Given an archipelago around 40 degrees south latitude, and slightly farther out west than New Zealand from a North-South continent roughly similar to the Americas, how could such an archipelago experience a humid continental climate? Looking at the climate of similar islands on Earth, I see that there aren't seasonal temperature swings, probably because they are beset by water on all side, and experience strong winds year round. Without changing the geography, what could I do to explain the warm summers?
Note that there is an Antarctica analogue, though with another archipelago connecting it to the North-South continent. The other continents are much farther away. The planet is more or less similar to Earth.