Australia: The Land Where Time Began
Totten Ice Shelf Melt and Acceleration caused by Wind
The Totten Glacier in Eastern Antarctica could potentially raise the global sea level by 3.5 m, at least, but the sensitivity of the glacier to climate change has not been well understood. The Totten Ice Shelf, which has exhibited variable speed, thickness, and position of the grounding line in recent years, couples the Totten Glacier to the ocean. In this study the velocity of the ice is compared to the oceanic wind stress in order to understand the drivers of the interannual variability and it was found that there is a consistent pattern of acceleration of the ice shelf 19 months after upwelling anomalies occur at the continental shelf break nearby. The sensitivity of climate forcing observed by Greene et al. is a response to the redistribution of oceanic Heat that is wind-driven and is independent of warming of the atmosphere and ocean on a large scale. A link between the Totten Glacier and upwelling near the coast of East Antarctica, where surface winds have been projected to intensify over the next century as a result of greenhouse gas concentrations increasing has been established by this study that was carried out by Greene et al.
An ice basin of 550,000 km2 area, with a base primarily below sea level, is drained by the Totten Glacier, which indicates a potential vulnerability to rapid collapse (Weertman, 1974; Schoof, 2007). The grounding line at which the Totten Glacier begins to float to become the Totten Ice Shelf (TIS) has retreated recently (Li et al., 2015) while the velocity of the ice shelf and terminus position have been unstable (Li et al., 2016; Roberts et al., 2017). From 2002 to 2008 a marked lowering trend was shown by the Totten Ice Shelf (Pritchard et al., 2012; Rignot, Jacobs, Mouginot & Scheuchl, 2013) which continued to at least 2012 in the part of the glacier that was grounded (Young et al., 2015), though it is suggested by longer records of the Totten Ice Shelf surface elevation that the subdecadal trends may represent only part of a variability on a longer term (Roberts et al., 2017; Paolo, Fricker & Padman, 2015).
A variable supply of warm, salty, modified circumpolar deep water (mCDW) has been believed to be a driver of changes in the TIS that have been observed (Li, Rignot, Mouginot & Scheuchl, 2016; Miles, Stokes & Jamieson, 2016; Rignot & Jacobs, 2002), which can access the water cavity beneath the Totten Ice Shelf by a network of bathymetric troughs (Greenbaum et al., 2015). Along the outer continental shelf it has been repeatedly shown by ship-based observations that mCDW is present (Wakatsuchi et al., 1994; Bindoff, Rosenberg & Warner, 2000; Williams et al., 2011; Nitsche et al., 2017), and it was confirmed by a recent survey that the mCDW can traverse the continental shelf and fill the troughs near the Totten Ice Shelf ice front (Rintoul et al., 2016; Silvano et al., 2017). Interannual variability of the Totten Ice Shelf melt rate has been linked by ocean models to the production of sea ice, which generates water that is cold and dense with the potential to displace mCDW and quench melt (Khazendar et al., 2013; Gwyther et al., 2014); though at the time of the only survey that was conducted on the continental shelf no such cold, dense water was detected (Rintoul et al., 2016; Silvano et al., 2017; Silvano, Rintoul & Herráiz-Borreguero, 2016). It is suggested by both models and observations that the melt rate of the Totten Ice Shelf is modulated by a supply of mCDW that is variable, though the mechanism that drives the mCDW exchange across the continental shelf break has not yet been explained, and no links between forcing mechanisms and the response of the Totten Ice Shelf have been observed directly. Insights into the drivers of the variability of the Totten Ice Shelf may be present in West Antarctica, where it has been hypothesised that similar behaviours that have been observed at the Pine Island Ice Shelf results from a variable supply of CDW, which is forced onto the shelf by wind processes at the shelf break (Wåhlin et al., 2013; Dutrieux et al., 2014; Webber et al., 2017; Kim et al., 2017).
In this paper Greene et al. report the results of their investigation into the causes of recent Totten Ice Shelf acceleration and deceleration by comparing a 14-year time series of the velocity of the ice shelf to oceanic wind stress. They used the velocity of the ice surface as a proxy for changes in ice thickness that were driven by melt as well as a direct measure of the response of the Totten Ice Shelf to variable forcing. The velocity time series of the Totten Ice Shelf was driven by a template-matching algorithm applied to 629 satellite image pairs that were obtained between February 2001 and September 2014. Reanalysis data of surface wind and sea ice data were used to calculate zonal and meridional components of wind stress. They focused their study on local regional upwellings, which develop at locations where surface waters are caused to diverge by the wind. Surface water is transported 90o to the left of the wind direction in the Southern Hemisphere as a result of the rotation of the Earth, so divergence of surface water is given by the mathematical curl of the wind stress. Greene et al. defined upwelling as the vertical water velocity at the bottom of the surface layer, which they estimated from the wind stress curl.
In order to assess the response of the Totten Ice Shelf to interannual forcing from the ocean Greene et al. limited analysis of velocity to a region where the ice shelf is bounded laterally by shear margins, between 20 km and 40 km from the ice front, where they expected minimum influence from the pinning points, processes of calving, or velocity anomalies that are associated with the lateral motion near the ice front. They report an increase of 5 % in surface velocity from 2001-2006 which was followed by an immediate trend reversal, slowing by 6 % by 2013. In 2005 and 2009 they observed minor velocity minima, and in 2010 and 2014 they observed minor maxima.
Totten Ice Shelf velocity is correlated negatively, with a 19-month lag, with zonal wind through the domain, which indicates that Totten Ice Shelf accelerates in response to weakening of the eastward winds that drive the Antarctic Circumpolar Current (ACC) or strengthening of the westward winds that drive the Antarctic Coastal Current. Greene et al. assumed that the velocity of the Totten Ice Shelf is linked to a variable supply of mCDW, with lag times primarily being due to the time required for melt rate anomalies to integrate and cause sufficient thinning to produce an observable response in surface velocity (Christianso et al., 2016). To the north of the continental shelf break, over the deep ocean, the negative correlation with the zonal wind contrasts with the notion derived from classical Ekman dynamics, that upwelling of warm deep water near 63oS should be induced by positive zonal wind anomalies. Over the continental shelf, westward winds were expected to induce southward transport of surface water, depress isotherms, and could therefore prevent the surmounting of the continental shelf (mCDW) (Oshima et al., 1996; Hayakawa et al., 2012), yet evidence is presented by Greene et al. that competing processes prevail.
Prevailing westward winds over the continental shelf serve as the southern component of the wind stress curl, which causes upwelling along the continental slope. The negative correlation between the velocity of the Totten Ice Shelf and the zonal wind over the continental shelf is associated with a positive correlation between the Totten Ice Shelf velocity and the upwelling along the continental slope. The zonal winds over the deep ocean maintain their negative correlation with Totten Ice Shelf velocity, in particular where downwelling occurs in compensation for upwelling along the continental slope. It is suggested by these observations that the meridional gradient of wind stress contributes more to the variability of the Totten Ice Shelf velocity than do zonal wind stress anomalies that are uniform.
In the region the mean coastal wind flow is oriented in such a way that its meridional component is small or nil, except for the northward flow diversion around Law Dome to the west of the Totten Ice Shelf front. Throughout most of the region of the study, where the meridional component of the mean velocity field in almost zero, Totten Ice Shelf shows a weak, though slightly negative, relationship with meridional wind stress. Totten Ice Shelf shows a weak (r2<0.2) positive correlation with meridional wind where coastal wind directs north around Law Dome. Totten Ice Shelf velocity is correlated positively with the concentration of sea ice throughout much of the domain, with small regions that have negative correlation over the continental shelf and over the deep ocean. The relationship between the concentration of sea ice and Totten Ice Shelf velocity is quite weak (r2<0.15) everywhere in the region.
It is revealed by linear regression of upwelling and Totten Ice Shelf velocity that the Totten Ice Shelf accelerates at times of strong upwelling along the continental slope. Bathymetric contours are roughly followed by the relationship, which indicates the role of the topography of the seafloor in blocking mCDW intrusions at times of weak upwelling. Along the Antarctic Coastal Current an upwelling zone lies upstream which exhibits a particularly strong relationship with Totten Ice Shelf velocity (r2<0.85). It is implied by this that Totten Ice Shelf accelerates in response to increased melt following strong upwelling anomalies along the continental slope. Correlation is maximised with a lag of 19 months, which indicates the time required for upwelled mCDW to traverse the continental shelf, enter the water cavity below Totten Ice Shelf, induce melt, and lead to ice shelf acceleration by lateral shear stress that is reduced.
Upwelling is implicated further by oceanographic observations as a driver of mCDW variability on the continental shelf, where temperature anomalies of 2oC or more between 450 m and 650 m depth can result from shoaling of the thermocline associated with upwelling along the nearby continental slope. In the region of the study of the velocity time series observations 550 m is the mean depth of the Totten Ice Shelf, and at that depth a +2oC temperature anomaly represents a 6-fold increase in thermal driving potential relative to temperature minima of 0.4oC above the in situ freezing point that has been observed. It has been indicated by models that ice shelf melt rates scale superlinearly to quadratically with thermal driving potential (Holland, Jenkins & Holland, 2008; Little, Gnanadesikan & Oppenheimer, 2009; Gwyther et al., 2015), which suggests that some areas of the base of the Totten Ice Shelf can experience more than a 10-fold increase or decrease in melt rate depending on the availability of mCDW that has been upwelled.
Widespread presences of mCDW on the continental shelf is shown by profiling ship-based and float observations, and the thickness of the mCDW layer is linked to upwelling along the continental slope. Greene et al., suggest that after mCDW surmounts the continental slope, the westward winds driving the coastal current may enhance the delivery of mCDW to the water cavity beneath the Totten Ice Shelf, where the base of the ice shelf is highly sensitive to small changes in thermal forcing. Also, cold meltwater from the cavity below the ice shelf may be flushed out or cavity circulation may be intensified and increase the melt (Gwyther et al., 2016).
Surface velocity that is averaged over the main trunk of Totten Ice Shelf reached a maximum in early 2007 corresponding to a minimum (Roberts et al., 2017) ice thickness that was reported, at the time of a minimised lateral shear stress restraining the Totten Ice Shelf flow. The linear trend of slowdown of Totten Ice Shelf amidst ongoing acceleration of surrounding grounded ice is similar to a pattern seen at the Pine Island glacier, where the ice shelf has shown a response to ocean forcing by accelerating about 9 months after arrival at the ice front of thermal anomalies (Christianson et al., 2016). The 19 month lag that was observed by Greene et al. from the time of upwelling along the continental slope to acceleration of the Totten Ice Shelf includes the time for the mCDW to traverse the continental shelf. Totten Ice Shelf is much thicker than the Pine Island Ice Shelf, on average, and is therefore expected to respond more slowly to anomalies of basal melt (Christianson et al., 2016).
The region along the continental slope where upwelling is highly covariant with the velocity of the Totten Ice Shelf is close to a persistent eddy feature where it has repeatedly been revealed by dissolved silicate measurements that upwelling (Wakatsuchi et al., 1994); warm, saline CDW has been detected (Bindoff, Rosenberg & Warner, 2000; Williams et al., 2011); and it has been shown that upwelling is correlated positively with the Southern Annular Mode (SAM) (Hayakawa et al., 2012). The SAM is the leading mode of climate variability in the Southern Hemisphere, it is influenced seasonally by various natural and anthropogenic drivers (Fogt et al., 2009), and its positive mode is associated with an intensification of the eastward winds around Antarctica (Thompson et al., 2011). The SAM has been tending towards its positive phase in summer in recent decades, which is primarily due to effects of substances that deplete the ozone, though an increasing influence of greenhouse gas in the atmosphere is expected to dominate the SAM in the coming century and continue its positive bias as the ozone hole recovers (Fyfe et al., 2007; Sigmond et al., 2011). It is shown by projections that an intensification of the Antarctic Circumpolar Current (ACC), that is driven by the wind, and an upwelling increase, in particular along the Antarctica continental slope (Fyfe et al., 2007; Wang, 2013; Spence et al., 2014). Greene et al. suggest it is possible that westward winds along the coast could weaken in conjunction with a migration to the south of the divergence zone (Spence et al., 2014), In which case the delivery of the mCDW to the Totten Ice Shelf could be weakened by a weakened coastal current; projections of coastal westward winds near Totten Ice Shelf are few, however, and their relationships to SAM or atmospheric greenhouse gas have not been validated.
According to Greene et al. they have confirmed the role of upwelling that is driven by the wind as a primary mechanism of delivery of the mCDW on the continental shelf of East Antarctica and have shown that upwelling of mCDW is directly correlated with the velocity of Totten Ice Shelf that is melt driven. Over the Southern Ocean wind patterns are expected to evolve throughout the 21st century, and a shifting regime of upwelling could precipitate a response that is marked in the Totten Glacier, unlocking the door to at least 3.5 m eustatic sea level potential (Greenbaum et al., 2015) in the vast ice basin it drains.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|