mean

import numpy as np

import xarray as xr

ds = xr.open_dataset(fn) # fn contains netcdf file name for geopotential height (zg)

z500=ds['zg'].sel(plev=50000, \

time=slice('1955-01-15','2005-12-16'))

z500_av = z500.mean('time') # time average as xarray as DataArray

z500_av2=np.array(z500).mean(axis=0) # time average as numpy array