On the representation of major stratospheric warmings in reanalyses

Major sudden stratospheric warmings (SSWs) represent one of the most abrupt phenomena of the boreal wintertime stratospheric variability, and constitute the clearest example of coupling between the stratosphere and the troposphere. A good representation of SSWs in climate models is required to reduce their biases and uncertainties in future projections of stratospheric variability. The ability of models to reproduce these phenomena is usually assessed with just one reanalysis. However, the number of reanalyses has increased in the last decade and their own biases may affect the model evaluation. Here we compare the representation of the main aspects of SSWs across reanalyses. The examination of their main characteristics in the pre- and post-satellite periods reveals that reanalyses behave very similarly in both periods. However, discrepancies are larger in the pre-satellite period compared to afterwards, particularly for the NCEP-NCAR reanalysis. All datasets reproduce similarly the specific features of wavenumber-1 and wavenumber-2 SSWs. A good agreement among reanalyses is also found for triggering mechanisms, tropospheric precursors, and surface response. In particular, differences in blocking precursor activity of SSWs across reanalyses are much smaller than between blocking definitions

Future changes in the Brewer-Dobson circulation under different greenhouse gas concentrations in WACCM4

The climatology and future changes of the Brewer–Dobson circulation (BDC) in three climate change scenarios are studied using the latest version of the Whole Atmosphere Community Climate Model (WACCM4), which is fully coupled to an ocean model. The results show an acceleration in both the shallow and deep branches of circulation in response to increasing greenhouse gases (GHGs) together with an upward displacement of the tropical upwelling in the deep branch near the stratopause. The downward control principle reveals that different waves are involved in forcing the acceleration of the upper and lower branches. Climatological-mean tropical upwelling in both the lower and upper stratosphere is dominated by explicitly resolved, planetary-scale waves. Trends in the tropical upwelling in the lower stratosphere are mainly attributed to explicitly resolved, planetary-scale waves. However, in the upper stratosphere, despite the fact that resolved waves control the forcing of the climatological upwelling, their contribution to the long-term trend diminishes with increasing GHGs, while the role of gravity waves associated with fronts increases and becomes dominant in the model scenario with the largest GHG increases. The intensification and upward displacement of the subtropical tropospheric jets due to climate change leads to filtering of the westerly part of the frontal gravity wave spectrum, leaving the easterly components to reach the upper stratosphere and force the changes in the circulation there

Comparing Sudden Stratospheric Warming Definitions in Reanalysis Data

Sudden stratospheric warmings (SSWs) are characterized by a pronounced increase of the stratospheric polar temperature during the winter season. Different definitions have been used in the literature to diagnose the occurrence of SSWs, yielding discrepancies in the detected events. The aim of this paper is to compare the SSW climatologies obtained by different methods using reanalysis data. The occurrences of Northern Hemisphere SSWs during the extended-winter season and the 1958-2014 period have been identified for a suite of eight representative definitions and three different reanalyses. Overall, and despite the differences in the number and exact dates of occurrence of SSWs, the main climatological signatures of SSWs are not sensitive to the considered reanalysis.The mean frequency of SSWs is 6.7 events decade^-1, but it ranges from 4 to 10 events, depending on the method. The seasonal cycle of events is statistically indistinguishable across definitions, with a common peak in January. However, the multidecadal variability is method dependent, with only two definitions displaying minimum frequencies in the 1990s. An analysis of the mean signatures of SSWs in the stratosphere revealed negligible differences among methods compared to the large case-to-case variability within a given definition.The stronger and more coherent tropospheric signals before and after SSWs are associated with major events, which are detected by most methods. The tropospheric signals of minor SSWs are less robust, representing the largest source of discrepancy across definitions. Therefore, to obtain robust results, future studies on stratosphere-troposphere coupling should aim to minimize the detection of minor warmings