This dissertation compares high‑resolution marine multi‑proxy data sets from sediment archives in the Southern Ocean offshore Dronning Maud Land (DML) with proxy data and model results from the subtropical Atlantic to reconstruct atmosphere–ocean–cryosphere interactions and dynamics on millennial to orbital timescales during the last 75,000 years. The present study examines trans-regional linkages and feedbacks between subtropical and polar changes in ocean and atmospheric circulation, which had particular impacts on the variability of Antarctic meltwater input, ocean stratification, the formation of Antarctic bottom water, and the position and dynamics of the Atlantic subtropical gyres. The variability of these components reveals a close relationship, on the one hand, with cyclical fluctuations in solar radiation, and on the other, with a pattern of climate variability that closely resembles today's Southern Annular Mode (SAM). During the last glacial period, changes in properties of the upper water column off the coast of DML above Bungenstock Plateau imply that variations in the thermohaline structure between Antarctic Surface Water and modified Circumpolar Deep Water (mCDW) may have resulted in either strengthening the stratification of the upper water column or promoting polynya formation (convective overturning). Subsurface ocean warming, elevated salinity and nutrient content indicate reduced density stratification and recurrent polynya formation, particularly during Antarctic stadials and intervals of low obliquity. Orbital- to millennial-scale shifts in upper-ocean stratification are driven by coupled atmosphere–ice–ocean dynamics that are very similar to the primary patterns of modern SAM. In the period from ~38–20 ka, when the Antarctic ice masses advanced to the edge of the continental slope, reconstructions suggest a hybrid polynya mode that combines different formation characteristics of coastal polynyas and open ocean polynyas. The polynya-driven increase in oceanic heat loss, moisture supply and snowfall may have contributed to the thickening of the ice sheet along the continental margin. For the period of the last deglaciation, the reconstruction of upper ocean salinity changes offshore DML at Bungenstock Plateau indicates a two-phase pattern of increased upper-ocean freshening: from ~15.5 – 14 ka (terminating with MWP-1A) and even more pronounced from 13.5 – 11.5 ka (terminating with MWP-1B). The associated minima in upper ocean salinity have been ascribed to Antarctic meltwater input, produced by iceberg calving and basal melting at the ice shelf/ocean interface, while the advection of warm mCDW onto the continental shelves and cavities beneath the ice shelf is considered the main driver of basal melting. The two phases of increased meltwater discharge most likely intensified upper ocean density stratification in the Antarctic Zone, thereby inhibiting the formation of Antarctic Bottom Water (AABW) and deep Southern Ocean overturning. In the Holocene, from 11.5 - 8 ka, the proxy data then indicate further shallowing and strengthened presence of mCDW, which may have helped to revive AABW formation. These cryospheric, atmospheric and oceanic changes in the Atlantic sector of Antarctica and the Southern Ocean have been compared with low-latitude meridional shifts of the North and South Atlantic subtropical gyres (NASG, SASG) over the past 22 ka in order to reveal possible trans-regional linkages, forcing mechanisms and feedbacks. Both gyres migrated poleward since the last ice age, with the SASG shifting ~1.5 kyr ahead of the NASG , consistent with an early response of the South Atlantic in the context of the ‘bipolar seesaw theory’. North Atlantic data show a shift of the northern NASG boundary by more than >6° northward. Complementary model experiments link these gyre migrations to a reorganization of meridional temperature gradients, which is consistent with changes in upper ocean stratification, meltwater input, and Southern Ocean overturning circulation inferred from the sediment archive offshore DML. Orbital variability modulates the climate background state and thus also shapes trans-regional changes and linkages between subtropical and polar variations in oceanic and atmospheric circulation. This study used climate model simulations to show that small variations in Earth–Sun distance reorganize seasonal insolation and meridional temperature gradients, producing systematic circulation shifts: closer Earth–Sun distance drives poleward displacement of atmospheric and oceanic circulation, whereas greater distance drives equatorward contraction. In the present configuration, perihelion during boreal winter corresponds to a poleward‑shifted winter circulation. Orbital precession alters the season of perihelion, imposing a seasonally varying circulation drift that can reach ~10° under high eccentricity. The results show how orbital geometry sets the latitude of large‑scale circulation, providing a mechanistic context for Southern Ocean hydrographic and cryospheric variability. My current studies also suggest that a high obliquity state (large tilt of the Earth’s axis) leads to a strong seasonal poleward shift of the SASG, which has intensified the transport of heat and moisture to the study area offshore DML, especially during the southern summer. The extent to which this intensified freshwater input and stratification of the upper ocean influenced the absence of polynyas remains to be clarified. At low obliquity, a contracted, equatorward SASG still enhances moisture transport but shifts it to winter, promoting snow accumulation and ice‑sheet growth, which in turn favor glacial polynya formation.
Tainã M. L. Pinho (Fri,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: