On the strongly negative cloud feedback over the satellite period implied by observational SST reconstructions

<p>Clouds strongly modulate Earth's radiative budget, and uncertainties in numerical model simulations of the global cloud field contribute substantially to uncertainties in future warming. In coupled atmosphere-ocean General Circulation Model (GCM) simulations, the global cloud field and its radiative effect are well correlated with global average surface temperature. However, GCM simulations with prescribed Sea Surface Temperatures (SSTs) from observational SST reconstructions over the historical period show time-varying relationships between the cloud field and average surface temperature (known as the "pattern effect"). We show that CERES/EBAF observational data confirms the presence of a second mode (in addition to mean SST) in particular in low cloud amount (and correspondingly SWCRE) that is consistent with variations in tropical atmospheric stability in ERA-Interim reanalysis data. This second mode in observations is tied to ENSO, and evolves in quadrature to ENSO indexes. It arises from differences in surface temperature change between regions of tropical deep convection and the tropical (or global) average. In contrast to the multidecadal trends over the full historical period, trends in this second mode since the year 2000 are small. The PCMDI/AMIPII SSTs recommended for CMIP6 stand out as having the largest trend over the full historical period. Different SST reconstructions agree on a trend over the satellite period - specifically the 1980s-90s - that is much larger than what coupled GCM simulations show: In forced coupled GCM simulations the regions of deep convection warm order 10% more than the tropical average, whereas over the satellite period the amplification is order +50%  in the AMIP simulations and in estimates using rainfall observations to identify regions of deep convection.</p>

Understanding the Equatorial Pacific Cold Tongue Time-Mean Heat Budget. Part II: Evaluation of the GFDL-FLOR Coupled GCM

The heat budget of the Pacific equatorial cold tongue (ECT) is explored using the GFDL-FLOR coupled GCM (the forecast-oriented low ocean resolution version of CM2.5) and ocean reanalyses, leveraging the two-layer framework developed in Part I. Despite FLOR’s relatively weak meridional stirring by tropical instability waves (TIWs), the model maintains a reasonable SST and thermocline depth in the ECT via two compensating biases: 1) enhanced monthly-scale vertical advective cooling below the surface mixed layer (SML), due to overly cyclonic off-equatorial wind stress that acts to cool the equatorial source waters; and 2) an excessive SST contrast between the ECT and off-equator areas, which boosts the equatorward heat transport by TIWs. FLOR’s strong advective cooling at the SML base is compensated by strong downward diffusion of heat out of the SML, which then allows FLOR’s ECT to take up a realistic heat flux from the atmosphere. Correcting FLOR’s climatological SST and wind stress biases via flux adjustment (FA) leads to weaker deep advective cooling of the ECT, which then erodes the upper-ocean thermal stratification, enhances vertical mixing, and excessively deepens the thermocline. FA does strengthen FLOR’s meridional shear of the zonal currents in the east Pacific, but this does not amplify either the simulated TIWs or their equatorward heat transport, likely due to FLOR’s coarse zonal ocean resolution. The analysis suggests that to advance coupled simulations of the ECT, improved winds and surface heat fluxes must go hand in hand with improved subseasonal and parameterized ocean processes. Implications for model development and the tropical Pacific observing system are discussed.