How to compare two netcdf file (present to past data) by means of subtraction

Hi Freinds

I have objective to find chlor conc. hotspot of particular month during successive year

For example I consider 2 year(2015 and 2016) nc data (Feb)month

I can extract the chlor_a data and plot with the below code

ncfile_1 = 'A20150322015059.L3m_MO_CHL_chlor_a_4km.nc';
lat_1 = ncread(ncfile_1,'lat') ;
lon_1 = ncread(ncfile_1,'lon') ;
chlor_a_1 = ncread(ncfile_1,'chlor_a');
[X_1,Y_1] = meshgrid(lon_1,lat_1) ;
nn = 1000;
xi_1 = linspace(30,100,nn) ;
yi_1 = linspace(0,30,nn) ;
[Xi_1,Yi_1] = meshgrid(xi_1,yi_1);
iwant_1 = interp2(X_1,Y_1,chlor_a_1',Xi_1,Yi_1)' ;

ind_1 = find(iwant_1>10 | iwant_1<0);
%ind = find(iwant_2 > 0 & iwant_1<10);
iwant_1(ind_1) = NaN;

%[min_val_1,min_ind_1] = min(iwant_1',[],'all','linear');
[min_val_1,min_ind_1] = min(reshape(iwant_1',[],1));
[r1_1,c1_1]=ind2sub(nn,min_ind_1);
x_min_1 = xi_1(c1_1);
y_min_1 = yi_1(r1_1);

%[max_val_1,max_ind_1] = max(iwant_1',[],'all','linear');
[max_val_1,max_ind_1] = max(reshape(iwant_1',[],1));
[r2_1,c2_1]=ind2sub(nn,max_ind_1);
x_max_1 = xi_1(c2_1);
y_max_1 = yi_1(r2_1);

figure(1)
pcolor(xi_1,yi_1,iwant_1'); shading interp;
c = colorbar;
cmap = jet(255);
cmap(:,2) = 0;
colormap(cmap)
caxis([0, 5])
hold on
plot(x_min_1,y_min_1,'k +', 'MarkerSize', 10);
plot(x_max_1,y_max_1,'m +', 'MarkerSize', 10);
hold off
legend('','min','max');

Image = getframe(gcf);
imwrite(Image.cdata, 'feb2015.tif');

--

ncfile_2 = 'A20160322016060.L3m_MO_CHL_chlor_a_4km.nc';
lat_2 = ncread(ncfile_2,'lat') ;
lon_2 = ncread(ncfile_2,'lon') ;
chlor_a_2 = ncread(ncfile_2,'chlor_a');
[X_2,Y_2] = meshgrid(lon_2,lat_2) ;
nn = 1000;
xi_2 = linspace(30,100,nn) ;
yi_2 = linspace(0,30,nn) ;
[Xi_2,Yi_2] = meshgrid(xi_2,yi_2);
iwant_2 = interp2(X_2,Y_2,chlor_a_2',Xi_2,Yi_2)' ;

ind = find(iwant_2>10 | iwant_2<0);
%ind = find(iwant_2 > 0 & iwant_2<10);
iwant_2(ind) = NaN;

%[min_val,min_ind] = min(iwant_2',[],'all','linear');
[min_val,min_ind] = min(reshape(iwant_2',[],1));
[r1,c1]=ind2sub(nn,min_ind);
x_min = xi_2(c1);
y_min = yi_2(r1);

%[max_val,max_ind] = max(iwant_2',[],'all','linear');
[max_val,max_ind] = max(reshape(iwant_2',[],1));
[r2,c2]=ind2sub(nn,max_ind);
x_max = xi_2(c2);
y_max = yi_2(r2);

figure(2)
pcolor(xi_2,yi_2,iwant_2'); shading interp;
c = colorbar;
cmap = jet(255);
cmap(:,2) = 0;
colormap(cmap)
caxis([0, 5])
hold on
plot(x_min,y_min,'k +', 'MarkerSize', 10);
plot(x_max,y_max,'m +', 'MarkerSize', 10);
hold off
legend('','min','max');

I find error while trying to execute below steps

need to save file in nc format for 1st file and same for next ...

by subtraction (present to past year) will spot seasonal trends

[+ve] value means max chlor_a conc. [-ve] value means less conc.

comparison will identify max persistence of chlor_a to identify hotspot

similary need to compare for weekly data to find the hotspot variation

data link

https://drive.google.com/file/d/1hAeVz54bKLA1dDyk5uxMyjJIppG-XS77/view?usp=sharing

https://drive.google.com/file/d/1XIcbaEbqLQOPgi0Ooq72J2elvZwfQnoO/view?usp=sharing

Kindly help, waitng for your suggestion and your valuable inputs

Thanks in advance

regards