ALBERT

Description: The subarctic North Pacific and its marginal seas constitute a key area in which rapid environmental changes over the past decades have been observed in instrumental records, like sea ice decreases, or alterations of nutrient inventories and oxygenation of mid-depth water masses. However, knowledge about the past climatic and oceanographic variability beyond instrumental time series in the subarctic North Pacific and its marginal seas is limited. Few temporally and spatially well-resolved high-resolution and spatially well datasets exist, with spatial and temporal coverage being insufficient to gain a detailed picture of past variations. Our proxydata-based study focuses on a collection of sediment records from the Okhotsk Sea as major source area for well ventilated North Pacific Intermediate Water (NPIW) that cover the last ca. 12,000 years with high temporal and adequate spatial resolution. We decipher rapid changes in NPIW ventilation patterns on centennial to millennial time scales and show that the current ventilation of the mid-depth North Pacific has likely only been prevalent for the last 2 ka. We further provide evidence for a Mid-Holocene shift in mid-depth NPIW ventilation characteristics. Additionally, changes in North Atlantic Deep Water flow speed and patterns are reflected in our records of North Pacific mid-depth water mass dynamics, thus indicating a hemispheric-wide connection between the Atlantic and Pacific regions during the Holocene. Planktic oxygen isotope data suggest a high variability in the stratification of local surface water masses and the formation of sea ice, influencing the formation of new, well ventilated water masses near to our core sites. We compare the main Holocene baseline changes evidenced in our proxy reconstructions to Early Holocene and Pre-Industrial time slice results from the fully-coupled MPI-ESM (COSMOS) Earth System Model, with a focus on the Pacific Ocean to better understand NPIW and upper ocean dynamic changes.

Description: Abrupt decadal climate changes during the last glacial-interglacial cycle are less pronounced during maximum glacial conditions and absent during the Holocene. To further understand the underlying dynamics, we conduct hosing experiments for three climate states: Pre-industrial (PI), 32 kilo years before present (ka BP) and Last Glacial Maximum (LGM). Our simulations show that a stronger temperature inversion between the surface and intermediate layer in the South Labrador Sea induces a faster restart of convective processes (32 ka BP 〉 LGM 〉 PI) during the initial resumption of the Atlantic meridional overturning circulation (AMOC). A few decades later, an AMOC overshoot is mainly linked to the advection of warmer and saltier intermediate-layer water from the tropical Atlantic into the South Labrador Sea, which causes a stronger deep-water formation than that before the freshwater perturbation. This mechanism is most pronounced during the 32 ka BP, weaker during the LGM and absent during the PI.

Description: Due to the lack of data, the extent, thickness and drift patterns of sea ice and icebergs in the glacial Arctic remains poorly constrained. Earlier studies are contradictory proposing either a cessation of the marine cryosphere or an ice drift system operating like present-day. Here we examine the marine Arctic cryosphere during the Last Glacial Maximum (LGM) using a high-resolution, regional ocean-sea ice model. Whereas modern sea ice in the western Arctic Basin can circulate in the Beaufort Gyre for decades, our model studies present an extreme shortcut of glacial ice drift. In more detail, our results show a clockwise sea-ice drift in the western Arctic Basin that merges into a direct trans-Arctic path towards Fram Strait. This is consistent with dated ice plow marks on the seafloor, which show the orientation of iceberg drift in this direction. Also ice-transported iron-oxide grains deposited in Fram Strait, can be matched by their chemical composition to similar grains found in potential sources from the entire circum-Arctic. The model results indicate that the pattern of Arctic sea-ice drift during the LGM is established by wind fields and seems to be a general feature of the glacial ocean. Our model results do not indicate a cessation in ice drift during the LGM.

Description: The climate during the last glacial-interglacial cycle exhibits distinct climate states and variability in various time scales with different spatial characteristics. These changes occur for natural reasons, but their mechanisms are not well understood. Compared to the research on present-day climate, which involves influences of human activity, the investigation of the climate during the last glacial-interglacial cycle can attribute to discover the underlying process of natural climate change, and assistant us to have a better prediction of future climate. Additionally, in comparison to studies on proxies, climate models provide a simplified numerical representation of dynamical and thermodynamical processes governing different components of the Earth’s climate system, which is not able to be recorded in proxy data. In this dissertation work, our first scientific focus is to clarify the mechanistic effects of a higher Northern Hemisphere ice sheet on large-scale North Atlantic Ocean surface circulation and Atlantic Meridional Overturning Circulation (AMOC) during glacial climate periods. We use the Community Earth System Models (COSMOS) to simulate five representative climate states during the last glacial-interglacial cycle: the Eemian interglacial, Mid Holocene, Pre-industrial (PI), stadial Marine Isotope Stage3 (MIS3), presented by 32 kilo years before present (ka B.P.), and Last Glacial Maximum (LGM). We have examined mean climatological states and variability of major large-scale North Atlantic Ocean surface circulation elements, including the Subtropical Gyre (STG), Subpolar Gyre (SPG), and Gulf Stream. Our results show that the existing Laurentide Ice Sheet and the elevated Greenland Ice Sheet induce increased surface winds over the North Atlantic Ocean during the LGM and MIS3, which subsequently enhance the North Atlantic gyres and the Gulf Stream. In addition, statistical analysis suggests that the correlation between AMOC and surface winds is increased during glacial climate states. The second part of our work is targeted at the explanation of the difference of abrupt decadal climate changes during the last glacial-interglacial cycle. As documented in Greenland ice cores, abrupt decadal climate changes are less pronounced during maximum glacial conditions and strongly suppressed during the Holocene. We conduct hosing experiments for three different climate states during the last glacial-interglacial cycle (PI, 32 ka B.P. and the LGM). Our results show that the freshening of the surface North Atlantic Ocean leads to a similar reduction of the AMOC due to the freshwater perturbation, independent of the background climate. However, the subsequent recovery stages show distinct tempo-spatial characteristics, with respect to the initial AMOC resumption and the strength of a superposed AMOC overshoot. During the initial AMOC resumption, a stronger temperature inversion between the surface and intermediate layer (200-800 m) in the South Labrador Sea induces a quicker restart of convective processes (32ka B.P. 〉 LGM 〉 PI). A few decades later, an AMOC overshoot is caused by the advection of warmer and saltier tropical Atlantic Ocean water into the South Labrador Sea. In case of a glacial climate background, this provides a strong positive feedback on the initial resumption. In comparison to the 32ka B.P. experiment, this feedback is noticeably weaker during the LGM, and completely absent during the PI. Furthermore, the temporal isolation of South Labrador Sea and Greenland-Iceland-Norwegian Sea contributions to the AMOC overshoot highlights the combined role of the tropical Atlantic Ocean and the South Labrador Sea response to the overshoot dynamics. The dependence of the AMOC overshoot and the associated climatic response on the climate state provides a coherent concept in agreement with pronounced rapid climate changes during glacial times, as recorded by proxy data. In addition to use fully coupled atmosphere-ocean model for the studies of different mechanistic processes in the Earth’s climate system, we employ a regional high-resolution ocean model (the North Atlantic/Arctic Ocean-Sea Ice Model) for further understanding of the hydrographic process of the surface Nordic Seas during the LGM, which has been reconstructed to be in different conditions by proxies in the CLIMAP (the Climate Long-Range Investigation, Mapping and Prediction) and GLAMAP (the Glacial Atlantic Ocean Mapping) projects. Using the atmospheric forcing corresponding to the CLIMAP and GLAMAP indicated surface ocean, our experiments successfully rediscovered the sea surface temperatures (SSTs) and sea ice cover, in agreement with the proxy reconstructions. Furthermore, the internal dynamics in our LGM experiments provide an intermediate cooling conditions in the Nordic Seas, colder than the GLAMAP reconstruction, but warmer than the CLIMAP reconstruction during the LGM. Furthermore, both the GLAMAP and CLIMAP atmospheric forcing lead to similar directions and magnitudes of surface ocean circulation in the Nordic Seas during the LGM, in spite of distinct features of the SSTs and sea ice cover.