You have the problem that you have 2D data for each of 12 months. You can only plot data for one month at a time
contourm(lat, lon, temp(:,:,MonthNumber))
The exception to this would be that in theory you could choose to "hold on" and plot all of the months on top of each other, using a different contour line color each time.
cmap = colormap(parula(12));
hold on
for MonthNumber = 1 : 12
contourm(lat, lon, temp(:,:,MonthNumber), 'EdgeColor', cmap(MonthNumber));
end
hold off
