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

• Main Author: Shi, Xiaoxu
• Other Authors: Lohmann, Gerrit, Rhein, Monika
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2016
• Subjects: Holocene; AMOC; NAO; sea ice; 500; 500 Science; ddc:500; Arctic; Arctic Ocean; Fram Strait; Ice Sheet; Labrador Sea; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://media.suub.uni-bremen.de/handle/elib/1137; https://nbn-resolving.org/urn:nbn:de:gbv:46-00105584-15

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. 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. 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 over mid and high latitudes compared to pre-industrial, with amplification over the continents; as well as a reduction of sea ice in the Arctic and Southern Oceans. A reduced sea ice transport through the Fram Strait leads to a stronger-than-present Atlantic Meridional Overturning Circulation (AMOC) in the early-Holocene. Including the early-Holocene topography and continental ice sheet over North America leads to an additional regional cooling year-round. The ...