Why is it so hot in Northeast China this summer (202 1)?

In fact, the climate is similar. In summer, it is very hot from south to north. The central line of subtropical high covers a wide area with a diameter of several thousand kilometers. The center is in the Sea of Japan, affecting the eastern part of Heilongjiang and Changbai Mountain in China. Japan and the Korean peninsula are also included. The weather in Japan and North Korea should also be hot, but their climate is maritime, regulated by the ocean, and the temperature is lower than that in the northeast, on the contrary, it is continental, so the northeast feels sultry.