Converting the practical salinity and potential temperature from ECCOv4r4 model output.

First, add the required dependencies

using Rasters, NCDatasets, Plots, Downloads
using OceanRasterConversions

and download model output from ECCOv4r4 [1], [2], [3]. This data is the daily average 0.5 degree salinity and temperature model output. To reproduce this example, an Earthdata acount is needed to download the data.

Read the data into a RasterStack

function download_ECCO()

    try
        Downloads.download("https://opendap.earthdata.nasa.gov/providers/POCLOUD/collections/ECCO%2520Ocean%2520Temperature%2520and%2520Salinity%2520-%2520Daily%2520Mean%25200.5%2520Degree%2520(Version%25204%2520Release%25204)/granules/OCEAN_TEMPERATURE_SALINITY_day_mean_2007-01-01_ECCO_V4r4_latlon_0p50deg.dap.nc4", "ECCO_data.nc")
    catch
        @info "dowloading from drive"
        Downloads.download("https://drive.google.com/uc?id=1MNeThunqpY-nFzsZLZj9BV8sM5BJgnxT&export=download", "ECCO_data.nc")
    end

    return nothing

end
download_ECCO()
stack = RasterStack("ECCO_data.nc")

RasterStack with dimensions: 
  X Mapped{Float32} Float32[-179.75, -179.25, …, 179.25, 179.75] ForwardOrdered Explicit Intervals crs: EPSG mappedcrs: EPSG,
  Y Mapped{Float32} Float32[-89.75, -89.25, …, 89.25, 89.75] ForwardOrdered Explicit Intervals crs: EPSG mappedcrs: EPSG,
  Z Sampled{Float32} Float32[-5.0, -15.0, …, -5461.25, -5906.25] ReverseOrdered Explicit Intervals,
  Ti Sampled{Dates.DateTime} Dates.DateTime[Dates.DateTime("1996-12-16T12:00:00")] ForwardOrdered Explicit Intervals,
  Dim{:nv}
and 6 layers:
  :THETA          Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)
  :time_bnds      Int32 dims: Dim{:nv}, Ti (2×1)
  :latitude_bnds  Float32 dims: Dim{:nv}, Y (2×360)
  :longitude_bnds Float32 dims: Dim{:nv}, X (2×720)
  :Z_bnds         Float32 dims: Dim{:nv}, Z (2×50)
  :SALT           Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)

with metadata Metadata{Rasters.NCDsource} of Dict{String, Any} with 63 entries:
  "product_time_coverage_end"   => "2018-01-01T00:00:00"
  "acknowledgement"             => "This research was carried out by the Jet Pr…
  "cdm_data_type"               => "Grid"
  "time_coverage_resolution"    => "P1M"
  "time_coverage_duration"      => "P1M"
  "history"                     => "Inaugural release of an ECCO Central Estima…
  "author"                      => "Ian Fenty and Ou Wang"
  "product_version"             => "Version 4, Release 4"
  "geospatial_bounds_crs"       => "EPSG:4326"
  "publisher_type"              => "institution"
  "geospatial_lat_units"        => "degrees_north"
  "geospatial_lon_max"          => 180.0
  "geospatial_lon_units"        => "degrees_east"
  "date_metadata_modified"      => "2021-03-15T22:05:59"
  "geospatial_vertical_max"     => 0.0
  "product_time_coverage_start" => "1992-01-01T12:00:00"
  "publisher_url"               => "https://podaac.jpl.nasa.gov"
  "comment"                     => "Fields provided on a regular lat-lon grid. …
  "naming_authority"            => "gov.nasa.jpl"
  ⋮                             => ⋮

Thanks to Rasters.jl we now have the dimensions of the data, the variables saved as layers and all the metadata in one data structure. From the metadata we can get a summary of the data which tells us more about the data

metadata(stack)["summary"]

"This dataset provides monthly-averaged ocean potential temperature and salinity interpolated to a regular 0.5-degree grid from the ECCO Version 4 Release 4 (V4r4) ocean and sea-ice state estimate. Estimating the Circulation and Climate of the Ocean (ECCO) state estimates" ⋯ 916 bytes ⋯ "dable bathythermographs (XBTs) from several programs [e.g., WOCE, GO-SHIP, Argo, and others] and platforms [e.g., research vessels, gliders, moorings, ice-tethered profilers, and instrumented pinnipeds]. V4r4 covers the period 1992-01-01T12:00:00 to 2018-01-01T00:00:00."

This tells us that the temperature variable is potential temperature and the salt variabile is practical salinity (for more information about this data see the user guide).

Converting all variables and plotting

To calculate seawater density using TEOS-10, we require absolute salinity, conservative temperature and pressure. This can be done by extracting the data and using GibbsSeaWater.jl or with this package,

converted_stack = convert_ocean_vars(stack, (Sₚ = :SALT, θ = :THETA))

RasterStack with dimensions: 
  X Mapped{Float32} Float32[-179.75, -179.25, …, 179.25, 179.75] ForwardOrdered Explicit Intervals crs: EPSG mappedcrs: EPSG,
  Y Mapped{Float32} Float32[-89.75, -89.25, …, 89.25, 89.75] ForwardOrdered Explicit Intervals crs: EPSG mappedcrs: EPSG,
  Z Sampled{Float32} Float32[-5.0, -15.0, …, -5461.25, -5906.25] ReverseOrdered Explicit Intervals,
  Ti Sampled{Dates.DateTime} Dates.DateTime[Dates.DateTime("1996-12-16T12:00:00")] ForwardOrdered Explicit Intervals
and 4 layers:
  :p  Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)
  :Sₐ Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)
  :Θ  Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)
  :ρ  Union{Missing, Float32} dims: X, Y, Z, Ti (720×360×50×1)

Note that this is a new RasterStack, so the metadata from the original RasterStack is not attached. As we have a returned RasterStack and plotting recipes have been written, we can, for example, look at the conservative temperature closest to the sea-surface (-5.0m)

contourf(converted_stack[:Θ][Z(Near(0.0)), Ti(1)]; size = (800, 500),
         color = :balance, colorbar_title = "ᵒC")

We can also take slices of the data to look at depth-latitude plots of the returned variables (note by default the in-situ density ρ is computed and returned)

lon = 180
var_plots = plot(; layout = (4, 1), size = (1000, 1000))
for (i, key) ∈ enumerate(keys(converted_stack))
    contourf!(var_plots[i], converted_stack[key][X(Near(lon))])
end
var_plots

As this is a RasterStack all methods exported by Rasters.jl will work. See the documentation for Rasters.jl for more information.

Converting chosen variables

It is also possible to convert only chosen variables from a RasterStack. If we just want to look at conservative temperature - absolute salinity vertical profiles, we can convert the practical salinity and potential temperature then extract vertical profiles and compute the potential density referenced to 0dbar

Sₐ = Sₚ_to_Sₐ(stack, :SALT)
Θ = θ_to_Θ(stack, (Sₚ = :SALT, θ = :THETA))
lon, lat = -100.0, -70.0
Sₐ_profile, Θ_profile = Sₐ[X(Near(lon)), Y(Near(lat)), Ti(1)],
                         Θ[X(Near(lon)), Y(Near(lat)), Ti(1)]
σ₀_profile = get_σₚ(Sₐ_profile, Θ_profile, 0)
profile_plots = plot(; layout = (2, 2), size = (800, 800))
plot!(profile_plots[1, 1], Sₐ_profile;
      title = "Sₐ-depth", xmirror = true, xlabel = "Sₐ (g/kg)")
plot!(profile_plots[1, 2], Θ_profile;
      title = "Θ-depth", xmirror = true, xlabel = "Θ (ᵒC)")
plot!(profile_plots[2, 1], Sₐ_profile, Θ_profile;
      xlabel = "Sₐ (g/kg)", ylabel = "Θ (ᵒC)", label = false, title = "Sₐ-Θ")
plot!(profile_plots[2, 2], σ₀_profile;
      title = "σ₀-depth", xmirror = true, xlabel = "σ₀ (kgm⁻³)")

Plotting with GeoMakie.jl

Rasters.jl also supports plotting with Makie.jl as of version 0.5.3. If using an older version we can write a method for convert_arguments to convert a Raster into a format that can be plotted by Makie.jl. For more information on implementing type recipes for plotting custom types in Makie.jl see the Makie.jl plot recipes documentation. The convert_arguments method extracts the longitude and latitude dims from a Raster as well as the values for the chosen variable. The SurfaceLike argument converts the data so we can use the contourf, heatmap or other SurfaceLike plotting functions.

using GeoMakie, CairoMakie

function Makie.convert_arguments(P::SurfaceLike, rs::Raster)

    lon, lat = collect(lookup(rs, X)), collect(lookup(rs, Y))
    plot_var = Matrix(rs[:, :])

    return convert_arguments(P, lon, lat, plot_var)

end

Now we can plot a Raster onto a GeoAxis and take advantage of the extra features GeoMakie.jl offers, like map projections (see the GeoMakie.jl documentation for more information about available projections and how to set them), automatic axis limits and coastlines.

date = lookup(converted_stack, Ti)[1] # get the date from the `Raster`
depth = 0.0                           # choose a depth to look at the ocean temperature
fig = Figure(size = (800, 500))
ax = GeoAxis(fig[1, 1];
             xlabel = "Longitude",
             ylabel = "Latitude",
             title = "Ocean conservative temperature at depth $(depth)m",
             subtitle = "$date",
             coastlines = true)
cp = CairoMakie.contourf!(ax, converted_stack[:Θ][Z(Near(depth)), Ti(At(date))];
                          colormap = :balance)
Colorbar(fig[2, 1], cp; label = "Θ (ᵒC)", vertical = false, flipaxis = false)
fig

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