Internal atmospheric variability of net surface heat flux in reanalyses and CMIP5 AMIP simulations

The internal atmospheric variability (IAV) of the net surface heat flux (NHF) in the observed 20th/21st century atmosphere is estimated as the residual after removing the sea surface temperature (SST) and externally forced atmospheric response derived from the four atmospheric models of the Atmospheric Model Intercomparison Project (AMIP) simulations under phase 5 of the Coupled Model Intercomparison Project (CMIP5). The mean NHF of four atmospheric reanalysis datasets is an estimate of the observed NHF. Although the AMIP models are forced with the same SST and external forcing, the forced responses differ significantly among AMIP models, suggestive of uncertainty in the models. Besides, the uncertainty of IAV in the reanalyses could also arise from the uncertainty in reanalyses as observations contain errors and reanalysis includes interpolation by models. It is concluded that: (a) The SST/NHF and SST/forced NHF correlations are significantly negative over most of world ocean in the AMIP models, indicating damping of the SST anomalies by the NHF. (b) The IAV of the AMIP models is not correlated with SST, while the positive IAV/SST correlations in the reanalyses suggests the role of IAV in forcing the SST variability in the extra-tropics. (c) The standard deviation (STD) of the IAV of AMIP models is indistinguishable from that of the mean reanalysis over a majority of world ocean, and the STD of the NHF of the AMIP models is larger than that of the mean reanalysis in the subtropics and midlatitudes. (d) The IAV in the mean reanalysis plays a role in forcing the SST variability in the extra-tropics (e.g., Atlantic Multidecadal Variability), while it may not be an important forcing in the tropical oceans (e.g., ENSO).