Impact of Model Resolution on Subtropical Eastern North Pacific Sea Surface Temperature Bias
Increasing model resolution allows for more accurate representations of coastal ocean processes.
Background and Impact
The subtropical eastern boundary regions in the global oceans play a key role in the climate of the Earth system. These are regions with shallow marine stratocumulus cloud decks that modulate the planet’s greenhouse gas forcing, and consequently, the radiative balance. Stratus clouds are maintained partly by cold sea surface temperatures (SSTs) associated with the eastern boundary currents that flow equatorward and form the eastern branch of subtropical gyres. Alongshore winds drive these currents and Ekman divergence along the coast, causing the upwelling of cold nutrient-rich waters making these regions some of the most biologically productive globally (Figure 1 C,D) Understanding these areas well, and accurately projecting their response under climate change is therefore important.
However, most coupled climate models incorrectly simulate various processes occurring in these regions and suffer from warm SST biases. Several previous studies examined the processes responsible for warm SST biases near eastern boundaries, however, they predominantly focused on the African coast in the Southeast Atlantic and the South American coast in the Southeast Pacific, prompting the need for an evaluation of the bias in the Subtropical Eastern North Pacific (SENP) region.
Science
A research team systematically evaluated the warm SST bias in the SENP, a problem plaguing most climate models, using the Energy Exascale Earth System Model (E3SM). At standard/low model resolution (1-degree atmosphere, 0.4-degree [30–60 km] ocean), the SST bias of several degrees is mainly concentrated along the coast between 25oN and 40oN (Figure 1A). In the high resolution (0.25-degree atmosphere, 0.1-degree [18–6 km] ocean) version of the model, the nearshore SST bias improves considerably with better representation of the California Current System and meridional winds along the coast (Figure 1 A,B). However, the offshore SST bias, centered at approximately 125oW and 25oN, is relatively stronger in the high-resolution version.
To better understand the model’s offshore warm bias, researchers performed a mixed layer heat budget analysis. While errors in the surface radiative fluxes due to cloud misrepresentations occur at both resolutions, positive biases in horizontal heat transport play a role in the SST bias at high-resolution.
Analysis of HighResMIP models (an intercomparison of climate models at low and high resolutions) indicates that the shift in the location of the prominent SST bias from nearshore to offshore with an increase in model spatial resolution is not unique to E3SM. A larger inter-model correlation of SST bias with bias in coastal upwelling in a suite of models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) highlights the important role of large-scale circulation biases in the SENP SST bias in coupled models.
Figure 1. A) Annual mean SST (oC) bias in E3SM_LR (E3SM at low or standard resolution). B) The difference in annual mean SST between E3SM_HR (high resolution E3SM) and E3SM_LR. C-D) The same as in A-B but for Ekman transport (m2s-1). In panels C and D, the color in the background represents the magnitude of vector differences.
Publication
- Balaguru, K, L Van Roekel, L Leung, and M Veneziani. 2021. “Subtropical Eastern North Pacific SST Bias in Earth System Models.” Journal of Geophysical Research: Oceans 126(8). https://doi.org/10.1029/2021jc017359.
Funding
- This work was supported by the Energy Exascale Earth System Model (E3SM) project, funded by the US Department of Energy, Office of Science, Office of Biological and Environmental Research.
Contact
- L. Ruby Leung, Pacific Northwest National Laboratory
