A recent paper by Straneo et al. (2011), has investigated the role that ocean circulation plays in bringing warm subtropical water into contact with the outlet glaciers of the Greenland ice sheet. As well as identifying the presence of these warm waters, the authors also show the complex factors which influence meltwater circulation and the impact that these warm waters have on the Greenland glaciers.
An initial paper by Straneo et al. (2010) identified the presence of subtropical waters flowing through the large glacial Sermilik Fjord in East Greenland. This initial study involved two extensive surveys of the fjord over the summer months, between July and September. These surveys found temperatures in the fjord as warm as 4oC. The use of seals tagged with temperature depth recorders also revealed that the fjord warms from July all the way to December, meaning warm temperatures in the winter. This longer temperature and depth record also revealed that the presence of these warmer subtropical waters can also be found throughout the year. This initial study was the first of its kind to identify the presence of these warm subtropical waters in glacial fjords, and also exposed the great unknown regarding interactions between the ocean and glaciers. This is particularly important, as the rate of ice mass loss for the Greenland ice sheet has increased and the contribution it makes to global sea level rise has more than doubled over the last decade. As Rignot et al. (2010) found, there has been a tripling of ice mass loss in Greenland between 1996 and 2007. Additionally, as Mernild et al. (2008) observed, the total amount of Greenland Ice Sheet freshwater input into the North Atlantic Ocean expected from 2071 to 2100 will be more than double what is currently observed as a result of climate change. Of this loss approximately 50-60 percent of ice loss is due to the speeding up (or acceleration) in the flow of outlet glaciers. As shown, this mass exodus of glacial water comes mostly from the acceleration of outlet glaciers into the fjord itself. As Rignot et al. (2010) note, the melt rate of studied glaciers in Greenland is over 100 times larger when looking under the surface. This submarine melting of the glacier front has been shown to contribute to glacier acceleration. Additionally, this acceleration of outlet glaciers coincided with a notable warming trend in the subpolar North Atlantic. Thus, the favoured hypothesis states that the changes in ocean circulation cause ice thinning and ungrounding of the glacier terminus which then leads to acceleration of the ice flow.
A similar paper by Holland et al. (2008) looked at the causes behind the acceleration of several outlet glaciers, in both Greenland and Antarctica. In one particularly strong acceleration, the Jakobshavn Isbrae glacier on the west coast of Greenland saw a doubling in glacier velocity. This, like the Sermilik Fjord was also linked to the presence of warm subtropical waters. This paper also gives a rather detailed atmospheric link for explaining the presence of these subtropical waters. To best detail this, the explanation made by the author is shown here:
“The warm, subsurface waters off the west Greenland coast are fed from the east by the subpolar gyre of the North Atlantic, via the Irminger current. Since the mid-1990s, observations show a warming of the subpolar gyre and the northern Irminger Basin. A key source of variability in the forcing of the subpolar gyre is the North Atlantic Oscillation (NAO). A major change in the behaviour of the NAO was observed during the winter of 1995-1996, when it switched from a prolonged positive phase with strong westerly winds to a negative phase with weaker winds. The net effect of the change was to weaken the subpolar gyre with the consequence of moving the subpolar frontal system (the boundary between cold polar waters and warm subpolar waters) from an easterly position to a more westerly one. Such a large-scale change in the subpolar gyre allowed warm subpolar waters to spread westward, beneath colder surface polar waters, and consequently on and over the west Greenland continental shelf.”
Evidently, the processes behind the arrival of warm subtropical water are very complex. Indeed, no attempt to link atmospheric phenomena to this observed warming is made by Straneo et al. and the root causes remain largely unknown and unproven. However, it can be seen from these studies that the entry of warm subtropical waters is having an effect on the melting and acceleration of the glaciers.
Moving on to the paper by Straneo et al (2011), further research was conducted one year on from the initial findings of subtropical waters in the fjord. This additional research allowed a winter survey of the fjord, with measurements taken much closer to the glacier. In all, the research in the Sermilik Fjord revealed a complex interaction between the Helheim glacier, freshwater melt and warm saline water. The expected circulation had been for the warm subtropical water to travel towards the glacier as deepwater and subsequently melt it. Then, the cooler mixture of subtropical and freshwater was expected to rise to the surface. However, as the authors have shown, the circulation proved to be much more complex. Whilst the waters do consist of a cold Arctic layer at the top and a warm layer beneath, the behaviour of the glacially modified water was different to what had been expected. Because of the density of the glacially modified water, whilst it was shown to rise, the majority does not come out on the surface and instead spreads horizontally between the polar cold and subtropical warm layers. This has the effect to prevent much greater melting, protecting the upper third of the glacier (acting as a vertical barrier to the transport of heat) and instead creating a floating tongue, below which submarine melting has taken place. The creation of this glacial tongue serves to alter the stability of the glacier and potentially increase acceleration into the fjord.
Figure 1. Survey locations within the Sermilik Fjord
The study shows that the process of quantifying the ocean’s impact on outlet glaciers is much more complex than just estimating temperature and velocity. Instead, more accurate estimates of heat transport to the glaciers will involve flow measurements over long periods of time, measure subglacial discharge and consider the effects of stratification and altering circulations in the fjord. Indeed, changes to the temperature of subtropical waters alone are not adequate indicators of the oceans impact on submarine melting. Thus, more research is needed into exactly how these processes occur and how we can better predict the effect on glaciers in the future. As was mentioned before, the contribution of the Greenland glaciers to global sea level rise has increased in the past decade and as yet we do not fully understand the processes governing this. The potential impact for many countries is great, and knowing how increases in melting and acceleration are occurring is key. Several studies looking at glacial fjords in Greenland have been shown, and the features and the effects of submarine melting are common to many other systems in Greenland, as observed in Holland et al. (2008) and Rignot et al. (2010). A final comment by David Holland shows the significance of these studies for the future and the need for greater research:
"The melting of the ice sheets is the wild card of future sea level, and our results hint that modest changes in atmospheric circulation, possibly driven by anthropogenic influences, could also cause future rapid retreat of the Antarctic Ice Sheet, which holds a far greater potential for sea level rise."
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