Mechanisms driving variability in the ocean forcing of Pine Island Glacier
Benjamin G. M Karen J David P E. Povl Stanley S Ho Kyung Sang Hoon Tae Wan
Pine Island Glacier (PIG) terminates in a rapidly melting ice shelf, and ocean circulation and temperature are implicated in the retreat and growing contribution to sea level rise of PIG and nearby glaciers. However, the variability of the ocean forcing of PIG has been poorly constrained due to a lack of multi-year observations. Here we show, using a unique record close to the Pine Island Ice Shelf (PIIS), that there is considerable oceanic variability at seasonal and interannual timescales, including a pronounced cold period from October 2011 to May 2013. This variability can be largely explained by two processes: cumulative ocean surface heat fluxes and sea ice formation close to PIIS; and interannual reversals in ocean currents and associated heat transport within Pine Island Bay, driven by a combination of local and remote forcing. Local atmospheric forcing therefore plays an important role in driving oceanic variability close to PIIS. Pine Island Glacier terminates in a rapidly melting ice shelf and ocean conditions are believed to influence its contribution to sea level rise. Here, the authors show that variability in these ocean conditions is driven by a combination of changes in ocean circulation and local surface heat fluxes.
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Nature
The ice shelves of the Amundsen Sea buttress a large portion of the West Antarctic ice sheet, protecting it from collapse1,2. The deep ocean temperatures close to the Amundsen Sea continental shelf are some of the warmest around Antarctica and have warmed by approximately 0.2 °C per decade since the early 1990s (refs 3, 4), although data on the Amundsen Sea continental shelf itself are sparse and no significant trend is evident5. The glaciers in the Amundsen Sea sector already produce the greatest mass loss contribution of all the major drainage basins in Antarctica, and this mass loss increased by 59% between 1992 and 2006 (refs 6, 7). Several studies have suggested that the irreversible retreat of some of the Amundsen Sea ice stream grounding lines is imminent or already under way8,9,10,11, due to a combination of ocean forcing5,12,13,14 and backwards-sloping bedrock9,10,12. Therefore, understanding the mechanisms that control ocean conditions within the Amundsen Sea is of paramount importance.

The ocean-driven melting of the Pine Island Ice Shelf (PIIS) arises as a result of the relatively warm, salty circumpolar deep water (CDW) that floods the lower part of the water column across the Amundsen Sea continental shelf. The thermocline between 300 and 700 m separates this CDW from the colder but fresher winter water (WW) layer above5,13. The water that reaches the grounding line of Pine Island Glacier (PIG) (where the ice shelf goes afloat) must pass between a ridge at around 700 m below sea level and the bottom of the ice shelf at around 350 m below sea level12. Therefore, the depth of the thermocline and the associated average temperature between these depths determines the heat available for melting within the cavity beneath PIIS. There is particular sensitivity to the ocean temperature at the grounding line, where the highest melt rates are observed12,15; the water reaching the grounding line is likely to originate from around 700 m depth as it enters the cavity5.

CDW reaches PIIS via a network of glacially carved troughs in the seabed13. CDW primarily flows onto the continental shelf through shelf-edge depressions at 113°W and 102–108°W (henceforth the central and eastern troughs, respectively; Fig. 1a; (refs 16, 17, 18, 19)). An undercurrent flowing eastwards along the continental slope20 beneath the surface-intensified westward Antarctic Slope Current turns onshore at these locations. Several processes have been suggested to explain the transport of water onshore at the continental shelf break; these include topographic steering, vortex stretching that pushes the current southwards on the downstream side of the depression21, and on-shelf flow due to upslope transport in the bottom Ekman layer22. The CDW that reaches PIIS appears to be a combination of water originating from both the central and eastern troughs19, which join paths and mix before flowing southwards into Pine Island Bay (74–75°S, 105–100°W).

The hydrographic conditions within the Amundsen Sea are known to vary seasonally17,23 and interannually5,13,14,24. Various model studies have found a relationship between wind forcing at the continental shelf edge and variability in the onshore flux of CDW17,18,25, and observational studies provide some support for this hypothesis5. However, the processes that modify the ocean advective heat transport between the shelf break and the ice shelf cavities (approximately 450 km apart) are not well understood. Furthermore there is growing evidence that intense surface heat and salt fluxes within coastal polynyas can modify the ocean forcing of PIIS26 and similar ice shelves27. Understanding of the variability of CDW transport into and within the Amundsen Sea has been limited by lack of observations, especially during winter. Various research cruises have provided snapshots of the hydrographic conditions during summer, but until several mooring recoveries in 2014 only a single 11.5-month near-bottom record had been obtained in Pine Island Bay28.

Here we present mooring observations spanning the continental shelf leading to PIIS. These include a 5-year record of temperature and salinity (the BSR5/iSTAR9 time series) located within 10 km of the southern edge of PIIS, where the main meltwater outflow has been observed5,13,29,30. We compare this with three other 2-year mooring records on the continental shelf and a combined time series from three consecutive moorings in the central trough at the continental shelf edge. Of these latter moorings, BSR12 (2009–2011) and iSTAR1 (2012–2014) were almost co-located (1.5 km apart and within one Rossby radius) on the continental shelf at approximately 605 m depth, while BSR13 (2011–2012) was 22.3 km away, on the continental slope at approximately 1,075 m depth. We find that there is considerable variability in the ocean conditions close to PIIS, which is not replicated in the shelf-edge moorings. Instead, it seems that local surface fluxes and changes in the ocean circulation within Pine Island Bay drive the changes observed close to the ice shelf.

Variability in ocean conditions within Pine Island Bay

The time series of temperature above in situ freezing point for BSR5 and iSTAR9 (green markers in Fig. 1b) shows considerable variability (Fig. 2c). There is a marked seasonal cycle evident in the depth of the thermocline (for example, the depth of the 2.5 °C above freezing isotherm, upper white line). This seasonal cycle is typified by a deep thermocline and cold temperatures in austral spring (October–December) and generally warmer conditions in all other seasons. Superimposed on this seasonal cycle are interannual changes in temperature, with a pronounced cold period extending from October 2011 to May 2013 (thick blue line above Fig. 2c). During the peak of this cold period, the seasonal cycle is amplified, extending to greater depths and with cold conditions persisting for longer than during the relatively warm period of February 2009 to February 2011 (thick red line above Fig. 2c). The depth of the 3.25 °C above freezing isotherm (lower white line, Fig. 2c) does not have a marked seasonal cycle and is dominated by relatively smooth interannual variability. The temperature minimum at all depths occurs between October 2012 and January 2013, with the deep temperatures lagging behind the shallow temperatures by up to 3 months.

We investigate whether the cold conditions observed in 2012–13 were unusual by calculating the average temperatures from all ship-based observations within Pine Island Bay (south of 74°S and east of 105°W), and comparing these with austral summer (January–February) mean temperature data from the BSR5/iSTAR9 time series (Fig. 3). The mooring data are consistent with the ship data for the same years, and it is apparent that temperatures between 350 and 700 m are colder during 2012 than any other summer when ship-based observations exist for this location. The mooring data show that the austral summer of 2013 (shortly after the peak of the cold period) was colder still.

The temperature from shorter mooring records in the region sheds light on the connections between the temperature variability observed at BSR5/iSTAR9 and broader oceanographic changes. The temperature time series at iSTAR8 (Fig. 2b, magenta line) matches very well to that at BSR5/iSTAR9 (Fig. 2b, green line) during its 2-year deployment. Similar variability is also seen at iSTAR7, on the edge of the bay (around 100 km away), and to a lesser extent at iSTAR6, almost 200 km away in Pine Island Trough (Supplementary Fig. 1). Therefore, the variability observed at BSR5/iSTAR9 represents large-scale changes that extend across Pine Island Bay.

At the peak of the cold period in spring 2012, the total ocean heat content above freezing (see Methods) between 400 and 700 m at BSR5/iSTAR9 was 1.25 GJ, a 62% reduction from the 3.28 GJ observed in spring 2009 (Fig. 2a). The reduction in temperature above freezing between spring 2009 and spring 2012 was 71% at 400 m, 70% at 500 m, 50% at 600 m, and 20% at 700 m. The average heat content between 400 and 700 m for the entire cold period was 2.37 GJ, a reduction of 32% relative to the 2009–10 average of 3.49 GJ. Due to the relationship between ocean temperature and buoyancy-driven flow along the ice boundary within the cavity, observed basal melt rates are assumed to respond quadratically to changes in temperature above freezing31,32, which would amplify the impact of these changes on the melt rate of PIIS. With only a single point measurement of temperature and salinity close to the depth of the meltwater outflow (430 m on BSR5), we cannot quantitatively assess the impact on melt. However, a ship-based survey in early 2012 showed that meltwater production had reduced to less than half of its 2009–10 value5, so it is reasonable to assume that the reduction in melt at the peak of the cold period was even greater.

The PIIS front advanced considerably between 2009 and 2013, almost reaching the location of iSTAR8 before a large iceberg calved in November 2013 (see coloured lines in Fig. 1b). This event was responsible for moving iSTAR9 downslope (visible in the time series of the depths of the moored sensors, black lines in Fig. 2c), but otherwise caused no major shift in the temperature (shading in Fig. 2c) or currents (Supplementary Fig. 2) observed at BSR5/iSTAR9. The changing shape of the ice tongue may have influenced the circulation around Pine Island Bay and the distribution of the polynyas. However, it seems unlikely that this alone could account for the deep ocean changes in temperature and velocity that are observed across Pine Island Bay.

The impact of local surface heat fluxes

One hypothesis for the variability within Pine Island Bay is changes in the heat flux at the ocean surface. Heat loss at the ocean surface will drive progressive cooling and deepening of the mixed layer, potentially explaining the near-freezing temperatures observed at 430 m during the cold period (Fig. 2c; Supplementary Fig. 2). In addition, localized intense heat loss could lead to small-scale deep convective chimneys that then mix laterally to cool the intermediate water masses33. To analyse quantitatively the impact of surface cooling we compare the change in heat content above freezing between 400 and 700 m with the total heat flux across the ocean surface due to atmosphere-ocean exchange and due to sea ice production and melt34,35 averaged over the blue box in Fig. 1 (approximately 101.5–103.2°W, 74.6–75.2°S). All reanalyses in this region are prone to errors and uncertainties36, as shown by the differences between the ERA-Interim37 and NCEP-38 derived heat fluxes in Fig. 2a; therefore, we use the average of the two products as the best guess. An evaluation of the importance of the additional heat flux derived from sea ice formation and melting is provided in Supplementary Fig. 3.

To compare the surface heat flux with the observed ocean heat content changes, we first remove the time-mean heat flux, assuming that the net heat loss at the surface is balanced by a steady supply of heat by ocean advection and mixing, an assumption that we investigate in the following section. In addition, we ignore the largely unobserved changes in heat content above 400 m; during the cold period the temperature at 400 m is close to the surface freezing point, so we assume that the layer above is isothermal, meaning that the unobserved changes will be negligible. However, when the mixed layer is shallow (during periods of warming or generally warmer conditions), the influence of surface fluxes on the deeper water masses will be less than our calculation suggests.

Despite these simplistic assumptions, the observed changes in heat content agree well (r=0.65) with those derived from accumulated surface heat fluxes (Fig. 2a), suggesting that the surface heat fluxes can explain most of the seasonal variability in ocean temperature, and a limited portion of the interannual variability. Between spring 2009 and 2012, the surface heat fluxes can account for a reduction in heat content of 0.7 GJ, one-third of the 2.1 GJ reduction observed (Fig. 2a). The simulated heat content in spring 2009 is lower than observed, probably due to the relatively shallow thermocline leading to heat content changes in the upper water column, at depths shallower than the observations. For similar reasons the warm peaks in 2010 and 2011 are over-estimated. The under-estimation of the cold conditions in 2012 suggests that additional processes, such as changes in ocean heat advection, may also have contributed to the cooling during 2011 and 2012.

The impact of changes in ocean circulation

The temperature variability at 600 m and deeper is characterized by a steady decline in temperature from mid-2010 to late 2012, followed by warming during 2013 into the start of 2014. This smooth variability suggests relatively little influence from surface heat fluxes locally, apart from during the peak of the cold period when the thermocline deepened to around 700 m. The other major source of variability in ocean temperatures in this region is likely to be from lateral advection or shifts in horizontal temperature gradients associated with fronts. As a proxy for this, the ocean current records at iSTAR7, iSTAR8 and BSR5/iSTAR9 are investigated, and compared with water mass properties (Fig. 4). There is a strong connection between the component of the current heading towards PIIS (PIG-wards currents) and water mass properties, with high temperatures across a range of depths (430–670 m) associated with flow towards the ice shelf at iSTAR7 and iSTAR8 and away from the ice shelf at BSR5/iSTAR9. During the cold period, the flow reversed, with the currents at BSR5/iSTAR9 being towards the ice shelf, and those at iSTAR7 and iSTAR8 being away from the ice shelf. While most ocean modelling studies14,26 and observational evidence13,29 suggest that the circulation around Pine Island Bay is cyclonic, with outflow at BSR5/iSTAR9, one modelling study39 produced an anticyclonic deep ocean circulation with flow into the cavity close to BSR5/iSTAR9, suggesting that this may be a stable alternative flow regime when the thermocline is deep. A schematic representation of these changes is shown in Fig. 4e,f. Note that the clockwise gyre in Pine Island Bay observed during previous cruises is associated with a doming of isopycnals and isotherms in the centre of Pine Island Bay by 100–200 m13,29; a reversal in this circulation will have been associated with deepening of these isopycnals by several hundred metres.

Changes in temperature and salinity are positively correlated, such that a drop in temperature corresponds to a decrease in salinity and potential density (Fig. 4a–d, Supplementary Fig. 4). The salinity of the WW increases by around 0.2 g kg−1 relative to ship-based summer observations (Fig. 4b,c), changing the mixing line between WW and CDW (Supplementary Fig. 5) such that water at all intermediate depths becomes colder (for example, Fig. 4a). The observed changes in WW properties are likely due to a combination of temporal changes in sea ice formation (Fig. 4e,f) and advection of WW from regions with climatologically higher rates of sea ice production. Therefore, the variability within the thermocline can be seen as a combination of isopycnal displacements (likely driven by changes in circulation and the volume of CDW) and changes to the vertical structure of the water column driven by surface forcing, sea ice formation and subsequent mixing.

The observed changes in circulation patterns could be driven by the changes in surface heat fluxes and sea ice production, which would alter the lateral density gradients in and around Pine Island Bay, and thus alter the currents, or by changes in local wind stress. Local zonal wind stress is weakly correlated (r>0.4) with changes in zonal velocity and temperature at BSR5; this correlation is the strongest observed across the continental shelf at any lag (Supplementary Fig. 6). It is also plausible, as has been suggested by several previous studies5,13,16,17, that changes in Pine Island Bay are driven by changes in the CDW flux onto the continental shelf 400 km to the north. The discontinuous mooring time series in the central trough (Fig. 2b, grey lines) show that the temperature of the inflowing CDW was relatively steady over the 5-year period of the moorings, and is not consistent with a major deepening of the thermocline at this inflow location. Even if the temperature remains unchanged, the volume flux of CDW could be altered through changes in on-shelf velocity. Measurements of on-shelf velocity at iSTAR1 and BSR12 offer an incomplete representation of the total on-shelf flux of water, but do show that the on-shelf flow was stronger and more variable during the relatively warm 2009–11 period compared with 2012–14 (Supplementary Fig. 7a). However, the overall agreement between the on-shelf velocity and temperatures in Pine Island Bay is weak (Supplementary Fig. 7b). It is therefore unlikely that variability in conditions in the central trough is the dominant driver of the seasonal or interannual variability that we observe in Pine Island Bay.

The other main pathway for warm CDW onto the continental shelf is the eastern trough. Changes in the zonal wind and associated wind stress curl to the north of the eastern trough have been hypothesized5 as a driving mechanism for the cold conditions in 2012. The zonal wind between 68–72°S and 100–115°W was very anomalous in 2011, and did not recover to the 30-year mean until late 2013 (Supplementary Fig. 8), which may have contributed to the anomalous conditions in Pine Island Bay during the cold period. If we assume that the shelf-edge wind stress in this region drives on-shelf CDW flux, then cumulative anomalies in this quantity should be related to variability in CDW volume in Pine Island Bay. The anomalously negative wind stress throughout the period is consistent with the longer-term decline in deep temperatures, although not the recovery to warm conditions in 2013–14 (Supplementary Fig. 8). Nevertheless, a reduction in inflow, such as that hinted at by the shelf-edge moorings (Supplementary Fig. 7), could have enhanced the impact of the surface heat flux variability and promoted the local changes in circulation that are responsible for the shorter-term variability.