Origin of the warm eastern tropical Atlantic SST bias in a climate model

Abstract

The substantial warm sea surface temperature bias in the eastern Tropical Atlantic reported in most CMIP5 climate simulations with various models, in particular along the coast of Namibia and Angola, remains an issue in more recent and CMIP6-ready versions of climate models such as EC-Earth. A complete and original set of experiments with EC-Earth3.1 is performed to investigate the causes and mechanisms responsible for the emergence and persistence of this bias. The fully-developed bias is studied in a historical experiment that has reached quasi-equilibrium, while retrospective prediction experiments are used to highlight the development/growth from an observed initial state. Prediction experiments are performed at both low and high resolution to assess the possible dependence of the bias on horizontal resolution. Standalone experiments with the ocean and the atmosphere components of EC-Earth are also analyzed to separate the respective contributions of the ocean and atmosphere to the development of the bias. EC-Earth3.1 exhibits a bias similar to that reported in most climate models that took part in CMIP5. The magnitude of this bias, however, is weaker than most CMIP5 models by few degrees. Increased horizontal resolution only leads to a minor reduction of the bias in EC-Earth. The warm SST bias is found to be the result of an excessive solar absorption in the ocean mixed layer, which can be linked to the excessive solar insolation due to unrealistically low cloud cover, and the absence of spatial and temporal variability of the biological productivity in the ocean component. The warm SST bias is further linked to deficient turbulent vertical mixing of cold water to the mixed layer. Our study points at a need for better representation of clouds in the vicinity of eastern boundaries in atmosphere models, and better representation of solar penetration and turbulent mixing in the ocean models in order to eliminate the Tropical Atlantic biases.

The Southeastern Tropical Atlantic SST Bias Investigated with a Coupled Atmosphere–Ocean Single-Column Model at a PIRATA Mooring Site

“…For the case of 2014 we eliminate solar surface radiation as the main cause of the warm SST bias. This is in line with other studies (Exarchou et al 2017;Voldoire et al 2019;Deppenmeier et al 2020). Forcing the ocean with observed surface shortwave radiation does not improve simulation of the SST.…”

“…The surface solar radiation bias, hence, cannot explain the warm SST bias, which is larger by almost two orders of magnitude. This conclusion is consistent with those of other studies using EC-Earth (Exarchou et al 2017;Voldoire et al 2019;Deppenmeier et al 2020).…”

“…In a gradual build-up, the SST bias grows to 1.18C at the end of the simulation. This bias is smaller than that of most state-of-the-art coupled GCMs, but it is only slightly smaller than the bias in the threedimensional version of EC-Earth (Exarchou et al 2017;Voldoire et al 2019, and Fig. 1b).…”

“…The specifics of the parameterization determine the strength of the mixing included in the model. The underrepresentation of this vital process is a strong candidate for producing the warm bias (Hazeleger and Haarsma 2005;Exarchou et al 2017;Planton et al 2018).…”

“…A recent multimodel study highlights the role of wind stress forcing in the bias development ), but also shows that it cannot explain the entire bias and sometimes even has limited effect (as is the case for EC-Earth, which we use here). Other studies have highlighted the contribution of the ocean model (Xu et al 2014b), its horizontal and vertical resolution (Seo et al 2006;Doi et al 2012;Small et al 2014), advection (Goubanova et al 2019), and turbulent processes (Hazeleger and Haarsma 2005;Exarchou et al 2017;Planton et al 2018) to the bias formation. The question of the southeastern tropical Atlantic warm bias is not yet resolved and more analysis is clearly necessary to trace its origins.…”