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Over recent years there have been concomitant advances in the development of stratosphere-resolving numerical models, our understanding of stratosphere–troposphere interaction, and the extension of long-range forecasts to explicitly include the stratosphere. These advances are now allowing for new and improved capability in long-range prediction. We present an overview of this development and show how the inclusion of the stratosphere in forecast systems aids monthly, seasonal, and annual-to-decadal climate predictions and multidecadal projections. We end with an outlook towards the future and identify areas of improvement that could\ud further benefit these rapidly evolving predictions.

The possibility that Arctic sea ice loss weakens mid-latitude westerlies, promoting more severe cold winters, has sparked more than a decade of scientific debate, with apparent support from observations but inconclusive modelling evidence. Here we show that sixteen models contributing to the Polar Amplification Model Intercomparison Project simulate a weakening of mid-latitude westerlies in response to projected Arctic sea ice loss. We develop an emergent constraint based on eddy feedback, which is 1.2 to 3 times too weak in the models, suggesting that the real-world weakening lies towards the higher end of the model simulations. Still, the modelled response to Arctic sea ice loss is weak: the North Atlantic Oscillation response is similar in magnitude and offsets the projected response to increased greenhouse gases, but would only account for around 10% of variations in individual years. We further find that relationships between Arctic sea ice and atmospheric circulation have weakened recently in observations and are no longer inconsistent with those in models.

The stratosphere and the troposphere are coupled in many ways. Because their interactions span days to weeks (or even longer), understanding these linkages and simulating them correctly in forecast models may provide a source of sub-seasonal to seasonal (S2S) prediction skill. This chapter reviews the tropical and extratropical coupling between the stratosphere and the troposphere and summarizes the most recent research showing how adequate simulation of the stratosphere may contribute to better prediction skill in the troposphere.

Fil: Tompkins, Adrian M.. The Abdus Salam; Italia Fil: Ortiz de Zarate, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina. Centre National de la Recherche Scientifique; Francia Fil: Saurral, Ramiro Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina. Centre National de la Recherche Scientifique; Francia Fil: Vera, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina. Centre National de la Recherche Scientifique; Francia Fil: Saulo, Andrea Celeste. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Defensa. Secretaria de Planeamiento. Servicio Meteorológico Nacional; Argentina Fil: Merryfield, William J.. Canadian Centre for Climate Modelling and Analysis; Canadá Fil: Sigmond, Michael. Canadian Centre for Climate Modelling and Analysis; Canadá Fil: Lee, Woo Sung. Canadian Centre for Climate Modelling and Analysis; Canadá Fil: Baehr, Johanna. Universitat Hamburg; Alemania Fil: Braun, Alain. Météo-France; Francia Fil: Amy Butler. National Ocean And Atmospheric Administration; Estados Unidos Fil: Déqué, Michel. Météo-France; Francia Fil: Doblas Reyes, Francisco J.. Institució Catalana de Recerca i Estudis Avancats; España. Barcelona Supercomputing Center - Centro Nacional de Supercomputacion; España Fil: Gordon, Margaret. Met Office; Reino Unido Fil: Scaife, Adam A.. University of Exeter; Reino Unido Fil: Yukiko Imada. Japan Meteorological Agency. Meteorological Research Institute. Climate Research Department; Japón Fil: Masayoshi Ishii. Japan Meteorological Agency. Meteorological Research Institute. Climate Research Department; Japón Fil: Tomoaki Ose. Japan Meteorological Agency. Meteorological Research Institute. Climate Research Department; Japón Fil: Kirtman, Ben. University of Miami; Estados Unidos Fil: Kumar, Arun. National Ocean And Atmospheric Administration; Estados Unidos Fil: Müller, Wolfgang A.. Max-Planck-Institut für Meteorologie; Alemania Fil: Pirani, Anna. Université Paris-Saclay; Francia Fil: Stockdale, Tim. European Centre for Medium-Range Weather; Reino Unido Fil: Rixen, Michel. World Meteorological Organization. World Climate Research Programme; Suiza Fil: Yasuda, Tamaki. Japan Meteorological Agency. Climate Prediction Division; Japón

Advances in seasonal forecasting have brought widespread\ud socio-economic benefits. However, seasonal forecast skill\ud in the extratropics is relatively modest, prompting the\ud seasonal forecasting community to search for additional\ud sources of predictability. For over a decade it has been\ud suggested that knowledge of the state of the stratosphere\ud can act as a source of enhanced seasonal predictability; long-lived circulation anomalies in the lower stratosphere that follow stratospheric sudden warmings are associated with circulation anomalies in the troposphere that can last up to two months. Here, we show by performing retrospective\ud ensemble model forecasts that such enhanced predictability\ud can be realized in a dynamical seasonal forecast system with\ud a good representation of the stratosphere. When initialized at the onset date of stratospheric sudden warmings, the model forecasts faithfully reproduce the observed mean tropospheric conditions in the months following the stratospheric sudden warmings. Compared with an equivalent set of forecasts that are not initialized during stratospheric sudden warmings, we document enhanced forecast skill for atmospheric circulation patterns, surface temperatures over northern Russia and eastern Canada and North Atlantic precipitation. We suggest\ud that seasonal forecast systems initialized during stratospheric sudden warmings are likely to yield significantly greater forecast skill in some regions.

The impacts of stratospheric ozone depletion on the extremes of daily precipitation in the austral summer are explored using two global climate models. Both models indicate that stratospheric ozone losses since the late 1970s may have increased the frequency and intensity of very heavy precipitation in austral summer over southern high and subtropical latitudes, and may have decreased the frequency and intensity over southern midlatitudes. This hemispheric wide pattern of extreme precipitation response projects strongly onto a previously identified pattern of seasonal mean precipitation response, both of which are shown to be likely of dynamic rather than thermodynamic origin.

Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity wave momentum flux, wavelengths, and frequencies of the waves. Until recently, global observations could not define the needed parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global data sets with improved resolution along with innovative analysis methods are now providing constraints for the parametrizations that can improve the treatment of these waves in climate prediction models. Research using very high resolution global models has also recently demonstrated the capability of resolving gravity waves and their circulation effects, and when tested against observations, these models are showing some very realistic properties. Here we review recent studies on gravity wave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity wave momentum fluxes, and results of the very high resolution model studies, and we outline some future research needs to improve the treatment of these waves in climate simulations. Fil: Alexander, M. J.. NWRA/Colorado Research Association; Estados Unidos Fil: Geller, M.. SUNY-Stonybrook; Estados Unidos Fil: McLandress, C.. University of Toronto; Canadá Fil: Polavarapu, S.. Environment; Canadá Fil: Preusse, P.. Forschungzentrum Jülich; Alemania Fil: Sassi, F.. Naval Research Laboratory; Estados Unidos Fil: Sato, K.. University of Tokyo; Japón Fil: Eckermann, S.. Naval Research Laboratory; Estados Unidos Fil: Ern, M.. Forschungzentrum Jülich; Alemania Fil: Hertzog, A.. UPMC Université Paris 06. Laboratoire de Météorologie Dynamique; Francia Fil: Kawatani, Y.. Japan Agency for Marine-Earth Science and Technology; Japón Fil: Pulido, Manuel Arturo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnologica; Argentina Fil: Shaw, T.. New York University; Estados Unidos Fil: Sigmond, M.. University of Toronto; Canadá Fil: Vincent, R.. University of Adelaide; Australia Fil: Watanabe, S.. Japan Agency for Marine-Earth Science and Technology; Japón

International audience; Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather-forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity-wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate-prediction models. Research using very-high-resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravity-wave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity-wave momentum fluxes and results of very-high-resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations. Copyright © 2010 Royal Meteorological Society and Crown in the right of Canada

We investigate the sensitivity of Northern Hemisphere polar ozone recovery to a scenario in which there is rapid loss of Arctic summer sea ice in the first half of the 21st century. The issue is addressed by coupling a chemistry climate model to an ocean general circulation model and performing simulations of ozone recovery with, and without, an external perturbation designed to cause a rapid and complete loss of summertime Arctic sea ice. Under this extreme perturbation, the stratospheric response takes the form of a springtime polar cooling which is dynamical rather than radiative in origin, and is caused by reduced wave forcing from the troposphere. The response lags the onset of the sea-ice perturbation by about one decade and lasts for more than two decades, and is associated with an enhanced weakening of the North Atlantic meridional overturning circulation. The stratospheric dynamical response leads to a 10 DU reduction in polar column ozone, which is statistically robust. While this represents a modest loss, it has the potential to induce a delay of roughly one decade in Arctic ozone recovery estimates made in the 2006 Scientific Assessment of Ozone Depletion.

Recent climate studies show that the northern-winter wave driving of the Brewer-Dobson circulation is enhanced if greenhouse gas concentrations increase. An explanation for this enhancement does not yet exist. In this study, the enhanced wave driving, as simulated in a doubled-CO2 experiment with the MA-ECHAM4 climate model, is analyzed in detail. The extratropical poleward eddy heat flux increases (decreases) in the stratosphere (troposphere) mainly due to the stationary (transient) heat-flux component. The heat flux at 100 hPa is a measure of the stratospheric wave driving, and is found to increase by 12% in the doubled-CO2 climate. This increase is dominated by the stationary-wave 1 heat flux, which is also enhanced in the midlatitude troposphere. The heat flux increase at 100 hPa is almost entirely due to an increase in the longitudinal temperature variability. The latter increase is mainly due to the well-understood sharpening of the lower-stratospheric meridional temperature gradient.

The separate climate effects of middle-atmospheric and tropospheric CO(2) doubling have been simulated and analyzed with the ECHAM middle-atmosphere climate model. To this end, the CO(2) concentration has been separately doubled in the middle-atmosphere, the troposphere, and the entire atmosphere, and the results have been compared to a control run. During NH winter, the simulated uniformly doubled CO(2) climate shows an increase of the stratospheric residual circulation, a small warming in the Arctic lower stratosphere, a weakening of the zonal winds in the Arctic middle-atmosphere, an increase of the NH midlatitude tropospheric westerlies, and a poleward shift of the SH tropospheric westerlies. The uniformly doubled CO(2) response in most regions is approximately equal to the sum of the separate responses to tropospheric and middle-atmospheric CO(2) doubling. The increase of the stratospheric residual circulation can be attributed for about two-thirds to the tropospheric CO(2) doubling and one-third to the middle-atmospheric CO(2) doubling. This increase contributes to the Arctic lower-stratospheric warming and, through the thermal wind relationship, to the weakening of the Arctic middle-atmospheric zonal wind. The increase of the tropospheric NH midlatitude westerlies can be attributed mainly to the middle-atmospheric CO(2) doubling, indicating the crucial importance of the middle-atmospheric CO(2) doubling for the tropospheric climate change. Results from an additional experiment show that the CO(2) doubling above 10 hPa, which is above the top of many current GCMs, also causes significant changes in the tropospheric climate.