How to Remove Seasonal Cycle?

%Point Barrow, Alaska
PB=readtable('monthly_flask_co2_ptb.csv');
%create time and CO2 variable
%time
PBdailyt=PB(:,4);
%CO2
PBdailyCO2=PB(:,7);
%take data out of table and put into array
PBt=table2array(PBdailyt);
PBCO2=table2array(PBdailyCO2);

%index to extract outliers
idx = PBCO2 >=450 | PBCO2 <=300 ;
%set outliers equal to NaN
PBCO2(idx) = NaN;

This is my code so far, I am now looking to extract the seasonal cycle. The two different codes I have tried(below) has been unsuccessful. Is there another way I can go about removing the seasonal cycle?

[~,mo,~]=datevec(PBt);
%find the mean of the data
for k=1:12
   PBCO2_mean(k) = mean(PBCO2(mo==k),'omitnan'); 
end
%subtract mean from data to remove seasonal cycle
for k = 1:12
   PBCO2_deseasoned(mo==k) = PBCO2(mo==k) - PBCO2_mean(k); 
end

and

[~,mo,~]=datevec(LJt);
%mean
PBCO2_mean=ones(12,1);
for i=1:12
    idx=find(mo==i);
    PBCO2_mean(i)=mean(PBCO2(idx),'omitnan');
end
%remove seasonal cycle
PBCO2_deseasoned=ones(636,1);
for i=1:12
    PBCO2_deseasoned(mo==i)=PBCO2(mo==i)-PBCO2_s(i);
end

Answer 1

[Star Strider]

If the seasonal cycle is a narrow frequency band, and the data are sampled at consistent regular intervals, use a bandstop filter to remove it.

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