Argo Delayed-Mode Salinity Data

Acknowledgments Taiyo Kobayashi, FORSGC, Japan Ron Perkin, IOS, Canada Brian King, SOC, UK Lars Böehme, IfM Kiel, Germany Rebecca McCreadie, BODC, UK

Introduction Salinity is one of the main measurements made by Argo profiling floats, but ...

How to calibrate without an absolute reference? … the sensors sometimes give measurements that have artificial trends. DRIFT OFFSET θ θ S S Problem: How to calibrate without an absolute reference?

Argo data quality control elements Real-time (RT) data stream Function: Apply agreed RT QC tests to float data. Assign quality flags. Users: Operational centres, data assimilation, researchers needing timely data. Timeframe: 24-48 hrs after transmission. Who/Where: Perform by National Data Assembly Centres. Data from floats Delayed-mode (DM) data stream Function: Apply accepted DM procedures to float data. Provide statistically justified corrections using accepted methods. Provide feedback to RT system. Users: All needing adjusted data with error estimates. Timeframe: 6-12 months after transmission. Who/Where: Perform by PIs with DM Agencies or Regional Centres.

What is done in real-time for salinity data? θ • Range test • Spike test • Gradient test • Density inversion test • • • • • • • • • • S Action: “Bad” float salinity data are flagged. Result: “Good” float salinity data have a “good” relative vertical profile, but may still contain artificial drifts and offsets.

What is done in delayed-mode for salinity data? Basic theory … θ-S spread in a 10°x10° box around 25°S,155°W • By using reference datasets and statistical methods, we can estimate what salinity should be at float locations. θ S from WOD 2001 34.6 - 34.8

• Confidence of statistical salinity estimates is θ-S spread in a 10°x10° box around 55°N,45°W • Confidence of statistical salinity estimates is dependent on reference data and regional variability. θ S from WOD 2001

Action: Float salinity are checked against the least • Weighted least squares fits are obtained from the “good” float salinity data and the statistical salinity estimates. θ S Action: Float salinity are checked against the least squares fits for artificial drifts and offsets. Significant drifts and offsets are adjusted. Result: Delayed-mode salinity data with error estimates.

What are the reference datasets? Need datasets with: • good quality data; • good spatial coverage; • recent in time. All post-1985 CTD data from WOD 2001 World Ocean Database 2001 + Additional recent CTD data

… depends on what region the float is in. What is the confidence of delayed-mode salinity data? … depends on what region the float is in.

Example in the tropical Pacific.

• Float WMO ID 3900059 salinity data have significant artificial drift • Float WMO ID 3900059 salinity data have significant artificial drift. Drift is removed in delayed-mode by weighted least squares fit to statistical salinity estimates from reference data. Before delayed-mode adjustment After delayed-mode adjustment Profile no

Time series along θ = 5.5°C (~800 dbar) • Artificial salinity drift is 0.25 PSS-78 in one year. Delayed-mode adjustment uncertainty is less than ±0.006 PSS-78. Time series along θ = 5.5°C (~800 dbar) drift = 0.25 S uncertainty < ±0.006 Profile number

Example in the subarctic North Pacific.

Time series along θ = 2°C (~1700 dbar) • Float WMO ID 4900083 has artificial salinity drift of 0.05 PSS-78 in two years. Delayed-mode adjustment uncertainty is less than ±0.005 PSS-78. Time series along θ = 2°C (~1700 dbar) uncertainty < ±0.005 S drift = 0.05 Profile number

Example near Japan.

Time series along θ = 2°C (~1900 dbar) • Float WMO ID 2900186 was recovered. Deployment CTD confirmed artificial salinity offset of 0.02 PSS-78. Delayed-mode adjustment uncertainty is less than ±0.006 PSS-78. S Profile number For more Pacific examples, see Kobayashi et al. “New Pacific historical dataset for the Argo standard scheme of delayed-mode quality control and its performance of salinity correction”

Modifications for the North Atlantic (Boehme/Send) Complex water mass structure and basin bathymetry, rapid timescales of Change require different approach to salinity mapping using historical data Do not mix different water masses in vertical (even if Temp homogeneous) and account for Temp inversions  Database not saved on isotherms windowing in the vertical Only specific isotherms are used in mapping Avoid mixing different regimes (here boundary current and basin interior) in historical data selection  Selection based on spatial distance D and fractional distance in potential vorticity f/H Use the same distance weighting for covariance functions and mapping

In North Atlantic, deep salinities change interannually In North Atlantic, deep salinities change interannually. Thus a very recent CTD cast must get much larger weight than 1 year old one  Determine timescales from appropriate data, e.g. moorings (here 250 days) Can also obtain better (and f/H weighted) spatial scales for salinity mapping 0 0.5 1 1.5 years Decision process: start with hypothesis that float is good and correct only if ΔS > 2 ×Error Assume a float has either constant offset or linear drift Apply linear fit ΔS=a+bt Calculate uncertainty of a and b estimates and correct only if a or b larger than 2× their uncertainty

In the South Indian Ocean • Looking at salinity anomaly at different depths can help discern artificial drift; • King, “The detection of subsurface θ-S changes”

Salinity anomaly on Theta levels

In the Southern Ocean • Isolating water masses that are common across the Polar Front; • McCreadie et al, “Delayed-mode QC of Argo float salinity in the Southern Ocean”

There is more than one salinity value for the same q level There is more than one salinity value for the same q level. Temperatures warmer than the sub-surface maximum can be associated with a wide range of salinities which interferes with the calibration. Float 3900069

Conclusion • With improving float technology, float sensors can be expected to remain stable for the lifetime of the float. • With improving reference datasets and statistical methods, Argo delayed-mode salinity data can achieve accuracy of 0.01 PSS-78 or better in most oceanic regions.

Where to find Argo delayed-mode salinity data? A novel feature of the Argo data format … PSAL PSAL_QC 30.00 1 30.00 1 30.00 1 30.00 1 30.00 1 PSAL_ADJUSTED PSAL_ADJUSTED_ERROR PSAL_ADJUSTED_QC 30.10 0.01 1 Real-time data Delayed-mode data

Argo Global Data Assembly Centres Coriolis at IFREMER USGODAE at Monterey