How to plot a spatial correlation with a p-value threshold?

Hi there!

I am trying to set a p-value threshold and plot along with my spatial correlation. The spatial correlation is between temperature and precipitation over a 40 year time series across all latitudes and longitudes in my study. This is what I have so far:

precip = double(ncread('precip_1979_2019.nc','precip'); % size = 316x332x40 (latxlonxyears)
temp = double(ncread('temp_1979_2019.nc','t2m'); % size = 316x332x40 (latxlonxyears)
lat = double(ncread('temp_1979_2019.nc','latitude')); % size = 316x332 (latxlon)
lon = double(ncread('temp_1979_2019.nc','longitude')); % size = 316x332 (latxlon)

% here I run the correlation for each latxlon point to get a spatial correlation plot (316x332)
for p = 1:316
    for q = 1:332
        X(p,q)=corr2(squeeze(precip(p,q,:)),squeeze(snowfall(p,q,:)));
    end
end

This gives me a great map of correlation values between temperature and precipitation.

My question is, how can I then find the areas in this correlation plot that have a p-value < 0.05?

I have tried this:

XX = [];
    for s = 1:316
        for q = 1:332
            [r,p]=corrcoef(squeeze(annual_snowfall_matrix(s,q,:)),squeeze(annual_mean_sie(s,q,:)));
            %XX = [XX;p(1)];
            XX = cat(3,XX,p);
        end
    end
    xxreshape = reshape(XX,[316 332]);

but all of the values come out as 1 for both r and p.

Any suggestions or help is greatly appreciated!

Thank you

Answer 1

[Jeff Miller]

After you get X (bottom of your first code segment):

rCrit = 0.312; % Based on N=40 and alpha = 0.05, 2-tailed, from a table of critical r values
sigLoc = abs(X)>rCrit;

sigLoc(lat,long) will be true if the correlation was signficant

[James Karden]

Thanks Jeff! This works and is extremely hepful!