PlioMIP2 simulations with NorESM-L and NorESM1-F

Abstract

As a continuation of the Pliocene Model Intercomparison Project (PlioMIP), PlioMIP Phase 2 (PlioMIP2) coordinates a wide selection of different climate model experiments aimed at further improving our understanding of the climate and environments during the late Pliocene with updated boundary conditions. Here we report on PlioMIP2 simulations carried out by the two versions of the Norwegian Earth System Model (NorESM), NorESM-L and NorESM1-F, with updated boundary conditions derived from the Pliocene Research, Interpretation and Synoptic Mapping version 4 (PRISM4). NorESM1-M is the version of NorESM that contributed to the Coupled Model Intercomparison Project Phase 5 (CMIP5). NorESM-L is the low-resolution of NorESM1-M, whereas NorESM1-F is a computationally efficient version of NorESM1-M, with similar resolutions and updated physics. Relative to NorESM1-M, there are notable improvements in simulating the strength of the Atlantic meridional overturning circulation (AMOC) and the distribution of sea ice in NorESM1-F, partly due to the updated ocean physics. The two NorESM versions both produce warmer and wetter Pliocene climate, with a greater warming over land than over ocean. Relative to the preindustrial period, the simulated Pliocene global mean surface air temperature is 2.1 ∘C higher with NorESM-L and 1.7 ∘C higher with NorESM1-F, and the corresponding global mean sea surface temperature enhances by 1.5 and 1.2 ∘C. The simulated precipitation for the Pliocene increases by 0.14 mm d−1 globally in both model versions, with large increases in the tropics and especially in the monsoon regions and only minor changes, or even slight decreases, in subtropical regions. The intertropical convergence zone (ITCZ) shifts northward in the Atlantic and Africa in boreal summer. In the simulated warmer and wetter Pliocene world, AMOC becomes deeper and stronger, with the maximum AMOC levels increasing by ∼9 % (with NorESM-L) and ∼15 % (with NorESM1-F), while the meridional overturning circulation slightly strengthens in the Pacific and Indian oceans. Although the two models produce similar Pliocene climates, they also generate some differences, in particular for the Southern Ocean and the northern middle and high latitudes, which should be investigated through PlioMIP2 in the future. As compared to PlioMIP1, the simulated Pliocene warming with NorESM-L is weaker in PlioMIP2 but otherwise shows very similar responses.