Cause of warm and cold winters. Computer illustration showing how the North Atlantic oscillation (NAO) affects the temperature experience by Europe in winter. In years of positive NAO values (left), both low pressure in the north and high pressure in the south is strong. This provides a strong flow of warmer air from the west into Northwestern Europe, leading to mild winters in northern Europe and cooler conditions in southern Europe. In years with negative NAO values, both low and high pressure are weak. The flow of warmer air from the west is weakened, leading to colder winters in northern Europe, while southern Europe enjoys milder conditions.