Coupling of surface air and sea surface temperatures in the CERA-20C reanalysis

ECMWF has recently produced its first ocean-atmosphere coupled 20th century reanalysis CERA-20C, with 10 ensemble members. CERA-20C is based on a coupled climate model with data assimilation implemented individually for the atmosphere and ocean components. This paper focuses on the relationships between 2-m air temperature (T2m) and sea surface temperature (SST) in this new coupled climate reanalysis from the perspective of ensemble statistics. The results show where the atmosphere or the ocean are driving the ensemble variability of temperature at the air-sea interface on varying timescales. In CERA-20C, the T2m-SST ensemble relationships are changing on diurnal, seasonal and longer timescales and also within regions. The T2m ensemble spread is much larger than the SST ensemble spread at high-frequencies (3-hour), normally with low correlations due to different timescales of oceanic and atmospheric dynamics, showing the atmospherically driven variability of T2m. On monthly timescales, the T2m spread is mostly slightly lower than the SST spread, with high correlations and strong coupling, reflecting the SST-forced variability. T2m-SST coupling is strongest where atmospheric and oceanic boundary layers are shallow, e.g. in the summer hemisphere, and weakest where the atmospheric boundary layer becomes deep, e.g. in the ITCZ region. As the 20th century progresses, ensemble spreads decline, while T2m-SST coupling becomes stronger as both atmosphere and oceans are better constrained to observations. In the tropics, strong ENSO-related variability is found in T2m-SST coupling as atmospheric convection centres move inter-annually to break the SST-dominated relationships