Subsurface heat storage and transport in the tropical Atlantic during the last Deglaciation
Final Report Abstract
Numerical models predict that reduced interhemispheric oceanic heat transport during a weakened Atlantic Meridional Overturning Circulation (AMOC) will lead to massive heat accumulation in the subsurface of the South Atlantic with a “hot spot” off East Brazil. These warm subsurface waters might also play a pivotal role for the resumption of the AMOC due to their high salinity and, hence, density when advected to the loci of deep-water formation in the North Atlantic. However, paleo-data from the proposed area of maximum subsurface warming in the western tropical Atlantic are yet missing but crucial to ground-truth the fidelity of the model results. Such model-testing is urgently needed as even state-of-the art models are limited in their capability to simulate the complex dynamics of water mass transport by major boundary currents (such as the Brazil Current in the South Atlantic) as well as the heat and salt transfer from the Indian Ocean into the (sub)surface of the South Atlantic via large-scale Eddies (the so-called “Agulhas Leakage”). For this purpose, subsurface temperatures (subT) and subsurface salinities (subS) based on the deep thermocline dwelling foraminifera Globorotalia truncatulinoides (d) were generated on Core M125-35-3 from off East Brazil. This core locates within the maximum of the subsurface warm anomaly predicted by models in response to a weak AMOC. The subT and subS records, generated by combined Mg/Ca and δ18O measurements, cover the time frame 19–4 ka, including two periods with a strongly weakened AMOC, i.e. Heinrich Stadial (HS) 1 and the Younger Dryas (YD). Contrary to expectation we find that both, HS 1 and YD were accompanied by subsurface cooling, where subT follows the northern hemisphere temperature very closely until the early Holocene. This clear imprint of a northern-hemisphere climate signal runs contrary to the classical conceptual model of a thermal see-saw with heat accumulation during periods of weak AMOC, and stresses the need to refine the respective concepts as well as re-evaluate the numerical model output. Interestingly, Holocene climate variability mirrors the inflow of warm waters from the Indo-Pacific into the Atlantic via Agulhas Leakage. Given the absence of this connection before the Holocene, it raises the possibility that hitherto unnoticed thresholds exist. These thresholds may be associated with factors such as sea level fluctuations and shifts in oceanic frontal systems, which influence the regulation of subsurface heat transfer in the western tropical South Atlantic. To gain a deeper understanding of these processes, numerical modelling will be employed to simulate the South Atlantic subsurface heat budget under varying boundary conditions, HS 1, the Last Glacial Maximum, and the Holocene.
Publications
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Indian to Atlantic Ocean heat transport reinforced abrupt climate change during the last deglaciation. International Conference on Paleoceanography, Bergen, 2022.
Meier, K.J.F.; Jaeschke, A.; Rethemeyer, A.; Chiessi, C.M.; Faulkner, K.A.; Friedrich, O. & Bahr, A.
