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El Nino events (warm) are often stronger than La Nina events (cold). This asymmetry is an intrinsic nonlinear characteristic of the El Nino-Southern Oscillation (ENSO) phenomenon. In order to measure the nonlinearity of ENSO, the maximum potential intensity (MPI) index and the nonlinear dynamic heating (NDH) of ENSO are proposed as qualitative and quantitative measures. The 1997/98 El Nino that was recorded as the strongest event in the past century and another strong El Nino event in 1982/83 ne

In the late 1970s, the ENSO cycle exhibited frequency change. The oscillation period increased from 2-4 yr (high frequency) during 1962-75 to 4-6 yr (low frequency) during 1980-93. Observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled ENSO mode. In comparison with the high-frequency regime, the structure of the coupled mode in the low-frequency regime shows three distinctive features during the warm phase of ENSO: the eastward shi

The vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current are known to be essential for the equatorial SST anomaly associated with the El Nin o-Southern Oscillation (ENSO). In the coupled model, these two processes are referred to as the thermocline feedback and the zonal advective feedback, respectively. Using a version of a recharge oscillator model for ENSO obtained from the stripped-down

Abstract Using ocean data assimilation products, variability of eastern Pacific Ocean tropical instability waves (TIWs) and their interaction with the El Niño–Southern Oscillation (ENSO) were analyzed. TIWs are known to heat the cold tongue through horizontal advection. Conversely, variability of the cold tongue influences TIW variability (TIWV). During La Niña, TIWs are more active and contribute to anomalous warming. During El Niño, TIWs are suppressed and induce an anomalous cooling. TIWV thu

The peaks of El Niño in the Cane–Zebiak (CZ) model tend to appear most frequently around November when the ocean Rossby waves, which were amplified during the previous unstable season (February–May), turn back to the eastern Pacific and when the local instability in the eastern Pacific is very weak. The peaks of La Niña in the CZ model occur most frequently in boreal summer, in contrast to the observed counterpart that usually occurs in boreal winter. Sensitivity experiments indicate that the ph

The mechanisms of interdecadal changes of El Niño‐Southern Oscillation (ENSO) modes are examined through an eigen analysis of a simple coupled ocean‐atmosphere model. It is shown that the observed interdecadal climate shift can effectively modify the strength of two major coupled feedbacks for the ENSO mode, namely, the zonal advection and thermocline feedbacks. These modifications lead to quantitative changes of the leading coupled mode in its frequency, growth rate, and spatial pattern, which

Abstract The El Niño–La Niña asymmetry was estimated in the 10 different models participating in the Coupled Model Intercomparison Project (CMIP). Large differences in the “asymmetricity” (a variance-weighted skewness) of SST anomalies are found between models and observations. Most of the coupled models underestimate the nonlinearity and only a few exhibit the positively skewed SST anomalies over the tropical eastern Pacific as seen in the observation. A significant association between the nonl

The SST anomalies (SSTA) over the past 148 years have been analyzed to describe the interdecadal change in the skewness of SSTA (ICS) in the tropical Pacific and possible consequence of this change. The first EOF mode of ICS represents the interdecadal changes in the El Nino‐La Nina asymmetry. The corresponding PC time series is related to the ENSO predictability, suggesting that ENSOs are more predictable during the positive ICS decades than during the negative ICS decades, and to the propagati

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period o

We present a numerical eigenmode analysis of an intermediate El Niño–Southern Oscillation (ENSO) model which is driven by present‐day observed background conditions as well as by simulated background conditions for the Last Glacial Maximum (LGM) about 21,000 years ago. The background conditions are obtained from two LGM simulations which were performed with the National Center for Atmospheric Research climate system model (CSM1.4) and an Earth system model of intermediate complexity (ECBilt‐CLIO

Abstract The reversibility of global mean surface temperature was examined by a transient CO 2 reversibility experiment using an Earth system model. The results showed that after CO 2 ramp‐up toward CO 2 quadrupling and ramp‐down returned to the present‐day level, the global mean surface temperature kept decreasing but stopped to change for ∼40 years in the early net‐zero CO 2 emission period. This period, referred to a cooling hiatus, resulted from a compensation between Southern Hemisphere coo

The El Niño Southern Oscillation (ENSO) is characterized by being irregular or nonperiodic and asymmetric between El Niño and La Niña with respect to amplitude, pattern, and temporal evolution. These observed features suggest the importance of nonlinear dynamics and/or stochastic forcing. Both nonlinear deterministic chaos and linear dynamics subject to stochastic forcing and/or to non-normal growth were introduced to explain the irregularity of ENSO, but no consensus has been reached to date gi