Observed data . Reanalysis . Satellite data . Model results  NAO  
NAO signal 
Statistical techniques 
POP 

(Hasselmann,1987; von Storch, 1987) For linear processes, where A is the system matrix and r is the noise:
Statistic approximation: POP modes are the eigenvectors p of the system matrix :
Complex eigenvalues: The period: The damping time: The associated time series:



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