Mean Climate State during the Last Interglacial

to salinity changes that were less than 0.002 practical salinity units (psu) in magnitude.

To confirm that the passive isotopic tracers are also in equilibrium, the deep oceanδ¹⁸O concentration is examined. It was found that the globally averagedδ¹⁸Osignature of the deep ocean changed by less than 0.002 ‰ over the 100 years used for evaluation.

3.1.2 Solar Insolation

The LIG is forced by changes in the orbital configuration, changes in GHG concentrations, and the climatic feedbacks that result from these changes. The prescribed changes to the incoming solar radiation distribution throughout the year are shown in Figure 3.1. An increase in incoming solar radiation in boreal summers for LIG-130 and LIG-125 over the Northern Hemisphere can be seen, and a slight decrease in radiation in autumn. Over the Southern Hemisphere, the insolation increase happens later in the year, between September and November. LIG-120 presents a very different pattern, with a decrease in insolation during the first half of the year, and an increase in the second half.

3.2 Mean Climate State during the Last Interglacial

In the following, the yearly mean climate state of the LIG is examined for each of the simulated time slices, followed by an examination of the seasonal changes in boreal summer and boreal winter (Section 3.4). This examination is divided into several parts:

First, an analysis of Surface Temperature (ST) changes and changes in precipitation to describe the state of the atmosphere state is shown. Furthermore, a description of the precipitation-weightedδ¹⁸Osignals is presented, and the analysis is concluded by presenting the simulatedδ¹⁸O signatures in ocean seawater.

J F M A M J J A S O N D J -90

-45 0 45 90

Latitude(◦N)

LIG-130

J F M A M J J A S O N D J Month

-90 -45 0 45

90 LIG-125

J F M A M J J A S O N D J -90

-45 0 45

90 LIG-120

-60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5-2.5 2.55 10 15 20 25 30 35 40 45 50 55 60 Anomalous Insolation wrt PI (W m⁻²)

Figure 3.1: Anomalous Incoming Solar Radiation at the top of the atmosphere relative to PI. A strong increase in boreal summer incoming radiation can be seen in both LIG-130 and LIG-125, with a decrease during Autumn and Winter. During LIG-120, the climate receives less energy in Spring, and more in Autumn.

3.2.1 Surface Temperature Response

The first examination shows changes in ST relative to PI. The yearly mean ST in LIG-130, shown in Figure 3.2-A, is generally cooler than PI. Particularly over the Sahara, temperature decreases by more than−3.0^◦C, and an equally strong cooling is seen over the Indian subcontinent. Slight localized warming, with a magnitude ranging from 1.5^◦C to 2.0^◦C is simulated in the Arctic and the Sea of Okhotsk. Another slight warming is simulated in the mid latitudes of the Pacific off the California coast, with temperature anomalies between 0.25^◦C and 0.5^◦C. The remainder of the global oceans demonstrate cooling signals ranging from−0.5^◦C to−2.0^◦C.

During LIG-125 the results differ greatly, as shown in Figure 3.2-B. Large scale warming is simulated, particularly in the high latitudes, with Arctic climatologically averaged temperature increases of beyond 2.0^◦C. Just as in LIG-130 there is again a cooling simulated over the Sahara and the Indian Subcontinent. A vast majority of the

3.2. MEAN CLIMATE STATE DURING THE LAST INTERGLACIAL

A ^LIG-130 B ^LIG-125 C ^LIG-120

-2.0 -1.5 -1.0 -0.5 -0.25-0.1 0.10.25 0.5 1.0 1.5 2.0

Yearly Mean Surface Temperature Anomaly wrt PI (°C)

Figure 3.2: Changes in ST relative to PI for the three simulated timeslices during the LIG. These anomalies are constructed from 100 year means. While cooling is seen for much of LIG-130 (panel A), LIG-125 demonstrates pronounced warming, especially over the Northern Hemisphere. LIG-120 is slightly cooler than PI, due to relatively reduced GHG concentrations.

tropical ocean temperatures changes are insignificantly different from PI (based upon a 95% Students T-Test), with the exception of a slight cooling of up to −1.0^◦C in the South Pacific, west of South America. A similar magnitude of cooling can be seen in the Agulhas region surround both the Atlantic and Indian ocean Basins around South Africa, while the southern North Atlantic displays anomalous warming of up to 1.0^◦C. In the northern North Atlantic COSMOS-WISOsimulates a cooling of up to−1.5^◦C, whereas over the continents of Europe, Asia, and North America, as well as over the North Pacific, the model produces a warming with anomalous temperature responses between 0.5^◦C to beyond 2.0^◦C.

In LIG-120, shown in Figure 3.2-C; ST is generally cooler than PI in the climatological mean, similar as was demonstrated for the simulation of LIG-130. In particular, the northern high latitudes over Greenland are up to−2.0^◦C cooler, whereas the response over the Arctic and the high latitudes over Asia is less pronounced, with temperature decreases of up to−1.0^◦C. A slight cooling over the tropical latitudes, with temperature responses ranging from−0.1^◦C to−0.5^◦C. Unlike the other two time slices of the LIG,

the Sahara cooling is less pronounced (with changes of only−1.5^◦C, whereas the other time slices exceeded−2.0^◦C), and there is no localized warming in the Arctic.

3.2.2 Sea Ice Response

When examining the sea ice extent, shown in Figure 3.3, it can be seen that the maximum sea ice remains very close to the PI case for all three simulations. Given the changes in prescribed insolation during the three simulations, this is not surprising, as boreal winter incoming radiation anomalies are close to zero. However, a very strong changes in maximum sea ice extent is simulated, with drastically reduced sea ice cover for the early (-46% decrease by area) and mid (-56%) LIG simulations. The reduction is most severe in the mid LIG simulation, were sea ice cover is constricted solely to the central Arctic Ocean. The late LIG simulation maintains a similar maximum sea ice extent as in the PI case.

LIG-130 LIG-125 LIG-120

Figure 3.3: Changes in sea ice area coverage relative to PI for the three simulated timeslices during the LIG. The red (black) and blue (gray) contour lines demonstrate maximum and minimum yearly sea ice extent for each individual examined LIG (PI control) simulation.

3.2. MEAN CLIMATE STATE DURING THE LAST INTERGLACIAL

3.2.3 Precipitation Changes

Next, changes to the amount of precipitation throughout the LIG are examined. During LIG-130, shown in Figure 3.4-A, a mean climatology is simulated that is in general wetter than the PI, particularly over the equatorial Atlantic, Africa, and the Indian Ocean and Indian Subcontinent. There, precipitation amounts increase by up to 50 mm month⁻¹. A drying occurs with a similar magnitude over Indonesia and the equatorial Pacific. A very slight drying signal is simulated over South America, with values up to−20 mm month⁻¹ less than in the PI case, and a slight drying is also simulated along the Gulf Stream, Eu-rope, and North Atlantic, with values between−2.5 mm month⁻¹ and−10 mm month⁻¹ less than in the PI simulation. Also, an increase in precipitation over southern part of the North Pacific is simulated, with anomalous precipitation values of up to 20 mm month⁻¹.

These anomalous precipitation patterns are largely repeated in the LIG-125 simula-tion (Figure 3.4-B), with an increase in precipitasimula-tion over the equatorial Atlantic, the Sahara, and the Indian Ocean, where rainfall values increase by up to 50 mm month⁻¹. Drying again occurs just south of the equatorial Pacific, over South America, and over South Africa, with anomalous values of up to −30 mm month⁻¹. A drying signal over Indonesia is also simulated, with up locally up to 50 mm month⁻¹ less rainfall than in the PI case. Contrary to the LIG-130 simulation, the increase in precipitation over the Pacific is not confined to the ocean basin, but also extends to the western coast of North America. Finally, a slight increase in rainfall over Europe and Asia can be seen, although this increase is only very slight, with anomalous precipitation values of only up to 10 mm month⁻¹relative to the PI simulation.

Finally, during the LIG-120 simulation (Figure 3.4-C), only minimal changes rel-ative to the PI simulation can be seen. The large scale increase in precipitation over the Atlantic, Sahara, and Indian subcontinent is either weaker or absent, and a slight drying throughout the North Atlantic and equatorial Pacific is simulated, with values of

A ^LIG-130 B ^LIG-125 C ^LIG-120

-70 -50 -30 -20 -10 -2.52.5 10 20 30 50 70

Yearly Mean Precipitation Anomaly wrt PI (mm/month)

Figure 3.4: Changes in precipitation relative to PI for the three simulated timeslices during the LIG

−20 mm month⁻¹less precipitation than in the PI simulation. A majority of the precipi-tation patterns that are simulated do not vary significantly compared to the control case, based upon a 95% Student T-Test.