Modeling the water masses of the Atlantic Ocean at the Last Glacial Maximum

Abstract

[1] We produced gridded monthly sea-surface boundary conditions for the Atlantic Ocean at the Last Glacial Maximum (LGM) based on the sea-surface temperature reconstruction of the GLAMAP project. We used an ocean general circulation model (OGCM), subject to these sea-surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model, to study the differences in the distribution of the main water masses between the LGM and the present. Our global OGCM is characterized by high vertical resolution, low vertical diffusion, and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern Atlantic Ocean. According to a series of LGM experiments with an increasing sea-surface salinity anomaly in the Weddell Sea, the ventilated thermocline was colder than today by 2-3°C in the North Atlantic Ocean and, in the experiment with the largest anomaly (1.0 beyond the global anomaly), by 4-5°C in the South Atlantic Ocean. Its depth was reduced by 50 m on average, most notably in the tropics. In the North Atlantic Ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5°-10°, and the ventilation increased. In the South Atlantic Ocean the mixed layer and thermocline water masses were dominated by cold water originating from Drake Passage, and the import of warm water from the Indian Ocean was reduced to about 4 Sv or 40% of its modern value. Antarctic Intermediate Water was colder by 3-4°C and could be traced as far as 10°N. The meridional overturning rates of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the Atlantic Ocean were similar to those of the present-day experiment (9-10 Sv and 4 Sv, respectively). However, NADW cooled by 2.5°C and AABW by 1°C. AABW was near the freezing point of seawater at the surface and the saltiest water mass in the Atlantic Ocean, even saltier than NADW. We show that the differences between the LGM and the present-day experiments can be traced back to the changes in the subpolar and interhemispheric sea-surface density gradients.

Can sparse proxy data constrain the strength of the Atlantic meridional overturning circulation?

“…Alternatively, longer integration periods for each iteration would be required. Paleo-data from regions where Antarctic Bottom Water is formed may become more important on longer timescales, because the relative densities of the North Atlantic and Southern Ocean source waters are expected to play a larger role for the meridional overturning circulation in the steady-state problem (e.g., Paul and Schäfer-Neth, 2003;Weber et al, 2007). However, in our experiments the adjustment of the AMOC was very fast; it typically occurred in the first 10 years of a 20-year experiment.…”

The Atlantic Ocean at the Last Glacial Maximum: 2. Reconstructing the Current Systems with a Global Ocean Model

“…The main difference between the LGM experiments discussed here and those presented in Paul and Schäfer-Neth (2003) is the use of the corebased as opposed to the isoline-based temperature reconstruction (see our accompanying publication Schäfer-Neth and Paul this volume). With respect to δ 18 O w , Experiments GA-GC are not true simulations of the LGM, but important sensitivity experi-ments that isolate the effect of only the circulation changes on the distribution at depth, without changes of the surface hydrology.…”

“…In contrast to our previous study (Paul and Schäfer-Neth 2003), we now use as model forcing the SST fields obtained with the most reliable gridding technique (i.e. variogram analysis and kriging) applied to the proxy-data at the core locations, to simulate the glacial-to-interglacial changes in the hydrology and circulation of the Atlantic Ocean.…”

“…To force the δ 18 O w at the sea surface, we used the values for the upper 100 m from the GISS δ 18 O database (Bigg and Rohling 2000;Schmidt et al 1999 Paul and Schäfer-Neth 2003). Differences to the SST maps used here are discussed in Schäfer-Neth and Paul (this volume).…”

Interactions between topographically and thermally forced stationary waves: implications for ice-sheet evolution

“…In addition, changes of greenhouse gas concentrations (Berger et al, 1998;Shackleton, 2000), sea-surface temperatures (CLIMAP, 1984;Paul and Schaefer-Neth, 2003;Kageyama et al, 2006;Colleoni et al, 2010) and vegetation cover (Claussen et al, 2006;Colleoni et al, 2009) need to be considered. Furthermore, features associated with ice-sheet dynamics and the atmospheric state remain sources of uncertainty.…”