Mitigating Climate Biases in the Midlatitude North Atlantic by Increasing Model Resolution: SST Gradients and Their Relation to Blocking and the Jet

Abstract

Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multi-model ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multi-model ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the stormtrack. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

The Northern Hemisphere Wintertime Storm Track Simulated in the High‐Resolution Community Earth System Model

“…The response of the 850-hPa storm track to model resolution is similar to the response of the surface. There is likely a role for the above surface processes also associated with the changes in the free troposphere, which is consistent with previous work (Athanasiadis et al, 2022;Small et al, 2014bSmall et al, , 2019C. Zhang et al, 2020a).…”

“…The results imply that increasing ocean resolution from 1° to 0.1° is still too coarse to fully resolve coastal dynamics in terms of air‐sea interactions and does not eliminate all the biases in the simulation of the Gulf Stream and North Atlantic Current system. The SST bias in the North Atlantic can induce further biases in the atmospheric circulations over the entire North Atlantic and Europe (Athanasiadis et al., 2022; Small et al., 2019). A detailed analysis of the impacts of model horizontal resolution on SST biases in this CESM1.3 can be found in Xu et al.…”

“…Additionally, a larger negative bias in precipitation (by up to 0.3–1.4 mm/d) is seen in HR than in LR (Figure 8g), which is co‐located with the reduction of both the SST gradient and sensible and latent heat flux. This is indicative of a combination of weakened instability (the deficit in sensible heat flux) and reduced moisture content (due to the insufficient latent heat flux) (Athanasiadis et al., 2022). These changes could be related to the resolution‐induced bias of the surface storm track, suggesting the importance of the diabatic processes for improving the North Atlantic storm track simulation (Schemm, 2023; Willison et al., 2013).…”

Climate model Selection by Independence, Performance, and Spread (ClimSIPS) for regional applications

“…Additionally, we include annual-average North Atlantic SST climatology because SST biases in the region have been linked to biases in European SAT and PR variability through interactions with atmospheric circulation (e.g. Keeley et al, 2012;Simpson et al, 2019;Borchert et al, 2019;Athanasiadis et al, 2022). As atmospheric circulation biases tend to be more pronounced in the winter than in the summer, we also explicitly incorporate mean state SLP in the North Atlantic sector in the winter predictor set.…”

The Euro-Mediterranean Center on Climate Change (CMCC) decadal prediction system

“…From another perspective, model errors may be mitigated by enhancing spatial resolution (both horizontal and vertical) in the oceanic and atmospheric model components, since a coarse resolution limits a realistic representation of key physical processes (e.g., realistic SST front in the Gulf Stream region), impacting the atmospheric circulation downstream (Athanasiadis et al, 2022;Paolini et al, 2022). For instance, an eddy-permitting ocean model (i.e., 0.25 • horizontal resolution) in a fully coupled system led to improved decadal predictions over the whole equatorial zone (Shaffrey et al, 2017).…”