Abstract
Abstract. This work aims to assess the performance of state-of-the-art global climate models in representing the upper-tropospheric Rossby wave pattern in the Northern Hemisphere and over the European–Atlantic sector. A diagnostic based on finite-amplitude local wave activity is used as an objective metric to quantify the amplitude of Rossby waves in terms of Rossby wave activity. This diagnostic framework is applied to a set of coupled historical climate simulations at different horizontal resolutions, performed in the framework of the PRIMAVERA project and compared with observations (ERA5 reanalysis). At first, the spatio-temporal characteristics of Rossby wave activity in the Northern Hemisphere are examined in the multimodel mean of the whole PRIMAVERA set. When examining the spatial distribution of transient wave activity, only a minimal improvement is found in the high-resolution ensemble. On the other hand, when examining the temporal variability of wave activity, a higher resolution is beneficial in all models apart from one. In addition, when examining the Rossby wave activity time series, no evident trends are found in the historical simulations (at both standard and high resolutions) and in the observations. Finally, the spatial distribution of Rossby wave activity is investigated in more detail focusing on the European–Atlantic sector, examining the wave activity pattern associated with weather regimes for each model. Results show a marked inter-model variability in representing the correct spatial distribution of Rossby wave activity associated with each regime pattern, and an increased horizontal resolution improves the models' performance only for some of the models and for some of the regimes. A positive impact of an increased horizontal resolution is found only for the models in which both the atmospheric and oceanic resolution is changed, whereas in the models in which only the atmospheric resolution is increased, a worsening model performance is detected.
Highlights
The European continent is located at the downstream end of the North Atlantic storm track
As we did for the whole NH, we examine the temporal behaviour of transient local wave activity (LWA), restricting our attention on the EAT sector, where weather regimes (WRs) are computed
Our approach combined the diagnostic for Rossby waves based on the LWA in isentropic coordinates of Ghinassi et al (2018), to quantify their amplitude and a weather regime analysis to subsequently compute WRs over the EAT sector
Summary
The European continent is located at the downstream end of the North Atlantic storm track. Barnes and Screen (2015) pointed out that the Arctic amplification is one of the processes which may influence the jet stream variability (and the variability related with RWPs and WR) and that the opposite situation found in the upper troposphere (i.e. a strengthening of the meridional temperature gradient due to the warming of the upper troposphere in the tropics and polar stratospheric cooling) can, on the other hand, intensify the jet stream These contrasting results motivate us to use a robust diagnostic based on finite-amplitude local wave activity (LWA), which is able to objectively identify Rossby waves, as we will discuss in the following paragraph.
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