Abstract:

A server drought occurred in the middle and lower reaches of the Yangtze River basin from midsummer to fall in 2019. The characteristics of this abnormal drought were analyzed based on the NCEP/NCAR reanalysis data and meteorological observation data. The results showed that the drought is characterized by a long duration, less precipitation, more rainless day, higher average air temperature, and higher temperature days. The stronger western Pacific subtropical high(WPSA)and its north and west position are the main reason for continuous drought in summer and autumn in the middle and lower reaches of the Yangtze River. The negative vorticity extending eastward and the eastward of the South Asia high are in favour of the western extending of the WPSA. The middle and lower reaches of the Yangtze River are located on the right side of the East Asia subtropical westerly jet exit area and the superposition of negative vorticity in the middle and upper troposphere, which makes the vertical subsidence and divergence significantly enhanced. The dam formed by a high-pressure ridge from the Ural Mountains and Lake Baikal and maintained anomalous middle and high latitude circulation patterns such as the weaker the polar vortex and East Asia trough make it uneasy for the cyclonic disturbance in the westerly belt to affect the subtropical region, which is conducive to the stability of the WPSA and maintenance of drought. In addition, the frequent activities of the tropical system make the subtropical high further strengthened under the negative vorticity in the north of the tropical convective heating area, and leads to the changes of the southwest, cross-equatorial and southeast airflows in the lower troposphere on the northward transport path, and results in the weaker water vapor convergence in the middle and lower reaches of the Yangtze River, thus the drought maintained.

Key words: Atmospheric circulation anomalies, Western Pacific Subtropical High, Tropical circulation anomalies