Seasonal sea ice changes in the Amundsen Sea, Antarctica, over the period of 1979–2014

Abstract

Recent attention has focused on accelerated glacial losses along the Amundsen Sea coast that result from changes in atmosphere and ocean circulation, with sea ice playing a mediating but not well-understood role. Here, we investigated how sea ice has changed in the Amundsen Sea over the period of 1979 to 2014, focusing on spatio-temporal changes in ice edge advance/retreat and percent sea ice cover in relation to changes in winds. In contrast to the widespread sea ice decreases to the east and increases to the west of the Amundsen Sea, sea ice changes in the Amundsen Sea were confined to three areas: (i) offshore of the shelf break, (ii) the southern Pine Island Polynya, and (iii) the eastern Amundsen Sea Polynya. Offshore, a 2-month decrease in ice season duration coincided with seasonal shifts in wind speed and direction from March to May (relating to later ice advance) and from September to August (relating to earlier retreat), consistent with reported changes in the depth/location of the Amundsen Sea Low. In contrast, sea ice decreases in the polynya areas corresponded to episodic or step changes in spring ice retreat (earlier by 1-2 months) and were coincident with changes to Thwaites Iceberg Tongue (located between the two polynyas) and increased southeasterly winds. Temporal correlations among these three areas were weak, indicating different local forcing and/or differential response to large-scale forcing. Although our analysis has shown that part of the variability can be explained by changes in winds or to the coastal icescape, an additional but unknown factor is how sea ice has responded to changes in ocean heat and freshwater inputs. Unraveling cause and effect, critical for predicting changes to this rapidly evolving ocean-ice shelf-sea ice system, will require in situ observations, along with improved remote sensing capabilities and ocean modeling.

Upper Ocean Distribution of Glacial Meltwater in the Amundsen Sea, Antarctica

“…The oxygen isotope ratios provide estimates of SIM and MWI to the water column. We detected a strong signature of sea ice growth across the continental shelf and SIM at the continental shelf edge, which agrees with satellite observations of sea ice concentrations in the Amundsen Sea (Stammerjohn et al., ). In particular, the MWI distribution highlights the increase in freshwater toward the western end of the eastern channel.…”

“…While the surface (<40 m) shows SIM, below 60 m shows a net effect of sea ice growth, with values reaching −8.7 g/kg. This sea ice growth component decreases with depth to negligible sea ice contributions at depths below 600 m. As the measurements were taken at the end of the austral summer, the high surface SIM content reflects the result of the seasonal heating of the upper ocean and also the strong stratification this produces, shown by the restriction of this signal to the top 40 m. The net sea ice growth throughout the rest of the water column is consistent with previous studies on the Amundsen Sea continental shelf (Randall‐Goodwin et al., ), where significant sea ice export results in higher sea ice growth rates than SIM rates (Stammerjohn et al., ).…”

Modeling Iron and Light Controls on the Summer Phaeocystis antarctica Bloom in the Amundsen Sea Polynya

“…This freshwater contribution, cumulated between 1 October and 1 January, varies between 0 and 0.27 m of meltwater, depending on station. This represents a small fraction of the typical sea ice thickness in the Amundsen Sea (~1.5 m in winter; Kurtz & Markus, ; Xie et al, ) and is consistent with the observations that the ASP is primarily formed by winds (Stammerjohn et al, ) and dominated by sea ice production over sea ice melt (Randall‐Goodwin et al, ). The model uses bulk formulas (Fairall et al, ) to derive surface wind stresses and surface heat fluxes from the observations described above.…”

Pelagic microbial heterotrophy in response to a highly productive bloom of Phaeocystis antarctica in the Amundsen Sea Polynya, Antarctica

“…Integrated nitrate depletion (DDIN 100 ) ranged from 58 to 740 mmol N m −2 (Table 1); it correlated significantly with longitude (R = 0.70; n = 13; p < 0.01) and sea ice concentration (R = −0.59; n = 13; p < 0.05) at the time of sampling (Table 1). These trends reflect the overall pattern of the polynya opening (from southeast to northwest; Stammerjohn et al, 2015) that resulted in larger blooms to the east at the time we sampled, particularly at the open-water stations (Stations 25,29,35,50,57) compared to ice-edge or marginal ice zone stations to the west (5, 66, 68).…”