An official website of the United States government
Abstract The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing. Significance StatementThis study explores the impact of seafloor bathymetry on the dynamics of subpolar gyres in the Southern Ocean. The subpolar gyres are major circulation features that connect the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. This work provides deeper insight for how the submarine ridges that exist along the northern periphery of these gyres shape the vertical distribution of tracers and overturning circulation in these regions. These findings highlight an underappreciated yet fundamentally important topographical constraint on the three-dimensional cycling of heat and carbon in the Southern Ocean—processes that have far-reaching implications for the global climate. Future work should explore how the presence of these ridges affect the time-evolving response of the Southern Ocean to changes in surface conditions.
more »
« less
Evaluation of MITgcm-based ocean reanalyses for the Southern Ocean
Abstract. Global- and basin-scale ocean reanalyses are becoming easily accessible and are utilized widely to study the Southern Ocean. However, such ocean reanalyses are optimized to achieve the best model–data agreement for their entire model domains and their ability to simulate the Southern Ocean requires investigation. Here, we compare several ocean reanalyses (ECCOv4r5, ECCO LLC270, B-SOSE, and GECCO3) based on the Massachusetts Institute of Technology General Circulation Model (MITgcm) for the Southern Ocean. For the open ocean, the simulated time-mean hydrography and ocean circulation are similar to observations. The MITgcm-based ocean reanalyses show Antarctic Circumpolar Current (ACC) levels measuring approximately 149 ± 11 Sv. The simulated 2 °C isotherms are located in positions similar to the ACC and roughly represent the southern extent of the current. Simulated Weddell Gyre and Ross Gyre strengths are 51 ± 11 and 25 ± 8 Sv, respectively, which is consistent with observation-based estimates. However, our evaluation finds that the time evolution of the Southern Ocean is not well simulated in these ocean reanalyses. While observations showed little change in open-ocean properties in the Weddell and Ross gyres, all simulations showed larger trends, most of which are excessive warming. For the continental shelf region, all reanalyses are unable to reproduce observed hydrographic features, suggesting that the simulated physics determining on-shelf hydrography and circulation is not well represented. Nevertheless, ocean reanalyses are valuable resources and can be used for generating ocean lateral boundary conditions for regional high-resolution simulations. We recommend that future users of these ocean reanalyses pay extra attention if their studies target open-ocean Southern Ocean temporal changes or on-shelf processes.
more »
« less
- PAR ID:
- 10571794
- Publisher / Repository:
- EGU
- Date Published:
- Journal Name:
- Geoscientific Model Development
- Volume:
- 17
- Issue:
- 23
- ISSN:
- 1991-9603
- Page Range / eLocation ID:
- 8613 to 8638
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract We analyze 15‐year of observational data and a 5‐year Southern Ocean model simulation to quantify the transformation rates of Circumpolar Deep Water (CDW) and the associated heat loss to the surface. This study finds that over the continental shelves of East Antarctica and the Weddell and Ross Seas, surface buoyancy fluxes transform ∼4.4 Sv of surface waters into CDW, providing a path for CDW to lose heat to the surface. In addition, ∼6.6 Sv of CDW are mixed with surface waters in the Weddell and Ross subpolar gyres. In contrast, enhanced stratification inhibits the outcropping of CDW isopycnals, reducing their transformation rates by a factor of ∼8 over the continental shelf and by a factor of ∼3 over the deeper ocean in the Amundsen and Bellingshausen Seas. The CDW retains its offshore warm properties as it intrudes over the continental shelves, resulting in elevated bottom temperatures there. This analysis demonstrates the importance of processes in subpolar gyres to erode CDW and to facilitate further transformation on the continental shelves, significantly reducing the heat able to access ice shelf fronts. This sheltering effect is strongest in the western Weddell Sea and tends to diminish toward the east, which helps explain the large zonal differences in continental‐shelf bottom temperatures and the melt rates of Antarctic ice shelves.more » « less
-
null (Ed.)Abstract Observations show that since the 1950s, the Southern Ocean has stored a large amount of anthropogenic heat and has freshened at the surface. These patterns can be attributed to two components of surface forcing: poleward-intensified westerly winds and increased buoyancy flux from freshwater and heat. Here we separate the effects of these two forcing components by using a novel partial-coupling technique. We show that buoyancy forcing dominates the overall response in the temperature and salinity structure of the Southern Ocean. Wind stress change results in changes in subsurface temperature and salinity that are closely related to intensified residual meridional overturning circulation. As an important result, we show that buoyancy and wind forcing result in opposing changes in salinity: the wind-induced surface salinity increase due to upwelling of saltier subsurface water offsets surface freshening due to amplification of the global hydrological cycle. Buoyancy and wind forcing further lead to different vertical structures of Antarctic Circumpolar Current (ACC) transport change; buoyancy forcing causes an ACC transport increase (3.1 ± 1.6 Sv; 1 Sv ≡ 10 6 m 3 s −1 ) by increasing the meridional density gradient across the ACC in the upper 2000 m, while the wind-induced response is more barotropic, with the whole column transport increased by 8.7 ± 2.3 Sv. While previous research focused on the wind effect on ACC intensity, we show that surface horizontal current acceleration within the ACC is dominated by buoyancy forcing. These results shed light on how the Southern Ocean might change under global warming, contributing to more reliable future projections.more » « less
-
Abstract The Southern Ocean plays a major role in controlling the evolution of Antarctic glaciers and in turn their impact on sea level rise. We present the Southern Ocean high‐resolution (SOhi) simulation of the MITgcm ocean model to reproduce ice‐ocean interaction at 1/24° around Antarctica, including all ice shelf cavities and oceanic tides. We evaluate the model accuracy on the continental shelf using Marine Mammals Exploring the Oceans Pole to Pole data and compare the results with three other MITgcm ocean models (ECCO4, SOSE, and LLC4320) and the ISMIP6 temperature reconstruction. Below 400 m, all the models exhibit a warm bias on the continental shelf, but the bias is reduced in the high‐resolution simulations. We hypothesize some of the bias is due to an overestimation of sea ice cover, which reduces heat loss to the atmosphere. Both high‐resolution and accurate bathymetry are required to improve model accuracy around Antarctica.more » « less
-
null (Ed.)A part of the Southern Ocean, the Ross Sea, together with the Ross Ice Shelf and the atmosphere over the region represent a coupled system with respect to the low-frequency (with the periods longer than 1 hour) wave processes observed in the three media. We study interconnections between them using a unique combination of geophysical sensors: hydrophones measuring pressure variations on the bottom of the open ocean, seismographs measuring vertical displacements of the surface of the Ross Ice Shelf, and the Jang Bogo Dynasonde system measuring wave parameters at the altitudes of the lower thermosphere. Analysis of a year-long data sets from Ross Ice Shelf-based instruments reveals presence in their average power spectra of the peaks in the 2-11 hours period range that may be associated with the low-order resonance vibrations of the system. More harmonics of the 24 hour tide (seven) are detected by the RIS seismographs compared to the sea floor sensor (where only two are clearly visible). This may be a consequence of the RIS resonance-related broadband amplification effect predicted by our model. There are several peaks in the RIS vibration spectrum (T = 8.37, 8.23, 6.3 and 6.12 hours) that are not detected by the hydrophone and may be directly related to RIS resonances. The prominent T = 25.81 hour peak is a likely candidate for the sub-inertial RIS resonance. The periods of lower RIS resonance modes predicted by our simple model and the observed spectral peaks are in the same general band. This is the first direct observation of the resonance effects in vibrations of the Ross Ice Shelf. Our results demonstrate the key role of the resonances of the Ross Ice Shelf in maintaining the wave activity in the entire coupled system. We suggest that the ocean tide is a major source of excitation of the Ross Ice Shelf’s resonances. The ice shelf vibrations may also be supported by the energy transfer from wind, swell, and infragravity wave energy that couples with the ice shelf. Overlapping 6-month-long data sets reveal a significant linear correlation between the spectra of the vertical shifts of the Ross Ice Shelf and of the thermospheric waves with the periods of about 2.1, 3.7, and 11.1 hours. This result corroborates earlier lidar observations of persistent atmospheric wave activity over McMurdo. We propose a theory that quantifies the nexus between the ocean tide and the resonance vibrations of the Ross Ice Shelf. It complements the theoretical model of the process of generating the atmospheric waves by the resonance vibrations of the Ross Ice Shelf published by us earlier.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gmd-17-8613-2024
