| 1. |
- de Boer, Agatha M., et al.
(författare)
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The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front
- 2013
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Ingår i: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 118:10, s. 5669-5675
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Tidskriftsartikel (refereegranskat)abstract
- In recent years the latitudinal position of the Subtropical Front (STF) has emerged as a key parameter in the global climate. A poleward positioned front is thought to allow a greater salt flux from the Indian to the Atlantic Ocean and so drive a stronger Atlantic Meridional Overturning Circulation. Here the common view that the STF aligns with the zero wind stress curl (WSC) is challenged. Based on the STF climatologies of Orsi et al. (1995), Belkin and Gordon (1996), Graham and De Boer (2013), and on satellite scatterometry winds, we find that the zero WSC contour lies on average ∼10°, ∼8°, and ∼5° poleward of the front for the three climatologies, respectively. The circulation in the region between the Subtropical Gyres and the zero WSC contour is not forced by the WSC but rather by the strong bottom pressure torque that is a result of the interaction of the Antarctic Circumpolar Current with the ocean floor topography. The actual control of the position of the STF is crucially dependent on whether the front is regarded as simply a surface water mass boundary or a dynamical front. For the Agulhas Leakage problem, the southern boundary of the so-called Super Gyre may be the most relevant property but this cannot easily be identified in observations.
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| 2. |
- Graham, Robert M., et al.
(författare)
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Southern Ocean fronts : Controlled by wind or topography?
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117
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Tidskriftsartikel (refereegranskat)abstract
- The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Here we explore the relative importance of bottom topography and winds for the location of Southern Ocean fronts, using 100 years of a control and climate change simulation from the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3 degrees in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current's (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the Subtropical Front (STF) shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography. An assessment of different methods for identifying jets reveals that maxima of gradients in the sea surface height field are the most reliable. Approximating the position of fronts using sea surface temperature gradients is ineffective at high latitudes while using sea surface height contours can give misleading results when studying the temporal variability of front locations.
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| 3. |
- Graham, Robert M., 1988-, et al.
(författare)
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The Dynamical Subtropical Front
- 2013
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Ingår i: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 118:10, s. 5676-5685
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and sea surface height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore, we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and is characterized by strong SST and SSH gradients and no seasonal cycle. At the center of each basin it merges with the Sub-Antarctic Front. On the eastern side of basins, the STF surface water mass boundary coincides with a separate region of multiple SST fronts. We call this the Subtropical Frontal Zone (STFZ). The fronts in the STFZ have a large seasonal cycle and no SSH signature. Despite lying close to the same water mass boundary, the DSTF and STFZ are completely unrelated. We therefore suggest the term STF only be used when referring to the surface water mass boundary. When studying the strong currents on the western side of basins the term DSTF is more relevant and, similarly, the term STFZ better describes the region of enhanced SST gradients towards the east.
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| 4. |
- Graham, Robert M., 1988-
(författare)
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The Location and Variability of Southern ocean Fronts
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. Moreover, the Subtropical Front (STF) is believed play a key role in the global climate system. Model simulations have shown that a wind induced poleward shift of the STF may strengthen the Atlantic Meridional Overturning Circulation by allowing a stronger salt flux from the Indian to the Atlantic Ocean. This hypothesis has important implications for our future climate, as global warming scenarios predict an intensification and southward shift of the Southern Hemisphere Westerlies. Nonetheless, confirmation of the theory has been limited by a lack of data and also our poor dynamical understanding of fronts. In this thesis we produce a new working dynamical definition of the STF and study the relation of this and other Southern Ocean fronts to the winds and topography.We first explore the relative importance of bottom topography and winds for determining the location and structure of Southern Ocean fronts, using 100 years of a control and climate change simulation on the high resolution coupled climate model HiGEM. Topography has primary control on the number and intensity of fronts at each longitude. However, there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography. The Southern Hemisphere Westerlies intensify and shift south by 1.3° in the climate change simulation, but there is no comparable meridional displacement of the Antarctic Circumpolar Current’s (ACC) path or the fronts within its boundaries, even over flat topography. Instead, the current contracts meridionally and weakens. North of the ACC, the STF shifts south gradually, even over steep topographic ridges. We suggest the STF reacts more strongly to the wind shift because it is strongly surface intensified. In contrast, fronts within the ACC are more barotropic and are therefore more sensitive to the underlying topography.We then use satellite sea surface temperature (SST) data to show that the traditional STF, as defined by water mass properties, is comprised of two distinct dynamical regimes. On the western side of each basin the traditional STF coincides with a deep current that has strong SST gradients and no seasonal cycle. We define this as the Dynamical STF (DSTF). Further east, the DSTF diverges from the traditional STF and tracks south-eastwards into the centre of each basin to merge with the Sub-Antarctic Front. The traditional STF continues to the eastern side of the basins where it coincides with the so-called Subtropical Frontal Zone, a zone of shallow SST fronts that have little transport and large seasonal cycles.Finally, we compare the position of our DSTF and previous STF climatologies to the mean wind stress curl field, from satellite scatterometry winds. We find that contrary to previous suggestions, the position of the STF does not coincide with the zero or maximum wind stress curl. Using output from the HiGEM model we show that instead of being controlled purely by the wind field, transport south of the subtropical gyre, including the latitude of the zero wind stress curl, is forced strongly by the bottom pressure torque that is a product of the interaction of the ACC with the ocean floor topography.Here in these studies we have provided a new simple and reproducible method for identifying fronts. We have also given new insights into the seasonal and decadal variability of fronts, as well as how fronts may respond to future climate change. This has highlighted previous misconceptions regarding the relationship between the position of fronts and winds. Finally we have provided a new framework to study the behaviour of the STF and interpret observations, paving the way for better predictions on the likelihood and impact of future STF changes.
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| 5. |
- Schuster, U., et al.
(författare)
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Measurements of total alkalinity and inorganic dissolved carbon in the Atlantic Ocean and adjacent Southern Ocean between 2008 and 2010
- 2014
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Ingår i: Earth System Science Data. - : Copernicus GmbH. - 1866-3508 .- 1866-3516. ; 6:1, s. 175-183
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Tidskriftsartikel (refereegranskat)abstract
- Water column dissolved inorganic carbon and total alkalinity were measured during five hydrographic sections in the Atlantic Ocean and Drake Passage. The work was funded through the Strategic Funding Initiative of the UK's Oceans2025 programme, which ran from 2007 to 2012. The aims of this programme were to establish the regional budgets of natural and anthropogenic carbon in the North Atlantic, the South Atlantic, and the Atlantic sector of the Southern Ocean, as well as the rates of change of these budgets. This paper describes in detail the dissolved inorganic carbon and total alkalinity data collected along east-west sections at 47 degrees N to 60 degrees N, 24.5 degrees N, and 24 degrees S in the Atlantic and across two Drake Passage sections. Other hydrographic and biogeochemical parameters were measured during these sections, and relevant standard operating procedures are mentioned here. Over 95% of dissolved inorganic carbon and total alkalinity samples taken during the 24.5 degrees N, 24 degrees S, and the Drake Passage sections were analysed onboard and subjected to a first-level quality control addressing technical and analytical issues. Samples taken along 47 degrees N to 60 degrees N were analysed and subjected to quality control back in the laboratory. Complete post-cruise second-level quality control was performed using crossover analysis with historical data in the vicinity of measurements, and data were submitted to the CLIVAR and Carbon Hydrographic Data Office (CCHDO), the Carbon Dioxide Information Analysis Center (CDIAC) and and will be included in the Global Ocean Data Analyses Project, version 2 (GLODAP 2), the upcoming update of Key et al. (2004).
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| 6. |
- Sime, Louise C., et al.
(författare)
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Southern Hemisphere westerly wind changes during the Last Glacial Maximum : model-data comparison
- 2013
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 64, s. 104-120
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Hemisphere (SH) westerly winds are thought to be critical to global ocean circulation, productivity, and carbon storage. For example, an equatorward shift in the winds, though its affect on the Southern Ocean circulation, has been suggested as the leading cause for the reduction in atmospheric CO2 during the Last Glacial period. Despite the importance of the winds, it is currently not clear, from observations or model results, how they behave during the Last Glacial. Here, an atmospheric modelling study is performed to help determine likely changes in the SH westerly winds during the Last Glacial Maximum (LGM). Using LGM boundary conditions, the maximum in SH westerlies is strengthened by similar to+1 m s(-1) and moved southward by similar to 2 degrees at the 850 hPa pressure level. Boundary layer stabilisation effects over equatorward extended LGM sea-ice can lead to a small apparent equatorward shift in the wind band at the surface. Further sensitivity analysis with individual boundary condition changes indicate that changes in sea surface temperatures are the strongest factor behind the wind change. The HadAM3 atmospheric simulations, along with published PMIP2 coupled climate model simulations, are then assessed against the newly synthesised database of moisture observations for the LGM. Although the moisture data is the most commonly cited evidence in support of a large equatorward shift in the SH winds during the LGM, none of the models that produce realistic LGM precipitation changes show such a large equatorward shift. In fact, the model which best simulates the moisture proxy data is the HadAM3 LGM simulation which shows a small poleward wind shift. While we cannot prove here that a large equatorward shift would not be able to reproduce the moisture data as well, we show that the moisture proxies do not provide an observational evidence base for it.
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| 7. |
- Thomas, M. D., et al.
(författare)
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Upper ocean manifestations of a reducing meridional overturning circulation
- 2012
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L16609-
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Tidskriftsartikel (refereegranskat)abstract
- Atlantic Meridional Overturning Circulation during the 21st century. Using a 100 year climate change integration of a high resolution coupled climate model, we show that a 5.3 Sv reduction in the deep southward transport in the subtropical North Atlantic is balanced solely by a weakening of the northward surface western boundary current, and not by an increase in the southward transport integrated across the interior ocean away from the western boundary. This is consistent with Sverdrup balance holding to a good approximation outside of the western boundary region on decadal time scales, and may help to spatially constrain past and future change in the overturning circulation. The subtropical gyre weakens by 3.4 Sv over the same period due to a weakened wind stress curl. These changes combine to give a net 8.7 Sv reduction in upper western boundary transport. Citation: Thomas, M. D., A. M. de Boer, D. P. Stevens, and H. L. Johnson (2012), Upper ocean manifestations of a reducing meridional overturning circulation, Geophys. Res. Lett., 39, L16609, doi:10.1029/2012GL052702.
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| 8. |
- De Boer, Agatha M., et al.
(författare)
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Control of the glacial carbon budget by topographically induced mixing
- 2014
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:12, s. 4277-4284
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Tidskriftsartikel (refereegranskat)abstract
- Evidence for the oceanic uptake of atmospheric CO2 during glaciations suggests that there was less production of southern origin deep water but, paradoxically, a larger volume of southern origin water than today. Here we demonstrate, using a theoretical box model, that the inverse relationship between volume and production rate of this water mass can be explained by invoking mixing rates in the deep ocean that are proportional to topographic outcropping area scaled with ocean floor slope. Furthermore, we show that the resulting profile, of a near-linear decrease in mixing intensity away from the bottom, generates a positive feedback on CO2 uptake that can initiate a glacial cycle. The results point to the importance of using topography-dependent mixing when studying the large-scale ocean circulation, especially in the paleo-intercomparison models that have failed to produce the weaker and more voluminous bottom water of the Last Glacial Maximum.
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| 9. |
- de Boer, Agatha M., et al.
(författare)
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Processes driving thunderstorms over the Agulhas Current
- 2013
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - : American Geophysical Union (AGU). - 2169-897X. ; 118:5, s. 2220-2228
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Tidskriftsartikel (refereegranskat)abstract
- Lightning occurs predominantly over land and is not common over the open ocean. We study here one oceanic region in which thunderstorms are frequently found, namely the warm Agulhas Current off the southeast coast of South Africa. The seasonal and interannual lightning variability is derived from satellite and terrestrial data sets. Favorable climatic conditions for lightning are investigated using both ERA-Interim and NCEP/NCAR reanalysis data. We find peak lightning in austral autumn over the Agulhas Current but with low seasonality (i.e., there is considerable lightning throughout the year). While the climatological wind direction varies strongly with latitude and season, the wind direction is predominantly northerly throughout the region during thunderstorms. A composite of sea level pressure during thunderstorm days indicates that thunderstorms are related to eastward-propagating synoptic-scale wave trains passing through the Agulhas Current region. The strong convective activity during thunderstorms occur in the warm sector of a cyclone and is associated with horizontal convergence and lifting of warm, moist surface air originating over the warm Agulhas Current.
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| 10. |
- Graham, Robert M., 1988-
(författare)
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The role of Southern Ocean fronts in the global climate system
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts.A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary.A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl.Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front.
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