Python : Cartopy and pcolormesh, do a loop

I an actually in internship, and I want to plot some temperature of the ocean surface data of a map for each months for on years (I want for 50 years but I will see later) and I want it to be focus on a precise area. I have succeed to do this except that when I plot my data, in each maps (12 maps for each months). I have the same data on every months, it seems that the temperature for January are in each maps. It is probably a problem with this line:

ax.pcolormesh(lon, lat, tos, data=dset, cmap=cmap)

But I don't know how to fix it. Maybe I have to do a other loop inside this code line, but is it possible ? Thanks for your answers !

Code :

from netCDF4 import Dataset
import matplotlib.pylab as plt
import cartopy.crs as ccrs
import numpy as np
import netCDF4 as nc

month_name=["Jan","Feb","Mar","Apr","May","June","Jul","Aug","Sep","Oct","Nov","Dec"]

data = "/Users/name/Desktop/Internship/tos_1979.nc" 
dset = Dataset(data)
fig = plt.figure(figsize=(24,24))
cmap = plt.cm.jet
for imonth in np.arange(1,13):  
ax = fig.add_subplot(4,3,imonth, projection=ccrs.PlateCarree())   
  tos = dset.variables['tos'][0, :, :]
  lon = dset.variables['longitude'][:]
  lat = dset.variables['latitude'][:]
  time = dset.variables['time'][:]

  ax.set_extent([-70,20,30,90])
  ax.coastlines();
  plt.title('Ocean Surface Temperature :'+ month_name[imonth-1],fontsize=12
  ax.stock_img()
  ax.gridlines()
  ax.pcolormesh(lon, lat, tos, data=dset, cmap=cmap)
plt.savefig('fig_tos_1979.png')
plt.show()

Just to know, I code with Python, on a MacBook (with Spyder).

Also I have problem with the scalebar tool, it's install on my Mac (thanks to the terminal) but when I import this tools it's says :

  ImportError: cannot import name 'scale_bar' from 'scalebar' (/Users/name/opt/anaconda3/lib/python3.9/site-packages/scalebar/__init__.py)

Thanks !