Can I make a geographical subset of ERA5 or Seasonal Forecast data?

Yes, you can by downloading data through the CDS API service and using the keyword 'AREA'.

• For ERA5, please see How to download ERA5 (Section 3 Step B) for instructions and a script example showing how to set the keyword 'AREA' to select a geographical area.
• For Seasonal Forecast data, please see Recommendations and efficiency tips for C3S seasonal forecast datasets

A facility for selecting a geographical subset of ERA5 or Seasonal forecast data will soon be implemented on the CDS web download interface.

The Toolbox allows you to define a geographical subset when downloading ERA5 data.

The code below allows you to retrieve a NetCDF containing ERA5 2m temperature data for the desired years, months, days and times.  Additionally the 'area' keyword allows you define the geographical subset you are interested in while 'grid' allows you to define the spatial resolution of the downloaded data.

import cdstoolbox as ct

@ct.application()
@ct.output.dataarray()
def application():
    
    data = ct.catalogue.retrieve(
        'reanalysis-era5-single-levels',
        {
            'variable': '2m_temperature',
            'product_type': 'reanalysis',
            'year': ['2017', '2018'],
            'month': ['01'],
            'day': ['01'],
            'area':'73.5/-27/33/45', #North/West/South/East
            'time': ['12:00'],
            'grid': ['3', '3'],
        }
    )    
    return data

You can also use area as a list instead of a string: 

'area': [73.5, -27, 33, 45], # [North, West, South, East]