How to separate monthly data from yearly?

hello, everyone. I have a matrix of 15x18x213 size.

15 is longitude

18 is latitude, and

213 is time (month), the year start 2000-2017

my problem is.. how I can grab standard deviation for every month so I get new a matrix with size 15x18x12.

I'm tried with this code to looking for mean and work

%the code, work for mean 
for k = 1:12
ind = month==k;
sst(:,:,k) = nanmean(data(:,:,ind),3);
end

%the code, not working 
for k = 1:12
ind = month==k;
sst(:,:,k) = nanstd(data(:,:,ind),3);
end

I need an advance for it.. tks