Hi,
If I am understanding correctly, then you anyway are defining time axis correctly, but not using it. You are defining your required time axis as tyear , but you are not telling ferret that you want your variable on that time axis. Perhaps according to your requirement you may do
let new_mean=tos_mean1[d=2,gt=tyear@asn,k=1,x=40E:100E@ave,y=20N:20S@ave]
plot/ov/thick=2/col=2/nolab new_mean
You may go through Ferret manual for time axis definition and understand regridding of variable using time axis e.g. gt=tyear@asn etc.