On the lack of stratospheric dynamical variability in low‐top versions of the CMIP5 models

Abstract

[1] We describe the main differences in simulations of stratospheric climate and variability by models within the fifth Coupled Model Intercomparison Project (CMIP5) that have a model top above the stratopause and relatively fine stratospheric vertical resolution (high-top), and those that have a model top below the stratopause (low-top). Although the simulation of mean stratospheric climate by the two model ensembles is similar, the low-top model ensemble has very weak stratospheric variability on daily and interannual time scales. The frequency of major sudden stratospheric warming events is strongly underestimated by the low-top models with less than half the frequency of events observed in the reanalysis data and high-top models. The lack of stratospheric variability in the low-top models affects their stratosphere-troposphere coupling, resulting in short-lived anomalies in the Northern Annular Mode, which do not produce long-lasting tropospheric impacts, as seen in observations. The lack of stratospheric variability, however, does not appear to have any impact on the ability of the low-top models to reproduce past stratospheric temperature trends. We find little improvement in the simulation of decadal variability for the high-top models compared to the low-top, which is likely related to the fact that neither ensemble produces a realistic dynamical response to volcanic eruptions.All supporting information may be found in the online version of this article.

Evaluation of wind and temperature profiles from ECMWF analysis on two hemispheres using volcanic infrasound

“…There is a current interest from the NWP community to validate model specifications at stratospheric altitudes using independent observations [e.g., Randel et al, 2004;Charlton-Perez et al, 2013]. This includes the analysis of potential satellite radiance measurement biases, the consideration of additional high-resolution measurements (gravity waves and 10.1002/2014JD021632 momentum flux) that are currently not resolved in the model runs, and validation of currently employed gravity wave model parameterization schemes [Charlton-Perez et al, 2013].…”

“…One example is the influence of the onset and duration of Sudden Stratospheric Warmings (SSWs) on the latitude of the tropospheric jet stream, as this has a strong impact on near-term weather forecasting [Gerber et al, 2009]. Weather and climate forecasters are moving toward a more comprehensive representation of the atmosphere, in order to capture the stratospheric-tropospheric interactions [Charlton-Perez et al, 2013]. The development toward such circulation models is facilitated by the increase in computational resources.…”

The effect of a well-resolved stratosphere on East Asian winter climate

“…Studies revealed that, with high-top model, the common cold bias of the stratospheric polar vortex is corrected and the strength of the polar jet is weakened to be comparable to observation (Charlton-Perez et al 2013). Figure 6 shows the difference of geopotential height at various vertical levels between HT and LT model under goy is based on the data between 1970 and 1999.…”

“…In the most recent fifth assessment report (AR5) of the IPCC, 54 models were used for the assessment, among which only half (27) models have full stratosphere with model lid top above 1 hPa. These low-top models have weaker polar vortex variability, and reduced planetary wave activity in stratosphere, leading to much less frequency of major sudden stratospheric warming events in the stratosphere (Charlton-Perez et al 2013;Osprey et al 2013). The Climate-system Historical Forecast Project using 16 coupled models (8 high-top models and 8 low-top models) shows that the high-top models have a more realistic stratospheric response to El Nino and the QBO, and have a possible stratospheric pathway that enhance the wintertime prediction skill over high latitudes and North Atlantic region (Butler et al 2016).…”

Reduced cooling following future volcanic eruptions

“…Further, general circulation models (including HadGEM2-ES) had been thought to under-estimate the observed posteruption northern hemisphere winter dynamical response (Driscoll et al 2012;Charlton-Perez et al 2013;Bittner et al 2016), but restricting the analysis to the first post-eruption winter and only the strongest volcanic eruptions alters this (Bittner et al 2016;Zambri and Robock 2016). Nevertheless, an improved understanding of the dynamical response to volcanic eruptions is required to clarify the possibility of enhanced winter warming in response to future eruptions.…”