Last Glacial Maximum ocean thermohaline circulation: PMIP2 model intercomparisons and data constraints

Abstract

[1] The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere-ocean simulations with four climate models that are also being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing. Citation:

Antarctic sea ice control on ocean circulation in present and glacial climates

“…In Fig. 5 we show output from two National Center for Atmospheric Research Community Climate System Model version 3 (CCSM3) model simulations run for modern and LGM climates (33). The solid black lines, which mark the quasi-permanent sea ice line, shifted in excess of 5°latitude at the LGM, together with the area where the air-sea buoyancy flux is negative.…”

Model sensitivity to North Atlantic freshwater forcing at 8.2 ka

“…Since these were idealized experiments, no comparison to observations was possible. Otto-Bliesner et al (2007) compared AMOC in four Last Glacial Maximum simulations from the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2). These models gave very different glacial circulations and a comparison to Goosse et al (2010) paleoclimate proxy evidence indicated serious mismatches for several of the simulations.…”

Palaeoceanography: motivations and challenges for the future

“…Similarly, efforts, such as CLIMAP and more recently the Multiproxy Approach for the Reconstruction of the Glacial Ocean surface (MARGO), have attempted global syntheses of sea-surface temperatures [128,129]. Both of these types of compilations have been widely used by the data and modelling communities [130], highlighting the benefit of this sort of approach. But even in these few cases where reasonable amounts of data exist, there are still key gaps, such as at intermediate depth or in the Southern Ocean.…”