This statement is a matter of basic geographic and climatological fact, as the equator receives much more direct solar radiation year-round than the polar regions, making winter temperatures there significantly higher. No formal citation is needed.
The claim is a simple, fundamental geographical fact about the Earth's latitudinal temperature distribution. It is common knowledge by direct observation and basic physics, so no citation is strictly needed, and none of the retrieved papers specifically address or test this exact comparison as their primary focus.