This paper describes the mean ocean circulation and the tropical variability simulated by the Max Planck Institute for Meteorology (MPI-M) coupled atmosphere–ocean general circulation model (AOGCM). Results are presented from a version of the coupled model that served as a prototype for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) simulations. The model does not require flux adjustment to maintain a stable climate. A control simulation with present-day greenhouse gases is analyzed, and the simulation of key oceanic features, such as sea surface temperatures (SSTs), large-scale circulation, meridional heat and freshwater transports, and sea ice are compared with observations. A parameterization that accounts for the effect of ocean currents on surface wind stress is implemented in the model. The largest impact of this parameterization is in the tropical Pacific, where the mean state is significantly improved: the strength of the trade winds and the associated...

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Ocean Circulation and Tropical Variability in the Coupled Model ECHAM5/MPI-OM

Roughly 90% of atmospheric ozone is found in the lower stratosphere in the ozone layer Since about the 1970s, anthropogenic emissions of ozone-depleting gases have led to depletion of ~3–4% of the total overhead ozone averaged over the globe 1  The strongest depletion is found over Antarctica during spring, when photochemical processes combine with a unique set of meteorological conditions to greatly increase the effectiveness of ozone-depleting gases, and more than half of the total overhead ozone is destroyed Characteristics of the resulting Antarctic ozone hole are reviewed in refs 1 and 2, and the identification and attribution of the phenomenon was recently celebrated in a special edition of Nature (http://wwwnaturecom/nature/focus/ ozonehole/) The Antarctic ozone hole is evident in ozone observations taken every spring since about the early 1980s 1  Its annual onset coincides with the return of sunlight to the cold polar stratosphere during September/October, and its decay with the collapse of the stratospheric vortex during November/December 1,2  The most obvious surface impact is an increase in ultraviolet radiation reaching the surface 1  Over the past decade, however, it has become clear that the ozone hole is also associated with widespread changes in the Southern Hemisphere tropospheric circulation and surface climate Our purpose here is to review the evidence that suggests that the Antarctic ozone hole has had a demonstrable effect on the surface climate of the Southern Hemisphere The ozone hole and Southern Hemisphere circulation Ozone absorbs incoming solar radiation Hence the depletion of ozone over Antarctica leads to cooling of the polar stratosphere 2,3

https://www.atmos.colostate.edu/~davet/ao/ThompsonPapers/Thompson_etal_NatureGeoscience2011.pdf

Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change

Iron controls phytoplankton growth in large tracts of the global ocean, and thereby influences carbon dioxide drawdown. Recent advances reveal the importance of iron-binding ligands and organic matter remineralization in regulating ocean iron levels.

/pdf/the-biogeochemical-cycle-of-iron-in-the-ocean-22ruuiyko1.pdf

The biogeochemical cycle of iron in the ocean

AbyssalglobalanddeepSouthern Oceantemperature trendsarequantifiedbetweenthe1990sand2000sto assesstheroleofrecentwarmingoftheseregionsin globalheatandsealevelbudgets.Theauthors1)compute warming rates with uncertainties along 28 full-depth, high-quality hydrographic sections that have been occupied two or more times between 1980 and 2010; 2) divide the global ocean into 32 basins, defined by the topography and climatological ocean bottom temperatures; and then 3) estimate temperature trends in the 24 sampled basins. The three southernmost basins show a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic, and eastern Indian Oceans. Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends. Excepting the Arctic Ocean and Nordic seas, the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0.027 (60.009) W m 22 applied over the entire surface of the earth. Deep (1000‐4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0.068 (60.062) W m 22 . The abyssal warming produces a 0.053 (60.017) mm yr 21 increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0.093 (60.081)mm yr 21 . Thus, warmingin theseregions,ventilatedprimarilyby AntarcticBottomWater,accounts for a statistically significant fraction of the present global energy and sea level budgets.

Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets*

The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.

/pdf/antarctic-climate-change-and-the-environment-3mdc1ymkfh.pdf

Antarctic climate change and the environment

[1] High resolution hydrographic sections and maps of the gradient of sea surface height (SSH) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. Here we use a 15 year time series of SSH observations to determine the circumpolar structure and distribution of the ACC fronts. The jets are consistently aligned with particular streamlines along the entire circumpolar path, confirming and extending the results of an earlier study restricted to the region south of Australia. The intensity of the fronts (as measured by the cross-front gradient of SSH) varies along the fronts and the individual branches merge and diverge, often in response to interactions with bathymetry. Maps of absolute velocity at 1000 m depth derived from Argo trajectories confirm the existence of multiple current cores throughout the Southern Ocean. High resolution hydrographic sections and profiles of temperature and salinity from Argo floats are used to show that the front locations derived from fitting SSH contours to maps of SSH gradient are consistent with locations inferred from the traditional criteria based on water mass properties, suitably modified to account for multiple frontal branches. Three regions are examined in detail: the Crozet Plateau, the Kerguelen Plateau and the Scotia Sea. These examples show how recognition of the multiple jets of the ACC can help resolve discrepancies between previous studies of ACC fronts.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JC005108

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths

http://www.vliz.be/imisdocs/publications/59014.pdf

Structure of Southern Ocean fronts at 140°E

[1] In Part 1 of this study, we showed that the Antarctic Circumpolar Current (ACC) consisted of multiple fronts, each of which was consistently associated with a particular contour of sea surface height (SSH) or approximate streamline. In Part 2 we have used maps of SSH to examine the variability of the ACC fronts between 1992 and 2007. The SSH label associated with each frontal branch is nearly constant around the circumpolar belt. The front labels are also nearly constant in time: the bands of enhanced SSH gradient (i.e., fronts) occur along the same streamlines throughout the 15 year period of observations. Both short- and long-period changes of the SSH frontal labels of the ACC are small. Based on a tight relationship between dynamic height and cumulative baroclinic transport of the ACC, the baroclinic transport variability of the individual branches of the ACC is also expected to be small. The major change in the total ACC baroclinic transport occurs in the Drake Passage. The streamline associated with the northern branch of the SAF (SAF-N) does not pass through Drake Passage and the waters carried by this branch are not observed there. Instead, the transport of the SAF-N turns north in the Pacific to supply the export of water to the Indian Ocean north and south of Australia. Strong eddy activity in the southeast Pacific acts to dissipate the hydrographic signature of the SAF-N there. In the Atlantic, the SAF-N reappears as the same streamline is again associated with enhanced SSH gradients to the east of the Brazil – Malvinas confluence zone. While the large changes in SSH have occurred in the Southern Ocean between 1992 and 2007, there are strong regional differences. Because the ACC fronts are robustly associated with particular SSH contours, the changes in SSH reflect shifts in the position of the ACC fronts. In the circumpolar average, each of the ACC fronts has shifted to the south by about 60 km. The changes in SSH in the Southern Ocean are largely due to changes in ocean circulation, rather than warming and freshening by atmospheric fluxes. Much larger changes in SSH are observed in some locations of the Southern Ocean, particularly where the fronts interact with large-scale topography. The northern branch of the PF (PF-N) near the Kerguelen Plateau is an extreme example, where the PF-N followed a path around the northern end of Kerguelen Plateau between 1992 and 1997, passed through the Fawn Trough after 2003, and oscillated between the two paths between 1997 and 2003.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JC005248

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 2. Variability and relationship to sea surface height

[1] We reexamine the relationship between circulation, bathymetry, and surface chlorophyll in the Southern Ocean, using new high-resolution maps of the frontal structure of the Antarctic Circumpolar Current (ACC) derived from satellite altimetry. The maps reveal that the ACC consists of multiple filaments or jets. By averaging surface chlorophyll measurements along streamlines, we show that the fronts define the limits of zones with similar concentrations and seasonality of surface chlorophyll. The overall pattern of surface chlorophyll is consistent with strongest upwelling of nutrient-rich deep water south of the Polar Front (PF). However, the distribution of chlorophyll in the Southern Ocean is concentrated in a number of persistent blooms, observed downstream of islands and bathymetric features. In contrast to previous studies, we find little evidence that the fronts of the ACC are associated with enhanced productivity, at least where the fronts are distant from topography. Rather, we find that most regions of elevated chlorophyll in the open Southern Ocean can be explained by upwelling of nutrients (both macronutrients and micronutrients) where the ACC interacts with topography, followed by downstream advection. The upwelling is shown to be the consequence of the bottom pressure torque established by the large-scale flow, rather than being due to small-scale instabilities of the jets. The interaction of the flow with the topography therefore establishes both the large-scale dynamical balance of the ACC and determines the productivity of the open Southern Ocean.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JC004072

On the relationship between fronts of the Antarctic Circumpolar Current and surface chlorophyll concentrations in the Southern Ocean

Baroclinic transport variability of the Antarctic Circumpolar Current (ACC) near 140°E is estimated from six occupations of a repeat section occupied as part of the World Ocean Circulation Experiment (WOCE section SR3). The mean top-to-bottom volume transport is 147±10 Sv (mean ±1 standard deviation), relative to a deep reference level consistent with water mass properties and float trajectories. The location and transport of the main fronts of the ACC are relatively steady: the Subantarctic Front carries 105±7 Sv at a mean latitude between 51° and 52°S; the northern branch of the Polar Front carries 5±5 Sv to the east between 53° and 54°S; the southern Polar Front carries 24±3 Sv eastward at 59°S; and two cores of the southern ACC front at 62° and 64°S carry 18±3 and 11±3 Sv, respectively. The variability in net property transports is largely due to variability of currents north of the ACC in particular, an outflow of 8±13 Sv of water from the Tasman Sea and a deep anticyclonic recirculation carrying 22±8 Sv in the Subantarctic Zone. Variability of net baroclinic volume transport is similar in magnitude to that measured at Drake Passage. In density layers, transport variability is small in deep layers, but significant (range of 4 to 16 Sv) in the Subantarctic Mode Water. Variability of eastward heat transport across SR3 is significant (range of 139°C Sv, or 0.57 x 10 15 W, relative to 0°C) and large relative to meridional heat flux in the Southern Hemisphere subtropical gyres. Heat transport changes are primarily due to variations in the westward flow of relatively warm water across the northern end of the section. Weak (strong) westward flow and large (small) eastward heat flux coincides with equatorward (poleward) displacements of the latitude of zero wind stress curl.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JC900107

Serguei Sokolov

Papers

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths

Structure of Southern Ocean fronts at 140°E

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 2. Variability and relationship to sea surface height

On the relationship between fronts of the Antarctic Circumpolar Current and surface chlorophyll concentrations in the Southern Ocean

Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3)