Perspectives from equilibrium simulations
C. J. Van Meerbeeck, H. Renseen, and D. M. Roche
Abstract: "Dansgaard-Oeschger events occurred frequently during Marine Isotope Stage 3 (MIS3), as opposed to the following MIS2 period, which included the Last Glacial Maximum (LGM). Transient climate model simulations suggest that these abrupt warming events in Greenland and the North Atlantic region are associated with a resumption of the Thermohaline Circulation (THC) from a weak state during stadials to a relatively strong state during interstadials. However, those models were run with LGM, rather than MIS3 boundary conditions. To quantify the influence of different boundary conditions on the climates of MIS3 and LGM, we perform two equilibrium climate simulations with the three-dimensional earth system model LOVECLIM, one for stadial, the other for interstadial conditions. We compare them to the LGM state simulated with the same model. Both climate states are globally 2 degrees C warmer than LGM. A striking feature of our MIS3 simulations is the enhanced Northern Hemisphere seasonality, July being 4 degrees C warmer than in LGM. Also, despite some modification in the location of North Atlantic deep water formation, deep water export to the South Atlantic remains unaffected. To study specifically the effect of orbital forcing, we perform two additional sensitivity experiments spun up from our stadial simulation. The insolation difference between MIS3 and LGM causes half of the 30–60degreeN July temperature anomaly (+6degreesC). In a third simulation additional freshwater forcing halts the Atlantic THC[thermohaline circulation], yielding a much colder North Atlantic region (−7degreeC). Comparing our simulation with proxy data, we find that the MIS3 climate with collapsed THC mimics stadials over the North Atlantic better than both control experiments, which might crudely estimate interstadial climate. These results suggest that freshwater forcing is necessary to return climate from warm interstadials to cold stadials during MIS3. This changes our perspective, making the stadial climate a perturbed climate state rather than a typical, near-equilibrium MIS3 climate."
"In our MIS3 climate simulations with the three-dimensional earth system model LOVECLIM, we find a warmer base climate than that of LGM simulated with the same model. Boundary conditions were different during MIS3 than at LGM, notably insolation, ice sheet configuration, atmospheric greenhouse gases and dust concentrations, all leading to a positive forcing. Our main findings are:
– With smaller Northern Hemisphere ice sheets, higher greenhouse gases and lower dust concentration, MIS3 mean annual temperatures are higher than LGM(globally +1.7degrees C for MIS3-sta and +2.0degrees C for MIS3-int).
– Orbital insolation forcing leads to enhanced Northern Hemisphere seasonality, with mainly warmer summers due to an increase of summer insolation, whereas winter insolation did not change substantially. Northern Hemisphere mean July temperature anomalies compared to LGM are +3.5degreesC for MIS3-sta (+5.7degrees C) between 30degrees N and 90degrees N) and +3.8degreesC for MIS3-int. The sensitivity of the MIS3 climate to insolation changes is relatively high (up to 1degrees C per 4Wm−2). June insolation is 39Wm−2 higher in MIS3-sta than in LGM, which explains about half (2.5degreesC between 30degrees N and 90degrees N) of the July temperature differences.
– If we add 0.3 Sv of freshwater to the North Atlantic Ocean in our stadial simulation, we shut down the Atlantic thermohaline circulation, leading to a much colder climate over Europe and the North Atlantic region. The annual mean temperatures in these two regions are 7.4 degrees C, respectively 6.9 degrees C colder than in MIS3-sta. The simulated cooling leads to a better temperature match with permafrost reconstructions over Europe regarding stadials than in our MIS3-sta simulation. This simulation compares to previous glacial simulations with shutdown thermohaline circulation, with freshwater forcing explaining most of the temperature difference."
Figure 1 of this paper, page 1149, merits the PDF download. It graphically illustrates the oscillatory nature of climate during MIS 3, compared to the LGM and the Holocene, which both present a much more stable climate.