Types Of El Nino-Southern Oscillation Research Articles

Decadal amplitude modulation of two types of ENSO and its relationship with the mean state

In this study, we classified two types of El Nino–Southern Oscillation (ENSO) events within the decadal ENSO amplitude modulation cycle using a long-term coupled general circulation model simulation. We defined two climate states—strong and weak ENSO amplitude periods—and separated the characteristics of ENSO that occurred in both periods. There are two major features in the characteristics of ENSO: the first is the asymmetric spatial structure between El Nino and La Nina events; the second is that the El Nino–La Nina asymmetry is reversed during strong and weak ENSO amplitude periods. El Nino events during strong (weak) ENSO amplitude periods resemble the Eastern Pacific (Central Pacific) El Nino in terms of the spatial distribution of sea surface temperature anomalies (SSTA) and physical characteristics based on heat budget analysis. The spatial pattern of the thermocline depth anomaly for strong (weak) El Nino is identical to that for weak (strong) La Nina, but for an opposite sign and slightly different amplitude. The accumulated residuals of these asymmetric anomalies dominated by an east–west contrast structure could feed into the tropical Pacific mean state. Moreover, the residual pattern associated with El Nino–La Nina asymmetry resembles the first principal component analysis (PCA) mode of tropical Pacific decadal variability, indicating that the accumulated residuals could generate the change in climate state. Thus, the intensified ENSO amplitude yields the warm residuals due to strong El Nino and weak La Nina over the eastern tropical Pacific. This linear relationship between ENSO and the mean state is strong during the mature phases of decadal oscillation, but it is weak during the transition phases. Furthermore, the second PCA mode of tropical Pacific decadal variability plays an important role in changing the phase of the first mode. Consequently, the feedback between ENSO and the mean state is positive feedback to amplify the first PCA mode, whereas the second PCA mode is a negative feedback to lead the phase change of the first PCA mode due to their lead-lag relationship. These features could be regarded as evidence that the decadal change in properties of ENSO could be generated by the nonlinear interaction between ENSO and the mean state on a decadal-to-interdecadal time scale.

Relationships between Extratropical Sea Level Pressure Variations and the Central Pacific and Eastern Pacific Types of ENSO

Abstract This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical Pacific (20°–60°N) and the eastern Pacific (EP) and central Pacific (CP) types of El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode of the extratropical SLP anomalies represents variations of the Aleutian low, and the second EOF mode represents the North Pacific Oscillation (NPO) and is characterized by a meridional SLP anomaly dipole with a nodal point near 50°N. It is shown that a fraction of the first SLP mode can be excited by both the EP and CP types of ENSO. The SLP response to the EP type is stronger and more immediate. The tropical–extratropical teleconnection appears to act more slowly for the CP ENSO. During the decay phase of EP events, the associated extratropical SLP anomalies shift from the first SLP mode to the second SLP mode. As the second SLP mode grows, subtropical SST anomalies are induced beneath via surface heat flux anomalies. The SST anomalies persist after the peak in strength of the second SLP mode, likely because of the seasonal footprinting mechanism, and lead to the development of the CP type of ENSO. This study shows that the CP ENSO is an extratropically excited mode of tropical Pacific variability and also suggests that the decay of an EP type of ENSO can lead to the onset of a CP type of ENSO with the aid of the NPO. This extratropical linking mechanism appears to be at work during the 1972, 1982, and 1997 strong El Niño events, which were all EP events and were all followed by strong CP La Niña events after the NPO was excited in the extratropics. This study concludes that extratropical SLP variations play an important role in exciting the CP type of ENSO and in linking the transitions from the EP to CP events.

Niño indices for two types of ENSO

[1] El Nino-Southern Oscillation (ENSO), which dominates variability on interannual timescale in the climate system, is known to exhibit various spatio-temporal characteristics. Recent studies show that in additional to a canonical El Nino with its major center of sea surface temperatures (SST) anomalies in the equatorial Pacific cold-tongue region, a different type of El Nino with its major action center shifted to the warm-pool edge has emerged and become more common during the past two decades. Because the SST patterns of these two types of El Nino events are highly correlated, neither of the traditional Nino3 and Nino4 SST indices alone is effective in representing the new-type El Nino. Through a simple transformation of the Nino3 and Nino4 indices, we devised two new indices that separately identify the two types of ENSO events. Unlike the Nino3 and Nino4 indices, the two new indices are of little simultaneous correlation. The SST patterns associated with these new indices capture SST characteristics of the two types of ENSO. Their running lagged-correlations capture different ENSO-phase propagations and ENSO regime changes associated with the climate shift in 1976/77.

Identification of Central‐Pacific and Eastern‐Pacific types of ENSO in CMIP3 models

Much understanding of the El Niño‐Southern Oscillation (ENSO) has been obtained from the analyses of the climate simulations produced for World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3). However, most of these analyses do not consider the existence of the Eastern‐Pacific (EP) and Central‐Pacific (CP) types of ENSO events, which have been increasingly recognized as two distinct types of interannual sea surface temperature (SST) variation in the tropical Pacific. This study uses a regression‐Empirical Orthogonal Function method to identify how well these two ENSO types are captured in the pre‐industrial simulations of nineteen CMIP3 models. It is concluded that most CMIP3 models (13 out of 19) can produce realistically strong CP ENSOs, but only a few of them (9 out of 19) can produce realistically strong EP ENSOs. Six models that realistically simulate both the EP and CP ENSOs and their intensity ratio are identified. By separating the SST variability into these two types, it is further revealed that the leading periodicity of the simulated EP ENSO is linearly related to the latitudinal width of SST variability and varies from 1 to 5 years. As for the simulated CP ENSO, its leading periodicity is either 2 or 4 years depending on whether its SST variability is located to the east of the dateline or in the western‐Pacific warm pool, respectively. The identification produced in this study offers useful information to further understand the two types of ENSO using the CMIP3 models.

Contrasting Eastern-Pacific and Central-Pacific Types of ENSO

Abstract Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Niño–Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. This type of ENSO appears less related to the thermocline variations and may be influenced more by atmospheric forcing. It has a stronger teleconnection with the southern Indian Ocean. Phase-reversal signatures can be identified in the anomaly evolutions of the EP-ENSO but not for the CP-ENSO. This implies that the CP-ENSO may occur more as events or epochs than as a cycle. The EP-ENSO has experienced a stronger interdecadal change with the dominant period of its SST anomalies shifted from 2 to 4 yr near 1976/77, while the dominant period for the CP-ENSO stayed near the 2-yr band. The different onset times of these two types of ENSO imply that the difference between the EP and CP types of ENSO could be caused by the timing of the mechanisms that trigger the ENSO events.

Variability of the impact of El Niño–southern oscillation on sea‐level pressure anomalies over the North Atlantic in January to March (1874–1996)

AbstractSea‐level pressure (SLP) anomalies over the North Atlantic and European (NAE) sector (25–70°N, 100°W–50°E) and over a larger domain encompassing the entire North Pacific domain are studied to demonstrate that SLP anomalies (SLPAs) during boreal winter (January–March) vary widely between years characterized by the same El Niño–southern oscillation (ENSO) phase. The typical cold ENSO signal tends to be more stable than the warm one during the 1874–1996 period. The typical cold ENSO pattern (e.g. positive SLPA south of 55°N across the North Atlantic and negative SLPA in the northern North Atlantic) is similar to the positive phase of the North Atlantic oscillation (NAO) and occurs throughout the 20th century, except during the 1950s and 1960s when the basinwide westerlies are particularly slow. On the contrary, the typical warm ENSO pattern (e.g. positive SLPA from central Canada to Scandinavia and negative SLPA from the southeastern USA to central Europe, corresponding to the negative phase of the NAO) occurs mainly from 1930 to 1970. Another robust warm ENSO pattern is associated with a large positive (negative) SLPA between Newfoundland and western Europe (between Greenland and Scandinavia), and occurs mainly at the beginning and the end of the 20th century when the basinwide North Atlantic westerlies are strengthened. All these patterns stay statistically significant when the multi‐decadal variability is removed from the North Atlantic SLPA. It is shown that the low‐frequency variability of the north tropical Atlantic sea‐surface temperature anomalies could exert a modulating effect on the ENSO teleconnection. NAE SLPAs tend to be strong during warm (cold) ENSO winters and consistent with a negative (positive) phase of the NAO when the north tropical Atlantic is anomalously warm (cold). Lastly, the magnitude of the SLPA patterns over the NAE sector appears poorly related to the intensity of sea‐surface temperature anomalies in the central and eastern equatorial Pacific. Copyright © 2003 Royal Meteorological Society

Mechanism of Kelvin and Rossby waves during ENSO events

¶The fields of sea-level height anomaly (SLHA) and surface zonal wind anomaly (SZWA) have been analyzed to investigate the typical evolution of spatial patterns during El Nino-Southern Oscillation (ENSO) events. Sea surface temperature (SST) changes during ENSO events are represented as an irregular interplay of two dominant modes, low-frequency mode and biennial mode. Cyclostationary principal component (PC) time series of the former variables are regressed onto the PC time series of the two dominant SSTA modes to find the spatial patterns of SLHA and SZWA consistent with the two SSTA modes. The two regressed patterns of SLHA explain a large portion of SLHA total variability. The reconstruction of SLHA using only the two components reasonably depicts major ENSO events. Although the low-frequency component of SST variability is much larger than the biennial component, the former does not induce strong Kelvin and Rossby waves. The biennial mode induces much stronger dynamical ocean response than the low-frequency mode. Further decomposition of the SLHA modes into Kelvin and Rossby components shows how these two types of equatorial waves evolve during typical ENSO events. The propagation and reflection of these waves are clearly portrayed in the regressed patterns leading to a better understanding of the wave mechanism in the tropical Pacific associated with ENSO. A close examination suggests that the delayed action oscillator hypothesis is generally consistent with the analysis results reported here. Rossby wave development in the central Pacific in the initiation stage of ENSO and the subsequent reflection of Kelvin waves at the western boundary seems to be an important mechanism for further development of ENSO. The development of Kelvin waves forced by the surface wind in the far-western Pacific cannot be ruled out as a possible mechanism for the growth of ENSO. While Kelvin waves in the far-western Pacific serve as an intiation mechanism of ENSO, they also cause the termination of existing ENSO condition in the central and eastern Pacific, thereby leading to a biennial oscillation over the tropical Pacific. The Kelvin waves from the western Pacific erode the thermocline structure in the central Pacific preventing further devlopment of ENSO and ultimately terminating it. It should be emphasized that this wave mechanism is clear and active only in the biennial mode.

Development of sea surface temperature, surface wind and divergence anomalies during a composite ENSO episode

The interannual variations in sea surface temperature (SST) in the equatorial east Pacific, which are dominated by the El Nino phenomenon, are shown for the period 1870–1983. Since 1870 25 significant warm events have occurred. These events are classified as weak, moderate, strong and very strong, according to the normalized SST anomalies in the region 130° W–80° W, 0°–5° S. The spatial and temporal development of a composite El Nino/Southern Oscillation (ENSO) episode, based on 10 very strong or strong events, is presented in terms of SST, surface wind and divergence anomalies for the tropical Pacific (10° N–30° S). During its evolution the following phases are distinguished: “Antecedent Conditions”, “Onset Phase”, “Peak Phase”, “Mature Phase” and “Dissipation Stage”. Some aspects of ocean-atmosphere interaction associated with this evolution and, more specifically, the initiation of the composite event, are described. Seasonally varying feedback processes between SST, surface wind and convergence anomaly patterns in the western Pacific/Indonesian region suggest a possible mechanism for the initiation of typical ENSO events.