The Southern Hemisphere experiences milder seasons than the Northern Hemisphere because... Wind and air circulation is obstructed by mountains preventing the transport of heat.

It takes much longer to heat and cool the oceans than the continents.

Most of the landmass of the Earth is in the Northern Hemisphere.

The Northern Hemisphere gets more sunlight than the Southern Hemisphere.

It takes much longer to heat and cool the oceans than the continents.

Explanation:

The Southern Hemisphere and Northern Hemisphere, despite being equal in size and receiving the same amount of sunlight, have big differences in climate. This is the case because the Northern Hemisphere has much more landmass, while the Southern Hemisphere has much more ocean water.

The land is changing temperature much faster because it can heat and cool off very quickly, thus the temperature variations are very large, not just from season to season, but also between day and night. The water, on the other hand, is heating up and cooling off much slower, so its temperature is never oscillating a lot. This results in much smaller temperature swings, so the climate tends to be much milder than the climate in the Northern Hemisphere.