Climate history from Mount Logan differs from Arctic ice cores

One hypothesis is that the Mount Logan ice core contains a record of atmospheric circulation in the past, rather than a simple temperature history. Differences in proportions of oxygen isotopes at different levels in the ice core probably represent natural changes in the large-scale atmospheric circulation of the Northern Hemisphere which delivers water vapor on Mount Logan. Clouds that form over different regions of the Pacific Ocean (for example the equatorial region vs the Gulf of Alaska) hold different proportions of heavy and light oxygen isotopes because the surface temperature of these waters are very different. These isotope proportions are conserved (in part) when the clouds are carried by winds and storms to the St. Elias Mountains. So the big swings that we see in the Mount Logan ice-core record probably reflect changes in the dominant source(s) of the water vapor that fed snowfall over time. These changes were probably related to shifts in the dominant winds and storm tracks over the Pacific Ocean.

The atmospheric circulation in the Northern Hemisphere oscillates naturally between periods when it is more "zonal" and periods when it is "meridional" (see left-hand scale on the top panel of the "Logan record" figure). When the circulation is more "zonal", the westerly winds are stronger, and the mid-latitude jet stream (see bottom panel on the "cyclone" figure) follows a straighter trajectory. This could be the situation during colder-than-present climate conditions, when the Arctic air mass expands and "pushes" the jet stream further south.

Under these conditions storms in the North Pacific would track mostly from west to east, bringing high-latitude (northerly) moisture to the Mount Logan region. In contrast, when the circulation is more meridional the jet stream follows a more winding course, like it does nowadays. North Pacific storms then track mostly from the southwest to northeast, and as a result there is a quasi-permanent low pressure center near the Gulf of Alaska where the cyclones converge. These conditions allow for Mount Logan to receive moisture from a much broader region of the North Pacific (including from equatorial latitudes), than when the circulation is more zonal. A simple way to sum this up would be to say that the Mount Logan ice-core record (shown at the top of the "Logan record" figure) describes the extent to which the summit of Mount Logan is climatically connected, or isolated, from the North Pacific Ocean as a whole.

So what drives these changes in atmospheric circulation ?. Presently, we do not know for certain. they are most probably linked to a natural "oscillation" of the Earth's climate system that involves both the oceans and the atmosphere through energy (heat) exchanges. We know that such oscillations exist on shorter time scales, for example the El Nino phenomenon (which "oscillates" every ~2 to 8 years) or the Pacific Decadal Oscillation (which switches "modes" every ~15-25 years). So the changes recorded by the Mount Logan ice-core are probably a longer-lived manifestation of such ocean-atmosphere "oscillations", but with a persistence of centuries, instead of a few decades or years.

One of the most remarkable aspects of the Mount Logan record is how "fast" some of the circulation changes occurred in the past. If we look only at the most recent part of the Mount Logan ice-core record, say the last 550 years (enlarged in the topmost panel of the "Logan record" figure) we see the latest of these "shifts" occurred in the middle of the 19th century, roughly at the end of the Little Ice Age cold climate interval.  It looks like this shift occurred in a few years. A change in atmospheric circulation that left such a strong imprint in the Mount Logan ice core record probably affected precipitation patterns all over western Canada when it took place, because it would have modified the transport of water vapor from the Pacific Ocean into the continental interior. This abrupt change in the mid-19th century is also seen in lake sediments of the southwestern Yukon. But because we do not have a good weather record for this time period in northwestern Canada, we can not be sure just how severe the impact was. If such a rapid change were to occur again today (and there is no reason to think it could not), we would have only a short time to adapt.