Simulated mid and early Holocene climate in ECHAM6-FESOM: focus on North Atlantic variability

• Main Author: Shi, Xiaoxu
• Format: Thesis
• Language: unknown
• Published: 2016
• Subjects: Arctic Ocean; Labrador Sea; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://epic.awi.de/id/eprint/42764/; https://epic.awi.de/id/eprint/42764/1/thesis_shi.pdf; https://hdl.handle.net/10013/epic.49904; https://hdl.handle.net/10013/epic.49904.d001

In the first part of the thesis, changes of the Atlantic meridional overturning circulation (AMOC) in the mid-Holocene compared to the pre-industrial state are explored in different coupled climate models. Using time-slice integrations by a newly developed global finite-element model ECHAM6-FESOM with unstructured mesh and high resolution, our simulations show an enhanced mid-Holocene AMOC, accompanied by an increase in the ocean salinity over regions of deep water formation. We identify two different processes affecting the AMOC: 1) a more positive phase of North Atlantic Oscillation (NAO) increases water density over the Labrador Sea through anomalous net evaporation and surface heat loss; 2) a decreased import of sea ice from the Arctic causes a freshwater reduction in the northern North Atlantic Ocean. Using the coupled model ECHAM6-MPIOM in T63GR15 and T31GR30 grids, we find that the simulated AMOC is strongly affected by the model resolution. In detail, stronger-than-present mid-Holocene AMOC is revealed by simulations with the T63GR15 grid, which resembles the result of ECHAM6-FESOM, while a decline of the mid-Holocene AMOC is simulated by the low resolution model with the T31GR30 grid. Such discrepancy can be attributed to different changes in Labrador Sea density which is mainly affected by 1) NAO-induced net precipitation, 2) freshwater transport from the Arctic Ocean, and 3) the strength of AMOC itself. Finally, we analyzed available coupled climate models showing a diversity of responses of AMOC to mid-Holocene forcings, most of which reveal positive AMOC changes related to northern high latitudes salinification. Sensitivity of the simulated climate to the early-Holocene (9k) insolation, greenhouse gases (GHGs) and topography is examined in the second part of the thesis, by performing timeslice experiments under pre-industrial and 9k regimes using ECHAM6-FESOM. Under the early-Holocene orbit and GHGs, the ECHAM6-FESOM simulation shows a warming in boreal summer and a cooling in boreal winter from ...