Abstract The main purpose of this study is to reveal the instability of sea‐surface‐temperature (SST) anomalies in connection with the ‘predictability barrier’ of El Niño established by Webster and Yang. the characteristics of linear instability of Pacific SST anomalies are computed using monthly COADS data and the method of Lyapunov exponents. Analysis is carried out separately for the northern and tropical part of the Pacific Ocean via approximating SST fields using a few leading empirical orthogonal functions. Changes of Lyapunov exponents throughout the year are computed and periods of the most unstable and stable seasons revealed. of most interest is the spring instability of SST anomalies both in the tropical and extratropical Pacific. This instability is crucial for the existence of a predictability barrier for an El Niño event.