Soon-Il An

Nonlinearity and Asymmetry of ENSO

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 nearly reached the MPI. During these superwarming events, the normal climatological conditions of the ocean and atmosphere were collapsed com- pletely. The huge bursts of ENSO activity manifested in these events are attributable to the nonlinear dynamic processes. Through a heat budget analysis of the ocean mixed layer it is found that throughout much of the ENSO episodes of 1982/83 and 1997/98, the NDH strengthened these warm events and weakened subsequent La Nina events. This led to the warm-cold asymmetry. It is also found that the eastward-propagating feature in these two El Nino events provided a favorable phase relationship between temperature and current that resulted in the strong nonlinear dynamical warming. For the westward-propagating El Nino events prior to the late 1970s (e.g., 1957/58 and 1972/73 ENSOs) the phase relationships between zonal temperature gradient and current and between the surface and subsurface temperature anomalies are unfavorable for nonlinear dynamic heating, and thereby the ENSO events are not strong.

Strong El Niño events and nonlinear dynamical heating

[1] We present evidence showing that the nonlinear dynamic heating (NDH) in the tropical Pacific ocean heat budget is essential in the generation of intense El Nino events as well as the observed asymmetry between El Nino (warm) and La Nina (cold) events. The increase in NDH associated with the enhanced El Nino activity had an influence on the recent tropical Pacific warming trend and it might provide a positive feedback mechanism for climate change in the tropical Pacific.

Thermocline and zonal advective feedbacks within the equatorial ocean recharge oscillator model for ENSO

Through the dynamical coupling between ocean and atmosphere, the vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current constitute the so-called the thermocline and zonal advective feedbacks that are essential for El Nino/Southern Oscillation (ENSO) coupled dynamics. In this paper, we demonstrate that these two feedbacks are dynamically linked because of the geostrophic balance between the upper ocean zonal current and the meridional gradient of the thermocline. Both feedbacks thus play important and similar roles in the growth and phase transition of ENSO. We further propose a new version of the conceptual recharge oscillator model for ENSO by including these two feedbacks. The new model retains the simplest possible form of a harmonic oscillator, yet presents a more complete description of slow physics for ENSO. Moreover, it also provides the reconciliation between the biased emphases in the ENSO theories oh the thermocline and zonal advective feedbacks.

Warm Pool and Cold Tongue El Niño Events as Simulated by the GFDL 2.1 Coupled GCM

Recent studies report that two types of El Nino events have been observed. One is the cold tongue (CT) El Nino, which is characterized by relatively large sea surface temperature (SST) anomalies in the eastern Pacific, and the other is the warm pool (WP) El Nino, in which SST anomalies are confined to the central Pacific. Here, both types of El Nino events are analyzed in a long-term coupled GCM simulation. The present model simulates the major observed features of both types of El Nino, incorporating the distinctive patterns of each oceanic and atmospheric variable. It is also demonstrated that each type of El Nino has quite distinct dynamic processes, which control their evolutions. The CT El Nino exhibits strong equatorial heat discharge poleward and thus the dynamical feedbacks control the phase transition from a warm event to a cold event. On the other hand, the discharge process in the WP El Nino is weak because of its spatial distribution of ocean dynamic field. The positive SST anomaly of WP El Nino is thermally damped through the intensified evap- orative cooling.

Why the properties of El Niño changed during the late 1970s

Following the abrupt North Pacific climate shift in the mid-1970s, the period, amplitude, spatial structure, and temporal evolution of the E1 Nifio notably changed. Theory is needed to explain why the coherent changes in several El Nifio characteristics are nearly synchronized with the decadal climate shift. While the decadal variation in the equatorial thermocline could potentially change El Nino behavior, observation indicates that from the pre-shift (1961 - 1975) to the post-shift (1981 - 1995) period the change of equatorial eastern Pacific thermocline is insignificant. Our numerical experiments with a coupled atmosphere-ocean model illustrate that the observed changes in ENSO properties may be attributed to decadal changes in the surface winds and associated ocean surface layer dynamics without changes in the mean thermocline. A theoretical analysis is presented to elucidate the mechanisms by which the decadal variations in winds and upwelling modify the structure and propagation of the E1 Nifio and amplify and prolong the E1 Nifio-La Nifia cycle. The dominant period of the E1 Nifio increased from 2-3 years during 1960s and 1970s to 4-5 years during 1980s and 1990s; during this time the amplitude of E1 Nifio also increased (An and Wang, 2000). These changes were accompanied by a notable modification in the evolution pattern and spatial structure of the coupled ocean-

Collective Role of Thermocline and Zonal Advective Feedbacks in the ENSO Mode

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 Nino-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 approximation of the Cane-Zebiak-type model, it is demonstrated that these two feedbacks, which are linked dynamically through the geostrophic approximation, tend constructively to contribute to the growth and phase transition of ENSO. However, these two feedbacks control the leading coupled mode in different ways. The thermocline feedback leads to a coupled mode through the merging of the damped SST mode and ocean adjustment mode, whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode. With both of these feedbacks, the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup. The main consequence of these sensitivities is that the growth rate and frequency of the ENSO mode may be sensitive to slight changes in basic-state parameters, which control the strength of these feedbacks.

Mechanisms of Locking of the El Niño and La Niña Mature Phases to Boreal Winter

The peaks of El Nino 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 Nina 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 phase locking of the La Nina to boreal summer is primarily caused by seasonal variations of the tropical convergence zone, which regulate convective heating through atmospheric convergence feedback. The observed thermocline and the wind anomalies in the western Pacific exhibit considerable seasonal variations. These were missed in the original CZ model. In a modified CZ model that includes the seasonal variations of the western Pacific wind anomalies and the basic-state thermocline depth, the peaks of La Nina preferably occur in boreal winter, suggesting that the seasonal variation of the western Pacific surface wind anomalies and the mean thermocline depth are critical factors for the phase locking of the mature La Nina to boreal winter. The mechanisms by which these factors affect ENSO phase locking are also discussed.

An eigen analysis of the interdecadal changes in the structure and frequency of ENSO mode

The mechanisms of interdecadal changes of El Nino-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 are consistent with the observation.

Interannual Variations of the Tropical Ocean Instability Wave and ENSO

Abstract Using ocean data assimilation products, variability of eastern Pacific Ocean tropical instability waves (TIWs) and their interaction with the El Nino–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 Nina, TIWs are more active and contribute to anomalous warming. During El Nino, TIWs are suppressed and induce an anomalous cooling. TIWV thus acts as negative feedback to ENSO. Interestingly, this feedback is stronger during La Nina than during El Nino. To investigate this negative/asymmetric feedback, a simple parameterization for the horizontal thermal flux convergence due to TIWs was incorporated into a simple ENSO model. The model results suggested that asymmetric thermal heating associated with TIWs can explain the El Nino–La Nina asymmetry (with larger-amplitude El Ninos).

El Niño–La Niña Asymmetry in the Coupled Model Intercomparison Project Simulations*

The El Nino–La Nina 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 nonlinear dynamical heating (NDH) and asymmetricity in the model–ENSO indices is found, inferring that asymmetricity is caused mainly by NDH. Among the 10 models, one coupled GCM simulates the asymmetricity of the tropical SST realistically, and its simulation manifests a strong relationship between the intensity and the propagating feature of ENSO—the strong ENSO events moving eastward and the weak ENSO events moving westward—which is consistent with the observation. Interestingly, the coupled general circulation models, of which the ocean model is based on the one used by Bryan and Cox, commonly showed the reasonably positive skewed ENSO. The decadal changes in the skewness, variance, and NDH of the model-simulated ENSO are also observed. These three quantities over the tropical eastern Pacific are significantly correlated to each other, indicating that the decadal change in ENSO variability is closely related to the nonlinear process of ENSO. It is also found that these decadal changes in ENSO variability are related to the decadal variation in the tropical Pacific SST, implying that the decadal change in the El Nino–La Nina asymmetry could manifest itself as a rectified change in the background state.