Possible influence of ENSO on annual maximum streamflow of the Yangtze River, China

Summary

Variability and possible teleconnections between annual maximum streamflow from the lower, the middle and the upper Yangtze River basin and El Niño/Southern Oscillation (ENSO) are detected by continuous wavelet transform (CWT), cross-wavelet and wavelet coherence methods. The results show that: (1) different phase relations are found between annual maximum streamflow of the Yangtze River and El Niño/Southern Oscillation (ENSO) in the lower, the middle and the upper Yangtze River basin. In-phase relations are detected between annual maximum streamflow of the lower Yangtze River and anti-phase relations are found in the upper Yangtze River. But ambiguous phase relations occur in the middle Yangtze River, showing that the middle Yangtze River basin is a transition zone. Different climatic systems control the upper and the lower Yangtze River. The upper Yangtze River is mainly influenced by the Indian summer monsoon and the lower Yangtze is mainly influenced by the East Asian summer monsoon; (2) as for the individual stations, different phase relations are found in the longer and the shorter periods, respectively. In the longer periods, the annual maximum streamflow is more influenced by climatic variabilities, while in the shorter periods, it is influenced by other factors, e.g. human activities. The results of the study provide valuable information for improving the long-term forecasting of the streamflow using its relationship with ENSO and the Indian Monsoon.

Introduction

Flood hazards cause enormous economical, social and environmental damages and loss of lives. Floods usually include three factors: peak flood discharge, water level and flood duration; extreme flood discharge usually plays the key role in the occurrence of flood hazards and is likely to have a greater potential to impact water resources in many regions than the mean annual discharge does. More frequent or larger floods could lead to increased expenditures for flood management. It is why more and more researchers draw concerns on the study of extreme flood events (Jain and Lall, 2001, Camilloni and Barros, 2003). The intensifying human activities (e.g. urbanization, forestation/deforestation, construction of water reservoir) will exert tremendous influences on flood frequency, and temporal and spatial distributions of water resources. Furthermore, climatic variability combined with human-induced emission of green-house gases result in an increase in mean global temperature (IPCC, 2001), which in turn, leads to higher evaporation rates and makes the atmosphere transport larger amounts of water vapor. The global hydrological cycle is accelerated (Menzel and Bürger, 2002). Influence of the slowly changing climate on flood frequency has attracted interest (e.g. Robson et al., 1998, Jain and Lall, 2000, Jain and Lall, 2001, Olsen et al., 1999, Zhang et al., 2005). The El Niño/Southern Oscillation (ENSO) represents the dominant coupled ocean–atmosphere mode of the tropical Pacific (Cane, 1992). On inter-annual timescales the significant part of the global climatic changes can be linked to ENSO (Trenberth et al., 1998). The ENSO extreme phases are usually in linkage with major episodes of floods and droughts (e.g. Barlow et al., 2001) in many locations worldwide (Jain and Lall, 2001, Aceituno, 1988, Amarasekera et al., 1997).

Many scholars try to detect possible connections between ENSO and precipitation and streamflow. Lan et al. (2002) suggested that ENSO contributed to the runoff in the upper reaches of the Yellow River in China; the occurrence of El Niño is usually accompanied by high probability of low flow, while flood events in the Yellow River usually accompanied by the occurrence of La Niña event. Cardoso and Silva Dias (2006) also investigated the relationship between the Paraná River (27.36°S, 55.90°W) flow and the ENSO mode, and statistical forecasts of river flow are made using the relationship. An evaluation of the relationship between the Pacific sea surface temperature and the Paraná River flow indicates an ENSO pattern over the equatorial Pacific. Gong and Wang (1999), however, studied the teleconnection between ENSO and precipitation in China with the help of statistical analysis (χ² test), suggesting that the decreasing precipitation in China usually matches the El Niño events and there exists a significant relationship between winter and autumn rainfall and the ENSO in eastern China. Some other scientists have also detected strong correlations between flood events and ENSO events (e.g. Chang and King, 1999, Dilley and Heyman, 1999).

Many researches were performed on streamflow changes of the Yangtze River. Zhang et al. (2006) analyzed the changes of trends and periodicity of the annual maximum streamflow and water level at different stations along the Yangtze River during the past 130 years, indicating that the annual maximum streamflow in the upper Yangtze River is in a decreasing trend while the opposite is true in the middle and the lower Yangtze River. Annual maximum streamflow in the middle Yangtze River has a significant upward trend, which shows that the flood hazard in the middle Yangtze River is of a serious concern. Jiang et al. (2006) analyzed the teleconnections between flood/drought events in the Yangtze River basin and ENSO events during 1868–2003 with the help of χ² test and spectral analysis, suggesting that ENSO events and flood/drought variations are significantly correlated at a 5.04-year period and a ∼10- to 12-year period. These researches are greatly helpful for understanding and controlling the floods and droughts problems in the Yangtze River basin. However, what are the possible connections between ENSO and annual maximum streamflow of the Yangtze River, especially in terms of periods? To what degree does the ENSO impact the annual maximum streamflow? These questions are remaining unanswered and are seldom studied, especially with the help of the powerful cross and coherence wavelet analysis methods.

The main objectives of the present study are: (1) to explore the changes of variance and in-phase linkages between Niño3 (sea surface temperature) and annual maximum streamflow of the three major monitoring hydrologic stations along the main Yangtze River, i.e. Datong station, Hankou station and Yichang station; and (2) to evaluate the possible impacts of ENSO on flood hazards in the Yangtze River Basin. This study uses the wavelet transform (WT) approach, the cross-wavelet power and coherence wavelet analyses methods to detect the relations between Niño3 SST and annual maximum streamflow of the Yangtze River.