El Niño Modoki and its possible teleconnection

Abstract

[1] Using observed data sets mainly for the period 1979-2005, we find that anomalous warming events different from conventional El Niño events occur in the central equatorial Pacific. This unique warming in the central equatorial Pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly (SSTA) on both sides along the equator. empirical orthogonal function (EOF) analysis of monthly tropical Pacific SSTA shows that these events are represented by the second mode that explains 12% of the variance. Since a majority of such events are not part of El Niño evolution, the phenomenon is named as El Niño Modoki (pseudo-El Niño) (''Modoki'' is a classical Japanese word, which means ''a similar but different thing''). The El Niño Modoki involves ocean-atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution, analogous to the Southern Oscillation in the case of El Niño. Hence the total entity is named as El Niño-Southern Oscillation (ENSO) Modoki. The ENSO Modoki events significantly influence the temperature and precipitation over many parts of the globe. Depending on the season, the impacts over regions such as the Far East including Japan, New Zealand, western coast of United States, etc., are opposite to those of the conventional ENSO. The difference maps between the two periods of 1979-2004 and 1958-1978 for various oceanic/atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline. This appears to be a cause of more frequent and persistent occurrence of the ENSO Modoki event during recent decades.

Coherent Tropical Indo-Pacific Interannual Climate Variability

“…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).…”

“…Although there is no one-to-one relationship between the Seychelles dome SST variations and ENSO, a better understanding of this connection will provide a valuable additional ingredient for ENSO prediction at longer lead times and for understanding the behavior of ENSO on longer time scales. It would be interesting to investigate whether the frequent occurrence of CP El Niños in recent decades (Ashok et al 2007) is linked to the strong warming of the southwestern IO since the 1970s. If so, the southwestern IO temperature trends might also influence the changes in ENSO under global warming.…”

“…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.…”

“…Many studies have revealed that the CP ENSO can affect global and regional climate differently from the EP ENSO (e.g., Larkin and Harrison 2005;Hu et al 2012;Yu et al 2012b). The CP ENSO has occurred more frequently in the past few decades (Ashok et al 2007;Kao and Yu 2009;Kug et al 2009). The intensity of El Niño in the central Pacific region, for example, has doubled in the past three decades (Lee and McPhaden 2010), and the El Niño events in the twenty-first century so far have been mostly of the CP type (Yu et al 2012b).…”