These FAQs are a compilation of questions that we have received from our users since the release of World Ocean Database 2018.
The questions-answers are split into three categories: netCDF ragged array format which follows Climate-Forecast conventions.
If you do not see the answer to your question, please e-mail OCL.firstname.lastname@example.org.
- How do I cite the World Ocean Database (WOD)?
- Is there an easy way to extract header information from historical temperature profiles without actually processing all the data?
- How can I get the WOD data in NetCDF format?
- How can I visually read the WOD ASCII data?
- How can I download the WOD18 software and User's Manual?
- Why there is a drop in number of profiling float casts from WOD09 to WOD18?
- How do I download WOD18 data for a specific region?
- Is there any way to download the whole World Ocean Database?
- How long are my requested files available on the ftp site?
- How do I read WOD18 data with the Ocean Data View (ODV) software?
- How can I calculate the apparent oxygen utilization (AOU) in the deep ocean?
- How do I download Sea Surface Temperature (SST) data?
- How can I download surface data (SUR) and how are they organized?
- Is there a time variable in WOD?
- Does the WOD netCDF file contain information about the chlorophyll measuring method used?
- Is there an easy way to identify the corresponding project name, vessel, and other cruise attributes from WOD cruise ID number?
- Where can I look for references for XBT Bias Depth and Temperature Corrections?
- Where can I find information about the XBT correction method used for a profile?
- Where can I find information about the calibration of Chlorophyll CTD profiles?
- What do large negative values at particular depths mean? Why do these data have QC code '0', for good data?
- How are WOD depth values calculated?
- Explain the output format of WOD-reading programs.
- Am I allowed to use and reproduce WOD/WOA data/figures in my publications?
- Which methods are used for nutrient data?
Boyer, T.P., O. K. Baranova, C. Coleman, H. E. Garcia, A. Grodsky, R. A. Locarnini, A. V. Mishonov, T.D. O'Brien, C.R. Paver, J.R. Reagan, D. Seidov, I. V. Smolyar, K. Weathers, and M. M. Zweng, 2018: World Ocean Database 2018 (in preparation). In the meanwhile you may see the introduction to the publication.
Yes, if you have some programming skills. In particular data reading programs on www.nodc.noaa.gov/OC5/WOD/wod_programs.html can be adapted to retrieve only header information from WOD native ASCII files. Alternatively, you can use WODselect: set up your subset and obtain a graphic of the geographic distribution of the data.
You can get the World Ocean Database data in netCDF format from WODselect.
The WOD native ASCII format was constructed to save space and is not easily readable by the human eye. We have programs which will convert the native format into formats which are more easily readable and useable in common software. Please see: www.nodc.noaa.gov/OC5/WOD18/wod_programs.html. Another alternative is to download Ocean Data View, which is freeware for the display of oceanographic profile data. Please see the WOD18 tutorial for full details of obtaining Ocean Data View and uploading WOD native ASCII data. Another alternative is to use WODselect make a data selection, then from the download page, choose csv (comma-separated value format) or netCDF format, if either of these formats is easier for you to use.
The main reason is, there were a number of floats in the North Atlantic which had a pressure offset problem which was not correctable. These floats were removed by the Argo program from the Argo dataset, and we followed their example. The problem is noted on the Argo website.
Data for any specific region can be accessed through the World Ocean Database Select (WODselect).
If you want the entire database in NATIVE ASCII format, you can use the following: wget -N -nH -nd -r -e robots=off --no-parent --force-html https://data.nodc.noaa.gov/woa/WOD/YEARLY/ . If you want the netCDF format, you can use WODselect marking all years (1773-2018), and requesting the data in netCDF. You will receive an email when your request is ready, listing all the files for your request.
Files are removed from the ftp site 3 days after they are created by the WODselect application.
The user can use the ODV software to read the WOD18 native format and display the data. The user will need to open a new ODV collection (or add to an existing ODV collection) and specify "World Ocean Database Variables" in the "Definition of Collection Variables" menu. The detailed instructions are available from WOD Tutorial (0.5 MB). Note that ODV will not be able to read surface only data, originator's quality flags, or plankton biomass and taxonomic data in the OSD file.
To calculate AOU at any depth, use the saturation oxygen content calculated using potential temperature with respect to the surface, minus the dissolved oxygen measured at depth.
We do have historical sea surface temperature anomaly data from in situ measurements back to 1955 available at Global Ocean Heat and Salt Content online product. If you download the temperature anomaly data, then add the temperature anomalies back to the climatological means (also available on the same page) you will have SSTs (as well as water temperature in the depth) for each year (or season) 1955 to present.
This downloadable file contains the entire surface-only dataset: SURF_ALL.gz. The surface-only dataset is arranged somewhat differently than the profile datasets. A full surface-only cruise is fit into the format of a single profile cast. The individual levels of the surface-only form are distinct surface measurement sets from a given date/position for the cruise. So the cruise will span a time period from days to months rather than the single date/time of a profile cast. For this reason, the surface-only data are in a single file rather than split out by year in the YEARLY directory, or by position in the GEOGRAPHIC directory.
Times in WOD are in UTC. However, there are some cases where the time zone is not stated. In these cases UTC is assumed. There are other cases where time zone is reported as UTC but upon comparison with other documents or data it is apparent that the time is actually local. In these cases we correct the time to UTC. Please be aware that time zone is not always reported correctly.
Second Header variables in the netCDF file identify collection and processing methods used for the various
parameters. For chlorophyll, we identify a whole suite of methods that range from in situ fluorometer to HPLC. The codes are listed below:
601 Fluorescence in-situ Turner fluorometer (Strickland and Parsons 1972)
602 Fluorescence in-vivo underway (Lorenzen 1966)
603 Fluorometer in-situ CTD
604 Fluorometer (Aiken 1981)
605 Fluorometric chl-a assay acetone extraction
606 Fluorometric chl-a assay methanol extraction
607 Fluorometric chl-a assay acetone extraction; Turner fluorometer (Yentsch and Menzel, 1963, Holm-Hansen et al. 1965)
700 HPLC (High Performance Liquid Chromatography)
701 HPLC (normal phase High Performance Liquid Chromatography)
702 HPLC (reverse phase High Performance Liquid Chromatography)
The easiest way to get cruise info is to go to WODselect.
1. Check CRUISE option, hit Build a query button.
2. Enter cruise number in the search box, for example US029567, hit 'Get an Inventory' button. Multiple cruises can be entered separated by comma.
3. Hit CRUISE LIST button. As a result, you will get a list of cruises with links to cruise and accession metadata.
4. Click on link with cruise that you are interested in. It will open info file with data distribution map on the top. Scroll down to get metadata. See below example of info for cruise US029567. If you click on accession# link associated with this cruise you also will be provided with link to NODC Archive System.
WOD CRUISE REFERENCE US029567
COUNTRY UNITED STATES (US)
NODC ACCESSION NUMBER (CTD) 59005
SHIP NAME ALPHA HELIX
INSTITUTE ALFRED-WEGENER-INSTITUTE (BREMERHAVEN)
PROJECT SHELF BASIN INTERACTION PROJECT (SBI)
DATE OF FIRST CAST 9/ 8/2001
DATE OF LAST CAST 9/12/2001
TOTAL NUMBER OF CASTS 54
The XBT References Table contains the list of relevant references.
Second header code 54 gives the XBT correction applied. Please see a list of the codes and the correction they represent.
Measurements of chlorophyll from the CTD dataset were all taken with a fluorometer. Most of these data are uncalibrated, many still in engineering units. Use of these data would have to be done with great care and attention on a cruise by cruise basis. There is an indicator (variable specific second header 14) which is set if the data came to NODC calibrated. But this information is rarely included with the data, so it is not readily ascertainable if the data were calibrated. (This information is noted on page 19 of the World Ocean Database 2018 Introduction (0.5 MB) but is not readily available elsewhere, such as the WODselect pages).
Missing data are replaced by -999.99 in the WOD. It would probably be more clear if the flag accompanying a missing value was not '0', but we already use all one digit flags 0-9 and we limited ourselves to a one-digit flag.
Some data already include depth. If the data do not include depth, but include pressure, we calculated depth using the UNESCO algorithm for standard ocean. The expendable bathythermograph (XBT) has depths which are calculated from a drop-rate equation and time since drop. Some older data have depth calculated from wire length and angle. The UNESCO equation is from Saunders and Fofonoff (1976), so data received at the US NODC prior to that publication may have depths calculated with a slightly different method. Regardless of how depths were calculated or measured, all profiles are interpolated to the standard depths used in the World Ocean Atlas.
In general, the output includes ocean parameters, number of significant digits stored, and QC flags. The value in parentheses is the number of significant figures in the measured value directly to the left. Note that the program only prints out the first 3 decimal places, but the full value to the given significant figures is stored in the array read from the data file. For second headers, the number in parentheses is the number of significant figures in the value directly to the left. The two numbers in brackets are two single digit quality flags. The first digit is the World Ocean Database quality flag, detailed in WOD User Manual (0.5 MB). The second is a flag set by the data originator. The value of this digit is dependent on which originator flag is used. This information (if flags are set) is in second header 96. Please see WOD codes or the above user manual, which has the flag values included.
The World Ocean Database and World Ocean Atlas are open for public use without restriction. Relevant citations are present in this list of frequently asked questions. Please let us know about WOD and WOA based publications by sending citations to Tim.Boyer@noaa.gov.
The information on methods used for nutrients and how it is handled in the World Ocean Database (WOD) is explained in the WOD Introduction (0.6 MB) on page 45, see quoted text below: "It is difficult to estimate the precision and reproducibility of the historical chemical data in part because (1) there has not been a generally accepted set of standard international analytical oceanographic methods; (2) there has been a continuous availability over time of new or improved analytical techniques for the sampling and determination of the concentration of dissolved and particulate constituents in seawater; (3) there is the practical difficulty of periodic comparison of the precision and accuracy of oceanographic data collected by oceanographic institutions worldwide. At present, we are not aware of a suitable monitoring program for the systematic comparison of analytical instruments, measurements, and certified reference standards used by international research Institutions or Universities to collect oceanographic observations." We do include information on method used for nutrient (and other) data when that information is available. Variable description for the second header 6 may be found on WOD codes page, or accessed directly through methods list (0.01 MB).
World Ocean Atlas FAQs3> top^
- How do I cite the World Ocean Atlas 2018?
- How do I read WOA18 quarter degree netCDF files?
- How do I retrieve WOA18 data in ArcGIS format?
- What procedures does WOA use for quality control of data and products?
- How can I download the WOA18 data for the requested geographic area and output them in a comma separated (csv) ASCII format or a shapefile format compatible with GIS software such as ArcMap?
- What vertical interpolation method is used in the WOA18?
- What is meant by decadal averaging (decav) in WOA18?
- Where is the ftp site for WOA18?
- What are the units for temperature and salinity in the WOA18?
- Which years are the WOA18 climatologies based on?
- What is the accuracy of the WOA18 climatological means?
- How can I get WOA18 and how much does it cost?
- How are temporal changes in data coverage accounted for in the WOA18?
- How can I get the geographical boxes for WOA18 basins?
- What is the separation line between northern and southern parts of basins in the WOA18?
- How can I select vertical profiles of temperature and salinity for a particular geographic area?
- What is the temporal and vertical resolution of the WOA18 climatologies?
Locarnini, R. A., A. V. Mishonov, O. K. Baranova, T. P. Boyer, M. M. Zweng, H. E. Garcia, J. R. Reagan, D. Seidov, K. Weathers, C. R. Paver, and I. Smolyar, 2018. World Ocean Atlas 2018,
Volume 1: Temperature. A. Mishonov Technical Ed.; in preparation
Salinity: Zweng, M. M., J. R. Reagan, D. Seidov, T. P. Boyer, R. A. Locarnini, H. E. Garcia, A. V. Mishonov, O. K. Baranova, K. Weathers, C. R. Paver, and I. Smolyar, 2018. World Ocean Atlas 2018, Volume 2: Salinity. A. Mishonov Technical Ed.; in preparation
Oxygen: Garcia, H. E., K. Weathers, C. R. Paver, I. Smolyar, T. P. Boyer, R. A. Locarnini, M. M. Zweng, A. V. Mishonov, O. K. Baranova, and J. R. Reagan, 2019. World Ocean Atlas 2018, Volume 3: Dissolved Oxygen, Apparent Oxygen Utilization, and Oxygen Saturation. A. Mishonov Technical Ed.; in preparation
Nutrients: Garcia, H. E., K. Weathers, C. R. Paver, I. Smolyar, T. P. Boyer, R. A. Locarnini, M. M. Zweng, A. V. Mishonov, O. K. Baranova, and J. R. Reagan, 2019. World Ocean Atlas 2018, Volume 4: Dissolved Inorganic Nutrients (phosphate, nitrate, silicate). A. Mishonov Technical Ed.; in preparation
Due to the size of the quarter degree files compression software was used. These files are in netCDF4. Classic netCDF3 cannot read netCDF4. To read the quarter-degree files you need to install netCDF4 on your system.
The WOAselect returns selected data in ArcGIS shapefile form. It takes a long time to tar and compress larger files, so the link may not be immediately available. Test case of global 1/4 degree salinity at the surface runs about 10 minutes. Note that even though only one depth is requested through WOAselect, all depths are included in the ArcGIS file, so one not need multiple requests for different depth for the same field.
The quality control procedures for the input data for the World Ocean Atlas are detailed in the World Ocean Database introduction and user manual, which can be found at WOA documentation.
On the WOAselect web page choose one of the variables, make choices in the various menus (or leave as is), then click show figure. Above the figure you will see 'Data Access'. Click on the ArcGIS format or other desired format.
If data coverage allows, we use the 4-point Reiniger-Ross interpolation method directly, no changes to their algorithm. Where we are different is in some of the details of implementation. For instance, we don't use variable values for depths which are outside a given envelope of distance from the target standard depth: see table 4 in the WOA18 temperature atlas (p 23). We also use three point Lagrangian or, as a last resort, linear interpolation, when there are not enough valid points for Reiniger-Ross. Finally, we do not use levels for which quality control tests have failed.
The World Ocean Atlas 2018 decav (decadal average) climatology is the average of the six decadal climatologies. The standard deviation (and standard error, and data distribution) for the decav fields are not means of the 6 decades, but calculated for all data. So, while the climatological mean (and the statistical mean) for January are average values for Jan. 1955-1964, Jan 1964-75, ... Jan 2015-18, the standard deviation values are the standard deviation of all Jan data regardless of year.
Here is the ftp site: ftp://ftp.nodc.noaa.gov/pub/data.nodc/woa/WOA18/DATA.
In situ temperatures used for WOA18 are not converted from their original scale, so there is a mix of IPTS-48, IPTS-68, and ITS-90 (and pre IPTS-48 temperatures). The differences between scales are small (on the order of 0.01°C) and should not have much effect on the climatological means, except, possibly at very deep depths. Values for salinity are on the Practical salinity scale (PSS-78). Pre-1978 salinity values converted from conductivity may have used a different salinity scale. Pre-conductivity salinities use the Knudsen metnod. A description of PSS-78 and its predecessors can be found here: Millero, F.J. 2010. History of the equation of state of seawater. Oceanography, 23(3):1833, doi:10.5670/oceanog.2010.21. Lewis, E. L., 1980, The Practical Salinity Scale and its Antecedents, IEEE Journal of Oceanic Engineering, vol. OE-5, No. 1.
For temperature and salinity, they are based on a mean climatological field from 1955-2017 (which is the average of 6 decadal climatological means). For oxygen, phosphate, silicate, and nitrate, the climatological mean uses all data regardless of time period. The time period of measurements is from the early 1900s to present.
The World Ocean Atlas climatologies are long-term means. So the question of accuracy and precision are hard to answer. The measurements which went into the calculation of the means have accuracy ranging from 0.3°C (Mechanical Bathythermographs) to 0.001 (well calibrated platinum resistance thermometers) for temperature (and a relatively similar range for salinity). But, the long-term mean is also affected by data sparsity and by time bias (some areas with more measurements in a specific subset of the overall time span). Observed temperature data are stored to 3 digits right of the decimal. This is at the limit of the most accurate measurement device. For climatological means we also provide standard deviation and standard error of the mean fields to give an idea of the spread of values in any geographic area over the long-term. When we use the climatologies to quality control data, we expect that the individual temperature values are within 3 standard deviations of the long-term mean at a given depth and location. We can not provide simple numbers for accuracy and precision, but with the long-term mean, there are more factors than simple measurement accuracy and precision.
The World Ocean Atlas 2018 (and all previous versions) are available free of charge. You can download the data, atlases, and figures from the WOA18 web site.
Temporal changes in data coverage are the main reason we compute six decadal climatologies and then average the six decades. We address this in our recent EOS article and its supplemental material. The whole southern hemisphere is skewed to the Argo period if all data are used together. By computing decadal climatologies and averaging them, even if there are 50 values in an area from one decade, and only one in each other decade, each decade is treated as 1/6 of the total average. Of course if there are 50 values in one decade and none in any other decade, there will still be a skewed value for the area. The analysis procedure itself ameliorates this possible problem by using a fairly large radius of influence (880km for the first pass) and using seasonal values as first guess for each month (annual values for each season) so the analyzed value and decadal means still reflect a six decadal balance in most cases. It must be noted here that the standard deviation, standard error of the mean, and data distribution fields for the decadal averages are not averages of the six decadal fields. We did not come up with a good way to combine the standard deviation for the six decades in a meaningful way, so the standard deviation field associated with the decadal averaged objectively analyzed field is actually an all-data (all time period) field. There are standard deviation fields for each of the six decades, so you can see the differences of the standard deviations between the decades - but the standard deviation field provided with the decadal averaged field can be skewed in some areas, especially the southern Hemisphere, toward the Argo era. This is detailed, with some additional information, in the World Ocean Atlas 2018 publications.
The basin masks for the Pacific, Indian, and Atlantic basins are available in the heat content netCDF files accessible from
Global Ocean Heat and Salt Content web site.
South encompasses all latitudes (box centers) -89.5 to -0.5.
If netCDF is not convenient, we have a full basin mask (all depths) in a comma-separated file for the World Ocean Atlas which follows the same boundaries,
but must be manipulated somewhat.
Use just the surface level. The list of codes corresponding to the basins are found in appendix one (page 11) of
WOA Documentation (0.1 MB).
The following basins/codes are for the Southern Hemisphere:
1 - Atlantic Ocean
10 - Southern Ocean, between 63°W and 20°E longitudes The Pacific:
2 - Pacific Ocean
10 - Southern Ocean, between 147°E and 63°W The Indian:
3 - Indian Ocean
10 - Southern Ocean, between 20°E and 147°E.
The line between Northern and Southern parts is the Equator.
To get temperature and salinity from our gridded data use WOAselect (World Ocean Atlas Select). WOAselect is a tool for visualizing subsets of the World Ocean Atlas. When you make your choices and a graphic is returned, on the upper left of the graphic are two links, one for comma delimited ASCII and one for ArcGIS shapefiles. These files will contain all standard depths for the geographic area you chose for the given variable and time period. You will have to download temperature and salinity climatologies separately, and download each month separately, so it may take a little work. Otherwise, download a full set (monthly, seasonal, annual) of mean fields for each variable in WOA18 data and select your area from these gridded fields.
Table 4 in the WOA18 documentation (0.1 MB) lists variables for each time compositing period (annual, seasonal, monthly) and the standard levels for which climatological means were calculated. For temperature, salinity, and oxygen, annual and seasonal time periods have 102 levels with climatological means calculated (surface-5500 m). The monthly time periods have 57 levels (surface-1500 m). This is due both to data sparsity below 1500 m and the surface forced seasonal cycle not penetrating (in most regions) to depths below 1500 m. For the quarter-degree WOA18 climatologies, temperature and salinity, the mean for 2015-2018 and the all-decade average include monthly fields, whereas the other decades (1955-64, 1965-74, 1975-84, 1985-94, 1995-2004, 2005-2014) include only annual and seasonal (3-month average) fields.
Global Ocean Heat and Salt Content FAQs3> top^
- What is the file format for Global Ocean Heat and Salt Content?
- Why is there an inconsistency between the 5-year and shorter average ocean heat anomalies?
- What are the details of the heat content calculations?
- Are yearly Salt Content data available?
- What are the units for 0-700 m heat content?
- What is the halosteric component of sea level rise?
See the documentation (0.1 MB) for the World Ocean Atlas 2018 climatological fields. The format for the ASCII data is the same for the temperature anomaly and global heat content fields. The format is described in section 5 of the document (page 8) with relevant example on page 12.
This is not a 5-year average, it is a 5-year composite. So it takes the mean anomaly in each one-degree grid over a 5-year period, using all measurements during those 5 years. Since our first guess field for the objectively analyzed temperature anomalies (from which ocean heat content anomaly is calculated) is 0.0 (no difference from mean) those areas without data are underestimated - closer to the 0.0 anomaly line. For 3-month periods, especially toward the beginning of the time series, there are many data sparse areas where the anomalies are underestimated in this way. So, the red curve (3-month mean anomalies) and the black curve (yearly average of 3-month anomalies) are usually underestimated (in magnitude) compared to the 5-year composite mean which encompasses a much longer time period and therefore most often has better data coverage and fewer underestimated areas.
First, temperature anomalies are calculated at each of 16 standard depths surface to 700m (or 26 standard depths surface to 2000m) by subtracting observed (interpolated) temperatures at each standard depth from the long-term (1955-2018) climatological monthly mean. The mean of all temperature anomalies is calculated at each standard depth for each one degree gridbox. The temperature anomaly is considered representative over a volume of water consisting of a vertical distance from halfway between the next shallower depth and the given standard depth to halfway between the next deeper depth and the given standard depth. The temperature anomaly is multiplied by the climatological mean density of the one-degree square and the heat capacity of water and the area and volume of the one-degree square for the given standard depth. The heat contents for each volume surrounding a standard depth are then summed to get the full ocean heat content anomaly for each one degree gridbox. The values for each gridbox are summed to get the global value. [Note this is a global integral, not an average.] The area of each one-degree gridbox is calculated similar to the attached FORTRAN subroutine (easily adaptable to any software language). The land/sea mask used to decide whether a one-degree gridbox is land or ocean (the one-degree, not quarter-degree) and is derived from the ETOPO2 altitude/bathymetry data set. This same land/sea mask is used to determine whether the volume of a given one-degree square extends to the bottom of the integration level (700 m or 2000 m) or to a shallower depth. Ocean temperature measurements are relatively sparse subsurface, as data are not found in each one-degree gridbox over the entire ocean. So, once mean temperature anomalies are calculated at each standard level for each one-degree gridbox for which there are measurements over the given time period, an objective analysis technique is used to calculate a complete set of one-degree temperature anomaly data at each standard depth. From there the heat content is calculated. The objective analysis technique is described in a number of publications, including WOA18 Temperature (0.5 MB).
We do have pentadal salinity anomalies back to 1955. See the Global Heat and Salt Content online product, but yearly data only spans the Argo time period (i.e., 2005 onward) and available on the salinity anomaly page.
The value of J/m**2 is multiplied by the grid area over which the heat content is calculated, resulting in final units of joules [J].
Halosteric change is the change in sea level due to the effects of salinity change on seawater density.