Using timing of ice retreat to predict timing of fall freeze-up in the Arctic

Reliable forecasts of the timing of sea ice advance are needed in order to reduce risks associatedwith operating in the Arctic as well as planning of human and environmental emergencies. This studyinvestigates the use of a simple statistical model relating the timing of ice retreat to the timing of ice advance,taking advantage of the inherent predictive power supplied by the seasonal ice-albedo feedback and oceanheat uptake. Results show that using the last retreat date to predict the first advance date is applicable insome regions, such as Baffin Bay and the Laptev and East Siberian seas, where a predictive skill is found evenafter accounting for the long-term trend in both variables. Elsewhere, in the Arctic, there is some predictive skillsdepending on the year (e.g., Kara and Beaufort seas), but none in regions such as the Barents and Bering seas orthe Sea of Okhotsk. While there is some suggestion that the relationship is strengthening over time, this mayreflect that higher correlations are expected during periods when the underlying trend is strong

Pathways and Supply of Dissolved Iron in the Amundsen Sea (Antarctica)

Numerous coastal polynyas fringe the Antarctic continent and strongly influence the productivity of Antarctic shelf systems. Of the 46 Antarctic coastal polynyas documented in a recent study, the Amundsen Sea Polynya (ASP) stands out as having the highest net primary production per unit area. Incubation experiments suggest that this productivity is partly controlled by the availability of dissolved iron (dFe).As a first step toward understanding the iron supply of the ASP, we introduce four plausible sources of dFe and simulate their steady spatial distribution using conservative numerical tracers. The modeled distributions replicate important features from observations including dFe maxima at the bottom of deep troughsand enhanced concentrations near the ice shelf fronts. A perturbation experiment with an idealized draw-down mimicking summertime biological uptake and subsequent resupply suggests that glacial meltwaterand sediment-derived dFe are the main contributors to the prebloom dFe inventory in the top 100 m of the ASP. The sediment-derived dFe depends strongly on the buoyancy-driven overturning circulation associated with the melting ice shelves (the ‘‘meltwater pump’’) to add dFe to the upper 300 m of the water column. The results support the view that ice shelf melting plays an important direct and indirect role in the dFe supply and delivery to polynyas such as the ASP

Modeling of the Influence of Sea Ice Cycle and Langmuir Circulation on the Upper Ocean Mixed Layer Depth and Freshwater Distribution at the West Antarctic Peninsula.

The Southern Ocean is chronically undersampled due to its remoteness, harsh environment, and sea ice cover. Ocean circulation models yield significant insight into key processes and to some extent obviate the dearth of data; however, they often underestimate surface mixed layer depth (MLD), with consequences for surface water-column temperature, salinity, and nutrient concentration. In this study, a coupled circulation and sea ice model was implemented for the region adjacent to the West Antarctic Peninsula, a climatically sensitive region which has exhibited decadal trends towards higher ocean temperature, shorter sea ice season, and increasing glacial freshwater input, overlain by strong interannual variability. Hindcast simulations were conducted with different air-ice drag coefficients and Langmuir circulation parameterizations to determine the impact of these factors on MLD. Including Langmuir circulation deepened the surface mixed layer, with the deepening being more pronounced in the shelf and slope regions. Optimal selection of an air-ice drag coefficient also increased modeled MLD by similar amounts and had a larger impact in improving the reliability of the simulated MLD interannual variability. This study highlights the importance of sea ice volume and redistribution to correctly reproduce the physics of the underlying ocean, and the potential of appropriately parameterizing Langmuir circulation to help correct for biases towards shallow MLD in the Southern Ocean. The model also reproduces observed freshwater patterns in the West Antarctic Peninsula during late summer and suggests that areas of intense summertime sea ice melt can still show net annual freezing due to high sea ice formation during the winter

The Amundsen Sea Polynya International Research Expedition (ASPIRE)

In search of an explanation for some of the greenest waters ever seen in coastal Antarctica and their possible link to some of the fastest melting glaciers and declining summer sea ice, the Amundsen Sea Polynya International Research Expedition (ASPIRE) challenged the capabilities of the US Antarctic Program and RVIB Nathaniel B. Palmer during Austral summer 2010–2011. We were well rewarded by both an extraordinary research platform and a truly remarkable oceanic setting. Here we provide further insights into the key questions that motivated our sampling approach during ASPIRE and present some preliminary findings, while highlighting the value of the Palmer for accomplishing complex, multifaceted oceanographic research in such a challenging environment

Seasonal Dynamics of Dissolved Iron on the Antarctic Continental Shelf: Late-Fall Observations From the Terra Nova Bay and Ross Ice Shelf Polynyas

Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water-column DFe distributions has not been previously documented. We collected hydrographic data and water-column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April-May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from similar to 250 to similar to 600 m over lateral distances \u3c10 km; there vertical mixing was just starting to excavate the dense, iron-rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were \u3c300 m deep, and DFe distributions were similar to previous, late-summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid-depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid-late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production

Environmental controls on pteropod biogeography along the Western Antarctic Peninsula

Pteropods are abundant zooplankton in the Western Antarctic Peninsula (WAP) and important grazers of phytoplankton and prey for higher trophic levels. We analyzed long-term (1993-2017) trends in summer (January-February) abundance of WAP pteropods in relation to environmental controls (sea ice, sea surface temperature, climate indices, phytoplankton biomass and productivity, and carbonate chemistry) and interspecies dynamics using general linear models. There was no overall directional trend in abundance of thecosomes, Limacina helicina antarctica and Clio pyramidata, throughout the entire WAP, although L. antarctica abundance increased in the slope region and C. pyramidata abundance increased in the South. High L. antarctica abundance was strongly tied to a negative Multivariate El Nino Southern Oscillation Index the previous year. C. pyramidata abundance was best explained by early sea ice retreat 1-yr prior. Abundance of the gymnosome species, Clione antarctica and Spongiobranchaea australis, increased over the time series, particularly in the slope region. Gymnosome abundance was positively influenced by abundance of their prey, L. antarctica, during the same season, and late sea ice advance 2-yr prior. These trends indicate a shorter ice season promotes longer periods of open water in spring/summer favoring all pteropod species. Weak relationships were found between pteropod abundance and carbonate chemistry, and no long-term trend in carbonate parameters was detected. These factors indicate ocean acidification is not presently influencing WAP pteropod abundance. Pteropods are responsive to the considerable environmental variability on both temporal and spatial scales-key for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions

Sea-ice production and air/ice/ocean/biogeochemistry interactions in the Ross Sea during the PIPERS 2017 autumn field campaign

The Ross Sea is known for showing the greatest sea-ice increase, as observed globally, particularly from 1979 to 2015. However, corresponding changes in sea-ice thickness and production in the Ross Sea are not known, nor how these changes have impacted water masses, carbon fluxes, biogeochemical processes and availability of micronutrients. The PIPERS project sought to address these questions during an autumn ship campaign in 2017 and two spring airborne campaigns in 2016 and 2017. PIPERS used a multidisciplinary approach of manned and autonomous platforms to study the coupled air/ice/ocean/biogeochemical interactions during autumn and related those to spring conditions. Unexpectedly, the Ross Sea experienced record low sea ice in spring 2016 and autumn 2017. The delayed ice advance in 2017 contributed to (1) increased ice production and export in coastal polynyas, (2) thinner snow and ice cover in the central pack, (3) lower sea-ice Chl-a burdens and differences in sympagic communities, (4) sustained ocean heat flux delaying ice thickening and (5) a melting, anomalously southward ice edge persisting into winter. Despite these impacts, airborne observations in spring 2017 suggest that winter ice production over the continental shelf was likely not anomalous