Climate Variability and Predictability at Various Time ScalesView this Special Issue
Ocean Cooling Pattern at the Last Glacial Maximum
Ocean temperature and ocean heat content change are analyzed based on four PMIP3 model results at the Last Glacial Maximum relative to the prehistorical run. Ocean cooling mostly occurs in the upper 1000 m depth and varies spatially in the tropical and temperate zones. The Atlantic Ocean experiences greater cooling than the rest of the ocean basins. Ocean cooling is closely related to the weakening of meridional overturning circulation and enhanced intrusion of Antarctic Bottom Water into the North Atlantic.
The cooling at the Last Glacial Maximum has been extensively studied geologically and numerically (e.g., [1–5]) where proxy data and numerical modeling were both employed to explore the climate sensitivity and mechanism. The Paleoclimate Modeling Intercomparison Project (PMIP2) presented large-scale features (e.g., ). However, the spatial pattern of cooling at that period was little studied previously. In this paper we use the newly released PMIP3 data to study the cooling pattern at the Last Glacial Maximum.
We analyze ocean potential temperature anomaly, ocean heat content (OHC) change, and meridional overturning mass stream function based on four available PMIP3 models of IPSL-CM5A-LR, MIROC-ESM, MPI-ESM-P, and MRI-CGCM3 so far (http://cmip-pcmdi.llnl.gov/cmip5/), which follow the PMIP3 21ka experimental design (Table 1; http://pmip3.lsce.ipsl.fr/) and make a comparison between the last 50 years of Last Glacial Maximum (LGM) experiments relative to the base period of the last 50 years of the preindustrial control run.
We calculate the temperature anomaly, meridional overturning mass stream function anomaly, and ocean heat content change of each model by regridding all the temperature data into a common grid.
3.1. Geographical Distribution
Ocean cooling shows pronounced spatial variations (Figure 1), both horizontally and vertically. The Northern Hemisphere exhibits a stronger cooling than the Southern Hemisphere of IPSL-CM5A-LR, MIROC-ESM, and MPI-ESM-P except MRI-CGCM3 with stronger cooling in the Southern Ocean revealing a notable north-south asymmetry.
(a) 10 m
(b) 100 m
(c) 500 m
(d) 1000 m
(e) 1500 m
(f) 2000 m
(g) 3000 m
At the surface (Figures 1(a) and 1(b)), significant ocean cooling (<−5°C) occurs in the North Pacific and North Atlantic of the first three models. All these three models have shown the maximum cooling regions in the North Pacific around 40°N and in the Nordic Seas. But, MRI-CGCM3 demonstrates a significant cooling around 60°S in the Southern Ocean. It is of note that south of Greenland the model even shows a slight warming in shallow water.
At the subsurface 500 m layer (Figure 1(c)) cooling in the Pacific around 40°N and the North Atlantic still maintain the surface cooling pattern. However, the South Atlantic demonstrates the strongest cooling in IPSL, MIROC, and MRI models. Strong cooling in the Indian and Southern Oceans is also presented in the MRI model.
The water body between 1000–2000 m experiences a moderate cooling of −2°C (Figures 1(d), 1(e) and 1(f)). All the four models have shown that north of the Gulf Stream region and South Atlantic experience more significant cooling than the rest of the regions. The Southern Ocean around 40°S has a larger amount of cooling as well. It is of note that the eastern boundary of the North Atlantic shows the slightest cooling and even warming in the MPI model.
The four models differ remarkably below 2000 m (Figures 1(f) and 1(g)). The IPSL model only shows a cooling of about −1.0°C in the large expanse with a maximum cooling of −2.5°C south of the Greenland whilst the Southern Ocean shows merely slight cooling. But, the other three models of MIROC, MPI, and MRI show a much stronger cooling of greater than −2°C. The MIROC shows the strongest cooling in the Southern Ocean. But, MRI shows strong cooling in the Indian and Southern Oceans.
3.2. Latitudinal Cross Sections and Depth Variations
Figure 2 reveals that large cooling occurs in the upper 1000 m. The temperate and tropical zones between 40°S–40°N are marked by subsurface cooling. The northern high latitude north of 40°N experiences maximum surface cooling. It is of note that the deep Arctic Ocean experiences the largest cooling in MIROC and MRI whereas IPSL and MPI only show merely a subsurface cooling.
(b) Atlantic + Arctic
(c) Pacific + Indian
Ocean basins act differently. The cooling in the Atlantic (Figure 2(b)) penetrates to 3000 m with a strikingly subsurface cooling center between 20°S and 20°N. 40°N–70°N shows a surface cooling.
Compared to the Atlantic and Arctic, the Pacific, and Indian demonstrate a shallower cooling. All four models show a maximum cooling around 40°N. However, the cooling only penetrates to 600 m in IPSL and MPI, but the other two models have a deeper penetration depth.
Profiles of temperature anomaly in different ocean basins indicate that the largest surface cooling occurs in the North Atlantic (Figure 3). IPSL and MRI show a global average cooling of −2.5°C whilst MIROC and MPI have a cooling of −2°C. The North and South Atlantic in IPSL and MIROC show a similar pattern with a maximum of −5°C at 400−500 m. The Arctic cooling has a similar pattern in the four models where the coolest zone lies in the deep ocean. The other basins all exhibit a rapid subsurface cooling variation in the upper ocean.
3.3. Ocean Heat Content
Ocean heat content (OHC) change reveals the integrated cooling. Figure 4 reveals the OHC change of the four models. The IPSL model shows most cooling happens in the Atlantic, parts of the Pacific and Indian. But MIROC, MPI, and MRI models demonstrate a much more significant cooling in the Pacific and Indian. Figure 5 lists the average and total OHC change. The North Pacific and Southern Ocean have the strongest change of average OHC.
More than half of OHC change occurs above 2000 m. Like temperature change in oceans, OHC change also takes place mainly in the upper ocean to 2000 m (Table 2). On a global scale, the upper 2000 m accounts for 73%, 61%, 54%, and 56% in IPSL, MIROC, MPI, and MRI, respectively.
OHC change has its individual feature in the five Ocean basins. The OHC change in Atlantic, Arctic, and Indian is almost the same for the four models but differs in the North Pacific and Southern Ocean, which also affect the total OHC change (Figure 5(b)). It is of note that the 2000 m OHC change is similar for all the four models and, therefore, the OHC change difference in the Pacific and Southern Ocean lies in the deep ocean.
Unfortunately, not all PMIP3 models output ocean meridional overturning mass stream function. Among the four released model results, only MPI and MRI have meridional overturning variables. Meridional overturning circulation (MOC) changes are usually the responses to climate change. Here we explore the links between the MOC change and temperature anomaly on the global scale and regional scale at the Last Glacial Maximum.
Previous studies have revealed that cooling at the Last Glacial Maximum is closely related to meridional overturning circulation (MOC) [7–9]. They simulated a shallower and weaker North Atlantic Deep Water circulation and an enhanced intrusion of Antarctic Bottom Water (AABW) into the North Atlantic . Ganopolski and Rahmstorf  proposed a cold mode MOC to explain the cooling mechanism at the Last Glacial Maximum.
On a global scale the upper 2000 m witnesses a weakening trend of the MOC although the magnitude varies in the two models (Figure 6(a)). MPI presents a negative MOC anomaly whilst MRI only shows a slight decrease of MOC compared to the preindustrial run. In the deep part we can observe the enhancement of AABM. Both models exhibit the weakening of the Atlantic meridional overturning circulation (Figure 6(b)). Figures 1 and 2 have different temperature distributions with regard to MPI and MRI. Cooling in the upper Pacific and Indian Oceans simulated by MPI is not as strong as cooling by MRI, which is closely related to the enhanced MOC change in MPI upper ocean while MRI upper part shows a weakening MOC change.
(b) Atlantic + Arctic
(c) Pacific + Indian
Based on the MOC change and temperature anomaly in these two models, we can see that weakening of MOC in the upper ocean is closely related to upper cooling and enhancement of AABW in the deep part is associated with cooling in the deep ocean.
Although the four models differ in cooling magnitude, we see that cooling at the Last Glacial Maximum varies both horizontally and vertically. Cooling mostly occurs between 40°S and 40°N in the upper 1000 m. More than half of OHC change happens in the upper 2000 m.
All the four models are in agreement that ocean basins, except the Arctic, are featured by surface to subsurface cooling. Cooling in the Atlantic and Arctic is much deeper than the Pacific and Indian. The Atlantic experiences the greatest cooling at the Last Glacial Maximum. Ocean cooling at the Last Glacial Maximum is closely related to the MOC change.
We acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s Working Group on Coupled Modeling (WGCM) for their roles in making available the WCRP CMIP5 multimodel dataset. Support of this dataset is provided by the Office of Science, US Department of Energy.
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