Quantifying synoptic scale errors in CMIP5 simulation of the South Pacific Convergence Zone

The South Pacific Convergence Zone (SPCZ) remains poorly simulated in current generation climate models, including those in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). While prior studies have quantified SPCZ simulation errors in climatology as well as interannual variability associated with the El Niño-Southern Oscillation (ENSO), less work has been done on simulation errors on synoptic time scales. The latter are particularly important in this region, not only because the SPCZ has been shown to interact with synoptic disturbances but also because such errors may subtly manifest in the climatological mean. As a first step in examining potential differences between observations and CMIP5 models on synoptic timescales, spectral analysis of outgoing longwave radiation (OLR) is applied over three distinct subdomains of the SPCZ (in the equatorial region, the tropics, and subtropics) in order to determine the relative strength of synoptic variability compared to other time scales. This analysis reveals that synoptic variability is of comparable magnitude to observations, although its relative magnitude compared to ENSO time scales varies greatly; additionally, few CMIP5 models manifest variability on Madden-Julian Oscillation (MJO) timescales with a magnitude comparable to observations, consistent with the findings of recent studies. As an important factor in tropical-extratropical interactions in the SPCZ, the position and intensity of negative zonal stretching deformation near the SPCZ, i.e. the storm graveyard, is also examined in each model. These results show a distinct longitudinal spread in the placement of the storm graveyard, which is only marginally reduced when the models are forced with prescribed SSTs. These findings suggest that further work is needed to understand how the choice of model parameterization, such as the convection scheme, may perturb SPCZ simulation on the synoptic time scale, and consequently the climatological mean.