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

Abstract

A new climate model, HadGEM3 N96ORCA1, is presented that is part of the GC3.1 configuration of HadGEM3. N96ORCA1 has a horizontal resolution of ~135 km in the atmosphere and 1° in the ocean and requires an order of magnitude less computing power than its medium‐resolution counterpart, N216ORCA025, while retaining a high degree of performance traceability. Scientific performance is compared to both observations and the N216ORCA025 model. N96ORCA1 reproduces observed climate mean and variability almost as well as N216ORCA025. Patterns of biases are similar across the two models. In the northwest Atlantic, N96ORCA1 shows a cold surface bias of up to 6 K, typical of ocean models of this resolution. The strength of the Atlantic meridional overturning circulation (16 to 17 Sv) matches observations. In the Southern Ocean, a warm surface bias (up to 2 K) is smaller than in N216ORCA025 and linked to improved ocean circulation. Model El Niño/Southern Oscillation and Atlantic Multidecadal Variability are close to observations. Both the cold bias in the Northern Hemisphere (N96ORCA1) and the warm bias in the Southern Hemisphere (N216ORCA025) develop in the first few decades of the simulations. As in many comparable climate models, simulated interhemispheric gradients of top‐of‐atmosphere radiation are larger than observations suggest, with contributions from both hemispheres. HadGEM3 GC3.1 N96ORCA1 constitutes the physical core of the UK Earth System Model (UKESM1) and will be used extensively in the Coupled Model Intercomparison Project 6 (CMIP6), both as part of the UK Earth System Model and as a stand‐alone coupled climate model.

Preindustrial Control Simulations With HadGEM3‐GC3.1 for CMIP6

“…Here we present a selection of ENSO metrics giving an overview of model performance (Table ). These can be compared to Williams et al () and Kuhlbrodt et al () for recent HadGEM3 configurations and Bellenger et al () for CMIP3 and CMIP5 models.…”

“…A power spectrum analysis of east Pacific SST shows that, in agreement with observations, variability lies within the 2‐ to 7‐year timescale for both N96ORCA1 and N216ORCA025 (Figure and M3). Similar to the present‐day control, there is perhaps a little more power in the 2‐ to 3‐year timescale than observed (Kuhlbrodt et al, ). ENSO variability is closely phase locked to the annual cycle with variability at a maximum in early boreal winter and a minimum in spring.…”

“…How to implement such a parameterization at 0.25 ∘ ocean resolution is under debate, and in N216ORCA025 such a parameterization is not used. In the atmosphere, there are a limited set of parameter differences, as detailed in Kuhlbrodt et al (2018, their Table 1). throughout the spin-up and piControl phases, except that only preliminary versions of some of the forcings were available for the early stages of the spin-up runs.…”

“…M6 is the annual mean SST for Niño3 (K), and M7 is the annual mean zonal wind stress for Niño4 (−1,000 Nm −2 ). SST observations are HadISST (1911HadISST ( -2010Rayner et al, 2003), precipitation is GPCP v2.2 (1979Adler et al, 2003) and CMAP (in brackets, 1979CMAP (in brackets, -2015Xie & Arkin, 1997), and wind stress is the SOC climatology (Josey et al, 1998) can be compared to Williams et al (2018) and Kuhlbrodt et al (2018) for recent HadGEM3 configurations and Bellenger et al (2014) for CMIP3 and CMIP5 models.…”

“…How to implement such a parameterization at 0.25° ocean resolution is under debate, and in N216ORCA025 such a parameterization is not used. In the atmosphere, there are a limited set of parameter differences, as detailed in Kuhlbrodt et al (, their Table 1).…”