1. TAIYO KOBAYASHI and Shinya Minato
What observation scheme should we use for profiling floats to achieve the Argo goal on the accuracy of salinity measurement?
TAIYO KOBAYASHI and Shinya Minato
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?

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

4. Procedures of PMEL-DMQC (Refereed to Wong et al., 2003)
Dataset of salinity on preset isotherms are prepared.
Select several hundred salinity data around float locations from the dataset.
Calculate the climatological salinity and its errors for the reference of data correction with objective mapping.
Calculating correcting factors to minimize salinity difference from the reference. As considering mapping errors, the references in deeper (colder) layers are more effective for data correction.
Correcting factors are restricted to its variations not to be largely influenced by the short-term variations (e.g. eddies).
Area of data selection
Area of float measurements
Climatological T-S profile at float location. → reference for salinity correction

5. Examples of salinity correction by Wong’s method
Salinity correction on 2C isotherms
Corrected Salinity
Measured salinity
Local climatology (Reference)
Poor results
Good results
Correction errors ± below psu
Nearby CTD casts and post-calibration in the laboratory
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.
Correction errors ±0.02psu

6. 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.
East Indian WMO
Tropical Pacific WMO
Subarctic WMO
Subtropical WMO
Mixed Water Region WMO 29043

7. Correction Error: 0.005psu (0.005-0.006)
Subtropical
Profile: dbar
Correction Error: psu ( )
Profile: dbar
0.007psu ( )
Profile: dbar
0.010psu ( )

8. Correction Error: 0.006psu (0.004-0.007)
Subarctic
Profile: dbar
Correction Error: psu ( )
Profile: dbar
0.008psu ( )
Profile: dbar
0.016psu ( )

9. Correction Error: 0.014psu (0.007-0.025)
Mixed Water Region
Profile: dbar
Correction Error: psu ( )
Profile: dbar
0.045psu ( )
Profile: dbar
0.020psu ( )
Similar to XCTD
What is “observed”?

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

11. Correction Error: 0.004psu (0.004-0.005)
Profile: dbar
Correction Error: psu ( )
East Indian
Profile: dbar
0.007psu ( )
Profile: dbar
0.008psu ( )

12. New observation scheme developed by Prof. S.C. Riser1 2 3 4 6
Profile #
Deployed 5
Surface
2000dbar
1000dbar
Sometimes dive to 2000dbar to measure deeper layers for quality control
Normal measurements: Upper 1000dbar only
Advantage of Riser scheme: Mechanical troubles will occur less frequently due to the reduction of movements under higher pressure.

13. Evaluation of Riser scheme performance
0.0085psu ( )
Subarctic region
Riser scheme
0.0202psu ( )
Mixed Water Region
Averaged accuracy is similar to that of the constant 1500dbar case
Maximum error is larger than that of the constant 1500dbar case
Constant 1500dbar
0.0196psu ( )
0.0083psu ( )

14. 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, or 1 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.

15. All results At least 2000dbar 1500dbar or more
2000dbar constant 1500dbar constant 1000dbar constant Riser scheme
1500dbar or more
At least 2000dbar
Within 0.01psu errors by 1000dbar profiles

16. 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.

17. Basic concept of PMEL QC
“Water-mass structure” exists in the ocean stably.
They can be expressed as S = S(θ)
They are calculated with optimum interpolation based on climatological dataset.
S = S(θ, x, y, t)
Scales of water-mass (needs to discuss further) Climatological (x, y) :Basin scale (10-20degrees) Long-term variation (x, y, t) x, y :Smaller than basin scales (some –10degree) t :Time of water ventilation (some – decades years)

18. Basic concept of PMEL QC (continued)
Salinity is calculated from conductivity values, C.
C should be corrected for salinity correction.
Change of C will be caused from the change of inside of measuring cell.
Correction: whole profile is corrected by a cell factor, k. C_true = k X C_measured.
Salinity change due to sensor degrade will be much slower than that due to short-term ocean variations (e.g., eddies). Thus, the changes of correcting factor k are restricted not to be largely affected from such variations.