North Pacific storms typically form in the lee of large "waves" in the mid-latitude jet stream and they steer towards western North America. When they run against the barrier of coastal mountains (for example in the Gulf of Alaska), they often stall for several days, drawing moist Pacific air towards the coast and continental interior, until their energy is exhausted. When the cyclones are associated with deep low pressure systems (as shown on the figure), they can draw moisture from a broad area of the North Pacific Ocean, from subarctic to equatorial latitudes.

Ahead of the storm front, moisture is drawn mostly from nearby, northern latitudes (e.g., the Gulf of Alaska), while more distant moisture from southern latitudes (e.g., the central Pacific Ocean) is transported behind the front and feeds snowfall on the high parts of the mountains. This is why the composition of snow that falls on the high plateau of Mount Logan reflects atmospheric conditions over a larger part of the North Pacific Ocean. When climate changes, it modifies the strength and trajectory of the storms that deliver snow to the St. Elias Mountains. So at different times in the past, the snowfall high on Mount Logan includes a different mix of moisture from the North and central Pacific Ocean, each source having its own characteristic "signature". This snow gradually compacts to ice, preserving the compositional differences in frozen layers of the ice cap. Learn more about: ice cores hold secrets of long ago climate.

Because Mount Logan rises to 6 km above the ocean surface, it also sits right in the path of fast-moving winds that carry dust and pollution from East Asia to North America across the North Pacific Ocean. As a result, the snows of Mount Logan sometime get sprinkled by major Asian dust storms. Find out more...