Testing climate models using GPS radio occultation: A sensitivity analysis

We survey the IPCC AR4 models' responses to SRES A1B forcing in order to evaluate a prediction of climate change common to all models and testable using GPS radio occultation data over the coming decades. Of the IPCC AR4 models that submitted runs of the SRES A1B forcing scenario, we select twelve because of the timeliness of their submission. Trends in the global average surface air temperature show better overall agreement over the first 50 years than in the IPCC Third Assessment Report, but the patterns of global surface air temperature trends show little improvement in intermodel agreement. These same twelve models show qualitatively better agreement in their patterns of upper air temperature trends. All show maintenance of a moist adiabatic temperature profile in the tropics, making tropospheric temperature trends the greatest at 200 hPa in the tropics. We test the climate models' predictions using optimal fingerprinting. In order to do so, we use long preindustrial control runs of four of the IPCC AR4 models. Simulating trends in the log of the vertically integrated microwave refractivity, or “dry” pressure, of the atmosphere is nearly the same as measuring trends in geopotential heights of constant pressure surfaces. The first four EOFs of interannual variability of log‐dry pressure, as determined by two independent climate models, are ENSO, modes closely associated with the southern and northern annular modes, and a previously unidentified symmetric jet migration EOF. The latter is characterized by poleward migration of the eddy‐driven midlatitude jet correlated between hemispheres. The ENSO mode and especially the symmetric jet migration EOF contribute most to optimal detection and are similarly predicted by the IPCC AR4 models. The common prediction of all climate models will be tested with 95% confidence with GPS radio occultation data in 7 to 13 years. The fingerprint is dominated by symmetric poleward migration of the eddy‐driven midlatitude jets.