DIAGNOSTIC ANALYSIS OF PERSISTENT DROUGHT/FLOOD EVENTS IN SUMMER OVER THE TWO-LAKE REGION OF CHINA

Based on the daily regional mean rainfall, the Z-index method is used to identify persistent flood and drought events lasting for at least 10 days over a region where Dongting Lake and Poyang Lake sit (referred to as the "two-lake region" hereafter). The National Centers for Environmental Prediction (NCEP) reanalysis data are then utilized to perform a preliminary diagnostic analysis on these events. The results indicate that the composite standardized geopotential height at 500 hPa presents two different meridional wave trains from north to south over the East Asian-Pacific region, i.e., a "-+-" pattern for the droughts and a "+-+" pattern for the floods, respectively. The developing, maintaining and decaying phases in the drought and flood events are closely related to the intensity and location of a subtropical high and an extra-tropical blocking high. It is shown that the East Asian summer monsoon is strong (weak) with the occurrence of persistent drought (flood) events. Droughts (floods) are accompanied by a weak (strong) tropical convergent system and a strong (weak) subtropical convergent system. Furthermore, the persistent drought (flood) events are associated with a divergence (convergence) of vertically integrated water vapor flux. In the vertical profile of water vapor flux, divergence (convergence) in the mid- and lower-levels and convergence (divergence) in the higher levels are evident in the droughts (floods). Both the divergence in the droughts and the convergence in floods are strongest at 850 hPa.