Initial validation of an agile coupled atmosphere-ocean general circulation model

Mathematical models based on physics, chemistry and biology principles are one of the main tools to understand climate interactions, variability and sensitivity to forcings. Model performance must be validated checking that results are consistent with actual/observed climate. This work describes the initial validation of a new intermediate complexity, coupled climate model based on a set of existing atmosphere, ocean and sea-ice models. The model, developed and made available by the International Centre for Theoretical Physics (ICTP), is based on the widely used SPEEDY atmospheric model. Limited literature is available for its version, coupled to the NEMO ocean model referred to as SPEEDY-NEMO. The focus of this study is on the adaptation and validation of this model. A long-term spin-up run with constant present-day forcing has been performed to achieve a steady-state climate. The simulated climate has then been compared with observations and reanalyses of the recent past. The initial validation has shown that simulations spanning a thousand years can be easily run. The model does not require many h/w resources and therefore significant size samples can be generated if needed. Our results prove that long timescale, stable simulations are feasible. The model reproduces the main features of Earth’s mean climate and variability, despite the use of a fairly limited resolution grid, simple parameterizations and a limited range of physical processes. Ocean model outputs have not been assessed. However, a clear El Niño signal in the simulated Sea Surface Temperatures (SSTs) data and arctic sea ice extent show that the ocean model behaviour is close to observations. According to the results, the model is a promising tool for climate studies. However, to understand its full potential the validation should be improved and extended with an analysis of ocean variables and targeted simulations with modified conditions to evaluate model behaviour under different conditions