It is clear from your post that the peak in ice thickness (calculated as Volume/area) has shifted significantly earlier.
It is also clear from crandles' post that if you calculate ice thickness as Volume/extent that the shift is much reduced, but still there.
Well actually, what I have been tring to say is that there are two peaks.
Using Wipneus area and finding Jan to 31 jul peak and also 1Aug to 31 Dec peak, I get:
3.092361804 1979 207 21.281
3.128831497 1979 230 17.996
3.036558828 1980 205 20.931
3.032874754 1980 215 19.230
2.802673201 1981 208 17.534
2.827039146 1981 218 15.775
2.503914211 1982 176 24.171
2.620895658 1982 255 13.400
2.860465965 1983 201 20.703
2.834102544 1983 245 15.186
2.908341814 1984 190 22.284
3.090239468 1984 256 14.500
3.159945094 1985 209 18.129
3.101037998 1985 213 17.471
2.92154531 1986 200 21.236
2.936283655 1986 225 17.430
3.044844646 1987 206 20.166
3.016573892 1987 221 17.298
2.96417511 1988 206 19.161
2.989685742 1988 219 17.022
2.819695352 1989 200 20.281
3.010469726 1989 254 14.666
2.946993494 1990 200 18.785
2.952489574 1990 264 13.759
2.971922926 1991 193 20.635
3.020591297 1991 261 13.490
2.859749842 1992 202 19.670
2.949292771 1992 243 14.999
2.88542031 1993 191 19.740
2.73732818 1993 229 13.186
2.846941741 1994 196 19.991
2.967926293 1994 224 15.029
2.765426222 1995 200 16.407
2.612161264 1995 220 13.039
2.576331873 1996 185 21.667
2.596124209 1996 251 13.746
2.940792488 1997 206 17.232
2.833314658 1997 223 14.691
2.81055007 1998 201 17.440
2.775820448 1998 217 14.485
2.837047931 1999 182 21.757
2.749647039 1999 220 13.964
2.755376398 2000 182 20.785
2.616035427 2000 245 11.093
2.83031532 2001 202 16.783
2.708392504 2001 215 14.571
2.650614194 2002 197 17.019
2.695923739 2002 214 13.673
2.688218718 2003 184 19.353
2.493603879 2003 214 12.844
2.476061308 2004 183 19.476
2.415927938 2004 213 13.326
2.496950185 2005 191 16.399
2.490413855 2005 227 10.671
2.45932977 2006 169 20.488
2.335210146 2006 214 11.688
2.333668324 2007 172 17.813
2.342022239 2007 230 7.269
2.379133233 2008 173 19.387
2.504845221 2008 225 9.374
2.172280361 2009 153 22.147
2.028605291 2009 216 9.330
2.079175679 2010 176 14.666
1.649473401 2010 214 7.147
1.99884714 2011 146 20.088
1.696375421 2011 213 6.882
2.028808358 2012 165 16.329
1.787574146 2012 219 5.693
1.994332988 2013 167 17.489
1.581658026 2013 216 7.458
2.042485784 2014 199 11.393
1.9535078 2014 252 6.987
2.164799525 2015 165 18.925
2.050185725 2015 219 7.906
2.093139502 2016 154 18.898
1.887173372 2016 239 5.001
The first peak is moving earlier from about 205 to 160 over 37 year so similar to vol/Extent just over a day earlier per year.
The second peak is possibly also moving earlier but has wild swings from time to time. 213 is the 1 Aug the first possible date for second peak the way I have done it. As it is often very close to 213 this suggests I haven't got the date right for splitting the peaks. So don't rely on those second peak dates.
Probably ought to graph the vol/Area numbers having got this far ....