I've modified your code. Not sure how robust this solution is, but you can try this:
figure(2)
set(gcf,'color','w');
ax = polaraxes('Position', [0.25 0.25 0.6 0.6])
ax.ThetaDir = 'counterclockwise';
ax.ThetaZeroLocation = 'left';
rlim([70,120])
ax.RTickLabel = [];
ticks = [-flip(ax.RTick(2:end)) ax.RTick];
a = axes('Color', 'none',...
'XColor', 'none',...
'Position', [ax.Position(1),ax.Position(2)+ax.Position(4)/2,...
ax.Position(3),ax.Position(4)/2],...
'YLim', [ax.RLim(1) ax.RLim(2)],...
'YTick', ticks,...
'YTickLabel', abs(ticks));
a_m = copyobj(a,gcf)
a_m.Position = a.Position - [0 a.Position(4),0,0];
a_m.YAxis.Direction = 'reverse';
ylabel(a, 'r axis label')
a.YAxis.Label.Position = [a.YAxis.Label.Position(1),70,0];
