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.
(a)