Analysis of the predictability of stratospheric variability and climate indices based on seasonal retrospective forecasts of the INM RAS climate model

Ensemble numerical experiments for winter seasons of 1980–2014 were carried out with the use of the mathematical climate model of the Institute of Numerical Mathematics (INM) of the Russian Academy of Sciences developed initially for multi-year climate forecasts. Based on the results obtained in this research, a qualitative assessment of the reproduction of the North Atlantic (NAO) and Pacific-North American (PNA) oscillation indiceswas obtained. It was shown that the INM-CM5-0 climate model has a very high predictability of the winter NAO index and one, but not unique reason for this is the predictability of the stratospheric variability in the INM RAS model. The analysis of the quality of reproduction of the PNA index on a seasonal time scale for the INM-CM5-0 model has shown an acceptable result.

Exploring the Tropospheric Response to Stratospheric Variability Using Lagged Quantile Regression

<p>Stratospheric variability has become increasingly popular due to its potential impact on the tropospheric circulation. Extreme states of the stratospheric polar vortex have been associated with reoccurring tropospheric weather patterns more than 2-3 weeks after the initial stratospheric signal. Standard linear regression methods used to assess the statistical stratosphere-troposphere connection estimate the distribution's mean effect of a stratospheric variable as a predictor on a tropospheric response variable. However,  supplementary information of the impact of extreme stratospheric behavior is hidden in the tails of the distribution, revealing a different behavior than the mean. Therefore, we use quantile regression, a method that enables us to model the complete conditional distribution of the response variable. This presentation explores various quantiles of the conditional distribution to investigate the impact of stratospheric variability on the tropospheric circulation using the ERA5 reanalysis dataset. Comparison between (lagged) linear and (lagged) quantile regression reveals significant differences making the latter method a neat tool that offers valuable information about the statistical connection between the stratosphere and the troposphere.</p>

Signatures of the Madden-Julian Oscillation in Stratospheric Temperature from Aura MLS

<p>The Madden-Julian oscillation (MJO) is a major source of intraseasonal variability in the tropical troposphere. It refers to a recurring pattern of strong convection, which travels from the Indian ocean over the Maritime Continent to the Pacific ocean with time scales of 30 to 90 days.</p><p>Although some studies have recently indicated that the occurrence of tropospheric MJO events could also affect stratospheric parameters, the MJO is not very much recognized as a source of stratospheric variability. This bears the risk of mixing it up with other sources of variability on this time scale, e.g., with signatures of the solar 27-day variations. Many of the studies that have found MJO signatures in the stratosphere are, however, based on either modelled or reanalyzed data. Particularly, we are not aware of any purely observational studies related to the temperature response in the middle atmosphere.</p><p>To fill this gap, we analyze the signature of the MJO in stratospheric temperatures measured by the Microwave Limb Sounder (MLS) satellite instrument aboard the Aura satellite. Analyzing the period from about 2004 to 2018, we indeed identify corresponding temperature variations in various altitudes and locations with many of them being significant according to Monte Carlo tests. The amplitudes of these signatures are on the order of 0.5 K. Moreover, basic characteristics of signatures, which have been identified in the preceding publications, are confirmed in this study based on purely observational data.</p><p>Hence, our study supports the coupling of parts of the stratospheric variability on the intraseasonal time scale to anomalous tropospheric convection represented by the MJO.</p>

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.

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 than 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 fingerprint. In particular, differences in blocking precursor activity of SSWs across reanalyses are much smaller than between blocking definitions.

Influence of Arctic Stratospheric Ozone on Surface Climate in CCMI models

The Northern Hemisphere and tropical circulation response to interannual variability in Arctic stratospheric ozone is analyzed in a set of the latest model simulations archived for the Chemistry-Climate Model Initiative (CCMI) project. All models simulate a connection between ozone variability and temperature/geopotential height in the lower stratosphere similar to that observed. A connection between Arctic ozone variability and polar cap sea-level pressure is also found, but additional analysis suggests that it is mediated by the dynamical variability that typically drives the anomalous ozone concentrations. The CCMI models also show a connection between Arctic stratospheric ozone and the El Nino Southern Oscillation (ENSO): the CCMI models show a tendency of Arctic stratospheric ozone variability to lead ENSO variability one to two years later. While this effect is much weaker than that observed, it is still statistically significant. Overall, Arctic stratospheric ozone is related to lower stratospheric variability and may also influence the surface in both polar and tropical latitudes, though these impacts can be masked by internal variability if data is only available for ~ 40 years.

The Vertical Structure of Annular Modes

Stratosphere–troposphere interactions are conventionally characterized using the first empirical orthogonal function (EOF) of fields such as zonal-mean zonal wind. Perpetual-winter integrations of an idealized model are used to contrast the vertical structures of EOFs with those of principal oscillation patterns (POPs; the modes of a linearized system governing the evolution of zonal flow anomalies). POP structures are shown to be insensitive to pressure weighting of the time series of interest, a factor that is particularly important for a deep system such as the stratosphere and troposphere. In contrast, EOFs change from being dominated by tropospheric variability with pressure weighting to being dominated by stratospheric variability without it. The analysis reveals separate tropospheric and stratospheric modes in model integrations that are set up to resemble midwinter variability of the troposphere and stratosphere in both hemispheres. Movies illustrating the time evolution of POP structures show the existence of a fast, propagating tropospheric mode in both integrations, and a pulsing stratospheric mode with a tropospheric extension in the Northern Hemisphere–like integration.

Role of Finite-Amplitude Rossby Waves and Nonconservative Processes in Downward Migration of Extratropical Flow Anomalies

There is growing evidence that stratospheric variability exerts a noticeable imprint on tropospheric weather and climate. Despite clear evidence of these impacts, the principal mechanism whereby stratospheric variability influences tropospheric circulation has remained elusive. Here, the authors introduce a novel approach, based on the theory of finite-amplitude wave activity, for quantifying the role of adiabatic and nonconservative effects on the mean flow that shape the downward coupling from the stratosphere to the troposphere during stratospheric vortex weakening (SVW) events. The advantage of using this theory is that eddy effects (at finite amplitude) on the mean flow can be more readily distinguished from nonconservative effects. The results show (in confirmation of previous work) that the downward migration of extratropical wind anomalies is largely attributable to dynamical adjustments induced by fluctuating finite-amplitude wave forcing. The nonconservative effects, on the other hand, contribute to maintaining the downward signals in the recovery stage within the stratosphere, hinting at the importance of mixing and diabatic heating. The analysis further indicates that variations in stratospheric finite-amplitude wave forcing are too weak to account for the attendant changes and shapes in the tropospheric flow. It is suggested that the indirect effect of tropospheric finite-amplitude wave activity through the residual displacements is needed to amplify and prolong the tropospheric wind responses over several weeks. The results also reveal that the local tropospheric wave activity over the North Pacific and North Atlantic sectors plays a significant role in shaping the high-latitude tropospheric wind response to SVW events.