Loads the given 2D field at the given month from the given boundary file of a given length.
This is used for reading the SST anomalies from a particular month of a
  particular year. The SST anomalies are stored in a long multidecadal
  file and the total number of months in this file must be passed as an
  argument (length).
| Type | Intent | Optional | Attributes | Name | ||
|---|---|---|---|---|---|---|
| character(len=*), | intent(in) | :: | file_name | The NetCDF file to read from  | 
  
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| character(len=*), | intent(in) | :: | field_name | The field to read  | 
  
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| integer, | intent(in) | :: | month | The month to read  | 
  
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| integer, | intent(in) | :: | length | The total length of the file in number of months  |