Impact of Ocean Eddy Resolution on the Sensitivity of Precipitation to CO2 Increase. Issue 14 (20th July 2018) - He, Jie
Abstract: The past few years have seen a growing investment in the development of global eddy‐resolving ocean models, but the impact of incorporating such high ocean resolution on precipitation responses to CO2 forcing has yet to be investigated. This study analyzes precipitation changes from a suite of Geophysical Fluid Dynamics Laboratory models incorporating eddy‐resolving (0.1°), eddy‐permitting (0.25°), and eddy‐parameterizing (1°) ocean models. The incorporation of eddy resolution does not challenge the large‐scale structure of precipitation changes but results in substantial regional differences, particularly over ocean. These oceanic differences are primarily driven by the pattern of sea surface temperature (SST) changes with greater sensitivity in lower latitudes. The largest impact of ocean resolution on SST changes occurs in eddy‐rich regions (e.g., boundary currents and the Southern Ocean), where impact on precipitation changes is also found to various degrees. In the Gulf Stream region where previous studies found considerable impact of eddy resolution on the simulation of climatological precipitation, we do not find such impact from the Geophysical Fluid Dynamics Laboratory models, but we do find substantial impact on precipitation changes. The eddy‐parameterizing model projects a banded structure common to the Coupled Model Intercomparison Project (Phase 5) models, whereas the higher‐resolution models project a poleward shift of precipitation maxima associated with an enhanced Gulf Stream warming. Over land, precipitation changes are generally not very sensitive to ocean resolution. In eastern North America adjacent to the Gulf Stream region, moderate differences are found between resolutions. We discuss the mechanisms of land differences, which arise through the simulation of both climatological SST and SST changes. Plain Language Summary: Global coupled atmosphere‐ocean models have not included the explicit simulation of ocean mesoscale eddies (turbulent ocean flow with a horizontal scale of less than 100 km) until the recent few years. As a result, current estimates of precipitation responses to greenhouse gases increase have predominantly relied on models in which ocean eddies have to be parameterized. Given the importance of eddies on ocean heat transport and the strong dependence of precipitation on ocean surface temperature, the lack of ocean eddy resolution may have influenced the projection of precipitation changes. In this study, we compare precipitation changes from CO2 forcing in models with and without explicit ocean eddy simulation. It is found that the incorporation of ocean eddy resolution can substantially influence the simulation of precipitation changes in eddy‐rich regions and the deep tropics. Some land regions are also affected, albeit to a lesser degree compared to the oceanic regions. Key Points: The incorporation of ocean eddy resolution has substantial influences on regional precipitation changes The largest impact of ocean eddy resolution on precipitation projections is found in eddy‐rich regions and the deep tropics Land regions are generally not very sensitive to ocean eddy resolution