Antarctic Ice Shelves – Response of Pacific-Sector to the El Niño/Southern Oscillation (ENSO)

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Antarctic Ice Shelves – Response of Pacific-Sector to the El Niño/Southern Oscillation (ENSO)

Over the past 2 decades it has been revealed by satellite observations that loss of grounded ice in West Antarctica has been increasing, while the floating ice shelves have been thinning. When an ice shelf thins it becomes less effective at constraining the discharge of grounded ice, yet understanding of the climate processes diving mass discharge is limited. In this study ice shelf height data obtained from 4 satellite altimeter missions (1994 to 2017) were used to show that there was a direct link between the variability of ice shelf height in the Pacific sector of the Antarctic and changes in the regional atmospheric circulation that were driven by the ENSO. In the sector from the Dotson Ice Shelf to the Ross Ice Shelf this link was found to be the strongest and elsewhere the link was weaker. In years of intense El Niño the increase in height resulting from accumulation of snow exceeds the decrease in height that results from basal melting, though there is a decline in the mass of the ice shelf as the loss of basal ice exceeds the ice gain by snow, which has a lower density than the ice that is lost. It was demonstrated by the results of this experiment that there was a substantial response of the ice shelves of the Amundsen Sea to global and regional climate variability, with rates of height and mass loss on interannual time scales that can be comparable to the longer-term trend, with a significant fraction of basal melting changes being offset by mass changes from surface accumulation. It is implied by this that the variability of the ice shelf height and mass will increase as the interannual atmospheric variability increases in a climate that is warming.

As a result of insufficient understanding of the processes that cause mass loss from the ice sheets in Antarctica and Greenland projections of global changes in sea level over the next century are not certain (Church et al., 2013). The West Antarctic Ice Sheet (WAIS), which is mostly grounded below sea level (Fretwell et al., 2013), is therefore a marine ice sheet, contains enough ice above flotation to increase the global sea level by more than 3 m. It is suggested by ice shelf models (Favier et a;l., 2014; Joughin, Smith & Medley, 2014; DeConto & Pollard, 2016) that as glaciers and ice streams respond dynamically to mechanisms of internal instability (Joughin, Smith & Medley, 2014; Weertman, 1974) mass loss will accelerate. The ice loss from the WAIS that has been predicted on timescales of decades to centuries is about 1 m of global sea level equivalent, and full loss of ice sheets within a few millennia (Joughin, Smith & Medley, 2014; DeConto & Pollard, 2016).

In the Amundsen Sea sector of the West Antarctic Ice Sheet the acceleration of the loss of grounded ice has been attributed to a reduction of backstress as the fringing ice shelves are thinning and their grounding lines are retreating (Joughin, Smith & Medley, 2014; Rignot et al., 2014). The rapid and sustained thinning of the ice shelves in the Amundsen Sea (Paolo, Fricker & Padman, 2015; Pritchard et al., 2012) appears to be caused by increasing wind-driven flow of warm Circumpolar Deep Water (CDW) into the ocean cavities beneath the ice shelves, which enhances the basal melting (Pritchard et al., 2012; Jacobs et al., 2011). It is suggested by models (Thoma et al., 2008; Steig et al., 2012) and limited observations (Dutrieux et al., 2014; Jacobs et al., 2013) that ice shelves might be responding to changes in the circulation of the Circumpolar Deep Water on interannual timescales. A paucity of time series of ocean observations in the continental shelf offshore of the Amundsen Sea ice shelves and the sub ice cavities limits this ability to confirm this hypothesis, which led Paolo et al. to seek indirect measures of the sensitivity of the mass change of the ice shelf to large scale climate variability.

Antarctic Pacific sector – Climate variability

Well-known climate drivers of interannual changes in the Antarctic Pacific sector are the ENSO, the Southern Annular Mode (SAM or the Antarctic Oscillation), and variation of the Amundsen Sea Low (ASL) (Turner, 2004; Raphael et al., 2016; Hosking et al., 2013; Turner et al., 2017). The leading mode of ocean-atmosphere variability on timescales of 2-7 years in the tropical Pacific is the ENSO, and on a global scale it is the strongest interannual climate fluctuation (Philander, 1989). Much of the variability that is observed in the atmosphere, ocean and sea ice in the Amundsen Sea-Bellingshausen Sea sector (Thoma et al., 2008; Steig et al., 2013), which exhibits the largest fluctuations of climate around Antarctica (Raphael et al., 2016; Schneider, Okumura, & Deser, 2012). Regional responses to ENSO that have been observed include changes in snowfall (Sasgen et al., 2010; Genthon & Cosme, 2003; Medley et al., 2013), surface air temperature (Steig et al., 2013), extent of sea ice (Turner, 2004; Yuan, 2004; Raphael & Hobbs, 2014), upwelling of CDW near the ice front of the Pine Island Glacier ice shelf (Dutrieux et al., 2014) and variations in basal melting of the Getz Ice Shelf (Jacobs et al, 2013).

In the Southern Hemisphere the SAM is a major variability of climate, which influences precipitation and temperature patterns from the subtropics to Antarctica (Marshall, 2003; Abram et al., 2014). The southern spring and summer are the times when SAM is usually strongest. The effect of ENSO in Antarctica is influenced by the phase of SAM, with the strongest response of the Pacific sector to ENSO occurring when SAM is weak or in opposite phase (Fogt, Bromwich & Hines, 2011), i.e., with the combinations of La Niña/SAM+ and El Niño/SAM- strengthening the anomalies of atmospheric circulation in the mid- to high latitudes.

The Amundsen Sea Low is a low pressure system that is persistent within the Amundsen Sea and the Bellingshausen Sea, and it plays a dominant role in the determination of the regional-scale pattern of the circulation of the atmosphere across West Antarctica (Raphael et al., 2016; Hosking et al., 2013; Turner et al., 2017). The Amundsen Sea Low determines the wind anomalies in all seasons through changes in strength (i.e., central pressure) and position, strongly influencing snowfall, temperature distribution and the conditions of the sea ice near the ice shelves of the Amundsen Sea (19). Variations in the position and strength of the Amundsen Sea Low are driven by variability of the tropical Pacific ocean-atmosphere (ENSO) and fluctuations in the pressure of the Southern Hemisphere (Turner et al., 2017; 17-19). The central pressure of the Amundsen Sea Low tends to be lower during positive SAM conditions and La Niña years, and higher during higher El Niño years (Turner et al., 2017; 17,31,32),

In this study Paolo et al. show that the height of the ice shelf and its mass changes in the Pacific sector, in the Amundsen Sea sector in particular, are correlated with interannual variability in regional atmospheric and oceanic circulation driven by ENSO.

Climate controls on change of the ice shelf

As well as ENSO and the Amundsen Sea Low, there are other regional modes of interannual variability, e.g., the zonal wave three (Raphael, 2004) may also contribute to changes that are observed in the Pacific sector of ice shelves, even though their correlations with δh(t) might be negligible; e.g., SAM. These other modes interact with ENSO and with each other to drive complex patterns of changes in ice shelf height that cannot be explained solely by a linear combination of climate modes.

Studies (Turner et al., 2017; Turner et al., 2017; Raphael & Hobbs, 2014; Clem & Fogt, 2013) that correlated ENSO tropical forcing with climate indicators of the Pacific sector, such as the strength of the Amundsen Sea Low, the extent of sea ice and temperature of the Antarctic Peninsula, have found that correlations with ENSO are significant for some seasons though not with others, with in some cases sign reversals for the correlations from season to season. In the Amundsen Sea Low changes in the strength, location and extent, which all display large interannual variability, result in each distinct ENSO event yielding different responses in individual ice shelves, depending on their location. Assessments of the height of the ice shelves by Paolo et al. for the Amundsen Sea sector reflect the unique environmental conditions of the strong ENSO period of 1997-2000, during which there were exceptionally large changes in the Amundsen Sea Low between the El Niño and La Niña cycles. The strong El Niño event of 2015-2016, in contrast, was characterised by remarkably large changes in sea temperature at the surface of the tropical ocean, did not impact ice shelves in the Amundsen Sea in the same way. It is suggested by Paolo et al. that this probably resulted from the interaction of El Niño with a positive SAM during the development phase, a northwards displacement of the ASL relative to previous strong El Niños (Fogt, 2016) and the absence of a strong La Niña.

The increased basal melting that is associated with the onshore flow of the Cold Deep Water and upwelling along the coast as there is an intensification of the westerly wind stress is the dominant effect of El Niño on the mass of the ice shelf in the Amundsen Sea (Thoma et al., 2008; Steig et al. 2912; Dutrieux et al., 2014). This basal mass loss anomaly on interannual time scales, relative to the loss of mass trend (Paolo, Fricker & Padman, 2015), is offset partially by an increase in snowfall. This increase in precipitation is consistent with the northern wind anomaly that occurs during El Niño events, that possibly includes local uptake of moisture from the coastal ocean due to a reduction in the concentration of regional sea ice.

According to Paolo et al. the reversed pattern of meridional wind anomalies between the Amundsen Sea and the Bellingshausen Sea-Antarctic Peninsula (BS/AP) sector is consistent with the SMB for the WAIS as well as with the surface mass balance that has been obtained from satellite-gravity measurements over the Amundsen Sea and Antarctic Peninsula sectors (Sasgen, 2010). During El Niño in the Amundson Sea sector precipitation increases though it decreases in the BS/AP region and the opposite occurs during La Niña. The response of the Amundsen Sea Low pressure system to ENSO and SAM (Turner et al., 2017; Turner et al., 2017; Fogt, Bromwich & Hines, 2011) is the principal mechanism that affects the local winds, with the Amundsen Sea Low changing consistently in its intensity between El Niño and La Niña years. A weaker response to ENSO was shown by the Pine Island Glacier Ice Shelf, which is thinning rapidly, which is consistent with the weaker, and in some cases reversed, atmospheric and sea ice anomalies in Pine Island Bay relative to the broader Amundsen Sea.

It was shown by Paolo et al. how the height and mass of the ice shelves in the Amundsen Sea sector of the WAIS is affected by the ENSO. The response in height consists of the combined effect of 2 opposing processes, both of which intensified during El Niño events: accumulation of surface snow and basal melting that is driven by the ocean. The result is an overall increase in height, though not a loss of mass, as the ice that is lost from the base is of higher density than that of the snow being deposited on the surface. Given that the atmosphere of the Earth warms it is expected that there will be increases in precipitation and frequency of ENSO events (Cai et al., 2014), the results obtained by Paolo et al. imply that interannual variability of ice shelf height and mass will also increase, stressing the need to quantify accumulation at the surface relative to basal melting to project future changes in ice shelves in Antarctica.

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