Using ocean surface paleo-density to evaluate PMIP3 and PMIP4 Last Glacial Maximum climate simulations

Abstract. Quantitative reconstruction of ocean surface density during the Last Glacial Maximum (LGM) offers valuable insights into the ability of climate models to simulate past climate conditions, when global temperatures were about 4.5 °C to 6 °C colder than today. We assess the performance of the LGM climate simulations, as part of the 3^rd and 4^th phase of the Paleoclimate Modeling Intercomparisons Project, using a recent ocean surface density reconstruction based on the δ¹⁸O of foraminiferal calcite (δ¹⁸Oc). We consider the differences between the LGM and the preindustrial climates and each period separately, at both global and regional scales. Because surface density reflects the combined effects of temperature and salinity, we also examined sea surface temperature (SST) to better identify the processes underlying model–data differences.

Surface density reconstructions show greater variability than simulated surface density. Models therefore struggle to reproduce the spatial variability of the density difference (LGM – pre-industrial (PI)), but part of the mismatch may arise from the uneven spatial distribution of reconstructions, which are mostly located near coastal areas.

Density anomaly (LGM – PI) differences between data and models are largely controlled by sea surface salinity (SSS), with SST contributing to a lesser extent. This influence of SSS is directly linked to the reduction in tropical precipitation during the LGM: models that best match the large-scale density anomalies also simulate the strongest reductions in reconstructed low-latitude precipitation during the LGM, highlighting the key role of hydrological cycle changes in shaping surface density.

On a global scale, 100 % of model simulations show a statistically significant relationship with surface density reconstructions, looking at LGM and PI separately. However, on a regional scale, some features are poorly simulated, leading to weaker agreement between data and model simulations, particularly in the North Indian and Southern Oceans. Our analysis concludes with a focus in the Indo-Pacific Warm Pool. Past reconstructions indicate a LGM weakened Indian ocean west–east surface density gradient, but only 7 out of 14 models (50 %) reproduce this feature. These results highlight the need to better constrain regional hydrological cycle changes in models, as improving their representation is crucial to reduce uncertainties in both paleoclimate simulations and future climate projections.