Data table documentation (#551)

* Update forcing.rst

* add more detail to forcing.rst removing ambiguity about yaml format.

docs/forcing.rst

@@ -1,5 +1,33 @@
 Forcing
 =======
+Data Override
+-------
+When running MOM6 with the Flexible Modelling System (FMS) coupler, forcing can be specified by a `data_table` file. This is particularly useful when running MOM6 with a data atmosphere, as paths to the relevent atmospheric forcing products (eg. JRA55-do or ERA5) can be provided here. Each item in the data table must be separated by a new line, and contains the following information:
+
+| ``gridname``: The component of the model this data applies to. eg. `atm` `ocn` `lnd` `ice`.
+| ``fieldname_code``: The field name according to the model component. eg. `salt`
+| ``fieldname_file``: The name of the field within the source file. 
+| ``file_name``: Path to the source file.
+| ``interpol_method``: Interpolation method eg. `bilinear`
+| ``factor``: A scalar by which to multiply the field ahead of passing it onto the model. This is a quick way to do unit conversions for example. 
+
+| 
+The data table is commonly formatted by specifying each of the fields in the order listed above, with a new line for each entry.
+
+Example Format:
+    "ATM", "t_bot",  "t2m", "./INPUT/2t_ERA5.nc", "bilinear", 1.0
+
+A `yaml` format is also possible if you prefer. This is outlined in the `FMS data override <https://github.com/NOAA-GFDL/FMS/tree/main/data_override>`_ github page, along with other details. 
+
+Speficying a constant value:
+    Rather than overriding with data from a file, one can also set a field to constant. To do this, pass empty strings to `fieldname_file` and `file_name`. The `factor` now corresponds to the override value. For example, the following sets the temperature at the bottom of the atmosphere to 290 Kelvin.
+
+
+    "ATM", "t_bot",  "", "", "bilinear", 290.0
+
+Which units do I need?
+    For configurations using SIS2 and MOM, a list of available surface flux variables along with the expected units can be found in the `flux_exchange <https://github.com/NOAA-GFDL/FMScoupler/blob/f4782c2c033df086eeac29fbbefb4a0bdac1649f/full/flux_exchange.F90>`_ file. 
+
 
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