The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in MPI-ESM

 

Reconstructions based on d13C, Cd/Ca and d18O suggested that the interface between North Atlantic Deep Water (NADW) and Antarctic Bottom Water shifted upwards during the Last Glacial Maximum (LGM, 21ky before present). First studies indicated that the measured tracer distributions required a shallower and weaker upper overturning cell. While there is little debate about the shoaling, the AMOC strength is difficult to constrain by reconstructions. Also an AMOC as strong as or even stronger than today cannot be ruled out by the proxy data. Coupled climate models show large inconsistencies amongst themselves when it comes to simulating the state of the glacial deep ocean and the glacial AMOC.

 

We study the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) and the deep ocean water masses during the LGM to different sets of forcings in steady state simulations with the coupled climate model MPI-ESM.

 

The effects of the glacial GHG concentrations and ice sheets

The low glacial GHG concentrations lead to less NADW below 3000m, indicated by pronounced cooling in the deep North Atlantic (Fig. 1a). The salinity AABW increases (Fig. 1d).

The ice sheets on the other hand increase the percentage of NADW below 3000m, indicated by the pronounced warming and salinity increase in the North Atlantic (Fig. 1b and e). The effect of GHG concentration and ice sheets compensate in the deep ocean, so that no clear indication of a shift of the interface between NADW and AABW is found in the total LGM response which is defined as the difference between the glacial simulation LGM-ref and the preindustrial control run piCTL (Fig. 1c and f).

 

 

 

Figure 1 Section through the Atlantic as indicated in the upper right panel. Shown are changes of potential temperature (top row) and salinity (bottom row) in response to the GHG reduction (a and d), the glacial ice sheets (b and e) and the combined glacial forcings, i.e. GHG concentrations, ice sheets and orbit (c and f). Note that the ice sheets cause a global mean salinity increase of 1.21 g/kg, the salinity anomalies in (e) and (f) are shown relative to this mean increase to facilitate the comparison with the GHG effect.

 

The response of the AMOC (Fig. 2, left) is consistent with the changes of the deep Atlantic water masses. The GHG reduction causes a shoaling of the upper overturning cell and a reduction of the overturning strength. The ice sheets induce a deepening of the upper overturning cell and an increase of the overturning strength. Also here, the GHG and ice sheet effects cancel, so that no shoaling of the upper cell is simulated in the full glacial simulation (LGM-ref).

 

Figure 2 Profile of the AMOC at 30ºN in the different simulations. The left panel shows the effect of the individual glacial forcings, the right panel shows the sensitivity of the glacial AMOC to different constant GHG concentrations. Numbers in the simulation names indicate pCO2 in ppm.The prefix 'pi' indicates a set-up with preindustrial topography and ice sheets, the prefix 'LGM' indicates a set-up with glacial topography and ice sheets. The orbital parameters in piTOPO and the LGM-nnn experiments are set to glacial values.

 

The effect of different GHG concentrations on the glacial AMOC

As the analysis of the effect of the individual glacial forcings shows that the GHG effect induces a shoaling of the upper overturning, we perform experiments with glacial ice sheets and different GHG concentrations with linearly decreasing radiative forcing to study the GHG effect in more detail (Fig. 2, right). The applied pCO2 ranges from 353 to 149 ppm. At 353 and 284 ppm, the AMOC strength and vertical geometry are nearly identical. The weakening sets in below 284 ppm, the shoaling only below 230 ppm. GHG concentrations below the glacial level are necessary to cause a shoaling of the upper overturning cell with respect to the preindustrial state in MPI-ESM. The overturning state and deep ocean water masses (not shown) are in best agreement with reconstructions for a pCO2 of 149 ppm. Sensitivity studies confirm that brine release and shelf convection in the Southern Ocean are key processes for the shoaling of the upper overturning cell. Shoaling occurs only when the very salty Southern ocean shelf water contributes significantly to the formation of AABW.

 

Read the full story:

Klockmann, M., U. Mikolajewicz, M. Marotzke (2016) Climate of the Past, The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model (Opens external link in current windowlink)

 

Contact: Marlene Klockmann