The warm, ~400ppm CO2 world of the Pliocene supported a northern-sourced meridional overturning cell in the Pacific Ocean

Mainz, September 13, 2017

Press release of the George Mason University, Fairfax, VA, USA / adaption by Max Planck Institute for Chemistry

Appearing online in Science Advances on Wednesday September 13th. This manuscript, by Natalie Burls, Alexey Fedorov, Daniel Sigman, Samuel Jaccard, Ralf Tiedemann and Gerald Haug, presents compelling observational and modeling evidence that the ocean had an intense Pacific Meridional Overturning Circulation (PMOC) during the Pliocene – a counterpart of the well known and much discussed Atlantic Meridional Overturning Circulation (AMOC). The Pliocene, 5.3 – 2.5 million years ago, is a time interval that has been investigated quite extensively due to its potential similarity to the climate our planet will face in the context of future anthropogenic warming.

The modern ocean is characterized by a strong AMOC, which is of critical importance for nutrient cycles, and the air-sea exchange of carbon dioxide (CO2). The modern Pacific lacks such an overturning cell due to the presence of a layer of very fresh surface water (which oceanographers refer to as a halocline) preventing deep water formation, a crucial asymmetry between the two oceans that affects a broad range of climatic variables from the mean temperature of the North Atlantic to the position of the Intertropical Convergence Zone in the atmosphere. Why this asymmetry exists, and whether there were times in the past with different configurations of ocean deep cells, are fundamental questions in climate science.

Our fully coupled climate model simulations demonstrate that the large-scale sea-surface temperature patterns of the warm Pliocene, particularly the weak zonal and meridional sea surface temperature gradients, are capable of maintaining changes in the atmosphere’s hydrological cycle that act to increase the salinity of the subarctic North Pacific leading to deep water formation and an deep meridional overturning cell. On the data side, enhanced sedimentary calcium carbonate and biogenic opal accumulation provide direct evidence of deep water formation in a location that is remarkably consistent with the region in which our simulation predicts deep water formation. Taken together these results make a convincing argument that Pliocene conditions supported a strong Pacific meridional overturning cell (PMOC), comparable in strength to that existing today in the Atlantic.

The establishment of the PMOC is not a minor change in ocean circulation – in fact it is a major reorganization of ocean circulation with fundamental consequences for climate. The Pacific is by far the largest ocean basin and the presence of the deep meridional overturning in this basin would have had impacts, for example, on carbon and nutrient cycling globally with potentially important implications for the evolution of climate.

These findings might also provide some insight into the long-term (order thousands of years) trajectory of circulation changes in the Pacific under future global warming. While the oceans are expected to warm more at the surface than at depth initially, which will act to inhibit deep water formation in the Pacific, as the deep oceans slowly warms and if the waters of the subarctic Pacific become saline enough this may result in North Pacific deep water formation.