Analysis of a stronger Atlantic Meridional Overturning Circulation in mid-Pliocene simulations

Abstract

The mid-Pliocene is the most recent geological period with a greenhouse gas concentration of approximately 400 ppmv, similar to the present day. Proxy reconstructions indicate enhanced warming in the high North Atlantic in the mid-Pliocene, which has been suggested to be a response to a stronger Atlantic Meridional Overturning Circulation (AMOC). Ensemble results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show a stronger AMOC and North Atlantic sea surface temperatures (SSTs) that match reconstructions better than PlioMIP1. A major difference between PlioMIP1 and PlioMIP2 is the closure of the Bering Strait and Canadian Archipelago in PlioMIP2. Previous studies have shown that closure of these Arctic gateways leads to an intensified AMOC due to altered freshwater fluxes in the Arctic. For this study, we compared results from our Community Earth System Model (CESM1) simulations, using two pre-industrial runs with a CO2 concentration of 280 and 560 ppmv and three mid-Pliocene runs with PlioMIP2 boundary conditions and a CO2 concentration of 280, 400 and 560 ppmv. Our analysis shows that the simulated intensified AMOC in the mid-Pliocene is a result of the palaeogeographic boundary conditions, in particular the closure of the Arctic gateways. In the mid-Pliocene simulations, the AMOC intensifies by 2.6 - 4.2 Sv as a result of a higher salinity in the Labrador Sea. The stronger AMOC is accompanied by enhanced Atlantic northward ocean heat transport, which is the cause for amplified SST warming in the high North Atlantic in the mid-Pliocene. Further analysis on the variability of the AMOC suggests that the simulated mid-Pliocene AMOC shows fundamentally different behavior from the pre-industrial AMOC, where it appears that the mid-Pliocene AMOC is driven more strongly by salinity and has a weakened temperature feedback. This is supported a high sensitivity of the mid-Pliocene AMOC to the surface freshwater flux in the Labrador Sea.