The atmospheric anomalies associated with the drought over the Yangtze River basin during spring 2011

The middle and lower reaches of the Yangtze River basin (MLRY) suffered a once-in-a-50-year drought during the spring of 2011. The abnormality of the atmospheric circulation in the spring is characterized by the deeper-than-normal trough over East Asia and the northwest Pacific while stronger-than-normal high-pressure system over the west, which results in a stronger-than-normal meridional circulation. Meanwhile, the western Pacific subtropical high is weaker than normal and retreats to the east, so the spring monsoonal moist air from lower latitudes is relatively weak. The anomalous northerly wind in higher latitudes suppresses the northward motion of the moist air and brings dry air to the MLRY. The northerly wind also suppresses the northward motion of warm air and brings cold air to the MLRY. So finally, the air over the MLRY is drier, but colder than normal, which is different from many other droughts. The coldness of the air plays a negative role to the drought. Results from comparing the C-q and C-T, the measures of the changes in moisture and temperature between this spring and the normal spring defined based on the tight precipitation-relative humidity relation, indicate that the dryness of the air is much stronger than the coldness, and the air over the MLRY thus maintains a lower-than-normal relative humidity. The animation of daily Cq and CT shows that dry and cold air moves from the northwest to the MLRY in a manner of low-frequency oscillation.