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Global ECMWF IFS simulations with horizontal grid spacings of 1, 4, and 9 km are used to assess gravity wave forcing (GWF) in the extratropical stratosphere. Results with important implications for GWF parameterizations at high and intermediate resolutions are presented. A doubling in the zonal-mean resolved GWF is observed when the horizontal resolution is increased from 9 to 1 km. Small-scale gravity waves with horizontal wavelengths < 100 km dominate this increase. Over most regions, excluding the polar night jet in the Antarctic spring, the total (resolved + parameterized) GWF at 9 km (4 km) is underestimated by up to 30% (15%). This implies that the parameterization of GWF is still required at 9 and 4 km horizontal resolutions. Despite the small land area in the Southern Hemisphere (SH), the resolved orographic and nonorographic GWF contribute equally to the total GWF in the SH at 1 km resolution. This is not reflected in the partitioning of the parameterized GWF, which has a significantly larger nonorographic contribution at 9 km. As a result, a zonal-mean momentum budget analysis reveals that the total GWF contributes one-third of SH springtime polar vortex deceleration at 1 km, whereas the contribution is as much as 50% at 9 km. This suggests that a rebalancing of the parameterized nonorographic and orographic GWF is required. [ABSTRACT FROM AUTHOR]

We have implemented a new stratospheric ozone model in the European Centre for Medium-Range Weather Forecasts (ECMWF) system and tested its performance for different timescales to assess the impact of stratospheric ozone on meteorological fields. We have used the new ozone model to provide prognostic ozone in medium-range and long-range (seasonal) experiments, showing the feasibility of this ozone scheme for a seamless numerical weather prediction (NWP) modelling approach. We find that the stratospheric ozone distribution provided by the new scheme in ECMWF forecast experiments is in very good agreement with observations, even for unusual meteorological conditions such as Arctic stratospheric sudden warmings (SSWs) and Antarctic polar vortex events like the vortex split of year 2002. To assess the impact it has on meteorological variables, we have performed experiments in which the prognostic ozone is interactive with radiation. The new scheme provides a realistic ozone field able to improve the description of the stratosphere in the ECMWF system, as we find clear reductions of biases in the stratospheric forecast temperature. The seasonality of the Southern Hemisphere polar vortex is also significantly improved when using the new ozone model. In medium-range simulations we also find improvements in high-latitude tropospheric winds during the SSW event considered in this study. In long-range simulations, the use of the new ozone model leads to an increase in the correlation of the winter North Atlantic Oscillation (NAO) index with respect to ERA-Interim and an increase in the signal-to-noise ratio over the North Atlantic sector. In our study we show that by improving the description of the stratospheric ozone in the ECMWF system, the stratosphere–troposphere coupling improves. This highlights the potential benefits of this new ozone model to exploit stratospheric sources of predictability and improve weather predictions over Europe on a range of timescales. [ABSTRACT FROM AUTHOR]

This study offers an overview of the low-frequency (i.e., monthly to seasonal) evolution, dynamics, predictability, and surface impacts of a rare Southern Hemisphere (SH) stratospheric warming that occurred in austral spring 2019. Between late August and mid-September 2019, the stratospheric circumpolar westerly jet weakened rapidly, and Antarctic stratospheric temperatures rose dramatically. The deceleration of the vortex at 10 hPa was as drastic as that of the first-ever-observed major sudden stratospheric warming in the SH during 2002, while the mean Antarctic warming over the course of spring 2019 broke the previous record of 2002 by ∼50% in the midstratosphere. This event was preceded by a poleward shift of the SH polar night jet in the uppermost stratosphere in early winter, which was then followed by record-strong planetary wave-1 activity propagating upward from the troposphere in August that acted to dramatically weaken the polar vortex throughout the depth of the stratosphere. The weakened vortex winds and elevated temperatures moved downward to the surface from mid-October to December, promoting a record strong swing of the southern annular mode (SAM) to its negative phase. This record-negative SAM appeared to be a primary driver of the extreme hot and dry conditions over subtropical eastern Australia that accompanied the severe wildfires that occurred in late spring 2019. State-of-the-art dynamical seasonal forecast systems skillfully predicted the significant vortex weakening of spring 2019 and subsequent development of negative SAM from as early as late July. [ABSTRACT FROM AUTHOR]

Planetary waves (PWs) and gravity waves (GWs) are the key drivers of middle atmospheric circulation. Insufficient observations and inaccurate model representation of GWs limit our understanding of their stratospheric contributions, especially during the Antarctic polar vortex breakdown. This study employs the strength of the high‐resolution ERA5 reanalysis in resolving a broad spectrum of GWs in southern midlatitudes and its ability to estimate their forcing during the breakdown period. Most of the resolved southern hemisphere GWs deposit momentum around 60°S over the Southern Ocean. Further, a zonal momentum budget analysis during the breakdown period reveals that the resolved GW forcing in ERA5 provides as much as one‐fourth of the necessary wind deceleration at 60°S, 10 hPa. The parameterized GW drag, mostly from non‐orographic sources, provides more than half of the wind deceleration. Both findings highlight the key role of GWs in the vortex breakdown and discuss possibilities for further stratospheric GW analysis. Plain Language Summary: Strong flow over mountains during winters and instabilities in the southern hemisphere troposphere can excite gravity waves that propagate from near‐surface all the way to atmospheric heights of 50–80 km. At these heights, they dissipate momentum and decelerate the strong eastward winds. Knowing the structure and extent of the forcing by these waves can help to better understand their role in driving the stratospheric and mesospheric circulation. However, the net forcing by these waves is not accurately known on account of limited observations and because global climate models cannot sufficiently resolve them. This study illustrates that the high‐resolution ERA5 reanalysis, which forms a natural bridge between observations and free running climate models, can be used to estimate the mean forcing due to such gravity waves and can help assess their role in springtime deceleration of polar vortex. Such an analysis was not possible using previous reanalysis datasets due to low resolution. The findings show that, indeed, gravity wave forcing can provide a large fraction of the deceleration needed to slow down the strong westerly winds in late winters. Key Points: ERA5 partially resolves gravity waves allowing evaluation of their contribution to the springtime polar vortex breakdownClimatology of the gravity wave forcing highlights relevance of gravity wave refraction and oblique propagation and suggests deceleration of up to −8 m/s/day around 60°SDeceleration of the vortex due to gravity wave dissipation can be as high as the total necessary deceleration for the vortex breakdown [ABSTRACT FROM AUTHOR]

Common model deficiencies in representing the large-scale circulation include an equatorward bias in the location of the midlatitude westerlies and an overly zonal orientation of the North Atlantic storm track. Orography is known to strongly affect the atmospheric circulation and is notoriously difficult to represent in coarse‐resolution climate models. Here we show that the effects of switching off the parameterization of drag from low‐level orographic blocking in one climate model resemble the biases of the Coupled Model Intercomparison Project Phase 5 ensemble: An overly zonal wintertime North Atlantic storm track and less European blocking events, and an equatorward shift in the Southern Hemispheric jet and increase in the Southern Annular Mode time scale. Experiments with a dry model with and without topography confirm that in the presence of topography, increased localized drag can shift the jet polewards.This suggests that typical circulation biases in coarse‐resolution climate models may be alleviated by improved parameterizations of low‐level drag.Simple models suggest that inter-model differences in the response to forcing, such as the jet shift in a warming climate, may be related to differences in present-day variability. If such a relationship holds in complex models and can be used as an emergent constraint remains unclear. Here, we show that including topography and a more realistic surface drag in a dry model substantially changes its variability and response to forcing. No universal relationship between annular mode timescale and forced response is found. In a high-top model, adding a stratospheric polar vortex in winter allows to reproduce a relationship between control climate jet latitude and jet shift found in complex climate models. Control climate jet latitude and the latitudes of maximum wind speed changes in CMIP models are strongly correlated in Southern Hemisphere summer, but hardly so in Southern Hemisphere winter. Our results suggest that the presence of the stratospheric polar vortex in winter acts to fix the latitudes of maximum windspeed changes independently from variations in the control climate jet latitude. [ABSTRACT FROM AUTHOR]