3 Remaining drift and imbalances in the pre-industrial control simulation

In the pre-industrial control simulation AWI-CM is in quasi-equilibrium: The 2 m temperature drift from year 150 to year 400 of the piControl simulation (the time period to which most of the historical, scenario, and idealized CO2 increase experiments need to be compared) amounts to 0.00022 °C/year. Furthermore, sea ice trends are ranging from -0.00069 to -0.00027 million km²/year for the Arctic and from -0.00044 to -0.00026 million km²/year for the Antarctic computed for the years 150 to 400 during March and September, respectively. This suggests that any residual drift of 2 m temperature and sea ice extent in the coupled system is much smaller than the changes anticipated in a warming word.
Fig. 2 shows the Hovmöller diagrams for the global average profiles of oceanic potential temperature and salinity for the last 400 years of the control simulation. The amplitude of the drift is less than 0.15 °C for temperature and 0.05 psu for salinity, respectively, indicating that the system is close to its quasi equilibrium state. The drift in temperature is concentrated at depths of 500, 1,500, 3,000 and 4,500 m, while the drift in salinity happens mainly at depths of 500 and 2,000 m. From inspecting the spatial distribution of the drift (not shown) we conclude that the upper drift zone at 500 m stems primarily from the overall cooling and freshening of the ocean. The drift between 1,500 m and 2,000 m is partly linked to the Mediterranean outflow which spreads into the southern North Atlantic. The simulated outflow is too warm and too salty. At 3,000 m, we observe that the Atlantic and Pacific Oceans become cooler while Indian and Southern Oceans show positive trends in temperature. Simultaneously, salinity in the North Atlantic shows a negative trend at this depth, partly compensating the warming signal there in terms of density. Everywhere else at this depth there is a positive drift in salinity, most pronounced in the Indian Ocean. Finally, the deepest zone of temperature increase at ~4,500 m stems from a warming trend in the Southern Ocean. Although the spatial pattern of non-zero temperature changes implies a small remaining redistribution of heat and salinity, we overall conclude that the system is close to a quasi equilibrium state. Simulated changes in response to greenhouse gas increases are clearly stronger than this residual drift as shown in section 5.4.
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