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

Abstract

Abstract:We use a global ocean general circulation model (OGCM) with low vertical diffusion and isopycnal mixing to simulate the circulation in the Atlantic Ocean at present-day and the Last Glacial Maximum (LGM). The OGCM includes δ 18 O as a passive tracer. Regarding the LGM sea-surface boundary conditions, the temperature is based on the GLAMAP reconstruction, the salinity is estimated from the available δ 18 O data, and the wind-stress is derived from the output of an atmospheric general circulation model. Our focus is on changes in the upper-ocean hydrology, the large-scale horizontal circulation and the δ 18 O distribution. In a series of LGM experiments with a step-wise increase of the 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; furthermore it was generally shallower. As the meridional density gradient grew, the Antarctic Circumpolar Current strengthened and its northern boundary approached Cape of Good Hope. At the same time the southward penetration of the Agulhas Current was reduced, and less thermocline-to-intermediate water slipped from the Indian Ocean along the southern rim of the African continent into the South Atlantic Ocean; the 'Agulhas leakage' was diminished by up to 60% with respect to its modern value, such that the cold water route became the dominant path for North Atlantic Deep Water (NADW) renewal. It can be speculated that the simulated intensification of the Benguela Current and the enhancement of NADW upwelling in the Southern Ocean might reduce the import of silicate into the Benguela System, which could possibly resolve the 'Walvis Opal Paradox'. Although δ 18 O w was restored to the same surface values and could only reflect changes in advection and diffusion, the resulting δ 18 O c distribution came close to reconstructions based on fossil shells of benthic foraminifera.

A dynamical reconstruction of the Last Glacial Maximum ocean state constrained by global oxygen isotope data

“…10). These density changes are, for example, in contrast to the results of Paul and Schäfer-Neth (2003) who found a stronger increase of density in the Weddell Sea compared to the North Atlantic and an AABW extending further north in their LGM simulation compared to their present-day simulation. The density changes might explain why the extend of the AABW into the Atlantic Ocean is very limited in our estimate.…”

Response of eastern tropical Atlantic central waters to Atlantic meridional overturning circulation changes during the Last Glacial Maximum and Heinrich Stadial 1

“…Thermocline waters of the western North Atlantic subtropical gyre were estimated to be 4°C cooler during the Last Glacial Maximum (LGM, 23–19 ka BP), which has been attributed to significantly cooler conditions in the subduction regions at that time [ Slowey and Curry , 1995]. A model study also predicts an equatorward shift of the outcrop regions of isopycnal surfaces in both hemispheres during the LGM, which led to a shoaling of the ventilated thermocline depths [ Paul and Schäfer ‐ Neth , 2003, 2004]. Therefore, changes in glacial climate boundary conditions might have significantly influenced tropical thermocline temperatures, and could obscure effects associated with past changes in the AMOC.…”

Quaternary oceans and climate change: lessons for the future?

“…1), it appears that the temperature anomaly of the North Atlantic vanished beyond 40°N (as expected from the southward advance of the polar front) but that heat transfer from south to north persisted in the tropical regions (aided by strong southeasterly winds off the Amazon; see Paul and Schäfer-Neth 2003, their Fig. 2).…”

“…More recently, these topics have generated much interest in modeling, as computing power has grown (e.g. Marotzke 2000;Rahmstorf 2001;Paul and Schäfer-Neth 2003). Traditionally, the Gulf Stream is cited as a crucial element in the heat transport system, especially with a view to subarctic regions in the North Atlantic, and this connection is deeply engrained in popular thinking about climate (e.g., in describing the warming of the coast of Norway).…”

“…Much progress has been made since, especially with regard to incorporating geologic information (e.g., Paul and Schäfer-Neth 2003), but error bars are still substantial.…”