Australia: The Land Where Time Began

A biography of the Australian continent 

Tidewater Glaciers – Scalings for Submarine Melting from Buoyant Plume Theory

Concerns have been raised by rapid dynamic changes at the margins of the Greenland Ice Sheet, that are synchronous with the warming of the ocean, that tidewater glaciers can respond sensitively to ocean forcing. Nevertheless understanding of the processes involved in ocean forcing has remained embryonic. Slater et al. used buoyant plume theory to study the dynamics of proglacial discharge plumes that arise from the subglacial discharge into a fjord at the grounding line of a tidewater glacier, deriving Scalings for the submarine melting that has been induced. Slater et al. focused on the parameter space relevant for high discharge tidewater glaciers, suggesting that in an unstratified fjord the relationship between total submarine melt volume and subglacial discharge raised to 1/3 power is appropriate whatever the plume geometry, as long as discharge lies below a critical value. It is then possible to formulate a simple equation that estimates the total submarine melt volume as a function of discharge, fjord temperature and calving from height. When linear stratification is introduced, however, as Slater et al. suggest may be more relevant for fjords in Greenland, the total melt rate discharge may be as large as ¾  (²/₃) for a point (line) source plume and display more complexity. A guide for more advanced numerical models is provided by the scalings, and they also inform understanding of the processes involved in ocean forcings, and facilitate the assessment of the variability in the marine melting in recent decades as well as under atmospheric and oceanic warming that have been predicted.

Conclusions

In this paper Slater et al., used buoyant plume theory to investigate the dynamics of proglacial plumes that arise from the subglacial discharge input at the grounding line of tidewater glaciers, focused on the submarine melting that is induced on the calving front. The study aimed in particular on deriving the scalings for variation in rates of submarine melt in terms of subglacial discharge, fjord properties, and height of the calving front.

The study found there is no simple relationship between the rate of submarine melt, subglacial discharge and stratification of the fjord. Slater et al. suggest the relationship between subglacial discharge and rate that is prevalent in the literature (i.e., submarine melt rate scales with subglacial discharge raised to the ⅓ power) is appropriate for local or total melt rates in a fjord that is uniformly stratified, whether or not the plume source geometry as long as discharge does not exceed a critical value. It is possible, in these cases, to formulate simple equations for total melt induced. Once linear stratification is introduced, however, the total melt rate discharge exponent may be as large as ¾ (²/₃) for a point (line) source plume, though stratification in temperature complicates the exponent, the exponent possibly being reduced somewhat. Slater et al. suggest these higher exponents are likely representative for large glaciers that terminate in deep water in Greenland where plumes are rarely seen, and where submarine melt rates could possibly be more sensitive to the magnitude of subglacial discharge than was believed previously. The range of values found in the literature can also be explained by the findings of this study.

Slater et al. estimated, based on the melt rates scalings from the study, that submarine melt rates may have increased by 50 % in recent decades, being driven by a combination of a warming atmosphere and warmer ocean. It remains uncertain if this is sufficient to explain the dynamic changes that have been observed at tidewater glaciers in Greenland over the same period of time; if this proves to be the case it would indicate a sensitive coupling between submarine melting and calving dynamics. Slater et al. say it is clear there is potential for further dynamic response in the future of tidewater glaciers to submarine melting and therefore the need for further research into ice-ocean interaction in Greenland, as it is likely submarine melting will increase in response to the predicted warming of the atmosphere and the ocean.

Sources & Further reading

Slater, D. A., D. N. Goldberg, P. W. Nienow and T. R. Cowton (2016). "Scalings for Submarine Melting at Tidewater Glaciers from Buoyant Plume Theory." Journal of Physical Oceanography 46(6): 1839-1855.