Quality Control for the World Ocean Database GSOP Quality Control Workshop June 12, 2013

Purpose Define aim of quality control for the World Ocean Database (WOD) Brief outline of quality control procedures for data in the WOD. Active areas of investigation into quality control checks. Presentation of quality control flags to users.

Aim of Quality Control in WOD WOD is the input data for the World Ocean Atlas (WOA) climatology series. Quality control of WOD aims at producing the definitive long-term mean gridded ocean fields. Quality flags represent pass/fail of a given set of tests, not good/bad judgement. WOD with quality flags are presented to ensure reproducibility of WOA

4 Quality Control Step 0: Check for format errors, duplicate data, incorrect units, incorrect metadata. Red: Mar. 11, 1961, 21:00GMT, °E, 41.39°N Green: Mar. 11, 1961, 12:30GMT, °E, 41.65°N Z (m) Temp Temp

5 Quality Control Step 1: Automatic Checks - reveal systematic errors in incoming data and metadata - eliminate most non-representative data from consideration Eliminates ~3% of temperature measurements from consideration. Checks include: Range check Spike check Density inversion Speed check (no flag) Land/ocean bottom check (no flag) Standard deviation Profiles ( x 10 5 ) Depth (m)

Example of data flags in WOD not being used A user showed this T-S plot as an example of problems in WOD. The vertical lines centered on S=36.5 is clearly not a feature of the real ocean. However, when we tried to reproduce the plot on the bottom left we found the user had included data that we had flagged as being erroneous. Bottle data for WMO Sq °N, 60-70°W

Red: Apr. 23, 2002, 06:22GMT, °E, °S Green: Apr. 23, 2002, 05:22GMT, °E, °S Depth (m) Temperature (°C) Keep all data with flags Choices for Disseminating Quality Controlled Data Provide only realistic data XBT example: Two datasets, same data, different choices for disseminating quality control information Dashed line: start of qc flag “bad” for red data. Green “bad” data removed.

Red: Apr. 23, 2002, 06:22GMT, °E, °S Green: Apr 23, 2002, 05:22GMT, °E, °S Depth (m) Temperature (°C) Keep all data with flags Choices for Disseminating Quality Controlled Data Provide only realistic data XBT example: Two datasets, same data, different choices for disseminating quality control information Dashed line: start of qc flag “bad” for red data. Green “bad” data removed.

Gross Range Checks by areas [basins/latitude belts/coastal] Additional areas: Sulu Sea NW Pacific, Japan Sea, Yellow Sea, Seto Inland Seas How narrow to make range envelopes? Too narrow = throw out good but anomalous data. Too wide=keep too many bad data [At least one measurement flagged in 178,041 temperature profiles – 1.6%]

Excessive Gradient and Inversion Checks Excessive Gradients – an excessive decrease in value over depth Temperature: 0.7°C/m [523,934 profiles, 4.8%] Excessive Inversion – an excessive increase in value over depth Temperature: 0.3°C/m [269,500 profiles, 2.5%] Combination (spike) – excessive gradient followed by excessive inversion or vice versa. Temperature: [20,536 profiles, 0.2%] (Also monotonic/zero value profile checks)

Quality control checks after interpolation to standard levels: 515,885 temperature profiles completely eliminated from use (4.7%) 1, profiles with at least one level flagged (10.6%) Green -> standard deviation outliers (>= 2 in a profile) Yellow -> density inversion (>=2 in a profile) Red/orange -> individual measurements/profiles/cruises subjectively flagged

Density Inversion Check: Sufficiently large negative stability between adjacent standard levels. >=2 in a single profile flags entire profile lower depth 3 x g/cm 3 lower depth > 30m 2 x g/cm 3 lower depth > 400m instability > g/cm 3 Standard deviation outlier check: >= 2 in a single profile flags entire profile Means and standard deviations in 5° lat/lon boxes Coastal: Outlier > 5 standard deviations from mean above 50 m Near Coastal: Outlier > 4 standard deviation from mean above 50m Near bottom: Outlier > 4 standard deviations from mean Open Ocean: Outlier > 3 standard deviations from mean Coastal=adjacent to designated land box (1° grid) Near coastal= 2 grids from designated land box or <=200m depth Near bottom: last standard depth above ocean bottom Open Ocean=all other ocean grid boxes

13 Quality Control Step: Climatologies Climatology after Automatic quality control January temperature at 800m depth Final Climatology

REXAMINATION OF QUALITY CONTROL PROCEDURES AT NODC BASED ON RESULTS USING ARGO DATA 14

Extensive and intensive quality control work done Argo profile data by Data Assembly Centers (DAC) Additional quality control work done upon arrival of data at NODC What are the effects of Argo quality control and NODC quality control on ocean heat content calculations? Based on work done by Mathieu Hamon [PhD thesis] with Karina von Schuckmann and Gilles Reverdin [heat content discrepancy in southern hemisphere using WOD05 + Argo qc data vs WOD09 with NODC qc Argo data] 15

YEAR m OCEAN HEAT CONTENT ANOMALY (OHCA) RELATIVE TO WOA09 ABOVE: FULL NODC QUALITY CONTROL BELOW: ARGO QC FLAG 4 ONLY ABOVE: NODC QC OHCA MINUS ARGO QC FLAG 4 OHCA RED=POSITIVE OHCA BLUE=NEGATIVE OHCA 16

1985 [5%]2009 [22%] Global Subsurface Temperature Coverage: 60S to 30S compared to Global Ocean

Profiles with Two or More Depth Levels with Temperature Standard Deviation Outliers: Global Ocean compared to 60S – 30S latitudes. Left Panel: Number of Profiles Right Panel: Percent of Total Profiles 18

Float – Cycle 25 September 9, °S, 140.5°E Black line=Temperature profile World Ocean Atlas Annual Mean Temperature Climatology at 600 m depth. Solid Grey Line=5° mean temperature. Dashed Grey Lines=Mean ± 3 x Standard Deviation 19

One possibility: standard deviation check is flagging too many good data New more inclusive climatology (WOA13), shorter time period climatology (WOA13) or float-only climatology (Roemmich and Gilson) may help Change or remove check 20

Second possibility: standard deviation check flagged data represent limited time/space features and should not be used for OHCA calculation Continue as before Change checks for known frequent anomalous regions (Agulhas retroflection/rings) and anomalous time periods ( El Niño/La Niña) 21

Presentation of quality control flags for WOD WOD qc flags represent pass/fail of given set of tests Originators flags also included in WOD – passing on previous quality information For WOD13, there will be an option for IODE standard flag scheme: two flags 1) WOD pass/fail qc flag 2) IODE good/bad flag based on 1)

Summary 1.Many non-standard quality control tests/decisions are made during conversion/upload to WOD. 2.About 3% of all measurements are flagged in WOD automatic qc process. 3.Important to decide whether to present all data with flags, or remove obviously bad data. 4. > 80% of all quality control flags in WOD are standard deviation outliers. This procedure needs to be examined/changed. 5.Subjective checks important: for specific purpose or general? 6.Important to decide how to present quality control flag information