Ross Gyre Research Articles

Tropical teleconnections through the Amundsen Sea Low impact Antarctic toothfish recruitment within the Ross Gyre

Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region are believed to spawn predominantly in the northern parts of the Ross Gyre during the austral winter with fluctuations in their recruitment observed. This Lagrangian modelling study attempts to explain these fluctuations and shows how sea-ice drift impacts the buoyant eggs and the overall recruitment of juveniles reaching the Amundsen shelf break. Interannual variations in the Amundsen Sea Low, linked to tropical sea surface temperatures, cause modulations in the sea-ice drift and subsequent recruitment. When the Amundsen Sea Low is weaker, consistent with El Niño conditions, the northward sea-ice drift reduces, and more eggs remain within the Ross Gyre leading to a larger recruitment success. Conversely, recruitment success reduces during La Niña conditions. The sea-ice drift may explain about 80% of the interannual Antarctic toothfish recruitment variability over the period 1975–2016 and is of particular importance during the first year after spawning. These results enable future interannual changes in Antarctic toothfish recruitment success based on remote observations to be anticipated. Our findings suggest that ongoing climate change strengthening of the Amundsen Sea Low, will likely contribute to a long-term toothfish recruitment decline in the Ross Gyre region.

On the dynamics of the Ross Gyre: the relative importance of wind, buoyancy, eddies, and the Antarctic Circumpolar Current

On the dynamics of the Ross Gyre: the relative importance of wind, buoyancy, eddies, and the Antarctic Circumpolar Current

Impact of Ross Gyre on surface circulation in the Amundsen Sea, Antarctica

The east limb of the Ross Gyre approaches the western Amundsen Sea, thereby exerting an influence on the surface circulation dynamics. This study investigates the impact of the zonal movement of the eastern boundary of the Ross Gyre on the surrounding surface geostrophic circulation in January. Specifically, a zonal shift of the eastern boundary of the Ross Gyre induces a substantial shelf-basin circulation within the Amundsen Sea. The findings of this study provide valuable insights into the influence of the Ross Gyre on the exchange processes between the Amundsen Sea shelf and basin.

Ross Gyre variability modulates oceanic heat supply toward the West Antarctic continental shelf

West Antarctic Ice Sheet mass loss is a major source of uncertainty in sea level projections. The primary driver of this melting is oceanic heat from Circumpolar Deep Water originating offshore in the Antarctic Circumpolar Current. Yet, in assessing melt variability, open ocean processes have received considerably less attention than those governing cross-shelf exchange. Here, we use Lagrangian particle release experiments in an ocean model to investigate the pathways by which Circumpolar Deep Water moves toward the continental shelf across the Pacific sector of the Southern Ocean. We show that Ross Gyre expansion, linked to wind and sea ice variability, increases poleward heat transport along the gyre’s eastern limb and the relative fraction of transport toward the Amundsen Sea. Ross Gyre variability, therefore, influences oceanic heat supply toward the West Antarctic continental slope. Understanding remote controls on basal melt is necessary to predict the ice sheet response to anthropogenic forcing.

Going with the floe: Sea-ice movement affects distance and destination during Adélie penguin winter movements.

Seasonal migration, driven by shifts in annual climate cycles and resources, is a key part of the life history and ecology of species across taxonomic groups. By influencing the amount of energy needed to move, external forces such as wind and ocean currents are often key drivers of migratory pathways exposing individuals to varying resources, environmental conditions, and competition pressures impacting individual fitness and population dynamics. Although wildlife movements in connection with wind and ocean currents are relatively well understood, movements within sea-ice fields have been much less studied, despite sea ice being an integral part of polar ecology. Adélie penguins (Pygoscelis adeliae) in the southern Ross Sea, Antarctica, currently exist at the southernmost edge of their range and undergo the longest (~12,000 km) winter migration known for the species. Within and north of the Ross Sea, the Ross Gyre drives ocean circulation and the large-scale movement of sea ice. We used remotely sensed sea-ice movement data together with geolocation-based penguin movement data to test the hypothesis that penguins use gyre-driven sea-ice movement to aid their migration. We found that penguins traveled greater distances when their movement vectors were aligned with those of sea ice (i.e., ice support) and the amount of ice support received depended on which route a penguin took. We also found that birds that took an eastern route traveled significantly further north in two of the 3 years we examined, coinciding with higher velocities of sea ice in those years. We compare our findings to patterns observed in migrating species that utilize air or water currents for their travel and with other studies showing the importance of ocean/sea-ice circulation patterns to wildlife movement and life history patterns within the Ross Sea. Changes in sea ice may have consequences not only for energy expenditure but, by altering migration and movement pathways, to the ecological interactions that exist in this region.

Projected West Antarctic Ocean Warming Caused by an Expansion of the Ross Gyre

AbstractWe use United Kingdom Earth System Model simulations from the Coupled Model Intercomparison Project 6 to analyze the Ross Gyre (RG) dynamics during the historical 1850–2014 period and under two contrasting future climate‐change scenarios. The modeled RG is relatively stable, with an extent and strength that agree with observations. The projections exhibit an eastward gyre expansion into the Amundsen‐Bellingshausen Seas that starts during the 2040s. The associated cyclonic ocean circulation enhances the onshore transport of warm Circumpolar Deep Water into the inner regional shelf, a regime change that increases the local subsurface shelf temperatures by up to 1.2°C and is independent of future forcing scenario. The RG expansion is generated by a regional ocean surface stress curl intensification associated with anthropogenic sea ice loss. If realised in reality, such a warming would strongly influence the future stability of the West Antarctic Ice Sheet.

An Assessment of the Oceanic Physical and Biogeochemical Components of CMIP5 and CMIP6 Models for the Ross Sea Region

AbstractThe physical and biogeochemical performance of 16 CMIP5 and 16 CMIP6 Earth System models (ESM) are examined relative to present day (1976–2005) observational data sets for a Ross Sea Region (RSR) containing the Ross Gyre (RG) and the Ross Sea Continental Shelf. A relative ranking scheme using error metrics and published ESM properties (including climate sensitivity parameters and anomalous deep ocean convection statistics) enables identification of a “best” ensemble of models for the RSR. Over the RSR the CMIP6 models are generally found to have improved physical representations compared to the CMIP5 set (based on our metrics for sea ice concentration, sea surface temperature, salinity, and height, and mixed layer depth), but the CMIP5 and CMIP6 biogeochemical representations remain similar. Examination of mean properties for the period 2081 to 2100 for RCP8.5 and SSP585 for CMIP5 and CMIP6, respectively, shows significant surface temperature increases, with significant decreases in sea surface salinity, sea ice concentration, and mixed layer depth across the RSR. Biogeochemically, there are generally small increases in surface values for chlorophyll, integrated primary production, and zooplankton carbon concentrations. The projections also have robust reductions in surface nitrate, phosphate, and silicate across the RSR. For that part of the RG circulation to the east of 180°E—which we refer to as the “inner RG”—significant barotropic transport increases are found by the end of century.

Ocean‐Forced Instability of the West Antarctic Ice Sheet Since the Mid‐Pleistocene

AbstractEvidence on West Antarctic Ice Sheet (WAIS) instability through Pleistocene glacial/interglacial cycles can provide fundamental constraints on interactions between the climate system and cryosphere. To explore such ice sheet‐ocean‐climate processes on orbital timescales over the last ∼770 ka, we provide continuous records of iceberg‐rafted debris (IRD) content and clay mineralogy, supported by detrital Sr‐Nd isotopes from the pronounced IRD peaks, in gravity core ANT34/A2‐10 from the Amundsen abyssal plain. The IRD record reveals interglacial WAIS instability since ∼770 ka, while comparison to the clay mineralogy record and published records of regional oceanic and atmospheric forcing suggests a temporal link with a strengthened Antarctic Circumpolar Current, enhanced deepwater ventilation, and poleward‐shifted southern westerly winds. In addition, the Sr‐Nd isotope signature of the detrital sediments indicates a shift in provenance around Marine Isotope Stage (MIS) 16, potentially linked to regional oceanic circulation changes. We suggest that an expanded Ross Gyre was important for controlling iceberg trajectories and sediment transport to the site before MIS 16, whereas modern‐like iceberg trajectories were established after MIS 16, probably related to a poleward shift of the Amundsen Sea Low after the end of the Mid‐Pleistocene Transition. This reorganization of the ocean and atmospheric circulation was followed by an interval of enhanced WAIS variability during MIS 15 to 13, which was linked to strong orbital and ocean forcing. These insights into the role of ocean‐atmosphere forcing on the past behavior of the WAIS may improve our framework for understanding future changes in this region.

Tying policy to system: Does the Ross Sea region marine reserve protect transport pathways connecting the life history of Antarctic toothfish?

A central objective of the Ross Sea region Marine Protected Area (MPA) is to protect areas important to the life cycle of Antarctic toothfish (Dissostichus mawsoni), a top fish predator and by far the region’s most important commercial species. Juvenile toothfish predominate in deep basins along the inner continental shelf, whereas adults are found mostly along the continental slope and spawning areas on the Pacific-Antarctic Ridge. The inner basins connect to the continental slope via glacial troughs and predictable transport along each trough results in exchange with the Antarctic Slope Current as it flows westward. From the slope, two transport pathways, an eastern one from Iselin Bank and a western one that turns cyclonically along the flank of the Southeast Indian Ridge, connect northward to the Pacific-Antarctic Ridge, where the northern arm of the Ross Gyre and the Antarctic Circumpolar Current flow eastward. Using a circulation model to compare transport pathways connecting toothfish life history areas, we consider which inshore basins are likely most important in contributing to adult spawning aggregations; how transport pathways from each may be expected to influence distributions along the continental slope and Pacific-Antarctic Ridge; and how zonal transport pathways may promote export to areas downstream of the marine reserve. Although the MPA protects some critical life history pathways for toothfish, others remain vulnerable to commercial fishing, and we argue that those in adjacent areas along the Iselin Bank, Pacific-Antarctic Ridge and the Amundsen Sea might usefully be protected, discussing the range of policy instruments available. We also recommend consideration of transport pathways in deliberations for a proposed network of Southern Ocean MPAs, introducing a system-based tool using chemical tracers in otoliths that can test for toothfish movement between areas connected along the Antarctic Slope Current and Antarctic Circumpolar Current.

Ocean Model Formulation Influences Transient Climate Response

AbstractThe transient climate response (TCR) is 20% higher in the Alfred Wegener Institute Climate Model (AWI‐CM) compared to the Max Planck Institute Earth System Model (MPI‐ESM) whereas the equilibrium climate sensitivity (ECS) is by up to 10% higher in AWI‐CM. These results are largely independent of the two considered model resolutions for each model. The two coupled CMIP6 models share the same atmosphere‐land component ECHAM6.3 developed at the Max Planck Institute for Meteorology (MPI‐M). However, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice‐ocean model developed at MPI‐M and the FESOM sea ice‐ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is related to ocean heat uptake in response to greenhouse gas forcing. Specifically, AWI‐CM simulations show stronger surface heating than MPI‐ESM simulations while the latter accumulate more heat in the deeper ocean. The vertically integrated ocean heat content is increasing slower in AWI‐CM model configurations compared to MPI‐ESM model configurations in the high latitudes. Weaker vertical mixing in AWI‐CM model configurations compared to MPI‐ESM model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell and Ross Gyres and the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI‐CM model configurations and the presence of a warming hole in MPI‐ESM model configurations. All these differences occur largely independent of the considered model resolutions.

The Impact of Sea‐Ice Drift and Ocean Circulation on Dispersal of Toothfish Eggs and Juveniles in the Ross Gyre and Amundsen Sea

AbstractKnowledge about the early life history of Antarctic toothfish (Dissostichus mawsoni) is still incomplete, particularly on the spatial and temporal extent of spawning and the subsequent transport of eggs, larvae, and juveniles from the offshore spawning areas to the continental shelf. This study used a high‐resolution hydrodynamic model to investigate the impact of ocean circulation and sea‐ice drift on the dispersal of eggs, larvae, and juvenile Antarctic toothfish. The virtual eggs were released on seamounts of the Pacific‐Antarctic ridge in the Ross Gyre and advected using hydrodynamical model data. Particles were seeded annually over the years 2002–2016 and tracked for 3 years after their release. Recruitment success was evaluated based on the number of juveniles that reached known coastal recruitment areas, between the eastern Ross and Amundsen Seas, within 3 years. Sensitivities to certain juvenile behaviors were explored and showed that recruitment success was reduced by around 70% if juveniles drifted with sea‐ice during the second winter season as this carried them into the open ocean away from the shelf region. Recruitment success increased during the second winter season if juveniles were entrained in the Ross Gyre circulation or if they actively swam toward the shelf. These modeling results suggest that the ecological advantage of sea‐ice association in the early life cycle of toothfish diminishes as they grow, promoting a behavior change during their second winter.

Thermohaline Suppression of Upper Circumpolar Deep Water Eddies in the Ross Gyre

AbstractTemperature and salinity measurements of a warm‐core eddy at the northern flank of the Ross Gyre are analyzed for dominant mixing mechanisms. The eddy is centered at the depths of the Circumpolar Deep Water and carries heat towards the gyre. Vertical and horizontal heat fluxes out of the eddy associated with internal wave turbulent mixing and thermohaline intrusions are estimated. Upward internal wave turbulent heat flux is W, whereas, the lateral intrusive heat flux is of the order of W. The horizontal flux due to intrusions is suggested to be the dominant mechanism for eddy decay and yields an eddy lifetime of about 6 months. The thermohaline intrusion‐eddy suppression mechanism is proposed and shown to be effective in suppressing the eddy field at the northern flank of the Ross Gyre. This effect has important implications for setting the basin‐wide heat budget and regulating sea‐ice cover.

The Regulation of Sea Ice Thickness by Double‐Diffusive Processes in the Ross Gyre

AbstractNew fine‐scale observations from the central Ross Gyre reveal the presence of double‐diffusive staircase structures underlying the surface mixed layer. These structures are persistent over seasons, with more developed mixed layers within the double‐diffusive staircase in winter months. The sensitivity of the ice formation rate with respect to mixing processes within the main pycnocline (double‐diffusive versus purely turbulent mixing) is investigated with the 1‐D model. A scenario with purely turbulent mixing results in significant underestimates of sea ice thickness. However, a scenario when double‐diffusive mixing operates in the presence of weak shear yields plausible ranges for sea ice thickness that agrees well with the observations. The model results and observations suggest a peculiar feedback mechanism that promotes the self‐maintenance of double‐diffusive staircases. Suppression of the vertical heat fluxes due to the presence of a double‐diffusive staircase, compared to purely turbulent case, allows Upper Circumpolar Deep Water to be more exposed to surface buoyancy fluxes. Our results shed light on the process—double diffusion—that might account for estimated rates of winter water mass transformation in the central Ross Gyre.

Exchange of Water Between the Ross Gyre and ACC Assessed by Lagrangian Particle Tracking

AbstractTo reach upwelling and downwelling zones deep within the Southern Ocean seasonal sea ice cover, water masses must move across the Antarctic Circumpolar Current and through current systems including the Ross Gyre, Weddell Gyre, and Antarctic Slope Current. In this study we focus our attention on the Lagrangian exchange between the Ross Gyre and surrounding current systems. We conducted numerical experiments using five‐day 3‐D velocity fields from the Southern Ocean State Estimate with a particle tracking package to identify pathways by which waters move from near the Antarctic coastal margins or Antarctic Circumpolar Current into the interior of the Ross Gyre, and to identify the time scales of variability associated with these pathways. Waters from near the Antarctic margins enter the Ross Gyre along the western and northern boundaries of gyre until the gyre separates from the Pacific‐Antarctic Ridge near fracture zones. At this juncture, Antarctic Circumpolar Current‐derived inflow dominates the across‐gyre transport up to the Antarctic margin. Transport and exchange associated with different time‐average components of flow are calculated to determine the relative contributions of high‐ and low‐frequency and time‐mean components.

Are the Near-Antarctic Easterly Winds Weakening in Response to Enhancement of the Southern Annular Mode?

AbstractPrevious studies have highlighted the sensitivity of the Southern Ocean circulation to the strengthening, poleward-shifting westerlies, associated with the increasingly positive southern annular mode (SAM). The impacts of the SAM have been hypothesized to weaken momentum input to the ocean from the easterly winds around the Antarctic margins. Using ERA-Interim data, the authors show that the circumpolar-averaged easterly wind stress has not weakened over the past 3–4 decades, and, if anything, has slightly strengthened by around 7%. However, there has been a substantial increase in the seasonality of the easterlies, with a weakening of the easterly winds during austral summer and a strengthening during winter. A similar trend in the seasonality of the easterlies is found in three other reanalysis products that compare favorably with Antarctic meteorological observations. The authors associate the strengthening of the easterly winds during winter with an increase in the pressure gradient between the coast and the pole. Although the trend in the overall easterly wind strength is small, the change in the seasonal cycle may be expected to reduce the shoreward Ekman transport of summer surface waters and also to admit more warm Circumpolar Deep Water to the continental shelf in summer. Changes in the seasonal cycle of the near-coastal winds may also project onto seasonal formation and export of sea ice, fluctuations in the strengths of the Weddell and Ross Gyres, and seasonal export of Antarctic Bottom Water from the continental shelf.

Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves

Melting of West Antarctic ice shelves is enhanced by Circumpolar Deep Water (CDW) intruding onto the Amundsen and Bellingshausen Seas (ABS) continental shelves. Despite existing studies of cross-shelf and on-shelf CDW transports, CDW pathways onto the ABS originating from further offshore have never been investigated. Here, we investigate CDW pathways onto the ABS using a regional ocean model. Simulated CDW tracers from a zonal section across 67°S (S04P) circulate along the Antarctic Circumpolar Current (ACC) and Ross Gyre (RG) and travel into ABS continental shelf after 3–5 years, but source locations are shifted westward by ~900 km along S04P in 2001–2006 compared to 2009–2014. We find that simulated on- and off-shelf CDW is ~0.1–0.2 °C warmer in the 2009–2014 case than in the 2001–2006 case together with changes in simulated ocean circulation. These differences are primarily caused by lateral, rather than surface, boundary conditions, implying that large-scale atmospheric and ocean circulations are able to control CDW pathways and thus off- and on-shelf CDW properties.

Variability of the Ross Gyre, Southern Ocean: Drivers and Responses Revealed by Satellite Altimetry

AbstractYear‐round variability in the Ross Gyre (RG), Antarctica, during 2011–2015, is derived using radar altimetry. The RG is characterized by a bounded recirculating component and a westward throughflow to the south. Two modes of variability of the sea surface height and ocean surface stress curl are revealed. The first represents a large‐scale sea surface height change forced by the Antarctic Oscillation. The second represents semiannual variability in gyre area and strength, driven by fluctuations in sea level pressure associated with the Amundsen Sea Low. Variability in the throughflow is also linked to the Amundsen Sea Low. An adequate description of the oceanic circulation is achieved only when sea ice drag is accounted for in the ocean surface stress. The drivers of RG variability elucidated here have significant implications for our understanding of the oceanic forcing of Antarctic Ice Sheet melting and for the downstream propagation of its ocean freshening footprint.

The Role of Oscillating Southern Hemisphere Westerly Winds: Southern Ocean Coastal and Open-Ocean Polynyas

Abstract An oscillation in intensity of the Southern Hemisphere westerly winds is a major characteristic of the southern annular mode. Its impact upon the sea ice–ocean interactions in the Weddell and Ross Seas is investigated by a sea ice–ocean general circulation model coupled to an energy balance model for three temporal scales and two amplitudes of intensity. It is found that the oscillating wind forcing over the Southern Ocean plays a significant role both in regulating coastal polynyas along the Antarctic margins and in triggering open-ocean polynyas. The formation of coastal polynya in the western Weddell and Ross Seas is enhanced with the intensifying winds, resulting in an increase in the salt flux into the ocean via sea ice formation. Under intensifying winds, an instantaneous spinup within the Weddell and Ross Sea cyclonic gyres causes the warm deep water to upwell, triggering open-ocean polynyas with accompanying deep ocean convection. In contrast to coastal polynyas, open-ocean polynyas in the Weddell and Ross Seas respond differently to the wind forcing and are dependent on its period. That is, the Weddell Sea open-ocean polynya occurs earlier and more frequently than the Ross Sea open-ocean polynya and, more importantly, does not occur when the period of oscillation is sufficiently short. The strong stratification of the Ross Sea and the contraction of the Ross gyre due to the southward shift of Antarctic Circumpolar Current fronts provide unfavorable conditions for the Ross Sea open-ocean polynya. The recovery time of deep ocean heat controls the occurrence frequency of the Weddell Sea open-ocean polynya.

The mean properties and variations of the Southern Hemisphere subpolar gyres estimated by Simple Ocean Data Assimilation (SODA) products

Based on the Simple Ocean Data Assimilation (SODA) products, we study the mean properties and variations of the Southern Hemisphere subpolar gyres (SHSGs) in this paper. The results show that the gyre strengths in the SODA estimates are (55.9±9.8)×106 m3/s for the Weddell Gyre (WG), (37.0±6.4)×106 m3/s for the Ross Gyre (RG), and (27.5±8.2)×106 m3/s for the Australian-Antarctic Gyre (AG), respectively. There exists distinct connectivity between the adjacent gyres and then forms an oceanic super gyre structure in the southern subpolar oceans. And the interior exchanges are about (8.0±3.2)×106 m3/s at around 70°E and (4.3±3.1)×106 m3/s at around 140°E. The most pronounced variation for all three SHSGs occurs on the seasonal time scale, with generally stronger (weaker) SHSGs during austral winter (summer). And the seasonal changes of the gyre structures show that the eastern boundary of the WG and AG extends considerably further east during winter and the interior exchange in the super gyre structure increases accordingly. The WG and RG also show significant semi-annual changes. The correlation analyses confirm that the variations of the gyre strengths are strongly correlated with the changes in the local wind forcing on the semi-annual and seasonal time scales.

Southern Ocean deep convection in global climate models: A driver for variability of subpolar gyres and Drake Passage transport on decadal timescales

AbstractWe investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA‐UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water‐like water mass and affect the large‐scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a precondition for subsequent strong convection activity. The combination of enhanced‐gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport, and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport, and Ross Gyre strength variability are evident in most models, whether or not they host open ocean convection.