Modeling the Miocene climatic optimum: Ocean circulation

Abstract

Ocean circulation is investigated using the Community Climate System Model 3 (CCSM3) forced with early to middle Miocene (∼20–14 Ma) topography, bathymetry, vegetation and modern CO2. Significant bottom water formation is modeled in the Weddell Sea along with intermediate North Component Water formation in the North Atlantic. This is attributed primarily to stronger‐ and weaker‐than‐modern convective preconditioning in the Weddell and Labrador Seas, respectively. Global meridional overturning and gyre circulation is weaker in the Miocene due to weaker midlatitude westerlies in the southern hemisphere, caused by lowering of the meridional surface temperature gradient, in addition to regional influences on convection. Subsurface temperatures in the Miocene are significantly higher in the far North Atlantic, Greenland‐Norwegian Seas and Arctic basin compared to the present. Ocean heat transport is symmetrical about the equator and resembles that simulated for late Cretaceous and early Cenozoic climates, suggesting the northern hemisphere dominated ocean heat transport active today developed after the middle Miocene. Simulated deep water warming in the Miocene is more than an order of magnitude lower than indicated by proxies. This discrepancy is not reconciled by higher CO2 due to the persistence of sea‐ice at sites of deep water formation. This suggests that either the CCSM3 is insufficiently sensitive to Miocene boundary conditions, greater greenhouse forcing existed than is currently reconstructed, or that proxy records of warming are exaggerated. Given the diversity of global Miocene proxy records and their near‐unanimous estimate of a significantly warmer Earth, the first two options are more likely.

The role of eastern Tethys seaway closure in the Middle Miocene Climatic Transition (ca. 14 Ma)

“…Their experiments particularly depict an overestimated ACC, relatively low water transport in the Panama seaway and a net water flow in the Tethys seaway which is oriented from the Atlantic to the Indian Ocean (von der Heydt and Dijkstra, 2005). These results are consistent with an open Tethys experiment carried out with a later version of the model used in von der Heydt andDijkstra (2005, 2006) -CCSM3 - (Herold et al, 2012) but differ from ours, which depicts a weak eastward Mediterranean throughflow and a strong eastward transport through Panama (not shown). The diverging responses with the von der Heydt and Dijkstra (2005) study can be explained by important palaeogeography differences (they used idealised flat-bottom ocean (5000 m) and two geographic configurations corresponding to the late Oligocene and the early Miocene).…”

“…The diverging responses with the von der Heydt and Dijkstra (2005) study can be explained by important palaeogeography differences (they used idealised flat-bottom ocean (5000 m) and two geographic configurations corresponding to the late Oligocene and the early Miocene). However, our palaeogeography being the same as used in Herold et al (2012), differences with the latter are more likely linked to model parameterisation and ocean-atmosphere coupling. Using a three-box model, Karami et al (2009) found a water flow oriented from the west to the east in the eastern Tethys seaway.…”

“…In our experiments, the Mediterranean Basin is characterised by high density, mainly due to the high salinity. In the experiment from Herold et al (2012), salinity in the Mediterranean Using an ocean circulation carbon cycle model of intermediate complexity, Butzin et al (2011) performed sensitivity experiments on various eastern Tethys and Panama gateways configurations. Comparable with our study, water fluxes through the Tethys seaway are northward in surface and southward in depth.…”

Modelling global-scale climate impacts of the late Miocene Messinian Salinity Crisis

“…In terms of ocean circulation, both proxy-and modelbased research suggests that with an open CAS, Messinian North Atlantic Deep Water (NADW) formation would have been considerably weaker than for the present day (Böhme et al, 2008;Herold et al, 2012;Lunt et al, 2008;Molnar, 2008;Murdock et al, 1997;Prange and Schulz, 2004;Schneider and Schmittner, 2006;Steph et al, 2010;Zhang et al, 2012). We find that the maximum Atlantic Overturning Circulation is ∼ 17.5 Sv, compared to ∼ 18.5 Sv in the modern equivalent (e.g.…”

Late Eocene to middle Miocene (33 to 13 million years ago) vegetation and climate development on the North American Atlantic Coastal Plain (IODP Expedition 313, Site M0027)

“…Climate model results assessing differences between the MMCO interval and present-day conditions (e.g. Herold et al, 2011Herold et al, , 2012 show only minor differences between MMCO and presentday conditions for the eastern US coast (including the region around New Jersey), while the northern North American (Canadian) coast and the regions around the Gulf of Mexico reveal strong differences. However, the climate models used in the studies of Herold et al (2011Herold et al ( , 2012) also indicate minor changes for western Europe, which is not congruent with results from European pollen records showing very warm conditions during the middle Miocene (e.g.…”