Why Larsen C?

The Antarctic Peninsula, which juts out North from the continent into the tempestuous Southern Ocean, is an extremely important region to study. Not only is it unique and interesting, but it is also a canary in the coalmine. The polar regions are highly sensitive, and climate changes are most significant here, thanks to the phenomenon of polar amplification first proposed, or popularised at least, by Manabe & Stouffer in 1980. This is an effect whereby changes are more considerable at the poles because of the presence of the sea ice albedo feedback, which exacerbates temperature changes because when sea ice is lost, there is a larger area of dark open water with a lower albedo to absorb heat, which accelerates further ice loss in a positive feedback cycle.

The poles are vital in the global climate system because the meridional (in the North-South direction) temperature gradient between the equator and the poles drives atmospheric circulation. Air is colder and denser at the poles, whereas it is warmer and less dense at the equator. This means that air at the same pressure is at different heights at the poles and at the equator. These tilted pressure surfaces are part of what drives winds, and therefore meridional flow and fluxes of heat and momentum. Of course, atmospheric circulation is much more complex than this, but a reduction in this temperature gradient would have a considerable effect on atmospheric circulation across the planet, so studying the poles is very important.

The Antarctic Peninsula is one of the most rapidly warming regions on the planet, having warmed by around 3°C in half a century. Although this warming has slowed, and is even largely absent in the most recent years (Turner et al., 2016), this still has dramatic consequences for the region. Part of the reason that the peninsula has warmed so much in the last 50 years is because of changing atmospheric circulation, and the changing index of the dominant climatological mode in the Southern Hemisphere, the Southern Annular Mode, or SAM.

The SAM describes a pattern of pressure anomalies around the Antarctic continent, which affects the strength and position of a band of intense westerly winds that blow around Antarctica. Other climatological modes also have indirect influences on Antarctic atmosphere and climate, such as ENSO, via teleconnections. Teleconnections are structures made up of Rossby wave chains (a type of atmospheric wave) that transmit anomalies in sea surface temperature from the Pacific to the Antarctic. This means that what happens in the Pacific can have a remote and indirect effect on the climate of Antarctica, and in particular the peninsula.

Changes in ozone concentration and greenhouse gas forcing are thought to have contributed to the trend in the SAM towards more positive values. A more positive SAM causes a migration of the circumpolar westerly winds towards the poles, where they intersect the peninsula. This causes increased advection of warm, maritime air over the peninsula, which is then forced over the not-insignificant barrier presented by the tail end of the Transantarctic Mountains.

When this air is forced upwards, moisture is forced to condense out of the air mass, meaning that when it descends on the other side, it is warmer and drier. This generates what are known as foehn winds. These winds are much warmer and drier than the usual climatology over the eastern side of the peninsula, therefore they have a significant effect on its climate, particularly in winter when the contrast is most stark. They also clear clouds away, meaning that more energy from incoming solar radiation is absorbed at the surface.

This warming of course contributes to melt over the ice shelves on the Eastern side of the peninsula. Especially in summer, this effect can be pronounced enough to cause temperatures to rise above zero, causing meltwater ponds to form, which can lead to ice shelf fracturing and eventually break-up.

The Antarctic Peninsula contains numerous ice shelves, all fed by tributary glaciers which drain from the mountains. Larsen C is the largest remaining, at 51,000 km2. The loss of Larsen C would raise global sea level by around 27 cm, and much more water is contained in the other ice shelves around the peninsula. Further, these ice shelves have a buttressing effect, meaning they act like a plug on the glaciers that feed them.

Indeed, following the collapse of Larsen A and B in 1995 and 2003, respectively, the tributary glaciers that fed them accelerated massively.

This shows that the ice shelves around the peninsula are vital for maintaining ice sheet mass balance, and preventing the wholesale loss of vast amounts of water. Not only that, but the peninsula’s unique topography and climate make it a uniquely useful natural laboratory for scientific experiments on the atmosphere. However, we know very little about this intensely important region. Poorly resolved and complex processes are active here, so improving our knowledge of the climate over the peninsula will greatly improve our ability to project into the future and to apply this understanding to other cold environments.

New and improved methods for exploring this unique environment are essential to advance our knowledge in the region. My PhD will use high resolution modelling to examine the processes active over the peninsula, and determine how these interact with each other, and contribute to observed change. This may eventually allow us to project into the future and make a guess at what might be in store for Larsen C in the coming century.