Saving only one slice of 4d velocity field

[keli559]

In the demo, I saw using the following code to collect the output. How would I do it if I only want to save one slice at certain x and y ?
My velocities fields are 4d, time, depth, lat and lon. I only want to save time and depth with fixed lat and lon.
fields_to_output = merge(model.velocities, model.tracers, (; νₑ, dbdz, dcdz))

[glwagner]

Hi @keli559! This feature is described in our documentation:

https://clima.github.io/OceananigansDocumentation/stable/model_setup/output_writers/#JLD2-output-writer

To save at some specific x, y you need to identify the i, j indices that you'd like to extract, and pass this to the output writer. I'm describing data at a given x, y as a "column" in physical space:

# Selected indices are i, j = 3, 4
ii = 3
jj = 4
column_indices = (ii, jj, :) # indices of a column with all points in z, and one point in x, y

# syntax is the same for `NetCDFOutputWriter`
output_writer = JLD2OutputWriter(model, outputs; indices=column_indices, other_kw...)

To select indices corresponding to a particular location you can use something like

x = xnodes(grid, Center(), Center(), Center())
x_of_interest = 4.1 # for example
ii = searchsortedfirst(x, x_of_interest)

This might need additional work for the GPU (or just CUDA.@allowscalar).

PS I'm converting this to a discussion so we can continue there; I don't think this issue motivates any changes to the source code.