Over recent decades, the massive ice shelves along Western Antarctica have been thinning at an increasing rate. This is likely due in part to global warming, but there are complex climate and ocean systems at work there as well. The thinning and melting of the West Antarctic Ice Sheet (WAIS) is of particular concern as it contains sufficient ice volume above sea level to increase global sea-level by over 3 meters. Models predict that full ice-sheet loss may occur within a few millennia.
Unfortunately, projections of global sea-level change over the next century are uncertain due to a limited understanding of the complex processes causing mass loss from these ice sheets in the Amundsen Sea (AS) sector of the southern Pacific. Observations suggest that increasing wind-driven flow of warm Circumpolar Deep Water (CDW) into the ocean cavities beneath ice shelves may be important through enhancing basal melting (melting that happens where the glacier meets the seafloor).
Models and limited observations suggest that these ice shelves might respond to changes in CDW circulation on interannual (year to year) timescales. However, a lack of observations on the continental shelf offshore of AS ice shelves and in sub-ice cavities limits scientists’ ability to confirm this hypothesis. Researchers must instead seek indirect measures of the sensitivity of ice-shelf mass change to large-scale climate variability. They do this by researching interannual climate-driving forces, or modes, of the Pacific sector.
Loads of Modes
The most famous mode is the El Niño/Southern Oscillation (ENSO). It is the leading mode of ocean–atmosphere variability on timescales of 2–7 years in the tropical Pacific, and is the strongest interannual climate fluctuation on a global scale. In Antarctica, observed regional responses to ENSO include changes in snowfall, surface air temperature, sea-ice extent, upwelling of CDW near glacier ice shelves, and variation in basal melting under the Getz Ice Shelf.
Another dominant force is the Southern Annular Mode (SAM). SAM describes the north–south movement of the westerly wind belt that circles Antarctica, being a major driver of climate variability in the Southern Hemisphere. It is usually strongest in spring and summer. In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. A negative SAM event sees an expansion of the belt of strong westerly winds towards the equator. The phase of SAM can influence the effect of ENSO in Antarctica, with the strongest Pacific sector response to ENSO when SAM is weak or in opposite phase.
But wait – there’s more that comes into play. The Amundsen Sea Low (ASL) is a persistent, low atmospheric pressure zone located over the extreme southern Pacific Ocean, off the coast of West Antarctica and the AS. It plays the dominant role in determining the regional-scale pattern of atmospheric circulation across West Antarctica and the adjacent ocean. A change in the strength (pressure) of ASL influences snowfall, temperature distribution, and sea-ice conditions near AS ice shelves. Atmospheric variability in this region is larger than anywhere else in the Southern Hemisphere.
Which Mode is Strongest?
Researchers wanted to find out which of these modes influences grounded-sea-ice loss the most, and to what capacity. To do this, Paolo et. al. used data from four satellite altimeter missions (ERS-1, ERS-2, Envisat, and CryoSat-2) to create individual records of ice-shelf height in the region from 1994 to 2017.
They found that anomalies in ice-shelf height, δh(t), are strongly correlated with the oceanic Niño index (ONI) – a measure of ENSO that tracks sea surface temperatures in the Pacific. They also found a smaller negative correlation between δh(t) and the ASL pressure, but no significant correlation between δh(t) and SAM. Interestingly, the best fit between δh(t) and a linear combination of ONI and ASL indices showed high correlation values, which lagged by about six months. In simple terms, some large changes in ice-shelf height that are unexplained by ONI can be explained by the contribution from the ASL.
There was also revealed a strong link between ice-shelf height variability and local environmental controls. Zonal wind anomalies (along latitude) modulate ice-shelf basal melting through upwelling and downwelling of coastal waters (including CDW), and meridional wind anomalies (along longitude) modulate the transport of warm moist air and precipitation from the ocean to the AS ice shelves.
The largest observed change in ice-shelf height anomalies occurred between 1998 and 2001, which contained a very strong ENSO event: the El Niño of 1997–1998 and the subsequent strong La Niña of 1999–2000. During the El Niño, the westerly wind was stronger than normal, increasing the upwelling of ocean waters on the continental shelf, and the air temperature and precipitation were also higher on average than normal. This meant that while the ice shelf height increased due to snowfall, there was basal melting happening underneath that thinned the ice and decreased the volume. Not all strong ENSO effects happen exactly the same, however; the strong 2015–2016 El Niño, with large changes in tropical sea surface temperature, did not impact AS ice shelves in the same way. This was probably a result of El Niño interaction with a positive SAM phase, a northward displacement of the ASL relative to previous strong El Niños, and absence of a following strong La Niña.
Overall, this study showed that ice-shelf height and mass changes in the Pacific sector, in particular the AS sector, respond strongly to the combined effect of two opposing processes (which are both intensified during El Niño events): surface snow accumulation and ocean-driven basal melting. The result is an overall height increase, but net mass loss over interannual scales, since the ice lost from the base has higher density than the fresh snow being gained at the surface. ENSO is the biggest driving mode for this variability, though the strength of atmospheric pressure over AS affects its influence as well. With expected increases in total precipitation and frequency of extreme ENSO events as the Earth warms, the study implies that interannual variability of ice-shelf height and mass will also increase. Because of this, it is important to be able to quantify surface accumulation relative to basal melting to project future changes in Antarctic ice shelves.
Zoe has an M.S. in Oceanography and a B.S. in Geologic Oceanography from URI, with a minor in Writing and Rhetoric. She was recently a Knauss Marine Policy Fellow in the US House of Representatives, and now work at Consortium for Ocean Leadership. When not writing and editing, Zoe enjoys rowing, rock climbing, skiing, and reading.