Detection of ENSO Signal on the Climate of Korea
Jai-Ho Oh, Yong Hee Lee
Meteorological Research Institute
Seoul, Korea
jho@chaos.metri.re.kr
Introduction
Our climate system is highly non-linear, thus some characteristics are not easy to interpret with linear physics. Many studies on global atmospheric and oceanic variables have pointed out a distinguished low-frequency fluctuation in the global system. Further detailed investigations on climatic variables, such as temperature, precipitation, SST, etc., have presented evidence that there is a possible linkage between climatic variability and both extremes of the ENSO cycle (Bjerknes, 1969; Halpert and Ropelewski, 1992; Allen et al., 1995; Wang, 1995) and decadal scale fluctuations (Nitta and Yamada, 1989; Trenberth and Hurrel, 1994; Latif and Barnett, 1996; Yatagi and Yasunari, 1994; Zhang et al., 1997; Nakamura et al., 1997). Sometimes such a sudden shift of the mean is recognized as a step-like climatic shift. A climatic shift consists of an abrupt change in the average of regional climatic variables for a certain period of time (Vargas et al., 1995; Yamamoto et al., 1986).
In this study, we investigate the detection of the ENSO signal as well as a climate regime shift in precipitation over the Korean peninsula in the point of view of the time-frequency domain. A wavelet analysis has been applied and then investigated for possible events related to each identified temporal frequency.
Data and Methodology
A standardized precipitation index (SPI) was calculated to identify the wet and dry periods in the precipitation data set for several regions in Korea for the past 47 years (1951-1997). These data were provided by the Korea Meteorological Administration (KMA). In this study, monthly precipitation data have been analyzed using moving averages for periods of 3, 6, 12 and 24 months and then, fitted to the Gamma function using the maximum likelihood method (MLM) to define the probability distribution of precipitation. We defined SPI as a precipitation deviation for a normally distributed probability with a mean of zero and standard deviation of one which is easily calculated by the momentum characteristics of the Gamma distribution. It is a drought period when the SPI is below zero.
To evaluate the climatic shift in the SPI time series, we used the student "t"-test with a confidence level of 0.05 for the 'null' hypothesis. We accepted the difference between two non-overlapping sample means as evidence of an inconsistency or a climatic shift when the confidence level lies beyond 0.05.
Wavelet analysis was also used to resolve localized signals and scale interactions, whenever a climatic shift is detected. The complex Morlet wavelets, one of the most widely used continuous wavelets in geophysics, is used in this study. We also tried to find any relationship between SPI over the Korean peninsula and equatorial Pacific SSTs (NINO3, NINO4 and, NINO3.4).
Results
1) Climatic Shift in the SPI Time Series
SPI12, the standardized precipitation index of 12 months running averaged precipitation, is well described. The drought periods on the Korean peninsula reported by the KMA occurred in 1976, 1977, 1982, 1988 and 1994. The analyzed data shows clearly the existence of a decadal drought period in Seoul, but it is not clear in the southern part of the Korean peninsula (Fig. 1). By the student "t"-test, we have found two climatic regime shifts on the SPI12 time series for Seoul precipitation. These are identified on as having occurred August 1973 and January 1985. It is a significant jump in terms of the difference in the time mean of precipitation which are in the range of about 0.91-1.33. A similar change was found in precipitation at Inchon, Chunchun, and Suwon; however, it is not so clear in the southern part of the Korean peninsula including Taegu.
Table 1. The change of mean values of SPI12 in Seoul before and after the climatic shifts on August 1973 and January 1985.
Onset | Mean | |
August 1973 | 0.59 -0.74 0.72 0.19 | -1.33 |
2) Climatic Shifts on the Time-Frequency Domain-Wavelet Analysis
In the case of Seoul shown in Fig. 2a, the dominant cycle of SPI12 is 2-5 years. But this signal was diminished during 1974-1985, which is the same period we define as a climatic shift. Before 1974, the 2-3 year cycle was dominant in precipitation variations. However, this cycle diminished when the transition occurred and then switched to a longer 3-5 year cycle. It is also interesting that the 22-year cycle becomes dominant when the signal of 2-3 year cycle diminishes as a result of the climate shift. Another interesting feature is found in the case of Ulungdo, located in the middle of the East Sea, of which the dominant signal was the same cycle as in the middle of the Korean peninsula; however, the phase was opposite.
In the case of Taegu located in the southeastern part of the Korean peninsula (Fig. 2b), there is no distinguished signal of 2-3 year cycle during the period 1975-1985. Of course it is the same period as the climatic shift as in Seoul. Another dominant cycle, which has not appeared in Seoul, is the 6-year cycle. We also find the same trends of these cycles in the SST at NINO3, NINO4 and NINO3.4.
3) Typhoon as a Messenger of an ENSO Signal over Korean Peninsula
We pointed out that the number of major typhoons (MT) is abruptly decreased in 1973-1983. Baek and Kwon (1997) reported that there is a 22-year cycle in the variation of the number of MT. The area of origin and track of MT are also widely changed between the period of 1962-1972 and 1973-1983 as shown in Fig. 3. The track of the MT in the post-1973 period has been localized to the West Pacific region and shifted southward with respect to the pre-1973 period. This change might be the main source of the climatic shift and missing signal at the scale of 2-3 years. It means that the climatic shift in the precipitation data in Seoul is a decadal mode in global climate fluctuations.
The trends of the MT affecting the Korean peninsula is well matched with both the trend of Niños and the dominant cycle of the precipitation data in the southern part of Korean peninsula. Based on the signal periodicity of the variation of typhoons and precipitation, we may concluded that precipitation associated with typhoons might be one of possible source of the 3-6 year cycle signal in precipitation data over the Korean peninsula and its trend goes to a longer period variation on the time-frequency domain.
Conclusion
Through the wavelet analysis we may conclude that there is an identifiable ENSO signal in the precipitation records of many station on the Korean peninsula. However, the signal is very weak for the Ulungdo because the Ulungdo is an Island in the middle of the East Sea, a marginal sea isolated by the islands of Japan from the Pacific. Thus, the Ulungdo might be more under the influence of the East Sea, while most of the Korean peninsula is under the influence of the Pacific at least with regard to precipitation,
Another important factor in variations of precipitation in Korea is a decadal variation which is a climatic shift which appeared during the middle of the 1970s and in the 1980s, even though it is not quite so strong in the southern part of Korean peninsula. As a result of this decadal scale variation, the impact of El Niño or La Niña on the Korea climate has been switched either wet-to-dry or dry-to-wet by about every eleven years.
Acknowledgments
This study was carried out on a grant by the Research Project of the Ministry of Science and Technology under Project No. 97-ND-01-01-A-04.
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