Why is the air pressure difference bigger in winter?

This is the effect of thermal expansion and cold contraction.

In winter, the sun is biased towards the southern hemisphere, and the northern hemisphere receives less thermal radiation than in summer, so the air temperature is low, the gap between air molecules is smaller, and the air density is relatively high. From the perspective of the earth, the atmosphere is basically spherical. In other words, the atmospheric thickness is basically the same in winter and summer. So in the same area, the air quality in winter is more than that in summer, so the air pressure in winter is higher.