Technical note: TEOS10 Excel – implementation of the Thermodynamic Equation Of Seawater – 2010 in Excel
 MLA College, The Merchant, St Andrew Street, Plymouth, PL1 2AX, UK
 MLA College, The Merchant, St Andrew Street, Plymouth, PL1 2AX, UK
Correspondence: Carlos Gil Martins (carlos.martins@mla.ac.uk)
Hide author detailsCorrespondence: Carlos Gil Martins (carlos.martins@mla.ac.uk)
This paper and associated software implement the Thermodynamic Equation Of Seawater – 2010 (TEOS10) in Excel for an efficient estimation of Absolute Salinity (S_{A}), Conservative Temperature (Θ), and derived thermodynamic properties of seawater – potential density (σ_{Θ}), in situ density (${\mathit{\rho}}_{{S}_{\mathrm{A}},\mathrm{\Theta},p}$), and sound speed (c). Vertical profile template plots for these parameters are included as is an S_{A}–Θ diagram template, which includes plotting of the density field (computation of userselected σ_{Θ} lines is included). Absolute Salinity can be directly measured with the aid of a densimeter (IOC, SCOR and IAPSO, 2010: p. 82), but in TEOS10 its estimation relies on the interpolation of data from casts of seawater from the world ocean (IOC, SCOR and IAPSO, 2010), and the Excel workbook introduced here (TEOS10 Excel, available at https://doi.org/10.5281/zenodo.4748829) includes a subset of the TEOS10 lookup tables necessary for this estimation, namely the Absolute Salinity Anomaly [deltaSA_ref] and the Absolute Salinity Anomaly Ratio [SAAR_ref] lookup tables. As the user simply needs to paste new data into the spreadsheet to automatically compute the oceanographic parameters referred above, this tool may prove to be useful for all who are not comfortable using the fullfeatured TEOS10 programming language environments (e.g. MATLAB, FORTRAN, C) but rather need a simpler way of computing fundamental properties of seawater (e.g. density, sound speed) while adhering to current standards. Returned values are the same (up to 15 decimal places; i.e. difference = 0.000000000000000) as the ones obtained with the MATLAB version of the GSW (Gibbs Sea Water) toolbox (McDougall and Barker, 2011) available at the TEOS10 website (https://www.teos10.org, last access: 13 April 2022). This paper describes the Excel workbook, its use, and the included VBA (Visual Basic for Applications) functions. Quality control against the GSW toolbox is also addressed, namely issues detected with the interpolated values returned by the toolbox when there are missing values in the reference lookup table. In these situations, the GSW toolbox replaces missing values with a level pressure horizontal interpolation of neighbour points, while it is clear from the testing results that vertical interpolation, which was then implemented in TEOS10 Excel, returns a more robust solution.
The development of software to facilitate the efficient calculation of the properties of seawater has allowed users to better understand the marine environment, assisting members of the student, research, and industrial communities alike. One such initiative, the Gibbs Sea Water (GSW) toolbox (McDougall and Barker, 2011), implements the Thermodynamic Equation of Seawater – 2010 (TEOS10) into software that calculates required seawater properties through the utilisation of programming languages (e.g. MATLAB, FORTRAN, C) that require a working understanding and knowledge of computer programming. As such, the toolbox may not be as readily accessible to all practitioners within the field of marine data analysis (e.g. BuzzettoMore et al., 2010; Bosse and Gerosa, 2017). The aim of this paper is to present an implementation of TEOS10, within Microsoft Excel, a popular and readily available application. This new implementation requires no specialist knowledge to operate; it is therefore hoped that all groups interested in analysing sea water properties may benefit from free and open access to this new tool.
Seawater can be defined as a thermodynamic system with one liquid phase and two components: (i) pure water and (ii) dissolved salts. At the end of the 19th century, J. Willard Gibbs, established the Gibbs phase rule (Gibbs, 1874–1878), which states that, for a multiphase system in thermodynamic equilibrium, such as seawater, the degrees of freedom of the system, i.e. the number of independent variables needed to define it, equal the number of components subtracted by the number of phases plus two. For seawater this adds up to three ($\mathrm{2}\mathrm{1}+\mathrm{2}$), and the “chosen” variables are salinity, temperature and pressure.
As measurement technologies advance and our understanding of the oceanic environment evolves, standards relating to physical parameters frequently change in response. The definition of salinity has undergone several variations during the last century (Millero, 2010) and the temperature standard changed in 1989 from IPTS68 to ITS90 (PrestonThomas, 1990). The current TEOS10 has introduced a new salinity quantity, Absolute Salinity (S_{A}), defined as “the mass fraction of dissolved material in seawater” (IOC, SCOR and IAPSO, 2010: p. 3); however, Absolute Salinity is arguably more accurately defined as the mass fraction of dissolved material in Reference Composition Seawater of the same density as that of the sample (Wright et al., 2011). Accompanying S_{A}, a new temperature quantity, Conservative Temperature (Θ), was also introduced. Conservative Temperature is estimated from potential temperature (θ) and S_{A} and is 2 orders of magnitude more conservative than θ (IOC, SCOR and IAPSO, 2010: p. 5). These two new quantities, S_{A} and Θ, together with pressure (p), are now the arguments of the equation of state, and to compute any thermodynamic property of seawater (e.g. density, sound speed) they must be estimated first. Practical Salinity (S_{P}), which was used before in the Equation of State – 80 (EOS80), is, however, still required for the determination of S_{A} and remains the recommended way by which salinity should be archived in oceanographic databases (IOC, SCOR and IAPSO, 2010: p. 8).
The polynomial nature of EOS80 allowed the easy implementation of algorithms for the computation of seawater properties, which led to the proliferation of standalone applications, interactive websites, and Visual Basic for Applications (VBA) modules. Direct measurement of Absolute Salinity can be made with the aid of a densimeter (IOC, SCOR and IAPSO, 2010: p. 82), but in GSW it is estimated from interpolation of measured Absolute Salinity Anomalies stored in a world atlas lookup table. This difficulty might be a possible explanation for the absence of any previous application of TEOS10 in Excel, except for a tool (GSW_Sys_v1.0.xlsm)^{1} cited in Jiang et al. (2022). We have tested this Excel tool, using the two data sets included in TEOS10 Excel (TEOS10 Test Data and TS55), and, for both data sets (NW Pacific and NE Atlantic, respectively), there were differences in the estimation of Absolute Salinity, starting at the fourth decimal place (positive and negative). As discussed in Sect. 3., the results from TEOS10 Excel are the same (up to 15 decimal places), for every parameter, as the ones obtained with the GSW toolbox.
Section 2 of this paper introduces the new TEOS10 Excel workbook, explains its operation, and describes the world ocean lookup tables. Section 3 describes the translation of the original MATLAB code into VBA and discusses the interpolation method used for missing data in the reference lookup table, followed by the conclusion and summary in Sect. 4.
An Excel workbook file that implements a subset of the GSW toolbox accompanies this paper. The file includes sample data that can be easily replaced by new user data to obtain ocean vertical profiles and S_{A}–Θ diagrams. The computation algorithms are implemented as VBA functions and are used as any other standard Excel function. The TEOS10 world ocean lookup tables of measured Absolute Salinity Anomaly [deltaSA_ref] and Absolute Salinity Anomaly Ratio [SAAR_ref], essential to estimate Absolute Salinity, are included in the workbook and are described later in the paper. The desktop app version of Microsoft Office is needed to use the workbook, as VBA macros do not run in Microsoft Web Office. On opening the Excel file, authorisation for running macros must be granted.
The workbook (Fig. 1) contains four data spreadsheets (three light green tabs and one yellow), two plotting spreadsheets (blue tabs), six TEOS10 lookup tables (purple tabs), and an info tab (green). Pressing Alt + F11 (Windows) or Fn + Alt + F11 (Mac) opens the VBA environment allowing access to the 15 function modules (Table 1), although access to these is not required to make use of the Workbook nor is a working knowledge of VBA.
2.1 The light green data tabs
The structure of the three light green data tabs is identical, the only difference being the data sets incorporated in each. The “TEOS10 Test Data” spreadsheet includes a testing data set from the GSW toolbox, located in the NW Pacific at 33^{∘} N, 162.5^{∘} E. “TS55” data are a 1^{∘} longitude × 1^{∘} latitude historical average vertical profile in the NE Atlantic, off the Iberian Peninsula, centred at 40.5^{∘} N, 10.5^{∘} W, with pressure levels interpolated to standard “Levitus” levels (Levitus, 1982), and “CTD020” is a CTD cast in the same grid bin, at 40^{∘}05^{′} N, 10^{∘}01^{∘} W (Martins, 1998). Seawater properties in coloured columns are computed on the fly from user data input in white cells. The data included (in white cells) can be replaced by user data. Spreadsheet lines can be added (or deleted) and, if additional lines are required, the user need only copy down the coloured cells for the formulae to propagate over the extra rows, without any further adjustments being necessary. The only caution users should have, is to not move the data (white cells) to other locations, as the spreadsheet formulae will “follow” this operation, disrupting the original cell referencing. Users may also add new data spreadsheets to the Excel workbook, where they can then simply paste the whole content of one of the original data tabs for the new spreadsheet to become fully functional.
2.1.1 Data input

Location. The data template for the light green tabs was developed to process vertical casts located at a given location. Longitude and latitude must be input in cells “B1 : B2” in decimal format (degrees). Longitude can either be within the domain −180 to 180^{∘} or 0 to 360^{∘}; i.e. 10^{∘}30^{′} W can be input as −10.5 or 349.5^{∘}. The latitude domain is −90 to 90^{∘}; i.e. 30^{∘} S would be −30^{∘}. The input of the cast coordinates is essential, as Absolute Salinity is dependent on location (Sect. 3.6). If either the longitude or the latitude cells are left empty, the salinity anomaly is set to zero and Absolute Salinity becomes equal to Reference Salinity.

Pressure. Pressure (p) units are decibar. For seawater properties, pressure is always the pressure of the water column, i.e. absolute pressure subtracted by atmospheric pressure. Therefore, at the surface, p=0. For the upper ocean, 10 dbar ≈10 m.

Salinity. The user can toggle the input between Practical Salinity (the salinity quantity which continues to be the recommended quantity to be archived (IOC, SCOR and IAPSO, 2010)), conductivity (mS cm^{−1}) (i.e. measured by an in situ transducer), or the salinometer ratio (Rt) (i.e. ratio between the conductivities of the sample and of Standard Seawater, measured by a laboratory salinometer). Column D of the spreadsheet (“Practical Salinity (S_{P})”) either copies the S_{P} value if this was the salinity input or calculates S_{P} from conductivity using the function {SP_from_C(C,t,p)} or from the salinometer conductivity ratio using the function {SP_salinometer(Rt,t)}, depending on the radio button selected. In the latter option, t is the temperature of the thermostable bath of the laboratory salinometer.

Temperature. Temperature (^{∘}C) may be selected to be either ITS90 or IPTS68 (data sets before 1990 are in the IPTS68 standard, but recent data may still be applying this standard instead of the newer ITS90 – checking the instrument specifications and/or the metadata associated with the data is advisable). Column J of the spreadsheet (“Temperature ITS90”) either copies the temperature input if ITS90 is selected or converts the IPTS68 values to ITS90 (ITS90 = IPTS68/1.00024). All functions use temperature ITS90 as input.
2.2 The yellow Surface Data tab
The yellow Surface Data tab content differs from the other data spreadsheets on what refers to the input of the location coordinates. In this spreadsheet, longitude and latitude are input in the first two columns, allowing for the assignment of distinct coordinates for each line. This is useful if the data set is not a vertical cast at a given location but a set of measurements at different locations, typically at the same pressure level (e.g. surface measurements). Fictional data are included here to demonstrate the use of the template. The location of the first four data lines is in the Baltic Sea. Conditions in the Baltic are different from the open ocean (McDougall, 2010) and TEOS10 treats this adjacent sea as being a specific case. Whilst for the world ocean the estimation of S_{A} depends on the measured salinity anomaly values at that location (lookup tables), for the Baltic Sea it is estimated by Eq. (1).
Whenever the location is in the Baltic (which is checked by the {is_Baltic(lon,lat)} function), the salinity anomaly cells display “Baltic”. This spreadsheet also includes a line with data from line 1 of the TEOS10 Test Data tab (surface data from the NW Pacific) as well as a line of data without location coordinates (e.g. a sample from an estuary). In this case, salinity anomaly is set to zero and Absolute Salinity becomes equal to Reference Salinity.
2.3 Vertical Profiles tab
The Vertical Profiles tab includes five plots that use the TS55 and CTD020 data sets and one plot with the TEOS10 Test Data data set. Two of these plots are reproduced in Figs. 3 and 4. Changing the data will update the plots accordingly, and the user can add extra profiles by rightclicking the plot area, clicking “Select Data”, and then editing the data sources.
2.4 S_{A}–Θ Diagram tab
This is a template for plotting Absolute Salinity–Conservative Temperature diagrams. Since the introduction of TEOS10, S_{A}–Θ diagrams have replaced T–S diagrams (EOS80) for the characterisation of water masses in the ocean. The two diagrams represented (Fig. 5) are from the NW Pacific (TEOS10 Test Data) and NE Atlantic (TS55). Users can rightclick the plot area, click “Select Data” and edit the data sources. The plot also shows the pressure values at selected points along the two S_{A}–Θ diagram lines (the label of the points is retrieved from the pressure data column in the respective data tab). These points can be individually selected and edited. The density field is shown through a set of σ_{Θ} dashed lines obtained from S_{A}–Θ pairs that resolve to constant values of σ_{Θ} (i.e. 24, 25, …, 29). As in all data spreadsheets, white cells can be edited. In this case, S_{A} extends from the x axis minimum (33.0) to the x axis maximum (38.0) with a 0.05 increment. The Conservative Temperature (Θ) values (green columns) are obtained by the function {sigma_CT_line(SA,sigma,min_temp, max_temp}. If more σ_{Θ} lines are desired, additional column pairs can be inserted into the spreadsheet and new series added to the plot.
2.5 The TEOS10 lookup tabs (purple) a.k.a. the “Atlas”
Figure 6 shows the [ndepth_ref] lookup table which contains the number of pressure levels in each of the seawater samples that constitute the Atlas. On close inspection, it becomes apparent that the empty cells represent land, and the “white” shapes approximate to a map of the world land masses. The top of the spreadsheet is the South Pole, the bottom is the North Pole, and the Greenwich Meridian (0^{∘} longitude) is to the left. Longitude is positive to the right, so actually the “map” is a skewed mirror image of the earth surface. Nonetheless, mirrored continental shapes are identifiable. The longitude bins (or cells) are referenced in the [longs_ref] lookup table, so the longitude of the cell highlighted in green in the upper left of Fig. 6, for which the x coordinate is 4 (fourth column), is 12^{∘} E (value at the fourth line of the [longs_ref] table). This cell is at line 9 (y coordinate = 9) of the [ndepth_ref] table (Fig. 6), which, looking up in the [lats_ref] table, corresponds to −54^{∘} of latitude. The green cell in Fig. 6 corresponds though to a reference cast located at 54^{∘} S, 12^{∘} E, with 41 pressure levels. These 41 pressure levels correspond to the pressure values indicated in the [p_ref] table. The [p_ref] table has level 22 (1010 dbar) highlighted in green, as this 3D location is used and referenced as a debug point in the {LookUp_atlas(table_name,p,lon,lat)} function, which retrieves data from the Absolute Salinity Anomaly [deltaSA_ref] and the Absolute Salinity Anomaly Ratio [SAAR_ref] lookup tables. The [ndepth_ref] table (Fig. 6) has 45 lines by 91 columns and so [deltaSA_ref] and [SAAR_ref], which both have the same size, have 4095 columns (45×91) by 45 lines (pressure levels). An additional numbering line (which does not affect how data are located) was added to facilitate debugging. Position (column number) in these tables is given by Eq. (2).
In Eq. (2), x is the longitude bin, y the latitude bin, and nlat the number of latitude bins (45). For the green cell, x=4 and y=9 as previously mentioned, so the anomaly data for this cell are located at column 144, line 22 (which corresponds to 1010 dbar) of the [deltaSA_ref] table. The Reference Salinity anomaly at 54^{∘} S, 12^{∘} E, and 1010 dbar is 0.008323162 g kg^{−1} (also highlighted in green).
2.6 Info tab (green)
This tab lists all released versions of TEOS10 Excel, providing detailed information on the updates included in each version.
Table 1 lists all functions (VBA modules) and formulas included in TEOS10 Excel (v.2.1). Most modules are a direct translation into VBA of the GSW MATLAB counterpart (McDougall and Barker, 2011), and the original credit and references were kept in the code comments; however, due to the different way matrices are handled in MATLAB versus VBA, some functions needed to be redesigned, namely how accessing the Atlas lookup tables is managed. Returned values from TEOS10 Excel are the same, for every parameter, as the ones obtained with the GSW toolbox up to 15 decimal places, i.e. difference = 0.000000000000000 (error checking was performed against MATLAB GSW toolbox version 3.06.12 from 25 May 2020). As referred to before, access to the VBA project environment can be obtained by pressing Alt + F11 (Windows) or Fn + Alt + F11 (Mac). All functions (alphabetically listed in Table 1) are described next, following the spreadsheet's column sequence.
3.1 Practical Salinity (S_{P})
S_{P} is computed from conductivity using the function {SP_from_C(C,t,p)} or from the conductivity ratio (Rt) reading of a laboratory salinometer using the function {SP_salinometer(Rt,t)}, depending on the radio button selected. Practical Salinity is a dimensionless quantity, although PSU (Practical Salinity Unit) is commonly used. For reference, the calculation algorithm is designed so that the conductivity of Reference Composition Seawater at S_{P}=35, t_{68}=15, and p=0 is 42.9140 mS cm^{−1}, which can be used to validate the function. For the salinometer ratio function, a ratio = 1 will result in S_{P}=35, independently of the temperature. If S_{P}<2, both functions call the {Hill_ratio_at_SP2(t)} module which corrects the S_{P} value based on the Hill et al. (1986) algorithm. This algorithm is adjusted so that it is exactly equal to the Practical Salinity Scale−78 algorithm (Fofonoff and Millard, 1983) at S_{P}=2.
A VBA module to calculate Practical Salinity from the conductivity ratio (R), of a sample at temperature (t) and pressure (p) relative to SSW (Standard Seawater) at t=15 ^{∘}C and p=0 is also included as {SP_from_R(R,t,p)}, but it is not currently used in the template spreadsheets.
3.2 Reference Salinity (S_{R})
Reference Salinity (S_{R}) is assumed to be proportional to Practical Salinity (IOC, SCOR and IAPSO, 2010) and obtained by Eq. (3). Units for S_{R} are grams per kilogram.
3.3 The delta S_{A} Atlas
The function {LookUP_atlas(table_name,p,lon,lat)} interrogates the Atlas database and was developed specifically for TEOS10 Excel. The argument table_name can be one of the two lookup tables (“deltaSA_ref” or “SAAR_ref”) and the returned values are a 3D interpolation of the eight vertices of the cube around the location (Fig. 7). For the [deltaSA_ref] table, the result of the function is the Atlas Absolute Salinity Anomaly ($\mathit{\delta}{S}_{\text{A}}^{\text{atlas}})$. As the interpolation process is not clearly described in the GSW toolbox documentation, it is discussed next.
3.3.1 Interpolation
The function {LookUP_atlas(table_name,p,lon,lat)} begins by finding the grid point P1 of the 3D cube around the location (Fig. 7). P1 would be the grid point immediately before the latitude and longitude of a given location. The same applies to pressure. For example, if the spreadsheet data cell corresponds to a cast located at 1100 dbar, +13^{∘} longitude, −51^{∘} latitude, the grid position of P1(lon*, lat*, p^{*}) would be P1(4, 9, 22), obtained from the [longs_ref], [lats_ref], and [p_ref] tables. The other eight points are referenced to P1, by adding one unit to the grid position of P1 as shown in Fig. 1.
The standard basic 3D interpolation model assumes that the cube dimensions are $\mathrm{1}\times \mathrm{1}\times \mathrm{1}$ (Bourke, 1999), and the distances dx, dy and dz are obtained by subtracting, respectively, x, y, and z from the unit; however, in this case, the longitude and latitude grid space are 4^{∘} and the pressure difference between the upper and bottom points varies from grid level to grid level (e.g. 10 dbar between levels 1 and 2 but 101 dbar between levels 22 and 23). Distances x, y, and z are obtained from Eqs. (4, 5, and 6), and then dx, dy, and dz from Eq. (7).
The interpolated value (v) is obtained by weighing the contribution of the eight points according to Eqs. (8 to 16), where v(Pn) is the $\mathit{\delta}{S}_{\text{A}}^{\text{atlas}}$ value at Pn (from [deltaSA_table]).
3.3.2 Missing data
There are pressure levels in the Atlas reference casts where data are missing. Figure 8 illustrates this situation.
The GSW toolbox fills these gaps by averaging the neighbouring four points in the grid at the same pressure level. As the ocean is horizontally stratified, this is logical, but neighbour points themselves might also lack data at the same level, which may compromise the result. A $\mathit{\delta}{S}_{\text{A}}^{\text{atlas}}$ plot from [deltaSA_ref] column 8 (0^{∘} long, −54^{∘} lat) is shown in Fig. 9. The $\mathit{\delta}{S}_{\text{A}}^{\text{atlas}}$ output of the GSW toolbox and TEOS10 Excel is the same for the part where there are data (blue line), but the output of the GSW toolbox for the two pressure levels where data are missing is clearly off the profile (orange). In this situation, TEOS10 Excel implements a vertical interpolation within the vertical profile, which resolves the missing data situations better (green in Fig. 9). The second case where the GSW toolbox seems to be less consistent is when not all points of the 3D interpolation cube exist at the last pressure level. The GSW toolbox test data (included in the TEOS10 Test Data tab) are such an example (Fig. 10). The GSW toolbox approach for resolving missing data on the last pressure level is as before. In these situations, if one of the four bottom points of the interpolation cube (P5, P6, P7, or P8 in Fig. 7) is missing, TEOS10 Excel assigns to it the same value as the next upper point at that location (e.g. v(P7) = v(P3)). The subsequent resulting profile is potentially more consistent in TEOS10 Excel than would be available in the GSW toolbox (Fig. 10).
3.4 delta SAAR Atlas
The Atlas Absolute Salinity Anomaly Ratio (R^{δ}) is obtained by the function {LookUP_atlas(table_name,p,lon,lat)}, exactly as for the Absolute Salinity Anomaly (Sect. 3.3) but calling it with SAAR_ref as the table_name argument. R^{δ} is the quantity used to estimate the Absolute Salinity Anomaly (Sect. 3.5), which is then used for obtaining Absolute Salinity (Sect. 3.6).
3.5 Absolute Salinity Anomaly
The Absolute Salinity Anomaly (δS_{A}) is the product of the Atlas Absolute Salinity Anomaly Ratio and Reference Salinity (Eq. 17).
3.6 Absolute Salinity
Absolute Salinity (S_{A}) is the sum of Reference Salinity and salinity anomaly (Eq. 18).
If the location is in the Baltic Sea, the world atlas salinity anomalies do not apply (McDougall, 2010) and Absolute Salinity is computed algebraically from Practical Salinity with the function {SA_Baltic(SP)}, which applies Eq. (19).
Limits for the Baltic were taken from Fig. 2 of Feistel et al. (2010). The function {is_Baltic(lon,lat)} checks if the location is in the Baltic Sea by finding out if the coordinates lie within either of two rectangular areas defined by [52^{∘} N: 60^{∘} N, 9^{∘} E: 15^{∘} E] and [52^{∘} N: 67^{∘} N, 15^{∘} E: 30^{∘} E].
3.7 Temperature ITS90
The temperature standard used in TEOS10 as argument to all functions is ITS90 (PrestonThomas, 1990). If the ITS90 radio button is selected (column C of the spreadsheet), the temperature input values are copied; if IPTS68 is selected instead, the IPTS68 temperature will be converted to ITS90 using Eq. (20).
3.8 Potential temperature (θ)
Potential temperature (θ, ^{∘}C) is obtained by the function {pt0_from_t(SA,t,p)}. Three other functions – {Entropy_part(SA,t,p)}, {Entropy_part_zerop(SA,pt0)}, and {Gibbs_pt0_pt0(SA,pt0)} – are called within the computation process. These four functions are a VBA translation of the original GSW toolbox counterparts (IOC, SCOR and IAPSO, 2010; McDougall and Wotherspoon, 2013).
3.9 Conservative Temperature (Θ)
Conservative Temperature (Θ, ^{∘}C) is the temperature quantity used as argument in the Thermodynamic Equation Of Seawater – 2010 (IOC, SCOR and IAPSO, 2010). It is obtained with the function {CT_from_pt(SA,pt)}, which is a direct translation from the GSW toolbox counterpart and estimates Θ from S_{A} and θ.
3.10 Potential density (σ_{Θ})
Potential density (σ_{Θ}, kg m${}^{\mathrm{3}}\mathrm{1000}$) with reference to a sea pressure of 0 dbar is estimated by the function {sigma0(SA,CT)}, the arguments of which are Absolute Salinity (S_{A}) and Conservative Temperature (Θ). This function uses the TEOS10 75term equation and is a VBA translation of the original GSW toolbox counterpart (IOC, SCOR and IAPSO, 2010; McDougall et al., 2003; Roquet et al., 2015).
3.11 In situ density (${\mathit{\rho}}_{{S}_{\mathrm{A}},\mathrm{\Theta},p}$)
In situ density (${\mathit{\rho}}_{{S}_{\mathrm{A}},\mathrm{\Theta},p}$, kg m^{−3}) is estimated by the function {rho(SA,CT,p)}, whose arguments are Absolute Salinity (S_{A}), Conservative Temperature (Θ), and pressure (p). This function uses the TEOS10 75term equation and is a VBA translation of the original GSW toolbox counterpart (IOC, SCOR and IAPSO, 2010; McDougall et al., 2003; Roquet et al., 2015).
3.12 Sound speed (c)
Sound speed (c, m s^{−1}) is estimated by the function {Sound_Speed(SA,CT,p)}, whose arguments are Absolute Salinity (S_{A}), Conservative Temperature (Θ), and pressure (p). This function uses the TEOS10 75term equation and is a VBA translation of the original GSW toolbox counterpart (IOC, SCOR and IAPSO, 2010; McDougall et al., 2003; Roquet et al., 2015).
3.13 The [deltaSA_ref] table not required
The Atlas Absolute Salinity Anomaly (column F of the data spreadsheets) is not used for any calculation, as Absolute Salinity Anomaly (δS_{A}) is obtained from the product of the Absolute Salinity Anomaly Ratio (R^{δ}) and Reference Salinity (Eq. 17), and R^{δ} is retrieved from the [SAAR_ref] lookup table. However, the [deltaSA_ref] lookup table is not necessary for any computation and can eventually be deleted from the Excel workbook for brevity; however, this table was used as a debugging tool in the development of the VBA functions, and the error checking and interpolation improvements described in Sect. 3.3 refer to this lookup table, which is why it was included in TEOS10 Excel as a supporting element for this paper. Additionally, users might be interested in ascertaining the Absolute Salinity Anomaly for a given location or perform further error checking, comparing TEOS10 Excel output against the GSW toolbox for other data sets.
To our knowledge, TEOS10 Excel is the first implementation of the Thermodynamic Equation Of Seawater – 2010 outside the official GSW toolboxes. It does not aim to reproduce the fullfeatured GSW environment as it implements only a small subset of the TEOS10 functions. Opening the possibility of estimating a relevant set of seawater parameters within a wellknown and friendly environment (Excel), however, will hopefully democratise the compliance with current oceanographic standards among a large community of researchers and students who are not at ease with the use of highlevel programming languages. As discussed in the paper, some issues were detected with the GSW interpolation when there are missing data in the Atlas reference tables. In these cases, TEOS10 Excel adopts an alternative approach to the interpolation method, which has produced better results (Sect. 3.3.2). Nonetheless, this is perhaps a situation that deserves further research.
TEOS10 Excel is available for download at https://doi.org/10.5281/zenodo.4748829 (Martins, 2022). This DOI represents all versions and will always resolve to the latest one. This paper reflects the status of version 2.1.
CGM developed the code, tested the data, and prepared the original draft. JC critically reviewed and edited the initial and final versions of the paper.
The contact author has declared that neither they nor their coauthors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This paper was edited by Trevor McDougall and reviewed by Paul Barker and Trevor McDougall.
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