Comment on gmd-2021-322

Abstract

Ocean surface gravity waves have enormous effects on physical processes at the atmosphere–ocean interface. The effects of wave-related processes on global intraseasonal prediction were evaluated after we incorporated the WAVEWATCH III model into the Climate Forecast System model version 2.0 (CFSv2.0), with the Chinese Community Coupler version 2.0. Several major wave-related processes, including the Langmuir mixing, Stokes-Coriolis force with entrainment, air-sea fluxes modified by Stokes drift and momentum roughness length, were evaluated in two groups of 56-day experiments, one for boreal winter and the other for boreal summer. Comparisons were performed against in-situ buoys, satellite measurements and reanalysis data, to evaluate the influence of waves on intraseasonal prediction of sea surface temperature (SST), 2-m air temperature (T02), mixed layer depth (MLD), 10-m wind speed (WSP10) and significant wave height (SWH) in CFSv2.0. Overestimated SST and T02, as well as underestimated MLD in mid and high latitudes in summer from original CFSv2.0 are clearly improved, mainly due to enhanced vertical mixing generated by Stokes drift. The largest regional mean SST improvement reaches 35.89 % in the Southern Ocean. For WSP10 and SWH, the wave-related processes generally lead to reduction of biases in regions where wind speed and SWH are overestimated. The decreased SST caused by Stokes drift-related mixing stabilizes marine atmospheric boundary layer, weakens wind speed and then SWH. Compared with the NDBC buoy data, the overestimated WSP10 is improved by up to 13.52 % in boreal summer. The increased roughness length due to waves leads to some reduction in the originally overestimated wind speed and SWH, with the largest SWH improvement of 11.93 % and 20.05 % in boreal winter and summer respectively. The effects of Stokes drift and current on air-sea fluxes are investigated separately. Their overall effects on air-sea fluxes reduce the overestimated WSP10 by up to 17.31 % and 23.21 % in boreal winter and summer respectively. These cases are helpful for the future development of the two-way CFS-wave coupled system.