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I want to write a story on Earth as it is now, with the sole exception of one large island nation. I plan to introduce elements of mysticism into the story as well, but I would like to keep these subtle and explain the climate in non-magical ways if possible.

What I was wondering is if there was a location on Earth - or if I could design the island geographically or geologically in such a way - so that summers and winters are "shorter" in the sense that they are both not very intense, and for most of the year are indistinguishable from mild springs and autumns.

I plan to make this island's culture a blend of Korean/Japanese and Polynesian cultures, so a location somewhere between the two would be ideal.

Can it be done?

The ocean itself tends to moderate climate -- Vancouver, BC, gets much less severe winters than Toronto, despite being somewhat further from the equator. Further, the temperature of the water has a strong effect -- Sheffield (England) would have a climate like Yellowknife if not for the tail end of the Gulf Stream warming the winters.

So, that's what you need -- an island influenced by a warm current. Japan already enjoys some of this benefit; the northern islands are similar in latitude to the eastern end of Siberia, but (though they get snow and a genuine winter) lack the killing cold of north central Asia. Iceland is also similar in being warmed by the Gulf Stream -- without that, it would more resemble Greenland (only small, and punctured with volcanoes).

Generally, currents running from the equator toward the poles are warm, those running the other direction are cold. This is slightly complicated by some currents running at depth instead of on the surface, but find a chart of ocean currents and you'll have a map showing where to put your island.

Depending what kind of current you find at what latitude, you could get a climate similar to Bermuda (due east of North Carolina), Iceland, Ireland & Great Britain, Tasmania, New Zealand, or Madagascar.

Not only can it be done, but you're basically describing Vancouver Island. Vancouver Island has extremely mild winters and warm (but not hot) summers.

Most conveniently for the purposes of your story, you don't even have to add a new island to the Earth to make your story work. It's been often speculated (although not proven) that explorers from Korea and China might have reached the west coast of North America prior to the European colonization of the east. Certainly they were technologically capable of it, that they didn't was due to the fact that they didn't have any particular incentive to do so.

All you have to do is change history a little bit. Have Kublai Khan been a bit less successful in his campaigns resulting in a prolonged loss of China as both a trading partner and potential enemy during the eventual establishment of the Yuan Dynasty.

Without access to China's markets, Korea and Japan both would have had incentives to find resources and trade opportunities elsewhere, and it wouldn't have been difficult for an expedition to have found Vancouver Island and established a trading colony there. The climate is an even more pleasant version of what they had at home, with similar terrain, plants, and wildlife.

It's not hard to imagine a thriving colony growing up here made up of explorers, expatriates, and pirates from all over East Asia, more invested in their new home than any of their original motherlands, and more willing to band together to resist any attempts by said motherlands to throw their governmental weight around. You'd wind up with the Asian equivalent of the Wild West, but with more time for a sense of real political identity to arrive before the 19th century makes long distance travel so easy.

There's lots of ways you can play it politically from there, perhaps there's a land deal in the late 18th century with the nascent United States of America. Who knows?

The Canary Islands, the Insulae Fortunatae (Happy / Lucky Islands) of the ancients, are famous for their "eternal spring" climate -- average 24°C (75°F), winter 20°C (70°F), summer 26°C (80°F).

And they are real. Las Palmas (on Gran Canaria island) is a major tourist destination.

A good choice is a Mediterranean climate.

A Mediterranean climate or dry summer climate is characterized by dry
summers and mild, wet winters. The climate receives its name from the
Mediterranean Basin, where this climate type is most common.
Mediterranean climate zones are typically located along the western
sides of continents, between roughly 30 and 45 degrees north and south
of the equator. The main cause of Mediterranean, or dry summer
climate, is the subtropical ridge which extends northwards during the
summer and migrates south during the winter due to increasing
north-south temperature differences. (ref)

The wet winters and dry summers aren't necessary to your situation, but they come part and parcel with mild summers and winters and basically gorgeous year-round weather. Avoid the more extreme examples of the Mediterranean climate and aim for, say, coastal California.

In particular, try Catalina Island, not far from Los Angeles.

Santa Catalina Island has a very mild warm-summer Mediterranean
climate (Köppen Csb) with warm temperatures year-round...The average January temperatures are a maximum of
58.4 °F (14.7 °C) and a minimum of 47.6 °F (8.7 °C). Average July temperatures are a maximum of 78.1 °F (25.6 °C) and a minimum of 60.0
°F (15.6 °C). There are an average of 12.5 days with highs of 90 °F
(32 °C) or higher and an average of 0.3 days with lows of 32 °F (0 °C)
or lower. (ref)

This particular island is fairly small ("22 mi (35 km) long and 8 mi (13 km) across at its greatest width") so yours would need to be bigger in order to accommodate all but the smallest of nation states.

Mediterranean climates occur on the Western coasts of large land masses in the correct latitudes. Putting your island further east will completely change the climate. Between Korea and Polynesia is the Philippines (well, mostly the Philippine sea to the east, which has room for a good sized island invention).

The Climate of the Philippines is either tropical rainforest, tropical
savanna or tropical monsoon, or humid subtropical (in higher-altitude
areas) characterized by relatively high temperature, oppressive
humidity and plenty of rainfall. (ref)

Even if you go much further north, to a attitude closer to Southern California, you still have more summer heat.

The climate of Fukuoka, a Japanese city located on the north coast of
Kyushu (which is the southernmost of the major Japanese islands), is
temperate humid, with quite mild winters and hot, moist, and rainy
summers. Like the rest of Japan, the city is affected by the monsoon
circulation: in winter, the northwest cold currents prevail, while in
summer, they are replaced by hot and humid currents of tropical
origin. (ref)

It all depends what you want. If location is more important to you, then a humid temperate or tropical climate should still be within the range of more moderate winters (if not summers). But if you're okay with going further east, then nestle your island along the southern Californian coast and enjoy very mild, near perfect, weather year round.