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Nonlinearities in El Niño (EN) and La Niña (LN) teleconnections can arise for many reasons, involving nonlinearities in the two phases of El Niño Southern Oscillation (ENSO) itself and those occurring along the pathway of an atmospheric response. Here we search for nonlinearity in the ENSO stratospheric pathway. The ENSO stratospheric pathway is a teleconnection linking Equatorial Pacific SST anomalies to the North Atlantic and European regions, mediated by the stratospheric polar vortex. To address the question of nonlinear responses to ENSO, we use Large Ensembles (LEs, ~30 members) of historical simulations performed with six CMIP6 models. In addition, we consider the MPI-GE, a set of 95 members. In doing so, we assess the role of internal variability on ENSO related nonlinearities, with consistent atmosphere – ocean coupling. For the models with a stratospheric pathway, three key pathway metrics (tropospheric North Pacific low, polar night jet and North Atlantic pressure dipole) tend to show linearity in the response to moderate to strong EN events, but not so for LN events. Common to these models is also the tendency for asymmetry of strong events, with the EN response larger in amplitude. Instead, all metrics tend to be symmetric for weak and moderate events. The largest variety of outcomes across the models is found in the North Atlantic sea level pressure response, possibly because of the contribution of other factors, such as tropospheric processes, affecting the response., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Atmospheric circulation is one of the major drivers of Arctic sea ice and so, one of the key elements for its predictability. Most studies devoted to the atmosphere-sea ice coupling have so far only considered the effects of the troposphere. However, the low predictability of the polar tropospheric weather beyond two weeks hampers longer predictions of the sea ice. Further, changes in the stratosphere, such as sudden stratospheric warmings, have been found to affect surface conditions for up to several weeks, being the polar region one of the most affected areas. Thus, the consideration of the occurrence of these events may remarkably improve the predictability of the Arctic area.In this study, we aim to evaluate the predictive capacity of SSWs at sub-seasonal scale on Arctic sea ice concentration (SIC). To do so, we have used hindcast simulations of the sub-seasonal prediction model of the European Centre for Medium-Range Weather Forecasts (ECMWF) during the 20-year period of 1999-2018. Our results show that the model can reproduce the downward propagation of the stratospheric signal expected after an SSW event. It also simulates significant SIC anomalies after the occurrence of the downward-propagating SSWs. Finally, our results indicate an improvement in the prediction of Arctic sea ice after downward propagating SSWs compared to years without SSWs., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Stratospheric ozone is a key component of the climate system, playing an essential role in the radiative budget and protecting the Earth from harmful solar ultraviolet (UV) radiation. Past changes have had significant impacts on regional and global climate, so future ozone changes are also expected to have important effects on tropospheric and stratospheric climate. This work examines the long-term recovery trends in NH stratospheric ozone throughout the 21st century in the context of climate change using a set of chemistry-climate model simulations performed for the Chemistry Climate Model Initiative 1 and 2 (CCMI-1 and CCMI-2). While all simulations present a recovery of stratospheric ozone in the NH outside the tropics, there is a large spread in the rate of change, both among models and among ensemble members of individual models. We investigate to what extent this spread is attributed to uncertainties in the projected evolution of dynamical features, such as the strength of the polar vortex and the deep branch of the residual circulation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The stratosphere can be a source of predictability for surface weather on timescales of several weeks to months. However, the potential predictive skill gained from stratospheric variability can be limited by biases in the representation of stratospheric processes and the coupling of the stratosphere with surface climate in forecast systems. This study provides a first systematic identification of model biases in the stratosphere across a wide range of subseasonal forecast systems. It is found that many of the forecast systems considered exhibit warm global-mean temperature biases from the lower to middle stratosphere, too strong/cold wintertime polar vortices, and too cold extratropical upper-troposphere/lower-stratosphere regions. Furthermore, tropical stratospheric anomalies associated with the Quasi-Biennial Oscillation tend to decay toward each system's climatology with lead time. In the Northern Hemisphere (NH), most systems do not capture the seasonal cycle of extreme-vortex-event probabilities, with an underestimation of sudden stratospheric warming events and an overestimation of strong vortex events in January. In the Southern Hemisphere (SH), springtime interannual variability in the polar vortex is generally underestimated, but the timing of the final breakdown of the polar vortex often happens too early in many of the prediction systems. These stratospheric biases tend to be considerably worse in systems with lower model lid heights. In both hemispheres, most systems with low-top atmospheric models also consistently underestimate the upward wave driving that affects the strength of the stratospheric polar vortex. We expect that the biases identified here will help guide model development for subseasonal-to-seasonal forecast systems and further our understanding of the role of the stratosphere in predictive skill in the troposphere., Weather and Climate Dynamics Discussions, ISSN:2698-4024

24 pages, 13 figures., Las zonas polares, principales responsables del clima de nuestro planeta, están sufriendo cambios drásticos en su naturaleza debido a su alta sensibilidad al cambio global, que afectan directamente a la dinámica climática, oceanográfica y ambiental de latitudes extrapolares. El presente libro pretende dar una visión integral y multidisciplinar del estado del conocimiento de las zonas árticas y antárticas: su evolución geológica, los acuciantes problemas de contaminación de estos territorios, la caracterización de los diversos ecosistemas terrestres y marinos, así como la evolución pasada y futura del clima polar. El objetivo final es explicar, de forma clara y amena, las similitudes y diferencias entre ambos polos y concienciar sobre las alteraciones que están sufriendo debido al cambio global. Asimismo, se dan a conocer las investigaciones, de gran relevancia científica y social, realizadas por el personal científico y técnico del Consejo Superior de Investigaciones Científicas (CSIC), acercándonos a la historia y situación actual de la investigación polar en España.

19 pages, 1 figure, The Arctic and the Antarctic are the regions where temperature has raised most and faster than any other Earth’s place, producing a large number of impacts and feedback to the polar/climate system. Moreover, since polar oceans play a fundamental role in the Earth’s climate and global ecosystem, those changes produce climate consequences at mid latitudes . The study and monitoring of the poles from a global perspective and holistically is fundamental to better assess and understand the changes the polar regions are facing and its consequences on ocean circulation and climate, changes on the oceanic biogeochemistry composition and consequences on the oceanic living beings. Understanding the past to infer the future is another important leg to understand how the whole system is changing. The revision of the going on transformation and the continuous monitoring can be achieved with the combination of large amounts of observations (in situ and remote sensing) and numerical models

The stratosphere can have a signi_cant impact on winter surface weather on subseasonal to seasonal (S2S) timescales. This study evaluates the ability of current operational S2S prediction systems to capture two important links between the stratosphere and tropo sphere: (1) changes in probabilistic prediction skill in the extratropical stratosphere by precursors in the tropics and the extratropical troposphere and (2) changes in surface predictability in the extratropics after stratospheric weak and strong vortex events. Prob abilistic skill exists for stratospheric events when including extratropical tropospheric precursors over the North Paci_c and Eurasia, though only a limited set of models captures the Eurasian precursors. Tropical teleconnections such as the Madden‐Julian Oscillation, the Quasi‐Biennial Oscillation, and El Nin~o Southern Oscillation increase the probabilistic skill of the polar vortex strength, though these are only captured by a limited set of models. At the surface, predictability is increased over the USA, Russia, and the Middle East for weak vortex events, but not for Europe, and the change in predictability is smaller for strong vortex events for all prediction systems. Prediction systems with poorly resolved stratospheric processes represent this skill to a lesser degree. Altogether, the analyses indicate that correctly simulating stratospheric variability and stratosphere‐troposphere dynamical coupling are critical elements for skillful S2S wintertime predictions.

The stratosphere has been identified as an important source of predictability for a range of processes on subseasonal to seasonal (S2S) timescales. Knowledge about S2S predictability within the stratosphere is however still limited. This study evaluates to what extent predictability in the extratropical stratosphere exists in hindcasts of operational prediction systems in the S2S database. The stratosphere is found to exhibit extended predictability as compared to the troposphere. Prediction systems with higher stratospheric skill tend to also exhibit higher skill in the troposphere. The analysis also includes an assessment of the predictability for stratospheric events, including early and mid‐winter sudden stratospheric warming (SSW) events, strong vortex events, and extreme heat flux events for the Northern Hemisphere, and final warming events for both hemispheres. Strong vortex events and final warming events exhibit higher levels of predictability as compared to SSW events. In general, skill is limited to the deterministic range of one to two weeks. High‐top prediction systems overall exhibit higher stratospheric prediction skill as compared to their low‐top counterparts, pointing to the important role of stratospheric representation in S2S prediction models.

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 8 al 13 de abril de 2018, Stratospheric final warmings (SFWs) mark the final transition from the typical wintertime westerlies to summertime easterlies at high latitudes in the stratosphere. They occur in both hemispheres, but, due to the enhanced planetary wave activity, these events present a high inter-annual variability in the Northern Hemisphere. In particular, SFWs onset dates oscillate in a range of about two months and SFWs show different vertical temporal developments, i.e. events may occur first in the mid-stratosphere (10-hPa first SFWs) or in the upper stratosphere (1-hPa first SFWs). This variability has been shown to impact the polar ozone depletion, but also the tropospheric circulation, particularly over the North Atlantic sector. Thus, it is important to identify factors that influence SFW variability. However, most of the previous studies were essentially based on reanalysis data and so, the length of observations is relatively short to derive robust conclusions. In this study, we aim to improve the understanding of drivers, trends and surface impact of dynamical variability of boreal SFWs. To do this, we use multi-decadal integrations (145 years) of a fully coupled chemistry-climate model (CESM1(WACCM)) that ensure a large statistical sampling. Four sensitivity experiments are also analyzed to assess the impact of external factors such as quasi-biennial oscillation (QBO), sea surface temperature (SST) variability and anthropogenic emissions. Our results confirm previous reanalysis results regarding the differences in the time evolution of stratospheric conditions and surface impact between 10-hPa and 1-hPa first SFWs. Additionally, a tripolar SST pattern is, for the first time, identified over the North Atlantic in spring months related to the SFW variability. The analysis of the influence of remote modulators on SFWs revealed that the occurrence of stratospheric sudden warmings in the previous winter prioritizes the development of 10-hPa first SFWs later on. We further found that QBO and interannual SST variability significantly affect the ratio between 1-hPa first and 10-hPa first SFWs. Finally, the long term evolution of ozone depleting substances concentration seems to impact the timing of the occurrence of 1-hPa first SFWs. The results of this study might be useful for improving springtime seasonal forecast in the Euro-Atlantic sector.

Trabajo presentado en la Stratosphere-troposphere Processes And their Role in Climate General Assembly, celebrada en Tokyo (Japón) del 1 al 5 de octubre de 2018, Major sudden stratospheric warmings (SSWs) represent one of the most abrupt phenomena of the boreal wintertime stratospheric variability, and constitute the clearest example of the stratosphere-troposphere coupling. A good representation of SSWs in climate models is required to reduce their biases and uncertainties in future projections. 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 can affect the model evaluation. In this study, 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, the spread of results is wider in the pre-satellite period than afterwards, particularly for the NCEP/NCAR reanalysis. The good agreement among reanalyses is also found for triggering mechanisms, tropospheric precursors and surface fingerprint and all datasets reproduce similarly the specific signatures of wavenumber 1 (WN1) and wavenumber 2 (WN2) SSWs. Differences in blocking activity prior to WN1 and WN2 events between reanalyses are much smaller than between blocking definitions.

The stratospheric response to anthropogenic changes and ist feedback on tropospheric climate and weather are of growing interest in climate research and numerical weather prediction. The SHARP-2016 workshop discussed recent progress and future directions in the research on stratospheric change and its implications for climate and weather with focus on Brewer-Dobson circulation, Stratospheric ozone, Stratosphere-troposphere coupling, Stratospheric water vapour.