versión On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
The El Niño and Tropical Cyclogenesis
in the North-West Pacific
Vladimir Pudov, Sergei Petrichenko
Institute of Experimental Meteorology, 82, Lenin Str.,
Obninsk, Kaluga reg., Russia, 249039,
E-mail: email@example.com, firstname.lastname@example.org
The El Niño of 1997-98 was the second most intensive, after the 1982-83 El Niño, registered in the past century. It supported the interconnections of the El Niño phenomenon and the intensity of the tropical cyclogenesis. The effect of El-Niño/Southern Oscillation (ENSO) on TC formation in the Pacific as well as in the Indian and Atlantic oceans is studied with the use of a model of the atmospheric general circulation (Wu, Guo-Xiong, 1992; Wu, Guo-Xiong and Lau N.,1992). A high correlation between ENSO and the ocean surface temperature anomalies in the eastern equatorial part of the Pacific and in the North-West Atlantic has been revealed. In the Indian ocean this link is of a seasonal character.
The present paper gives an analysis of these interconnections for the period of 1970-1998. A working hypothesis is also given, the accounts of which are some what different from those in the paper of Pudov V.D., Petrichenko S.A., 1999.
Results & Discussion
To analyze the interconnections of the tropical cyclogenesis in the North-West Pacific with the El Niño phenomenon, a database of TC in this region for the period from 1970 to 1998, created by the authors, was used (Pudov V.D., Petrichenko S.A., 1999). Mean monthly indices characterizing the El Niño intensity (MEI) (Wolter, K., and M.S. Timlin, 1993), taken from the website of the NOAA (www.cdc.noaa.gov/enso), were also used, along with mean monthly values of the South Oscillation Index (SOI).
As El Niño begins its formation in the western part of the ocean by the second half of a year and stops in the south-east in the first half of the next year we shall employ the terms "the year of the El Niño beginning" and "the year of the El Niño ceasing".
Taking the El Niños of 1972-73, 1982-83 and 1997-98 as examples, we shall consider their interactions with tropical cyclogenesis in the North-West Pacific. In the year of the El Niño beginning, as follows from the hypothesis of Pudov V.D. & Petrichenko S.A., 1999, the ocean upper layer enthalpy in the Philippine Sea attains its maximum values. Therefore one should expect that the number of intensive TC and, correspondingly, of stormy and hurricane days during this year can be noticably greater than during the year of the El Niño ceasing.
It is also seen that the tropical cyclogenesis in the years of the El Niño beginning starts noticeably earlier than in years when the El Niño ceased. So in 1997, 1982 and 1972 TC began to originate in April, March and January, respectively. During the years when the El Niño ceased - 1998, 1983 and 1973 - TC originated in July, June and June, respectively. The ratios of the total number of days with storms and typhoons in the years when El Niño started and finished were correspondingly: 1997/98-242/123; 1982/83-200/152; 1972/73-206/158. For example, this is clearly seen in the figure, where, in the form of gistograms, the number of days with depressions (wind velocity does not exceed 17 m/s), with storms (wind velocity from 17 to 33 m/s) and typhoons (wind velocity more than 33 m/s) are plotted for the months of 1997/98.
Thus during the years of the El Niño beginning the number of days with storms and hurricanes is almost twice that when the El Niño comes to an end. Here it should be noted that the TC trajectories in the years of the phenomenon beginning and finish are significantly different. In the years of the El Niño beginning, the trajectories are mainly in the north, north-eastern direction along the Kuroshio stream. During the years when the El Niño finished, the majority of TC were directed towards the west north-west and fell on the South Korea, China and Vietnam coasts. Note also that the regions of TC origin in the years of the El Niño beginning envelope rather a larger surface area (up to 170 E). All this is caused by anomalously high amounts of heat accumulated in the upper ocean layer by the El Niño beginning, involving an ever increasing ocean area propagating towards the east. In the years of the El Niño termination, the upper ocean layer heat in the west is considerably less due both to a decrease in heat transport by the Trade-Wind current and to the intensive TC impact in the previous years. The cyclogenesis intensity decreases significantly and has a more pronounced seasonal character. It should also be mentioned that there is no link at all between South China Sea cyclogenesis and the El Niño events. This is likely to be caused by the closeness of the sea: the upper layer sensible heat does not undergo the effect of the Trade-Wind current variability, including the El Niño events.
Correlations of the SOI and El Niño have been analyzed. The highest correlation (-0.66) appeared between the years of the El Niño beginning and the SOI averaged values for July, August and September (the period of the upper ocean layer sensible heat in the North-West Pacific). During the years of the El Niño beginning-1972, 76, 87, 94 and 97-the SOI values were less than minus 12. This value could serve as one of the criteria of El Niño formation.