Initialization strategy for present day transient climate simulations

Abstract

A 500-year pre-industrial control (PICNTL) was run using standard resolution Community Earth System Model 1.0 (CESM1.0) in which the Community Atmosphere Model 4 (CAM4) contained a Eulerian spectral dynamical core. As is protocol, an ensemble of present day transient simulations were initialized from the PICNTL at various points in time and then run from the mid-19^th Century through to the present day. In anticipation of  next-generation climate models whose horizontal mesh sizes will be much finer than those in current climate models, an alternative protocol was tested whereby the transient ensemble members were initialized from multiple realizations of a forced ocean/sea ice simulation. Climatologies and trends from the resulting present day transient ensemble were compared to those obtained using standard protocol. We particularly focus on the issue of ocean model drift in the PICNTL and transient ensembles to assess the strengths and weakness of the alternative present day transient protocol.