How to subtract two different netcdf files after reading them

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 have extracted the chlor_a data and could plot with respective x and y coordinates

I used the below code for 1st nc file to read, the same code can be used for 2nd file

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');

I find error while trying to execute below steps

while try to save both files in nc format

while subtraction both files to spot seasonal trends

This comparison will identify max persistence of chlor_a

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

I need to get the plot as such after subtraction, please find the attached image

similary need to work on to compare weekly data

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 valuable inputs

Thanks in advance

regards