity Good results Local climatology (Reference) Correction errors ± below 0.005 psu Corrected Salinity Nearby CTD casts and post-calibration in the laboratory Poor results Correction errors±0.02psu We cannot always obtain the TRUE salinity data.Generally, we have no evidence whether the QCed salinity is rightly corrected or not. But, the results with smaller correction errors should be better than those with larger errors. In the above case, we can find whether the measured salinity seems to have a bias or not. But in the case with larger errors, we can obtain almost no information from the DMQC.

5. SubarcticWMO 2900055 Mixed Water RegionWMO 29043 SubtropicalWMO 2900175 Tropical PacificWMO 2900191 East IndianWMO 5900165 Test floats (total 32) * The most of the profiles are obtained to the depth of 2000dbar.* Observation cycle is constantly 10 days during its operation.* 30 profiles or more are measured.

6. Profile: 0-2000dbar Correction Error: 0.005psu (0.005-0.006) Profile: 0-1000dbar Profile: 0-1500dbar 0.010psu (0.009-0.012) 0.007psu (0.006-0.008) Subtropical

7. Profile: 0-2000dbar Correction Error: 0.006psu (0.004-0.007) Profile: 0-1500dbar Profile: 0-1000dbar 0.016psu (0.010-0.025) 0.008psu (0.007-0.012) Subarctic

8. Profile: 0-2000dbar Correction Error: 0.014psu (0.007-0.025) Profile: 0-1000dbar Profile: 0-1500dbar 0.020psu (0.009-0.029) 0.045psu (0.024-0.071) Mixed Water Region Similar to XCTD What is “observed”?

9. Profile: 0-2000dbar Correction Error: 0.004psu (0.002-0.006) Profile: 0-1000dbar Profile: 0-1500dbar 0.006psu (0.003-0.007) 0.007psu (0.003-0.009) Tropical Pacific The Argo goal of accuracy for salinity measurements can be achieved by the shallower parking floats.

10. Deployed Profile # 1 2 3 4 5 6 Surface Normal measurements:Upper 1000dbar only 1000dbar Sometimes dive to 2000dbar to measure deeper layers for quality control 2000dbar New observation scheme developed by Prof. S.C. Riser Advantage of Riser scheme:Mechanical troubles will occur less frequently due to the reduction of movements under higher pressure.

11. Subarctic region Mixed Water Region Riser scheme Constant 1500dbar Averaged accuracy is similar to that of the constant 1500dbar case 0.0083psu (0.007-0.012) 0.0085psu (0.005-0.021) Maximum error is larger than that of the constant 1500dbar case Riser scheme Constant 1500dbar 0.0202psu (0.009-0.033) 0.0196psu (0.009-0.029) Evaluation of Riser scheme performance

12. Do we have advantages to adopt Riser scheme for profiling float measurements? Averaged accuracy of Riser Scheme is as same as the scheme of the constant 1500dbar measurements. Its maximum error is larger than the latter scheme. => The Riser scheme has a potential risk to largely degrade data quality of float salinity measurements. Which strategy can reduce mechanical load for float hardware, 4 times 1500dbar dives, or1 time 2000dbar dive and 3 times 1000dbar dives ? => More studies are needed on the mechanical load. We can find almost no reasons to positively adopt the Riser scheme for the profiling float measurements in the present, at least from the view of the delayed-mode QC.

13. Within 0.01psu errors by 1000dbar profiles At least 2000dbar 1500dbar or more All results 2000dbar constant1500dbar constant1000dbar constantRiser scheme

14. Conclusions • The Argo goal of the accuracy for salinity measurement will be achieved generally by measuring profiles constantly to the depth of 1500dbar or more in the North Pacific. • In the tropical Pacific and the eastern Indian Ocean, the 1000dbar profiles give sufficient quality of salinity data. This conclusion is preferable for float hardware. • In Mixed Water Region, we MUST use the profiling floats programmed to measure the salinity profiles constantly down to 2000dbar in order to obtain the salinity data with sufficient quality for the present oceanography. • About a new observation scheme developed by Prof. Riser, we can find almost no reasons to positively adopt it for the profiling floats, at least from the viewpoint of the standard delayed-mode quality control.