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AcknowledgmentsThis work was made possible by a grant from the NOAA Climate and Global Change Program,which enabled the establishment of a research group at the National Oceanographic Data Center(now the National Centers for Environmental Information – NCEI). The purpose of this group isto prepare research quality oceanographic databases, as well as to compute objective analyses of,and diagnostic studies based on, these databases. Support is now from base funds and from theNOAA Climate Program Office.The data on which this atlas is based are in World Ocean Database 2018 and are distributed online by NCEI. Many data were acquired as a result of the IOC/IODE Global Oceanographic DataArchaeology and Rescue (GODAR) project, and the IOC/IODE World Ocean Database project(WOD).The WOD is a composite of publicly available ocean profile data, both historical and recent. Weacknowledge the scientists, technicians, and programmers who have collected and processed data,those individuals who have submitted data to national and regional data centers as well as themanagers and staff at the various data centers. We are working on a more substantive andformalized way to acknowledge all those who have collected and contributed to oceanographicmeasurements, which were used to calculate the fields in the WOA. Until we have such a systemin place, we direct the reader’s attention to lists of primary investigators, institutions, and projects,which contributed data (codes can be used to locate data in the World Ocean Database). We alsothank our colleagues at the NCEI. Their efforts have made this and similar works possible.We thank the thoughtful reviewers whose comments greatly improved this manuscript: ChristopheMaes (Institut de recherche pour le développement, Brest, France) and Yongsheng Zhang (NOAANational Centers for Environmental Information, Silver Spring, Maryland, USA).We dedicate this work to Carla Coleman who alwayscontributed with a smile and was taken from us too soon.v

WORLD OCEAN ATLAS 2018Volume 2: SalinityABSTRACTThis atlas consists of a description of data analysis procedures and horizontal maps ofclimatological distribution fields of salinity at selected standard depth levels of the World Oceanon a one-degree and quarter-degree latitude-longitude grids. The aim of the maps is to illustratelarge-scale characteristics of the distribution of ocean salinity. The fields used to generate theseclimatological maps were computed by objective analysis of all scientifically quality-controlledhistorical salinity data in the World Ocean Database 2018. Maps are presented for climatologicalcomposite periods (annual, seasonal, monthly, seasonal and monthly difference fields from theannual mean field, and the number of observations) at 102 standard depths.annual “all-data” climatology was calculatedusing observations from all months of allyears. Seasonal “all-data” climatologieswere calculated using only data from thedefined season (regardless of year). Theseasons are defined as follows: Winter isdefined as January, February, and March;spring as April, May, and June; summer asJuly, August, and September; and fall asOctober, November, and December. Monthly“all-data” climatologies were calculatedusing data only from the given month.1. INTRODUCTIONThis atlas is part of the World Ocean Atlas2018 (WOA18) series. The WOA18 seriesincludes analysis for temperature (Locarniniet al., 2019); salinity (this atlas); dissolvedoxygen (Garcia et al., 2019a); and dissolvedinorganic nutrients (Garcia et al., 2019b).This atlas presents annual, seasonal, andmonthly climatologies and related statisticalfields for salinity. Climatologies in this atlasare defined as mean oceanographic fields atselected standard depth levels based on theobjectiveanalysisofhistoricaloceanographic profiles and select surfaceonly data. A profile is defined as a set ofmeasurements for a single variable(temperature, salinity, etc.) at discrete depthstaken as an instrument drops or risesvertically in the water column. Temperatureand salinity climatologies are the average ofsix “decadal” climatologies for the followingtime periods: 1955-1964, 1965-1974, 19751984, 1985-1994, 1995-2004, and 20052017, as well as a “climate normal” period1981-2010 as defined by the WorldMeteorological Organization (WMO, 2017).Oxygen and nutrients climatologies use allavailable data regardless of the year of itsobservation (“all-data” climatology). TheThe salinity data used are available from theNational Oceanographic Data Center (NCEI)and World Data Service (WDS) forOceanography, Silver Spring, Maryland.Large volumes of data have been acquired asa result of the fulfillment of several datamanagement projects including:a) the Intergovernmental OceanographicCommission (IOC) GlobalOceanographic Data Archaeology andRescue (GODAR) project (Levitus etal., 2005);b) the IOC World Ocean Database project(WOD);c) the IOC Global Temperature SalinityProfile project (GTSPP) (IOC, 1998).1

The salinity data used in the WOA18 havebeen analyzed in a consistent, objectivemanner on one-degree and quarter-degreelatitude-longitude grids at standard depthlevels from the surface to a maximum depthof 5500m. The procedures for “all-data”climatologies are identical to those used inthe World Ocean Atlas 2013 (WOA13) series(Johnson et al., 2013, Locarnini et al. 2013,Zweng et al., 2013), World Ocean Atlas 2009(WOA09) series (Locarnini et al., 2010;Antonov et al., 2010; Garcia et al. 2010 a, b),World Ocean Atlas 2005 (WOA05) series(Locarnini et al., 2006; Antonov et al., 2006;Garcia et al. 2006 a, b), the World OceanAtlas 2001 (WOA01) series (Stephens et al.,2002; Boyer et al., 2002; Locarnini et al.,2002; Conkright et al., 2002) and WorldOcean Atlas 1998 (WOA98) series (Antonovet al., 1998 a, b, c; Boyer et al., 1998 a, b, c;Conkright et al., 1998, a, b, c; O’Brien et al.,1998, a, b, c). Slightly different procedureswere followed in earlier analyses (Levitus,1982; World Ocean Atlas 1994 series[WOA94, Levitus et al., 1994; Levitus andBoyer 1994a, b; Conkright et al., 1994]).WOA13 differed from WOA09 by increasingthe number of standard levels used from 33to 102, increasing the resolution with depth;WOA18 continues to use the same 102 depthlevels as WOA13.season, thus precluding any representativeannual analysis. In some areas there may be areasonable spatial distribution of data pointson which to base an analysis, but there maybe only a few (perhaps only one) data valuesin each one-degree latitude-longitude square.Objective analyses shown in this atlas arelimited by the nature of the salinity database(data are non-uniform in both space andtime), characteristics of the objective analysistechniques, and the grid used. The primarylimitation of the analysis is data coverage inspace and time. Since the publication ofWOA13, substantial amounts of additionalhistorical salinity data have becomeavailable.However, even with theseadditional data, we are still hampered in anumber of ways by a lack of data. In someareas, quality control is made difficult by thelimited number of data collected in theseareas. Data may exist in an area for only one2.1. Data sourcesThis atlas is divided into sections. We beginby describing the data sources and datadistribution (Section 2). Then we describethe general data processing procedures(Section 3), the results (Section 4), summary(Section 5), and future work (Section 6).After the references (Section 7), theappendices of this atlas (Section 8) includedescriptions and examples of the stabilizationof the temperature and salinity climatologies.Maps for each individual depth level for eachtime period are available online.2. DATA SOURCES AND DATADISTRIBUTIONData sources and quality control proceduresare briefly described below. For furtherinformation on the data sources used inWOA18 refer to the World Ocean Database2018 (WOD18, Boyer et al., 2019). Thequality control procedures used inpreparation of these analyses are describedby Garcia et al. (2019a).Historical oceanographic salinity profile mperature-Depth(CTD)packages, profiling floats, moored anddrifting buoys, gliders, and undulatingoceanographic recorder (UOR) profiles usedin this project were obtained from theNCEI/WDS archives and include all datagathered as a result of the GODAR and WODprojects.An important source of salinity data is shipmounted thermosalinographs, which measuresurface temperature and salinity as ships2

standard depths (see section 3.1 for details).The levels at which the climatologies werecalculated are given in Table 2. Table 3describes the datasets used to calculate theclimatologies. Table 4 shows the depths ofeach standard depth level.transit the ocean. The primary focus of WODand WOA is profile and subsurface data, butwe have included some thermosalinographdata as part of the SUR (surface) dataset. Forfurther information, please see Chapter 14 ofthe WOD18 documentation (Boyer et al.,2019).2.2. Data quality controlTo understand the procedures for takingindividual oceanographic observations andconstructing climatological fields, it isnecessary to define the terms “standard leveldata” and “observed level data”. We refer tothe actual measured value of anoceanographic variable in situ as an“observation”, and to the depth at which sucha measurement was made as the “observedlevel depth.” We refer to such data as“observed level data.”Before thedevelopmentofoceanographicinstrumentation that measures at highfrequencies along the vertical profile,oceanographers often attempted to makemeasurements at selected “standard levels” inthe water column. Sverdrup et al. (1942)presented the suggestions of the InternationalAssociation for the Physical Sciences of theOceans (IAPSO) as to which depthsoceanographic measurements should bemade or interpolated to for analysis.Historically the World Ocean Atlas used amodified version of the IAPSO standarddepths. However, with the increased globalcoverage of high depth resolutioninstrumentation, such as profiling floats,WOA has extended the standard depth levelsfrom 33 to 102. The current standard depthlevels include the original depth levelspresented up to WOA09, but have tripled theresolution in the upper 100 meters, more thandoubled the depth resolution of the upper1000 meters, and almost three and a halftimes the resolution for overall depth levels.For many purposes, including preparation ofthe present climatologies, observed level dataare interpolated to standard depth levels ifobservations did not occur at the desiredQuality control of the salinity data is a majortask, the difficulty of which is directly relatedto lack of data and metadata (for some areas)upon which to base statistical checks.Consequently, certain empirical criteria wereapplied - see sections 2.2.1 through 2.2.4, andas part of the last processing step, subjectivejudgment was used - see sections 2.2.5 and2.2.6. Individual salinity data, and in somecases entire profiles or all profiles forindividual cruises, have been flagged and notused further because these data producedfeatures that were judged to be nonrepresentative or questionable. As part of ourwork, we have made available WOD18 thatcontains both observed levels profile data andstandard depth level profile data with variousquality control flags applied. The flags markeither individual measurements or entireprofiles that were not used in the next step ofthe procedure-- either interpolation tostandard depth levels for observed level dataor calculation of statistical means in the caseof standard depth level data.Constantly improving knowledge of theworld ocean variability now includes agreater appreciation and understanding of theubiquity of mesoscale features such aseddies, rings, and lenses in some parts of theworld ocean, as well as interannual andmulti-decadal variability of water massproperties associated with modal variabilityof the atmosphere such as the North AtlanticOscillation (NAO) and El Niño SouthernOcean Oscillation (ENSO). Some of thesefeatures, especially in the region with densedata coverage like, for example theNorthwest Atlantic Ocean, can find their way3

into high-resolution

DRB Drifting Buoy ENSO El Niño-Southern Oscillation ERL Earth Research Laboratory ETOPO2 Earth Topography 2 arc minute EVR Extended Vertical Resolution GIS Geographic Information System GLD Glider GMT Greenwich Mean Time, or Generic Mapping Tools GODAR Global Ocean Data Archaeology and Rescue