Finger Prints of the Gulf Stream Intensity in the Cenozoic - A view on continental climate and biodiversity on both sides of the Atlantic Ocean.
Final Report Abstract
The cooling during the Late Cenozoic (the last 34 million years) has been attributed to different causes. For example, land-sea distribution (palaeogeography), orography (mountain uplift), atmospheric CO2-concentration and land cover have changed compared to present-day conditions. In our project we concentrated on the role of ocean heat transport, as in model experiments bathymetry changes and the closure of the Central American Seaway (CAS) have been shown to influence the Atlantic Meridional Overturning Circulation (AMOC) or the ocean gyres in the North Atlantic Ocean, i.e. the “Gulf Stream”. Geological records suggest that CAS closure took place between 22 and 6 Ma with intermittent closing and opening. The final closure took place during the Pliocene. Continental palaeoclimate data were derived from the palaeobotanical record (leaves, fruits and seeds, pollen and spores) by the Coexistence Approach, a taxonomy-based method using climate requirements of Nearest Living Relatives. While previous studies indicated that very high Eocene temperature anomalies recorded in the realm of the Tethyan Archipelago might be primarily referable to regional ocean currents, our current research detects the first possible evidence for Gulf Stream related temperature patterns for the Aquitanian (comparison of Atlantic and Pacific records of Eurasia). Concerning the fossil climates of Western Eurasia, there is evidence for Gulf Stream induced patterns in the middle and the late Miocene data. To simulate different states of the CAS closure and its effect on Atlantic climate, we performed simulation experiments with an Earth System Model of Intermediate Complexity, Planet Simulator. The model runs were executed on a T31 grid (ca. 3.75° by 3.75°) that represented possible AMOC intensities as different relative levels of ocean flux correction ranging from 0% to 120%, and modelled surface air temperature and precipitation fields were examined in detail. For the late Miocene (Tortonian, ca. 11 – 7 Ma) model data were compared to proxy-based palaeoclimate data. Using anomalies with respect to present-day conditions, a predominance on 100 to 120% AMOC intensity from MAT (mean annual temperature), WMT (warmest month temperature) and around 50% AMOC intensity from CMT (coldest month temperature) were obtained, thus pointing to a high impact of AMOC on the late Miocene climate of Western Eurasia. MAP (mean annual precipitation) failed to give the same response, with reasons suggested to be, inter alia, different response for convective showers and for large-scale precipitation, and relatively simple physics of the climate model. Model results for late Pliocene (3.6 – 2.6 Ma) with closed CAS did not show any sensitivity for the same AMOC intensities, with reasons remaining unclear. Sensitivity experiments with the present climate model cannot fully capture both the temperature and precipitation pattern for the late Miocene, an effect that also is observed with more comprehensive models with coupled ocean. Nevertheless, the experiments demonstrate a strong cyclonic anomaly in the wind field in the North Atlantic region in response to a stronger latitudinal temperature gradient due to a weaker poleward heat transport of the ocean currents which reduces Sea Surface Temperatures (SST) in the North Atlantic. These experiments suggest that changes in CAS depth led to increased climate variability in the North Atlantic and European region from the middle to late Miocene and onward. The lower modelled SST imply generally too cool conditions over Europe compared to the proxy data.
Publications
- (2012): Neogene aridification of the Northern Hemisphere, Geology, 40 (9), 823–826
Eronen, J. T.; Fortelius, M.; Micheels, A.; Portmann, F. T.; Puolamäki, K.; Janis, C. M.
(See online at https://doi.org/10.1130/G33147.1) - (2013): Vegetation change in Siberia and the northeast of Russia during the Cenozoic cooling: A study based on diversity of plant functional types, Palaios, 28(7), 418–432
Popova, S.; Utescher, T.; Gromyko, D. V.; Mosbrugger, V.; Herzog, E.; François, L.
(See online at https://doi.org/10.2110/palo.2012.p12-096r) - (2014): Miocene shift of European atmospheric circulation from trade wind to westerlies, Scientific reports, 4, 5660
Quan, C.; Liu, Y.-S. (C.); Tang, H.; Utescher, T.
(See online at https://doi.org/10.1038/srep05660) - (2014): Technical Note: Are large error bars desirable? A note on quantitative model-proxy comparison, Clim. Past Discuss., 10, 4535–4552
Liakka, J.; Eronen, J. T.; Tang, H.; Portmann, F. T.
(See online at https://doi.org/10.5194/cpd-10-4535-2014)
