Sailing through sea ice

The ice-covered Southern Ocean around Antarctica is a dynamic region that plays a critical role in global climate. It is characterized by substantial spatial and temporal variability in heat, salt, and biogeochemical fluxes. My current research focuses on several key processes within this region: sea ice growth and melt, snow accumulation on sea ice, offshore polynyas, and the transformation of deep waters.

Accessing the Southern Ocean by ship is often challenging, and so autonomous platforms have become increasingly important for collecting sustained observations within this region. These include under-ice hydrographic measurements from Argo profiling floats and instrumented seals, and my research takes advantage of these relatively new, year-round observational capabilities as well as remote sensing, atmospheric reanalysis, and numerical models.

Reconstructing Antarctic sea ice formation and melt rates from ocean salinity

Upper-ocean salinity in the seasonal ice zone from an Argo float

Southern Ocean sea ice thickness is an important climate variable due to its impact on freshwater fluxes, ocean-atmosphere heat exchange, and momentum transfer. Yet monitoring its evolution from satellites has proven challenging, in part owing to a sparsity of in situ measurements for validation purposes. In this ongoing project, I estimate Antarctic sea ice formation and melt using local mixed-layer salinity budgets constructed along the drift trajectories of under-ice Argo profiling floats. This enables the creation of an observationally-grounded circumpolar climatology of Antarctic sea ice production and melt rates, which together represent the rate of change of ice thickness.

Constraining the accumulation and loss of snow on Antarctic sea ice

Maximum potential loss of snow to the ocean through leads

Substantial uncertainties exist as to the fate of snow in the Antarctic seasonal ice zone. Snow deposited onto sea ice may experience erosion due to sublimation, loss to the ocean from wind transport, transformation into snow-ice, redistribution, compaction, metamorphism, and melt. The magnitude, timing, and geographic distribution of each of these processes are highly uncertain. To constrain these, I have developed a new numerical model that simulates the daily evolution of the depth and density of snow on Antarctic sea ice over an annual cycle. The model accumulates snowfall from reanalysis along Lagrangian sea ice parcel trajectories and incorporates parameterizations of key erosion and transformation processes. This work is in progress.

Characterizing the physical and biological impacts of offshore polynyas in the Weddell Sea

2017 Weddell polynya (credit: NASA Worldview)

The periodic appearance of large sea ice openings, known as polynyas, offshore of the Antarctic coast has been an enduring mystery in polar oceanography. In Campbell et al. (2019) in Nature (see also Swart et al., 2018 in BAMS), we explain why polynyas form near the Maud Rise seamount in the Weddell Sea in some years but not others. Two SOCCOM Project biogeochemical profiling floats were present during the unexpected 2016 and 2017 polynya events and collected measurements that suggest reduced haline stratification preconditioned the appearance of the polynyas. These in situ observations also indicate that the polynyas were sustained by deep convection, which had long been suspected for polynyas of this size but had not been previously observed. Using the float observations and a variety of other data sources, we show that storms were the proximal trigger of the recent openings. Both increased storminess and reduced upper-ocean stability in the eastern Weddell Sea region are favored by positive fluctuations in the Southern Annular Mode (SAM), which we identify as a common factor in the 2016-2017 events as well as past polynyas near Maud Rise. The long-term increasing trend in SAM raises questions of whether this effect will win out over the simultaneous trend toward a more stratified Southern Ocean, which would tend to limit polynya occurrences.

Phytoplankton blooms in the Antarctic seasonal ice zone are limited by light and iron. Offshore polynyas may impact biological productivity by alleviating light limitation or increasing subsurface iron fluxes. In von Berg et al. (2020) in GRL, we explore these relationships using biogeochemical profiling float observations and model data. We find that spring blooms in the Weddell Sea tend to immediately follow ice retreat, and that the Maud Rise offshore polynya in 2017 likely enhanced carbon export by allowing an early bloom initiation. While it appears that biological impacts attributable to deep mixing during the recent Maud Rise polynyas of 2016 and 2017 were minimal, it is likely that offshore polynyas can modify carbon fluxes through early bloom initiation or air-sea gas exchange during deep convection.

Understanding Southern Ocean sea ice–upper ocean feedbacks

Upper-ocean temperature in the seasonal ice zone from an Argo float

The Antarctic seasonal ice zone features strong regional variability in ice–ocean feedbacks, which affect the growth of sea ice and release of heat during winter. In Wilson et al. (2019) in JPO, we characterize these ice–ocean feedbacks using observations from Argo floats and instrumented seals. We find that the Weddell Sea is more susceptible to destabilization and polynya formation than other sectors of the Southern Ocean. Using idealized 1-D model simulations, we show that, to a large extent, ice–ocean feedbacks also determine the sensitivity of the upper ocean–sea ice system to strong wind-mixing events.

Decoding the N cycle of the South Atlantic using nitrate isotopes

Depth profiles of nitrate and nitrite, AOU, and N isotopes along the SAMBA line at 34.5°S

The South Atlantic is an important conduit for water masses of Southern Ocean, North Atlantic, and Indian Ocean origin. Analysis of the stable isotopes of N and O within seawater nitrate can illuminate aspects of the local and remote nitrogen cycle and the broader ocean circulation in this region. As reported in my Princeton University senior thesis (Campbell, 2016), I measured paired nitrate isotopes (δ15N and δ18O) from samples that I collected during a wintertime transect along 34.5°S, which reveal signals of distant denitrification, nitrogen fixation, and remineralization of organic matter, as well as local nutrient consumption and resupply. These measurements, some of the first from the South Atlantic, have contributed to collaborative work by Marconi et al. (2017) in GBC on Atlantic basin nitrogen fixation, Marshall et al. (2023) in JGR-Oceans on nitrogen cycling within the Agulhas Current, and Granger et al. (preprint) (in revision at G-Cubed) on Agulhas leakage signatures recorded in planktic foraminifera.

Future work

The following are projects that I am starting to work on (or have just been thinking about!). Contact me if any of these sound interesting or if you are engaged in similar research. I would be excited to collaborate.

  • The impact of storms on the coupled Antarctic ocean-ice-snow system
  • Changes in Southern Hemisphere subpolar gyre strength under anthropogenic forcing
  • Offshore pathways of Antarctic Bottom Water formation during glacial climates