Your recommendation led me to this article about undocumented properties. In particular, we are interested in the SecondaryLabel field of the Ruler field of the colormap object associated with any given axis. You may notice that that the article references a property PlaceSecondaryLabelAtLowerLimit, however this property seems to have been removed. You can check all hidden properties of ruler objects in your version of Matlab with:
c = colorbar
struct(c.Ruler)
We will modify the position of the SecondaryLabel manually instead.
for i = 1:2
subplot(1, 2, i)
contourf(peaks.*1e8)
c = colorbar;
% there is a bug with accessing the ruler before rendering has finished
drawnow
c.Ruler.SecondaryLabel.Units = 'normalized';
% -0.03 to force at the lower limit, 1.03 to force at the upper limit
c.Ruler.SecondaryLabel.Position = [1 (i-1)+0.03*(-1)^i];
end
The location of the exponent seems to be naturally skewed down if the data is centered below zero. The upper placement suffers most from this since the margin between it and the top tickmark is generally much smaller. If you do force upper placement, you may have to increase the percent offset from 0.03 to 0.08 if the data is centered below zero. You'll need to set this property for each colorbar/axis individually in any case.
Consider the option of centering it:
contourf(peaks.*1e8)
c = colorbar;
drawnow
c.Ruler.SecondaryLabel.Units = 'normalized';
c.Ruler.SecondaryLabel.Position = [3 0.5];
