10:45 AM - 12:15 PM
[ACC26-P08] Evaluation of the roles of surface flux changes during the Last Glacial Maximum in the Atlantic Meridional Overturning Circulation in climate models
Keywords:Atlantic meridional overturning circulation, last glacial maximum, OGCM, PMIP
Proxy data suggest that the Atlantic Meridional Overturning Circulation (AMOC) was shallower during the Last Glacial Maximum (LGM) than at present. On the other hand, many climate models participating in the Paleoclimate Model Intercomparison Project (PMIP) show a stronger and deeper AMOC, and there are significant differences among the models. The mechanisms of how the glacial climate changes the AMOC have been studied mainly in terms of changes in wind stress and temperature, but these mechanisms have not been discussed in a comparative manner.
In this study, we conducted numerical simulations to evaluate the relative impact of changes in sea surface boundary conditions on the AMOC during the glacial period by organizing them into three categories: thermal conditions, wind stress, and freshwater fluxes. The changes in sea surface boundary conditions at the LGM were obtained from the output of various PMIP models, and the changes in thermal conditions, wind stress, and freshwater fluxes were separately applied to an ocean general circulation model to investigate how they change the AMOC. The results showed that the thermal conditions of the LGM weakened the AMOC, while wind stress and freshwater flux strengthened it. It is also found that the differences in the model output used for thermal conditions and wind stress caused large variations in the strength of the AMOC. Thermal conditions in the LGM contributed to the weakening of the AMOC through the densification of Antarctic Bottom Water, and the different degrees of cooling of the Southern Ocean among the models caused a considerable variation in the strength of the AMOC. Wind stress affected deep water formation in the North Atlantic and hence the AMOC through both sea ice transport in the high latitudes and salinity transport in the mid-latitudes. However, no clear relationship between the strength of the AMOC and the glacial westerly wind enhancement was suggested, which has been focused on in previous studies. Our study indicates that differences in sea surface boundaries among models, such as surface air temperature and wind stress, can significantly affect the reproducibility of the AMOC in the LGM simulations by climate models.
In this study, we conducted numerical simulations to evaluate the relative impact of changes in sea surface boundary conditions on the AMOC during the glacial period by organizing them into three categories: thermal conditions, wind stress, and freshwater fluxes. The changes in sea surface boundary conditions at the LGM were obtained from the output of various PMIP models, and the changes in thermal conditions, wind stress, and freshwater fluxes were separately applied to an ocean general circulation model to investigate how they change the AMOC. The results showed that the thermal conditions of the LGM weakened the AMOC, while wind stress and freshwater flux strengthened it. It is also found that the differences in the model output used for thermal conditions and wind stress caused large variations in the strength of the AMOC. Thermal conditions in the LGM contributed to the weakening of the AMOC through the densification of Antarctic Bottom Water, and the different degrees of cooling of the Southern Ocean among the models caused a considerable variation in the strength of the AMOC. Wind stress affected deep water formation in the North Atlantic and hence the AMOC through both sea ice transport in the high latitudes and salinity transport in the mid-latitudes. However, no clear relationship between the strength of the AMOC and the glacial westerly wind enhancement was suggested, which has been focused on in previous studies. Our study indicates that differences in sea surface boundaries among models, such as surface air temperature and wind stress, can significantly affect the reproducibility of the AMOC in the LGM simulations by climate models.