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?

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.

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

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

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 0.005 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

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 5900165 Tropical Pacific WMO 2900191 Subarctic WMO 2900055 Subtropical WMO 2900175 Mixed Water Region WMO 29043

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

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

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

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

Correction Error: 0.004psu (0.004-0.005) Profile: 0-2000dbar Correction Error: 0.004psu (0.004-0.005) East Indian Profile: 0-1500dbar 0.007psu (0.006-0.008) Profile: 0-1000dbar 0.008psu (0.007-0.010)

New observation scheme developed by Prof. S.C. Riser 1 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.

Evaluation of Riser scheme performance 0.0085psu (0.005-0.021) Subarctic region Riser scheme 0.0202psu (0.009-0.033) 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 (0.009-0.029) 0.0083psu (0.007-0.012)

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.

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

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.

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)

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.