What you can try for this is to align the main peaks, using something like this:
[~,idxPeaks] = max(I_spectras,,2);
I_S_shifted = zeros(30,2048);
for i1 = 30:-1:1;
I_S_shifted(i1,1024-idxPeaks(i1)+[1:1024]) = I_spectras(i1,:);
Then you might get somewhere by interpolating I_S_shifted after which you might reverse-shift the spectras (how easy that is to do depends on how simple the shifts of the peaks are in the first place.) This works under the assumption that the shape of the peaks varies very slowly between neighboring spectra. This might be useful or not, If this is not good enough one might take a further step in scaling the spectra as well to line up additional features, but that's for later.