Abstract
Present work uses 1979–2005 monthly observational data to study the impacts of El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer. The El Niño Modoki phenomenon is characterized by the anomalously warm central equatorial Pacific flanked by anomalously cool regions in both west and east. Such zonal SST gradients result in anomalous two-cell Walker Circulation over the tropical Pacific, with a wet region in the central Pacific. There are two mid-tropospheric wave trains passing over the extratropical and subtropical North Pacific. They contain a positive phase of a Pacific-Japan pattern in the northwestern Pacific, and a positive phase of a summertime Pacific-North American pattern in the northeastern Pacific/North America region. The western North Pacific summer monsoon is enhanced, while the East Asian summer monsoon is weakened. In the South Pacific, there is a basin-wide low in the mid-latitude with enhanced Australian high and the eastern South Pacific subtropical high. Such an atmospheric circulation pattern favors a dry rim surrounding the wet central tropical Pacific. The El Niño Modoki and its climate impacts are very different from those of El Niño. Possible geographical regions for dry/wet conditions influenced by El Niño Modoki and El Niño are compared. The two phenomena also have very different temporal features. El Niño Modoki has a large decadal background while El Niño is predominated by interannual variability. Mixing-up the two different phenomena may increase the difficulty in understanding their mechanisms, climate impacts, and uncertainty in their predictions.










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http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml
http://www.data.kishou.go.jp/climate/elnino/tenkou/sekai1.html
http://www.data.kishou.go.jp/climate/cpdinfo/monitor/2004/2_1.html
http://en.wikipedia.org/wiki/2004_Atlantic_hurricane_season
http://www.japantimes.co.jp/cgi-bin/getarticle.pl5?nn20040724f3.htm
The definitions of some commonly used El Niño indices, such as Niño1+2, Niño3, Niño4, and Niño3.4 can be found in many references, e.g., http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/bulletin/table2.html.
Trenberth and Stepaniak (2001), based on the PC2 from a similar EOF analysis, defined an index called the Trans-Niño index (TNI) by the difference between the normalized SST anomalies averaged in the Niño1+2 and Niño4 regions. The TNI does not include the zonal SST gradient in the western equatorial Pacific. The main difference between the TNI and EMI is in their concepts. The TNI, just as its name implies, “capture the evolution of ENSO in the months leading up to the event and, with opposite sign, the subsequent evolution after the event”. As shown in this work, the El Niño Modoki phenomenon is not necessarily a stage of a developing or decaying ENSO cycle.
http://www2.chass.ncsu.edu/garson/pa765/partialr.htm
The PNA pattern is most pronounced in boreal winter, but very weak in boreal summer (e.g., Wallace and Gutzler 1981). Based on NOAA’s map for PNA patterns at 500 hPa (http://www.cpc.ncep.noaa.gov/data/teledoc/pna_map.shtml), the positive phase of July PNA features above-average heights to the west of Hawaii and the southeastern USA and below-average heights over the southern Aleutian Islands and the eastern Canada. The above-average heights over the western North America that are apparent in the positive phase of PNA in other seasons are hardly discerned in July. It is this component of the summer PNA that exerts great impact on the climate in the western USA during El Niño Modoki. Note that the strength and location of the PNA components have a seasonal dependence due to various forcings including SSTA in the tropical Pacific (Leathers and Palecki 1992).
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Acknowledgments
We are thankful to Y. He for the U.S. 102-division rainfall data, to S.-W. Wang and J.-B. Huang for the 160-station rainfall data in China, to R.-C. Zhang, H. Shibata and T. Maeda for their help in processing rainfall data in Japan. We appreciate very much for the constructive comments from three anonymous reviewers. Our special thanks go to P. Delecluse for her encouragement and inspiring remarks. HW also extends her thanks to S. Hakeem, Y.-M. Liu, J.-J. Luo, and D.-P. Shi for various discussions, to Z. Yu for her many critical remarks to improve the manuscript.
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Weng, H., Ashok, K., Behera, S.K. et al. Impacts of recent El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer. Clim Dyn 29, 113–129 (2007). https://doi.org/10.1007/s00382-007-0234-0
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DOI: https://doi.org/10.1007/s00382-007-0234-0
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