This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical and the eastern Pacific (EP) and central Pacific (CP) types of El Niñ o-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 508N. 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 Niñ o events, which were all EP events and were all followed by strong CP La Niñ a 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.

[1] In this study, evidence is presented from statistical analyses, numerical model experiments, and case studies to show that the impact on US winter temperatures is different for the different types of El Niño. While the conventional Eastern-Pacific El Niño affects winter temperatures primarily over the Great Lakes, Northeast, and Southwest US, the largest impact from Central-Pacific El Niño is on temperatures in the northwestern and southeastern US. The recent shift to a greater frequency of occurrence of the CentralPacific type has made the Northwest and Southeast regions of the US most influenced by El Niño. It is shown that the different impacts result from differing wave train responses in the atmosphere to the sea surface temperature anomalies associated with the two types of El Niño.

[1] Based on the simple framework of the recharge oscillator for El Niño-Southern Oscillation (ENSO), this paper describes the formulation of a coupled stability index for the large-scale tropical ocean-atmosphere interactions in the equatorial Pacific region. This index, referred to as the Bjerknes (BJ) stability index, takes its negative contributions from the mean upwelling and thermal damping and its positive contributions from the thermocline, the zonal advection, and the Ekman feedbacks. The validity of the BJ stability index formula is tested through a detailed eigenanalysis of an intermediate-coupled model, which shows that the BJ stability index captures the dependence of growth rate of the leading coupled ENSO-like mode on various climate conditions. The general formula of the BJ stability index may be useful for assessing the coupled stability of ENSO and its sensitivity to changes in tropical climate conditions.

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This study develops a pattern correlation method to determine the type of major El Niño events since 1870 from a reconstructed sea surface temperature dataset. Different from other identification methods, this method allows an El Niño event to be of the Central-Pacific (CP) type, the Eastern-Pacific (EP) type, or the Mixed type (i.e. the both types coexist). Application of this method to the 39 major El Niño events identified by the Ocean Niño Index during the period 1870-2010 results in 8 events that are categorized to be of the EP type, 16 of the CP type, and 15 of Mixed type. Before the 1910s, the El Niño events are mostly of the EP type, but are mostly the CP type after 2000, while in between both types occurred. The consistencies and inconsistencies between the El Niño types identified by this method and other three methods, which have been proposed recently for El Niño-type classification, are examined and discussed. All four methods consistently identify the El Niño events occurring in the following years to be of the EP