DOI: 10.18739/a22f7js58
title: Monthly climate fields in the Arctic in a CESM1 nudging run constrained by ERA5 6-hourly winds from 1979 to 2020

description: All stored fields are the ensemble means of 10-member CESM1 nudging simulations from 1979 to 2020 (42yr) conducted in Ding et al. (2022). Daily averages of these fields are stored on Ding's local data archive and monthly averages of daily values (from 30N to 90N) are available for download here. In addition, the data from individual members or other variables are available from Ding (qinghua@ucsb.edu) upon request. Please let me know if other fields are desirable. Ding, Q., A. Schweiger, and I. Baxter, 2022: Nudging Observed Winds in the Arctic to Quantify Associated Sea Ice Loss from 1979 to 2020. J. Climate, 35, 3197–3213, https://doi.org/10.1175/JCLI-D-21-0893.1. Field1: Northern Hemisphere (NH) (30N-90N) monthly sea ice fraction from 1979/1 to 2020/12- icefrac.monthly.1979-2020.nc Field2: NH (30N-90N) monthly surface temperature from 1979/1 to 2020/12- ts.monthly.1979-2020.nc Field3: NH (30N-90N) monthly longwave radiation at the top of the atmosphere (TOA) from 1979/1 to 2020/12- flnt.monthly.1979-2020.nc Field4: NH (30N-90N) monthly net shortwave radiation at the TOA from 1979/1 to 2020/12-fsnt.monthly.1979-2020.nc Field5: NH (30N-90N) monthly net longwave radiation at the surface from 1979/1 to 2020/12-flns.monthly.1979-2020.nc Field6: NH (30N-90N) monthly net shortwave radiation at the surface from 1979/1 to 2020/12-fsns.monthly.1979-2020.nc Field7: NH (30N-90N) monthly geopotential height(10 vertical levels) from 1979/1 to 2020/12-height.monthly.1979-2020.nc Field8: NH (30N-90N) monthly air temperature (10 vertical levels) from 1979/1 to 2020/12-tempt.monthly.1979-2020.nc Nudging wind forcing: To assess the role of the atmospheric circulation on sea ice trends, our nudging experiments nudge the CESM1’s zonal and meridional winds to observations in the Arctic’s atmosphere (north of 60°N) from the surface to the TOA. A buffer zone 2°–3° wide is set on the margin of the nudging domain to allow a smoother transition from full nudging within the domain to no nudging outside. The nudging procedure constrains simulated winds within the nudging domain to observed values at the corresponding time, by adding an additional tendency term in the momentum equations at every time step of 30 min. The 6-hourly ERA5 reanalysis observations are interpolated to model time steps (30 min) by linear interpolation. The magnitude of this tendency term is determined by a weighting parameter varying from zero (no nudging) to one (full nudging). In our experiments, this weighting parameter is selected as one (full nudging) over the entirety of each simulation to force zonal and meridional winds to vary exactly as observed within the Arctic. By doing so, the model will replay the observed circulation variability in the Arctic’s atmosphere while allowing for responses of other local systems and global climate to the specified wind changes in the Arctic. External forcing, initial condition, Spinup and ensemble members: To exclude variability directly related to external forcings (greenhouse gases, aerosols, solar) from nudging experiments, we set external forcings as constant values at the level of the year 2000, which roughly represent the climatological mean values over the 42 years period. In addition, a long spinup run is needed beforehand to ensure that the nudging simulation has no significant “numerical shock” in the early period when reanalysis winds are imposed in the Arctic since reanalysis winds in the Arctic may be very different from the model’s own winds there. To do so, a 150-yr anthropogenic forcing-fixed (at the level of the year 2000) nudging simulation is initially conducted with the model perpetually nudged to winds of year 1979 in the Arctic (the same nudging domain and buffer zone as those used afterward). In this spinup, many key indicators of climate stability, including Arctic sea ice and pan-Arctic and global mean net radiation at the TOA and surface, start to stabilize after the first 100 years and then vary around constant levels in the last 50 years. The model states on 1 January of each of the last 10 years of this spinup are then separately used as initial conditions to reinitiate a set of new 10 members of 42-yr nudging simulations with winds in the Arctic nudged to observations from 1979 to 2020.