A Thing of Vikings (HTTYD/Mulan/the 11th century): Eastward Winds.
https://archiveofourown.org/works/104089.../253430641
Sir Dr. Yosef ben Dror House Mordechai, PhD, Professor of Climatology, Presentation during the 111th Annual Symposium on Imperial History, Wrote:One question that has only recently been answered was the cause of the unseasonably warm weather in the Arctic during the First Berkian-Zhōngguó Expedition in AD 1044. Despite taking place only weeks after the winter solstice and therefore during the period of polar night at those latitudes, the expedition encountered atypically warm weather along the Arctic coastline, as recorded by numerous primary accounts.
The answer to this was only recently discovered and explained by Mac Lynn, (Journal of Historical Climatology, 1712, pg 83) through the collation of records from several Vestrilander cultures in the context of understanding the function of the polar vortex. According to these records, the winter of that year was noted as being particularly bitter and cold across a wide breadth of the northern Vestrilands. Climatological models based on the temperature proxies recorded indicate that the polar vortex had almost certainly shifted southwards over the Vestrilands, allowing the Berkian expedition easier access to their eastward flight path. Unfortunately, the Third and Fourth Expeditions met with a more typical polar vortex configuration and the associated cold and could not complete their journeys…
https://archiveofourown.org/works/104089.../253430641
