Evaluation of South Atlantic Thermohaline Properties from BESM-OA2.5 and Three Additional Global Climate Models

Abstract

Important global oceanic processes, such as the meridional overturning circulation, are governed by the temperature and salinity of the ocean. As such, it is essential that these properties be correctly represented in high-quality global climate models. This study aims to evaluate thermohaline properties both historically and under two simulations of the Brazilian Earth System Model BESM-OA2.5 in the South Atlantic Ocean (Representation Concentration Pathway (RCP) 4.5 and 8.5). Since error assessment in the global climate model (GCM) is fundamental to infer climate change projections, comparisons were made for thermohaline properties among four GCMs (HadGEM2-ES, MIROC-ESM-CHEM, MIROC5, and BESM-OA2.5) against data from ocean monitoring programs and from ORAS5-ECMWF. The results show common surface spatial pattern errors in all models, commonly related to mesoscale processes. Specific to BESM-OA2.5 over the Southern Ocean, we observed an increase in the temperature bias during autumn and summer, probably due to subsurface temperature overestimation linked to North Antarctic Deep Water (NADW) formation. With respect to salinity, the underestimations in the Subtropical/Subantarctic Zones and in the north of the South Atlantic subtropical gyre were linked to simulation errors in the Malvinas current. All models presented overestimated annual historical temperature rates, with BESM-OA2.5 being the closest to ORAS5. In the subsurface, the BESM-OA2.5 did not easily simulate the South Atlantic Central Water (SACW) formation, though in deep water, the model was able to better simulate the Antarctic Intermediate Water and NADW patterns. Statistically, the multi-model means performed better, while the BESM-OA2.5 performed worst among the models in both methodologies applied. In terms of projected scenarios, the models demonstrated sensitivity to variations in greenhouse gas emissions between the RCPs, with higher magnitude warming predicted in the equatorial zone, except for BESM-OA2.5.

Descriptors:
Climate models; Models evaluation; Thermohaline properties