The Met Office Global Coupled Model 3.0 and 3.1 (GC3.0 and GC3.1) Configurations

Abstract

The Global Coupled 3 (GC3) configuration of the Met Office Unified Model is presented. Among other applications, GC3 is the basis of the United Kingdom's submission to the Coupled Model Intercomparison Project 6 (CMIP6). This paper documents the model components that make up the configuration (although the scientific descriptions of these components are in companion papers) and details the coupling between them. The performance of GC3 is assessed in terms of mean biases and variability in long climate simulations using present‐day forcing. The suitability of the configuration for predictability on shorter time scales (weather and seasonal forecasting) is also briefly discussed. The performance of GC3 is compared against GC2, the previous Met Office coupled model configuration, and against an older configuration (HadGEM2‐AO) which was the submission to CMIP5. In many respects, the performance of GC3 is comparable with GC2, however, there is a notable improvement in the Southern Ocean warm sea surface temperature bias which has been reduced by 75%, and there are improvements in cloud amount and some aspects of tropical variability. Relative to HadGEM2‐AO, many aspects of the present‐day climate are improved in GC3 including tropospheric and stratospheric temperature structure, most aspects of tropical and extratropical variability and top‐of‐atmosphere and surface fluxes. A number of outstanding errors are identified including a residual asymmetric sea surface temperature bias (cool northern hemisphere, warm Southern Ocean), an overly strong global hydrological cycle and insufficient European blocking.

Increasing Atlantic Ocean Heat Transport in the Latest Generation Coupled Ocean‐Atmosphere Models: The Role of Air‐Sea Interaction

“…The spin‐up is included in the averaging period/run length (Table ). As indicated in Williams et al (), the model surface climatology takes around 30 years to reach a pseudo‐equilibrium, with a top‐of‐atmosphere radiative imbalance of about 0.6 W/m 2 , which is consistent with present‐day forcing. The adjustment timescale for the deeper ocean is clearly longer, with the largest global‐mean ocean temperature drift (0.11 K/decade 1 ) occurring at 563 m. However, the HRa minus LRa AOHT difference remains stable over the course of the run, indicating that both models adjust in similar ways.…”

The Low‐Resolution Version of HadGEM3 GC3.1: Development and Evaluation for Global Climate

“…Both the high‐ and low‐resolution models are in good agreement with the seasonal Arctic sea ice extent (Figure ) when compared with the HadISST.2 sea ice analysis (Titchner & Rayner, ). In the Southern Ocean, N96ORCA1 shows a better agreement because the Southern Ocean warm bias (Williams et al, ) is less severe than at high resolution (cf. section ).…”

Estimating the numerical diapycnal mixing in an eddy-permitting ocean model

“…It is used widely in forecasting and climate modelling: the current version of the UK Met Office's FOAM operational ocean forecasting system (Blockley et al, 2014) and the UK coupled climate model GC2 (Williams et al, 2015) use GO5.0 as their ocean component. The GC3 climate model (Williams et al, 2017) and the new UK Earth System Model UKESM1, both aimed at the IPCC Sixth Assessment Report, will both use an ocean component closely related to GO5.0, in particular sharing its horizontal and vertical grids (albeit with a southward extension in the more recent configurations to allow ice shelves to be simulated) and most of its physics choices.…”