Effects of mean state of climate models on the response to prescribed forcing: Sensitivity experiments with the SPEEDY general circulation model.

Abstract:

Understanding the variability of the general circulation of the atmosphere and its drivers is one of the grand challenges in climate science. It is therefore important to understand processes driving the variability of the circulation making use of numerical climate models and how these processes are affected by model bias. To characterize the effect of model bias on the response to a given forcing, several simulations were performed with an intermediate complexity model named Simplified Parameterizations, primitivE - Equation DYnamics (SPEEDY). Ten simulations are performed with a modified orography to obtain an atmospheric circulation at mid-latitudes characterized by different mean states. The main changes in the mean state are associated with a modulation of the jet intensity in both the North Atlantic Ocean and the North Pacific Ocean. We show that the resulting atmospheric mean states are broadly compatible with those of CMIP5 models. For each of these modified-orography experiments, we have studied the climatic response to idealized El Niño Southern Oscillation (ENSO) SST anomalies of different intensity. All the sensitivity simulations were performed with a large ensemble (200 members). Results from the forced experiments show that the typical ENSO late winter teleconnection pattern is influenced by the mean state of the model and reveals geographic areas where the sensitivity is large: Alaska and the Caribbean. Over Alaska, the surface temperature is strongly affected by the intensity of the North Pacific jet and stronger jet damps the El Niño signal. A similar result is found in the Caribbean where the total precipitation shows a clear sensitivity to the mean state of the North Pacific jet stream, since the total precipitation decreases with increasing set intensity. These results can be used to interpret and understand the multi-model spread in the atmospheric response to the aforementioned external forcing.