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A 250 year simulation of a strongly eddying global version of the Parallel Ocean Program (POP) model reveals a new mode of intrinsic multidecadal variability, the Southern Ocean Mode (SOM), with a period of 40-50 year. The peak-to-peak difference in the global ocean heat content within a multidecadal cycle is up to 60 ZJ. This change results from surface heat flux variations in the South Atlantic and propagation of temperature anomalies along the Antarctic Circumpolar Current and into the Weddell gyre around 30°E. The temperature anomalies propagate as deep as 5000 m along the isopycnals between 50°S and 30°S and induce multidecadal changes in the Atlantic Meridional Overturning Circulation. A positive feedback loop between the generation of eddies through baroclinic instability and the dynamics of the mean circulation is essential for the existence of the SOM. The dominant physics appears similar to that responsible for variability found in a three-layer quasi-geostrophic eddy-resolving model. This combined with the fact that the SOM is not found in a noneddying version of the same global POP model further suggests that eddy processes are crucial for its existence and/or excitation.

International audience; The variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1 year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs.

The viability of global climate models for forecasting the Indian monsoon is explored. Evaluation and intercomparison of model skills are employed to assess the reliability of individual models and to guide model selection strategies. Two dominant and unique patterns of Indian monsoon climatology are trends in maximum temperature and periodicity in total rainfall observed after 30 yr averaging over India. An examination of seven models and their ensembles reveals that no single model or model selection strategy outperforms the rest. The single-best model for the periodicity of Indian monsoon rainfall is the only model that captures a low-frequency natural climate oscillator thought to dictate the periodicity. The trend in maximum temperature, which most models are thought to handle relatively better, is best captured through a multimodel average compared to individual models. The results suggest a need to carefully evaluate individual models and model combinations, in addition to physical drivers where possible, for regional projections from global climate models.

Ponencia presentada en: VIII Congreso de la Asociación Española de Climatología celebrado en Salamanca entre el 25 y el 28 de septiembre de 2012. [EN]In this study we have analyzed the variability of explosive cyclones affecting Europe at different timescales. Cyclones have been identified and tracked through an automatic algorithm that has been applied to the MSLP NCEP reanalysis data. Subsequently, explosive cyclones affecting Europe have been selected from the whole climatology of extratropical North Atlantic cyclones (406 total cases from Oct-Mar 1950-2010). [ES]En este estudio se ha analizado la variabilidad en diferentes escalas temporales de las ciclogénesis explosivas que afectan Europa. La identificación y seguimiento de ciclones se ha realizado a través de la utilización de un algoritmo automático aplicado sobre los datos de MSLP del reanálisis de NCEP. En total, 406 casos de ciclogénesis explosivas que han afectado Europa han sido seleccionados (Oct-Mar 1950-2010). This study has been partially supported by the Spanish National projects DE VIAJE (CGL2009-06944), TRACS (CGL2009-10285) and the UCM-BSCH GR58/08 “Micrometeorology and Climate variability” group.

Observations of sea ice extent and atmospheric temperature in the Arctic, although sparse, indicate variability on multidecadal time scales. A recent analysis of one of the global climate models [the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (CM2.1)] in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change has indicated that Arctic Ocean variability on these time scales is associated with changes in basin-wide salinity patterns. In this paper the internal modes of variability in an idealized Arctic Basin are determined by considering the stability of salinity-driven flows. An internal ocean mode with a multidecadal time scale is found, with a spatial pattern similar to that obtained in the analysis of the CM2.1 results. The modes propagate as a "saline Rossby wave" induced by the background salinity gradient

Sea-level data from South Atlantic stations were quality-controlled and analysed. Eleven time series, longer than 20 years, were selected to study long-term variability. The main features of South Atlantic sea level were described by computing linear trends, mean seasonal cycles and power spectra for the selected time series. The data sparseness made it difficult to achieve a comprehensive description of the basin. The connection between sea-level variability and ENSO was studied using the composite time series obtained by merging data for Buenos Aires and Palermo, on the Argentinian side of Rio de la Plata. It was possible to recognize that sea-level variability is strongly connected with that of river discharge into Rio de la Plata, which in turn exhibits a lagged teleconnection with ENSO. The time lag between ENSO and sea-level anomalies is about 5 months.

International audience; Marine paleo-records acquired at high temporal resolution provide critical data for testing numerical climate models and help to understand processes underlying ocean variability. This study presents a unique 4500-year reconstruction of sea surface temperature (SST) obtained from alkenones in the North Atlantic Polar Front area off North Iceland, at an average temporal resolution of 4–5 years. Spectral analysis of this signal shows dominant multidecadal oscillations which occurred with a stronger amplitude between 2500 and 4200 years BP, hand in hand with fluctuations of bottom currents indicated by paleomagnetic proxies. Contemporaneous large excursions of the Inter-tropical Convergence Zone (ITCZ) are also recorded by the distant Cariaco titanium time series, suggesting a link with low latitude Atlantic climate. We speculate that the oscillations reflect changes of the Meridional Overturning Circulation (MOC) induced by increased ENSO activity

Studies based on data from the past 25–45 years show that irradiance changes related to the 11-yr solar cycle affect the circulation of the upper troposphere in the subtropics and midlatitudes. The signal has been interpreted as a northward displacement of the subtropical jet and the Ferrel cell with increasing solar irradiance. In model studies on the 11-yr solar signal this could be related to a weakening and at the same time broadening of the Hadley circulation initiated by stratospheric ozone anomalies. Other studies, focusing on the direct thermal effect at the Earth’s surface on multidecadal scales, suggest a strengthening of the Hadley circulation induced by an increased equator-to-pole temperature gradient. In this paper we analyse the solar signal in the upper troposphere since 1922, using statistical reconstructions based on historical upper-air data. This allows us to address the multidecadal variability of solar irradiance, which was supposedly large in the first part of the 20th century. Using a simple regression model we find a consistent signal on the 11-yr time scale which fits well with studies based on later data. We also find a significant multidecadal signal that is similar to the 11-yr signal, but somewhat stronger. We interpret this signal as a poleward shift of the subtropical jet and the Ferrel cell. Comparing the magnitude of the two signals could provide important information on the feedback mechanisms involved in the solar climate relationship with respect to the Hadley and Ferrel circulations. However, in view of the uncertainty in the solar irradiance reconstructions, such interpretations are not currently possible.

The influence of the Atlantic multidecadal variability (AMV) on the North Atlantic storm track and eddy-driven jet in the winter season is assessed via a coordinated analysis of idealized simulations with state-of-the-art coupled models. Data used are obtained from a multimodel ensemble of AMV± experiments conducted in the framework of the Decadal Climate Prediction Project component C. These experiments are performed by nudging the surface of the Atlantic Ocean to states defined by the superimposition of observed AMV± anomalies onto the model climatology. A robust extratropical response is found in the form of a wave train extending from the Pacific to the Nordic seas. In the warm phase of the AMV compared to the cold phase, the Atlantic storm track is typically contracted and less extended poleward and the low-level jet is shifted toward the equator in the eastern Atlantic. Despite some robust features, the picture of an uncertain and model-dependent response of the Atlantic jet emerges and we demonstrate a link between model bias and the character of the jet response. The authors are thankful to three anonymous reviewers for valuable comments on an early version of the manuscript. PR, AB, DN, SG, ND, RE, LH, DS, and SW were supported by the H2020 EUCP project (Grant GA 776613). ND, RE, LH, and DS were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. CC, ESG, and SQ would like to thank Marie-Pierre Moine for her help in model output post-processing and publication on CMIP6 database. BH acknowledges support from the NERC North Atlantic Climate System: Integrated Study (ACSIS) project. YRR was founded by the European Union’s Horizon 2020 Research and Innovation Programme in the framework of the Marie Sklodowska-Curie Grant INADEC (Grant Agreement 800154). The National Center for Atmospheric Research (NCAR) is a major facility sponsored by the U.S. National Science Foundation (NSF) under Cooperative Agreement 1852977. The NCAR contribution was supported by the U.S. NSF under the Collaborative Research EaSM2 Grant OCE-1243015 and the U.S. National Oceanic and Atmospheric Administration (NOAA) Climate Program Office under the Climate Variability and Predictability Program Grant NA13OAR4310138. SW has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement H2020-MSCA-COFUND-2016-754433. Peer Reviewed "Article signat per 20 autors/es: Paolo Ruggieri, Alessio Bellucci, Dario Nicolí, Panos J. Athanasiadis, Silvio Gualdi, Christophe Cassou, Fred Castruccio, Gokhan Danabasoglu, Paolo Davini, Nick Dunstone, Rosemary Eade, Guillaume Gastineau, Ben Harvey, Leon Hermanson, Saïd Qasmi, Yohan Ruprich-Robert, Emilia Sanchez-Gomez, Doug Smith, Simon Wild, and Matteo Zampieri"