Evolution of the characteristics of interannual climate variability associated with the El Nino and La Nina phenomena

Long-period variations in the amplitude and period of the El Nino/La Nina p henomena are examined in combination with an analysis of phase portraits an d their evolution for sea surface temperature (SST) in the NINO3 and NINO4 regions from 1903 to 1994. On the whole, the amplitude of the observed SST cycles in NINO3 increased noticeably over the last century, and the corresp onding periods decreased appreciably. The regime of the mutual evolution of amplitudes and periods of SST cycles has changed substantially in the last few decades. Considerable interdecadal variations were observed against th e background of long-period variations. A nonlinear delayed action oscillat or model was used to analyze the relationship between the period and the am plitude of El Nino/La Nina cycles. Two versions of the model were compared, and their parameters were estimated from empirical data. Negative feedback between the amplitude and the period of El Nino/La Nina cycles was found b y analyzing empirical data, and conditions for such feedback in these model s were obtained.