Evaluation of FESOM2.0 Coupled to ECHAM6.3: Preindustrial and HighResMIP Simulations

Abstract

A new global climate model setup using FESOM2.0 for the sea ice-ocean component and ECHAM6.3 for the atmosphere and land surface has been developed. Replacing FESOM1.4 by FESOM2.0 promises a higher efficiency of the new climate setup compared to its predecessor. The new setup allows for long-term climate integrations using a locally eddy-resolving ocean. Here it is evaluated in terms of (1) the mean state and long-term drift under preindustrial climate conditions, (2) the fidelity in simulating the historical warming, and (3) differences between coarse and eddy-resolving ocean configurations. The results show that the realism of the new climate setup is overall within the range of existing models. In terms of oceanic temperatures, the historical warming signal is of smaller amplitude than the model drift in case of a relatively short spin-up. However, it is argued that the strategy of "de-drifting" climate runs after the short spin-up, proposed by the HighResMIP protocol, allows one to isolate the warming signal. Moreover, the eddy-permitting/resolving ocean setup shows notable improvements regarding the simulation of oceanic surface temperatures, in particular in the Southern Ocean.

Description of the resolution hierarchy of the global coupled HadGEM3-GC3.1 model as used in CMIP6 HighResMIP experiments

“…A part of the deficit in the M ocean model is due to a strong countercurrent around the Antarctic shelf of about 20 Sv, together with changes to the density front, as dis-cussed more fully in Menary et al (2018) and . The ACC in MH and HH remains lower than observed (as also seen in Small et al, 2014) -these have negligible countercurrents, but perhaps they still have too much southward heat transport (Fig. 17a) and consequently weakened density gradient, and the H ocean resolution is still only marginally eddy resolving at these latitudes (Hallberg, 2013).…”

“…It is unclear how robust all of the results shown here will be across a multi-model dataset. Ongoing work within Horizon 2020 PRIMAVERA project (Grist et al, 2018;Vannière et al, 2018) suggests that at least for changes from 100 to 25 km ocean resolution there are robust reductions in SST and precipitation biases, while work by Griffies et al (2015) and Small et al (2014Small et al ( , 2019 does indicate some consistency in further changes to eddy-rich ocean resolutions. Further work using this multi-model ensemble within HighResMIP, in addition to comparing these simulations to their CMIP6 DECK equivalents, is ongoing and may reveal further insights into the impact of resolution and model complexity (Earth system processes such as interac- Figure 20.…”

“…The largest changes are found between the L and M resolution components, with smaller changes at higher resolutions (consistent with C. D. . For the L and M resolution ocean, a higher-resolution atmosphere tends to produce a cooler ocean SST, particularly in the ocean upwelling regions (as also seen in Gent et al, 2010 andSmall et al, 2014, 2015), the Southern Ocean and the North Atlantic. A higher ocean resolution, meanwhile, tends to produce a warmer SST particularly in boundary currents and other high gradient regions, with changes over 6 K (Griffies et al, 2015;.…”

“…For the global ocean, the temperature and salinity drifts differ mainly in their magnitude, while the Atlantic drifts have different patterns. These drifts may be compared to those in Small et al (2014), Griffies et al (2015) and Kuhlbrodt et al (2018).…”

“…There is now considerable evidence that enhancing model horizontal resolution can help to reduce systematic and longstanding climate model biases (Kinter et al, 2013;Small et al, 2014;Griffies et al, 2015;Hewitt et al, 2016; C. D. and hence potentially improve the robustness and trust in future projections. Some of the evidence for this comes from previous Coupled Model Intercomparison Project (CMIP) exercises (Meehl et al, 2000(Meehl et al, , 2007Taylor et al, 2012).…”