Study evaluates the E3SM simulation of the North Atlantic Oscillation and its effects on extremes.
Extreme value analysis reveals that high-resolution (0.25º) E3SMv0.3 better simulates the impact of the North Atlantic Oscillation (NAO) on precipitation extremes over parts of Western Europe. NAO-dependent upward mass fluxes are stronger in the high-resolution (high-res) model, intensifying precipitation. But, the high-res model simulates a weaker than observed impact of NAO on extra-tropical cyclone activity, underperforming the low-res model (1º).
Simulated NAO impacts on precipitation extremes are generally improved with an increase in E3SM resolution. However, NAO-associated large-scale drivers of these extremes are poorly reproduced. Alleviating these deficiencies could further improve teleconnections of large-scale climate variability modes.
DOE scientists evaluated a high-resolution (0.25°), four-member ensemble simulation of the global climate (1979-2005) with the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM) v0.3—forced with observed ocean surface temperatures and sea ice extent—for its ability to represent the North Atlantic Oscillation (NAO) teleconnections to winter precipitation extremes over Western Europe. As compared to the low-resolution model (1°), it simulates a stronger impact of NAO on daily precipitation extremes over the western slopes of mountain ranges over southwestern Norway, northwestern United Kingdom, and the Western Balkan states. Precipitation extremes and their linear relationship with NAO are quantified using the generalized extreme value distribution. NAO-dependent large-scale (stratiform) precipitation intensity strengthens in the high-resolution model on seasonal time scales and plays a dominant role during simulated daily precipitation extremes. Improvements in the high-resolution model over these varied-topography regions largely appear to be due to finer resolved scales of motion that amplify NAO-dependent seasonal vertical moisture fluxes and enhance stable condensation. However, the high-resolution model simulates a weaker than observed impact of NAO on extratropical cyclone activity and total precipitable water, generally underperforming the low-resolution model. These effects possibly offset the impact of enhanced vertical moisture fluxes on NAO-dependent precipitation extremes in the high-resolution model in these regions. Over the southwestern Iberian peninsula, the high-resolution model underperforms the low-resolution model simulating weaker than observed impact of NAO on precipitation extremes. This appears to be due to the reduction in total precipitable water despite an increase in NAO-dependent extratropical activity there.
Contacts (BER PM)
Earth and Environmental System Modeling
Department of Energy, Office of Science, Biological and Environmental Research
Oak Ridge National Laboratory
This research was supported by the Office of Science, Office of Biological and Environmental Research of the US Department of Energy under contract no. DE-AC02-05CH11231 as part of the Energy Exascale Earth System Model (E3SM) programs.
Mahajan, S., K. J. Evans, M. L. Branstetter and Q. Tang, “Model Resolution-Sensitivity of the Simulation of North Atlantic Oscillation Teleconnections to Precipitation Extremes.”Journal of Geophysical Research -Atmospheres 123(20), 11,392-11,409 (2018). [DOI:10.1029/2018JD028594]
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