In the Flood Season of 1998, the atmospheric circulation was abnormal, the characteristics of the polar vortex , the west wind zone, the subtropical zone and the tropical system were obviously different from those of the same year. In the summer,500hPa high altitude polar vortex position towarded the East and Its power was weak with the obviously development of the meridional circulation in high latitudes. The circulation situation of the two ridge and one trough in the Eurasian region continued to be stable and the blocking high existed for a long time. The Western Pacific subtropical high force was powerful and tipped the west and the south. Meanwhile, 1998 was the second year that strong EI Nino happened. Under the common influence of these abnormal factors, anomalous rainfall happened in Northeast China in the flood season of 1998 and the rain areas always hovered in the Nenjiang Basin. The general circulation of the weather in the flood season of 1998 was analyzed as follows.
Polar vortex and westerly zone circulation
In the summer of 1998 (June to August), On the 500hPa height field in the northern hemisphere, polar vortex subjected to the eastern hemisphere. its area was smaller than ever and power was weaker than ever. Near the Gulf of Alaska in the North American continent, Northeastern the Atlantic and Northern West Siberia were negative anomaly area. But Okhotsk and the northeast Pacific Region were controled by ridges. The middle and high latitude circulation were 4 wave type distributions and the meridional circulation continued to develop abnormally. High pressure ridge or blocking high pressure often existed in the middle and high latitudes in Ural mountain area and near the Okhotsk Sea, yet a wide low pressure trough or deep low pressure area between the two ridges. In this way, the cold air coming from polar regions continued to go down along the ridge front northwest air flow along the ridge of Ural mountain. It continuously replenished the trough of low pressure, which made the development of low pressure trough deepen. While the weat aera of the northeast part of China was located near the frontal zone in front of the big slot. Due to the high ridges or blocking often exist in the vicinity of the Okhotsk Sea, It would be cut off to vortex and become cold vortex when the deepened cold trough was hindered by the eastward movement. The cold vortex often maintain 2D ~ 5D or longer. Therefore, in the western part of the northeast region, a persistent local rainstorm or strong convective weather was formed.
Subtropical high
The subtropical high pressure of the Western Pacific was obviously stronger in the summer of 1998. The area index and intensity index of the subtropical high were 2 times more than the annual average. The West extension point leant to west by 20 to 30 longitude compared with the average year and the position of the subtropical ridge line leant to south by 2 or 1 latitude in July and August. Because the subtropical high leant to south, so the circulation in Northeast China was not adjusted to the circulation situation in midsummer. It had been cold vortex type rainfall often occurs in spring, so the rain always hovering in the northwest northeast region.
Sea water temperature and El Nino events
From May to November and December in 1997, warmer phenomena in the equatorial eastern Pacific reached the golden period, which was the strongest El Nino event in this century. The El Nino event tended to weaken in the first half of 1998 and until to June 1998, the warming phenomenon of the tropical Pacific collapsed rapidly. A sharp drop in the sea temperature in the equatorial and eastern Pacific Ocean and the Southern Oscillation Index also rose sharply. At last, the warming phenomenon in the Pacific Ocean end in July 1998. However, the El Nino event still had an important impact on summer rainfall. At the same time, the El Nino event was one of the factors affecting the abnormal summer rainfall in China in 1998. As a result of the impact of the El Nino event on the climate was complex and the impact mechanism was not yet clear. So it still needs to be further studied.
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