Temporal and Spatial Characteristics of Summer Extreme Precipitation in Eastern China and Possible Causalities

In the past decades, with the increasing frequency of extreme weather and climate events, the world has suffered huge losses. Based on NCEP/NCAR reanalysis data and China regional precipitation data provided by China Meteorological Administration, the extreme precipitation events in eastern China are defined by relative threshold method, and the temporal and spatial characteristics of summer extreme precipitation in eastern China from 1961 to 2016 are analyzed by empirical orthogonal function (EOF), and the reverse distribution of extreme precipitation in the middle and lower reaches of the Yangtze River and south China by Indian Ocean warm pool is revealed influence. The results show that the total amount and frequency of extreme precipitation in summer are concentrated in the Yangtze River Basin and south China. EOF1 decomposition of extreme precipitation reflects the interannual oscillation characteristics of reverse spatial distribution in the Yangtze River Basin and south China. The time series corresponding to EOF1 has significant interannual characteristics. The Pacific-Japan (PJ) teleconnection pattern is a circulation system that significantly affects the spatial-temporal pattern of extreme precipitation in southern China. When the PJ pattern is in the positive phase, the anticyclone controls the south China region, and restrains the convective activity, which results in the decrease of extreme precipitation. The anomalous southwest wind to the south of 30 ̊N and the anomalous northerly wind to the north of 30 ̊N converge in the middle and lower reaches of the Yangtze River. Combining with the sufficient water vapor carried by the anomalous southwest airflow at the edge of anticyclone, it is more conducive to the formation of extreme precipitation. The east propagating Kelvin wave in the warm pool of the Indian Ocean is an important reason for the formation of the PJ pattern and finally the formation of extreme precipiHow to cite this paper: Zhong, Y. H., Yang, M. Z., & Yuan, C. X. (2020). Temporal and Spatial Characteristics of Summer Extreme Precipitation in Eastern China and Possible Causalities. Journal of Geoscience and Environment Protection, 8, 36-46. https://doi.org/10.4236/gep.2020.86004 Received: May 25, 2020 Accepted: June 25, 2020 Published: June 28, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The fifth assessment report (AR5) of the first working group of the Intergovernmental Panel on Climate Change (IPCC) introduces and analyzes an unprecedented global climate change process in the earth system, which is characterized by global warming. Global warming is first manifested in the sharp rise of global surface temperature, which has a significant and far-reaching impact on the global climate system, not only in the rise of global sea surface temperature (SST) and global sea level, but also in the significant increase in the frequency and intensity of global extreme weather and climate events (Zhai et al., 2005).
In the past few decades, with the increasing frequency of extreme weather and climate events, with the continuous development of human society and economy, human beings are facing increasing losses of extreme natural disasters. East China is located in the East Asian monsoon region. Influenced by the East Asian summer monsoon, extreme weather and climate events occur almost every year, especially extreme precipitation. Extreme precipitation has become one of the important factors restricting the sustainable development of social economy in China. Therefore, it is an urgent need to carry out in-depth research on extreme precipitation events, comprehensively grasp the detection methods and spatial-temporal distribution characteristics of extreme precipitation events, and then analyze the physical mechanism of extreme precipitation events, which is of great practical significance.
The characteristics of extreme precipitation in China have been widely studied. In the past few decades, China's total precipitation has not changed much; the precipitation frequency is decreasing, but the intensity is increasing, and the regional and seasonal differences are very large (Wu et al., 2015). Most scholars use the absolute threshold to divide the precipitation into different levels of precipitation events. There are still many disputes about the trend of precipitation in China. For example, many studies show that the winter precipitation in northeast China is decreasing (Wang & Yan, 2009;Hu et al., 2003), but some studies also find that the winter precipitation in this area is increasing (Liu et al., 2005). Xu et al. (2011) showed that the total amount of extreme precipitation increased significantly in the lower and middle reaches of the Yangtze River and the northwest of China, but the increasing trend was smaller in the north and northeast of China. The different results of these studies show that the spatial and temporal variation of precipitation is very different, so it is necessary to fur- Eastern China is located in the East Asian summer monsoon region, which contains complex dynamic, thermal and hydrological processes (Wang & Lin, 2002;Ding & Johnny, 2005). The annual total precipitation is concentrated in summer, and there is a close relationship between summer precipitation and extreme precipitation. Many scholars have done a lot of work to reveal the precipitation anomaly in the East, and found that there is a relationship between the precipitation anomaly in the east and the intensity of the East Asian summer monsoon. As a member of the East Asian summer monsoon system, the subtropical high in the Northwest Pacific has also attracted a lot of attention. It has been found that the short-term changes of the shape and location of the subtropical high in the Northwest Pacific in summer and the seasonal advance and retreat significantly affect the occurrence of regional extreme precipitation in China. In addition, the study of Wang and Zhou (2005) found that the summer circulation in East Asia has a significant impact on the extreme precipitation in China. As an important forcing source, the tropical SST in summer plays a leading role in the large-scale circulation anomaly. The Pacific SST anomaly (Wang et al., 2000;Wang et al., 2003) and the Indian Ocean warm pool (Wu et al., 2010) are important factors affecting the large-scale circulation anomaly in East Asia. Wu et al. (2010) pointed out that the anomalous anticyclone in the Western Pacific was caused by the cold SST anomaly in the Northwest Pacific and the warming of the Indian Ocean basin. However, from June to August, the former was weakening while the latter was gradually increasing. The anomalous anticyclone in the Western Pacific may further stimulate the PJ pattern in summer to the north, which leads to the North-South inverse oscillation of the atmospheric circulation anomalies in the Philippines and the vicinity of mid latitude Japan through the anomalous meridional circulation (Li et al., 2017).
Previous studies on extreme precipitation have done a lot of work, but in terms of China's extreme precipitation research, the research area is mainly Jianghuai and south China, and the research on extreme precipitation in other areas is relatively small. In the past, the research on extreme precipitation was mostly focused on individual cases, and focused on the temporal and spatial distribution of extreme precipitation and its response to climate warming.
Based on the conventional reanalysis data, the temporal and spatial pattern of The fifth chapter is the summary and discussion of the whole paper.

Data and Methods
The data of precipitation in China in this study comes from the daily grid data of China's surface precipitation provided by China Meteorological Data Network (https://data.cma.cn/site/index.html), with a time span of 1961-2016 and a horizontal resolution of 0.5˚ × 0.5˚. The data set has been integrated daily precipitation of 2474 meteorological stations in mainland China. In order to facilitate the data processing, the precipitation on February 29 of all leap years is eliminated artificially. In addition, the NCEP/NCAR monthly reanalysis data set (Kalnay et al., 1996) is also used in this study. The monthly mean precipitation data of Global Precipitation Climate Center (GPCC) is used in the analysis of large-scale precipitation anomalies, and the resolution is 1˚ × 1˚. The National Oceanic and Atmospheric Administration (NOAA) extended reconstructed sea surface temperature V5 (Huang et al., 2017) for the same period are also used.
The main methods used in this study are relative threshold method, EOF analysis, PJ index.
In this paper, the relative threshold method is used to define the extreme pre- In addition, EOF is used to analyze the temporal and spatial distribution characteristics of summer extreme precipitation in eastern China. The PJ index is used to study the relationship between the P-J pattern and the extreme precipitation in eastern China. The definition of the PJ index follows Nitta (1987), only changing the cloud amount to the more commonly used and widely accepted precipitation: Where C(A, B) is the summer mean value of precipitation in latitude range A and longitude range B.

Temporal and Spatial Distribution of Summer Extreme Precipitation Frequency in Eastern China
After eliminating the number of days without precipitation, the overall sequence  It can be seen from the spatial distribution diagram of variance of summer extreme precipitation (Figure 1(c)) that the spatial distribution of summer extreme precipitation change in eastern China is very similar to that of summer extreme precipitation and extreme precipitation threshold, and also shows the spatial distribution characteristics of more in the south and less in the north, more in the east and less in the west.
In order to analyze the temporal and spatial distribution characteristics of summer extreme precipitation in eastern China, EOF analysis is carried out in this study. The first mock exam is north test, and variance is interpreted as 11.62%. The first EOF mode (EOF1) shows the spatial distribution of the north and south in the south of China. As shown in Figure 2

The Influence of Warm Pool in Indian Ocean on Summer Extreme Precipitation in Eastern China
In  After standardization, the correlation coefficient of 56-year time series is as high as 0.48 (Figure 4), passing the significance test with 99% reliability. In order to be closer to the general expression habit, we multiply the index by negative one.
Therefore, the following figures all reflect the connection between the PJ pattern of positive phase and the spatial-temporal distribution pattern of summer rainfall in the south of China. When PJ pattern with positive phase appears, the extreme precipitation in the middle and lower reaches of the Yangtze River appears positive anomaly, and the extreme precipitation in South China appears negative anomaly (Figure 3(b)).
The anomalies of large-scale circulation and precipitation in East Asia usually do not exist in isolation. There is a strong east wind anomaly near 10˚N on 850 hPa (Figure 3), which extends all the way to the North Indian Ocean. It is considered that the Kelvin wave response may be caused by the warm SST anomaly in the Indian Ocean. In order to verify the relationship between SST anomalies and the inverse distribution of precipitation in the middle and lower reaches of the Yangtze River and south China, we use extreme precipitation PC1 series and PJ index to regress the Global SST anomalies, and find that the most significant and direct signals are the warm SST anomalies in the Indian Ocean and the cold SST anomalies near Japan, as shown in Figure 5(a) and Figure 5

Conclusion
This paper focuses on the temporal and spatial characteristics of summer extreme precipitation in eastern China. It is found that the total amount and frequency of summer extreme precipitation are concentrated in the south of China.