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Increased ventilation of Antarctic deep water during the warm mid-Pliocene

The mid-Pliocene warm period is a recent warm geological period that shares similarities with predictions of future climate. It is generally held the mid-Pliocene Atlantic Meridional Overturning Circulation must have been stronger, to explain a weak Atlantic meridional δ13C gradient and large northern high-latitude warming. However, climate models do not simulate such stronger Atlantic Meridional Overturning Circulation, when forced with mid-Pliocene boundary conditions. Proxy reconstructions allow for an alternative scenario that the weak δ13C gradient can be explained by increased ventilation and reduced stratification in the Southern Ocean. Here this alternative scenario is supported by simulations with the Norwegian Earth System Model (NorESM-L), which simulate an intensified and slightly poleward shifted wind field off Antarctica, giving enhanced ventilation and reduced stratification in the Southern Ocean. Our findings challenge the prevailing theory and show how increased Southern Ocean ventilation can reconcile existing model-data discrepancies about Atlantic Meridional Overturning Circulation while explaining fundamental ocean features.


Ocean dynamics in the Southern Ocean/South Atlantic
In the mid-Pliocene experiment, due to the warm climate and the lower topography caused by smaller Antarctic ice-sheet, sea level pressure is decreased, especially along the coast of Antarctica, relative to the pre-industrial experiment. The changes in pressure system make the wind stress strengthened and slightly zonal shifted over the Southern Ocean/South Atlantic. In particular, there is a strengthening of easterlies along the coast of Antarctica, as well as westerlies over the ACC region (Fig. 3 in the main paper and Fig. S1).
The simulated mid-Pliocene sea surface temperature (SST) is warm 14 and the Southern Ocean is almost free of sea ice. The simulated sea ice only appears in small areas of the Weddell Sea and the Ross Sea (Fig. S2), which is consistent with recent sea ice reconstructions from Site ANDRILL AND-1B 42 that shows sea ice developed in the Ross Sea in the mid-Pliocene. This leaves the surface more sensitive to wind stress. Hence, the intensified surface wind stress in the mid-Pliocene efficiently mixes the upper layers of the ocean and increases the mixed layer depth off the coast of Antarctica by 200 m (Fig. S3), which contributes to the ventilation of the surface/subsurface waters in the Southern Ocean.
Due to the Ekman effect, stronger easterlies off Antarctica lead to increased mass convergence along the coast of Antarctica (Fig. 3 in the main paper). The increased Ekman convergence causes enhanced downwelling in the Southern Ocean along the coast of Antarctica, and anomaly upwelling occurs in the Southern Ocean/South Atlantic (Fig. S1c).
As a result, the Southern Ocean/South Atlantic is well ventilated, and water exchanges between surface/subsurface and deep ocean become faster. The efficient renewal of deep water makes water age younger in the deep Southern Ocean/South Atlantic in the mid-Pliocene, relative to the pre-industrial experiment ( Fig. S4 and S5). From the anomaly water age figures (Fig. S4), it looks like younger water subsides into deep ocean. In addition to the younger age, water masses between 2 and 4 km depth in the Southern Ocean become relatively fresher (Fig. S6c) and colder (Fig. S7c) compared to the pre-industrial experiment. Linkage between sea water age and δ 13 C Sea water age can be indicated by benthic foraminifera δ 13 C 24 , because newly formed (young) low nutrient deep water has high δ 13 C values which decrease with age due to mixing and the gradual respiration of low δ 13 C organic matter. There is a beautiful (linear) relationship between δ 13 C and water mass age (radiocarbon) in the modern ocean (CARINA dataset, http://cdiac.ornl.gov/oceans/CARINA/about_carina.html).
The mechanism behind this relationship can be understood by the oxidation of organic matters in deep ocean. Newly formed low nutrient deep water has high δ 13 C values and high organic matters. When these organic matters are gradually oxidized in the deep ocean, CO 2 is released and dissolved in deep water, thus leading to light (negative) δ 13 C in deep water. The oxidation of organic matters depends on time (water age). Old deep water has high content of CO 2 and nutrient, but low δ 13 C values and organic matters.
In this relationship, the main other effect causing "scatter" (deviation from the main trend) is overwhelmingly related to thermodynamic fractionation (equilibration of water masses at different temperatures). Since the Pliocene ocean temperature gradient is reduced, this effect will be even smaller in the Pliocene. It is already far too small to explain the much larger changes observed in the paleo data.

Supplementary Methods
The NorESM is an  39 . This is an appropriate parameterization for most of oceans, and is often used in ocean models 40  In the NorESM, an ideal water age tracer eg.41 is included. The tracer calculates sea water age-the time elapsed since a water parcel in direct contacts with the sea surface. It is also an indicator for experimental equilibrium. The simulated water age in the Atlantic section (from surface to 4 km deep) is younger than 1000 years, indicating that the ocean stratification has reached equilibrium in the Atlantic section in 1000-year integrations. Thus, the 1500-year integrations are long enough for the Atlantic section to reach equilibrium in the pre-industrial and the mid-Pliocene experiment.