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 Niñ o-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.

Coherent Tropical Indo-Pacific Interannual Climate Variability

“…It is not propagating and it has a shorter period than the other ENSO modes (viz., 2.5 yr). Both findings agree well with those of Kao and Yu (2009), but the spatial pattern of the EEOF8 and EEOF9 El Niño does not resemble the horseshoe pattern found in that paper. However, the CP Niño composites of Kug et al (2009) and Ashok et al (2007) do not show a pronounced horseshoe pattern either, but a warm anomaly centered at about 1708W (i.e., 208 farther west than for our EEOF8 and EEOF9).…”

“…First, EP El Niños need a large WWV to develop (Meinen and McPhaden 2000), while Kao and Yu (2009) suggest that CP events have a smaller vertical extent and are more dominated by winds and SST advection than by thermocline processes. Yu and Kim (2010) find that CP events can take place in a recharged, neutral, or discharged WWV state.…”

“…One type, which we will refer to as central Pacific (CP) El Niño, is characterized by warm anomalies in the central PO, whereas eastern Pacific (EP) El Niños have the strongest warm anomalies in the eastern PO. Kao and Yu (2009) report that the CP El Niño temperature signal reaches less deep than that of EP El Niños and suggest that CP El Niños might be more dominated by local winds and SST advection, whereas for EP El Niños thermocline processes and the WWV are important as well. Yu and Kim (2010) find that CP El Niños can occur during recharged, neutral, or discharged states of the WWV.…”

“…Two types of El Niño events have been distinguished (Ashok et al 2007;Larkin and Harrison 2005;Kug et al 2009;Kao and Yu 2009). One type, which we will refer to as central Pacific (CP) El Niño, is characterized by warm anomalies in the central PO, whereas eastern Pacific (EP) El Niños have the strongest warm anomalies in the eastern PO.…”

A Source of AGCM Bias in Simulating the Western Pacific Subtropical High: Different Sensitivities to the Two Types of ENSO

“…It has been increasingly recognized that there exists at least one more type of ENSO that onsets, develops, and decays locally in the equatorial central Pacific (Larkin and Harrison 2005;Yu and Kao 2007;Ashok et al 2007;Kao and Yu 2009;Kug et al 2009). This type of ENSO is often referred to as the central Pacific (CP) ENSO, while the conventional type of ENSO is referred to as the eastern Pacific (EP) ENSO.…”