关键词: 高温, 西太平洋副热带高压, 南亚高压, 海温, 异常反气旋

Abstract: Based on the daily maximum temperature from 98 weather stations of Shaanxi Province and monthly reanalysis data from NCEP/NCAR, sea surface temperature (SST) from NOAA, and circulation indexes from National Climate Center of China, the causes of extremely high temperature weather process in Shaanxi in July 2017 were analyzed, and the preceding signals were discussed. The results show that the extremely high temperature weather process lasted for nearly 20 days in July 2017 in Shaanxi, and its intensity was the second level since 1961. The process of high temperature event was divided into two stages. The high temperature at the first stage occurred in the north and central part of Shaanxi from 9 to 14 July 2017, while at the second stage it expanded southward to the south of Shaanxi from 19 to 27 July 2017, and the intensity reached the maximum. At the first stage of high temperature, the most areas of Shaanxi were in positive geopotential height anomaly field from Xinjiang to Hetao area. The anomalous strong sinking movement and northerly warm advection in low level caused the high temperature weather. At the second stage, the western Pacific subtropical high (WPSH) obviously moved to the north and the west, the upper-level jet stream and the South Asian high (SAH) travelled clearly eastward, which led to the enhancement of the anomalous sinking motion and moving southward, combined with the increase of warm advections intensity and range in low level, further made the high temperature to expand southward and its intensity reached the maximum. In addition, the study found that the anomaly of previous SST from May to June in the key area of tropical western Pacific (10°S-15°N, 160°E-140°W) was significantly positive correlated with high temperature in the south and central of Shaanxi in July, so it was a reliable preceding signal to forecast high temperature. When the previous SST from May to June was higher in the key area, SST connected with WPSH, SAH and zonal wind over East Asia via exciting wave train in teleconnection, which affected the temperature change in central and southern Shaanxi, it might be the main reason of extremely high temperature event in this area in July.

Key words: high temperature, WPSH, SAH, SST, abnormal anticyclonic