El Niño Modoki (Pseudo El Niño) and its Influences on the World Climate

* Climate Variations Research Program (CVRP), Frontier Research Center for Global Change (FRCGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan; Tel: 81-45-778-5502; Fax: 81-45-778-5707; E-mail: yamagata@jamstec.go.jp

** Department of Earth and Planetary Science (DEPS), Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; Tel: 81-3-5841-4297; Fax: 81-3-5841-8791; E-mai: yamagata@eps.s.u-tokyo.ac.jp

Using various ocean/atmosphere datasets mainly for the period 1979-2005, we suggest the existence of a new climate mode that is different from conventional El Niño in the central tropical Pacific. The unique central Pacific warming is associated with a horse-shoe SST pattern, and is flanked by a colder anomaly on both sides along the equator (Fig. 1). Such a zonal SST distribution results in anomalous two-cell Walker circulations over the tropical Pacific (Fig. 2). Both ITCZ and SPCZ expand poleward, forming a wet region in the central tropical Pacific. Conventional EOF analysis of monthly tropical Pacific SSTA shows that the new mode is represented by the second mode that explains 12% of the variance.

Since the mode cannot be described as one phase of El Niño evolution, we suggest that the phenomenon should be called El Niño Modoki (Pseudo-El Niño). The El Niño Modoki involves ocean-atmosphere coupled processes, indicating the existence of a unique atmospheric component during the evolution, which is analogous to the Southern Oscillation in case of El Niño. Thus the total entity should be called ENSO Modoki.

The Modoki's impact on the world climate is very different from that of ENSO (and IOD). Possible geographical regions for droughts and floods influenced by Modoki and ENSO are compared. Interestingly, the Modoki's influences over regions such as the Far East including Japan and the western coast of USA are almost opposite to those of the conventional ENSO (Fig. 3).

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 (Fig. 4). This appears to be a cause of more frequent and persistent occurrence of the Modoki event during recent warming decades; the ENSO Modoki has a large decadal background while ENSO is predominated by interannual variability.

Appreciating the two different phenomena in the tropical Pacific will enhance our understanding of the coupled ocean-atmosphere dynamics and thus contribute to reducing the uncertainty in the climate prediction.

Figure 1: Composite SSTA in ºC during strong positive El Niño Modoki events averaged over (a) seven boreal summers, namely JJAS seasons of 1986,1990, 1991, 1992, 1994, 2002 and 2004 and (b) 8 boreal winters, namely DJF seasons of 1979-80, 1986-87,1990-91, 1991-92, 1992-93, 1994-95, 2002-2003 and 2004-05. Significant values above 95% confidence level from a two tailed Student's t-test are shaded.

Figure 2: Anomalous Walker Circulations (10S -10N) between 90E and 60W based on partial regression for a) El Niño Modoki Index (EMI) introduced suitably using zonal SST differences and b) Niño3 Index. The vertical velocity at the pressure levels ismultiplied by a factor of -50 to give a better view. The regressed specific humidity is shaded. The contours are for the regressed velocity potential (unit: 10⁵m²s^-1). The units labeled in the regression patterns are actually the units per standard deviation of the index being regressed. The standard deviations for EMI and Niño3 in JJA are 0.504˚C and 0.553˚C, respectively.

Figure 3: Composite JJA rainfall patterns (anomaly percent of normal: %) for the three largest El Niño Modoki events (1994, 2002, and 2004) in a) China, b) Japan, and c) the United States, and those for the three larges El Niño events (1982, 1987, and 1997) in d) China, e) Japan, and f) the United States. The values with significant levels less than 80% are omitted.

Figure 4: Recent interdecadal changes (1979-2005 minus 1958-1978) in the global SST. Unit of the color bar is degree.