Answer:
The southern hemisphere is mostly occupied by the vast oceans. The water is generally characterized by their high specific heat capacity value. This is because the oceans warms and cools at a much slower rate. So the temperature of the ocean remains very high, during the summer and slowly cools down in the winter. The water content in the atmosphere also remains warm for a longer period of time, resulting in the warmer climate in the southern hemisphere.
On the otherhand, the northern hemisphere is comprised of land areas more than the oceans and it warms at the day time and releases the heat at night, maintaining a comparatively lower temperature in the northern hemisphere.