The weather system that caused the summer drought in the middle and lower reaches of the Yangtze River was () BC.

A, in the horizontal direction, the airflow flows counterclockwise from all sides to the center, which is a cyclone in the northern hemisphere and does not meet the meaning of the question;

B, in the horizontal direction, the airflow flows clockwise from the center to all directions, which is an anticyclone in the northern hemisphere, so it is correct;

C, in the horizontal direction, the airflow flows counterclockwise from the center in all directions, which is an anticyclone in the southern hemisphere and does not meet the meaning of the question;

D in the horizontal direction, the airflow flows clockwise from the periphery to the center, which is a southern hemisphere cyclone, which does not meet the meaning of the question.

Therefore, choose: B.