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Christophe Cassou and Laurent Terray

Abstract

The relationship between global sea surface temperatures (SSTs) and the North Atlantic–Europe (NAE) atmospheric circulation is investigated using an ensemble of eight simulations with the ARPEGE atmospheric global circulation model forced with prescribed SSTs over the 1948–97 period. The model mean state is first validated against NCEP reanalyses. The interannual SST-forced variability is then compared to the internal one using analysis of variance (ANOVA) techniques. Both components are maximum in winter over the Northern Hemisphere and the associated potential predictability shows weak but significant values located over the Icelandic low (IL) and the Azores high (AH).

The North Atlantic oscillation (NAO) is found to be the leading internal variability mode over the NAE sector as shown by principal component analysis of a control simulation with climatological SSTs. The noise imprint dominates the forced response estimated from the ensemble mean. The latter is related first to the El Niño–Southern Oscillation (ENSO) activity. During warm (cold) events in the Pacific, the AH shows negative (positive) pressure anomalies and weakened opposition with the IL. The AH fluctuations exhibit a 3.7-yr peak and result from changes in the activity of the Atlantic Hadley cell and from the eastward extension of the Pacific North America teleconnection pattern. Eliassen–Palm diagnostics show that eddy–mean flow interaction acts to maintain the anomalous Atlantic stationary wave pattern as described by Fraedrich in a review based on observational results. The simulated ENSO–NAE connection is, however, too strong in the model and this dominance may be related to the simulated mean state biases. Second, the North Atlantic atmospheric forced signal is associated with the Atlantic SSTs. A tripole structure over the North Atlantic basin with maximum loading in its tropical branch is linked to the phase of the simulated NAO. A local Hadley cell mechanism associated with Rossby wave excitation over the Atlantic is suggested to explain tropical–midlatitude interactions in the model.

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Christopher G. Fletcher and Christophe Cassou

Abstract

The northern annular mode (NAM) influences wintertime climate variability in the Northern Hemisphere, and understanding the processes controlling its sign and amplitude is of critical importance. Mounting evidence supports a robust teleconnection between the El Niño–Southern Oscillation (ENSO) and the NAM, while internal variability generated in the tropical Indian Ocean (TIO) may be associated with a NAM response of the opposite sign. This study uses a coupled ocean–atmosphere model to separate the influence on the NAM from teleconnections driven by ENSO and the TIO. In composites constructed using a long preindustrial control integration, increased December–February precipitation in the central/eastern Pacific drives a negative late-winter NAM response. When isolated from ENSO variability, increased precipitation over the western-central TIO drives a strong and persistent positive NAM response throughout the winter. Opposite linear interference of the anomalous wave teleconnections explains most of the opposite-signed planetary wavedriving of the NAM responses. The case with combined ENSO and TIO variability yields cancellation of the wave interference and a weak NAM response. This mechanism is confirmed using experiments where the tropical ocean is nudged separately over the Pacific and TIO to the large-amplitude 1997/98–1998/99 ENSO cycle. The phases of the Rossby wave and NAM responses in these two cases are of opposite sign, providing strong evidence that internal variability over the TIO can induce teleconnections independent of—and with opposite sign to—those associated with ENSO.

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Marie Drouard and Christophe Cassou

Abstract

Considerable uncertainties remain about the expected changes of ENSO and associated teleconnectivity as the climate is warming. Two ensembles of pacemaker experiments using the CNRM-CM5 coupled model are designed in a perfect model framework to contrast ENSO-forced teleconnectivity between the preindustrial period versus a warmer background state (obtained from a long stabilized simulation under late-twenty-first-century RCP8.5 constant forcing). The most notable sensitivity to the mean background state is found over the North Atlantic, where the ENSO–NAO teleconnection is considerably reinforced in a warmer world. We attribute this change to (i) a stronger and eastward-extended mean upper-level jet over the North Pacific, (ii) an eastward-shifted ENSO teleconnection over the North Pacific, and (iii) an equatorward-shifted and reinforced mean jet over the North Atlantic. These altogether act as a more efficient waveguide, leading to a better penetration of synoptic storms coming from the Pacific into the Atlantic. This downstream penetration into the North Atlantic basin forces more systematically the NAO through wave breaking. The reinforcement in the teleconnection is asymmetrical with respect to the ENSO phase and is mainly sensitive to La Niña events. Even though the Pacific jet tends to retract westward and move northward during cold events, mean changes are such that both Pacific and Atlantic jets remain connected in a warmer climate by contrast to the preindustrial period, thus ensuring preferred anticyclonic wave breaking downstream over the North Atlantic leading ultimately to NAO+ events.

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Christophe Cassou, Laurent Terray, and Adam S. Phillips

Abstract

Diagnostics combining atmospheric reanalysis and station-based temperature data for 1950–2003 indicate that European heat waves can be associated with the occurrence of two specific summertime atmospheric circulation regimes. Evidence is presented that during the record warm summer of 2003, the excitation of these two regimes was significantly favored by the anomalous tropical Atlantic heating related to wetter-than-average conditions in both the Caribbean basin and the Sahel. Given the persistence of tropical Atlantic climate anomalies, their seasonality, and their associated predictability, the suggested tropical–extratropical Atlantic connection is encouraging for the prospects of long-range forecasting of extreme weather in Europe.

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Saïd Qasmi, Christophe Cassou, and Julien Boé

Abstract

The response of the European climate to the Atlantic multidecadal variability (AMV) remains difficult to isolate in observations because of the presence of strong internal variability and anthropogenically forced signals. Using model sensitivity experiments proposed within the CMIP6/Decadal Climate Prediction Project Component C (DCPP-C) framework, the wintertime AMV–Europe teleconnection is here investigated in large ensembles of pacemaker-type simulations conducted with the CNRM-CM5 global circulation model. To evaluate the sensitivity of the model response to the AMV amplitude, twin experiments with the AMV forcing pattern multiplied by 2 and 3 (2xAMV and 3xAMV, respectively) are performed in complement to the reference ensemble (1xAMV). Based on a flow analog method, we show that the AMV-forced atmospheric circulation tends to cool down the European continent, whereas the residual signal, mostly including thermodynamical processes, contributes to warming. In 1xAMV, both terms cancel each other, explaining the overall weak AMV-forced atmospheric signal. In 2xAMV and 3xAMV, the thermodynamical contribution overcomes the dynamical cooling and is responsible for milder and wetter conditions found at large scale over Europe. The thermodynamical term includes the advection of warmer and more humid oceanic air penetrating inland and the modification of surface radiative fluxes linked to both altered cloudiness and snow-cover reduction acting as a positive feedback with the AMV amplitude. The dynamical anomalous circulation combines 1) a remote response to enhanced diabatic heating acting as a Rossby wave source in the western tropical Atlantic and 2) a local response associated with warmer SST over the subpolar gyre favoring an anomalous high. The extratropical influence is reinforced by polar amplification due to sea ice melting in all the subarctic seas. The weight between the tropical–extratropical processes and associated feedbacks is speculated to partly explain the nonlinear sensibility of the response to the AMV forcing amplitude, challenging thus the use of the so-called pattern-scaling technique to evaluate teleconnectivity and related impacts associated with decadal variability.

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Christophe Cassou, Clara Deser, and Michael A. Alexander

Abstract

Extratropical SSTs can be influenced by the “reemergence mechanism,” whereby thermal anomalies in the deep winter mixed layer persist at depth through summer and are then reentrained into the mixed layer in the following winter. The impact of reemergence in the North Atlantic Ocean (NAO) upon the climate system is investigated using an atmospheric general circulation model coupled to a mixed layer ocean/thermodynamic sea ice model.

The dominant pattern of thermal anomalies below the mixed layer in summer in a 150-yr control integration is associated with the North Atlantic SST tripole forced by the NAO in the previous winter as indicated by singular value decomposition (SVD). To isolate the reemerging signal, two additional 60-member ensemble experiments were conducted in which temperature anomalies below 40 m obtained from the SVD analysis are added to or subtracted from the control integration. The reemerging signal, given by the mean difference between the two 60-member ensembles, causes the SST anomaly tripole to recur, beginning in fall, amplifying through January, and persisting through the following spring. The atmospheric response to these SST anomalies resembles the circulation that created them the previous winter but with reduced amplitude (10–20 m at 500 mb per °C), modestly enhancing the winter-to-winter persistence of the NAO. Changes in the transient eddies and their interactions with the mean flow contribute to the large-scale equivalent barotropic response throughout the troposphere. The latter can also be attributed to the change in occurrence of intrinsic weather regimes.

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Nicolas Barrier, Christophe Cassou, Julie Deshayes, and Anne-Marie Treguier

Abstract

A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

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Robin Waldman, Joël Hirschi, Aurore Voldoire, Christophe Cassou, and Rym Msadek

Abstract

This work aims to clarify the relation between the Atlantic meridional overturning circulation (AMOC) and the thermal wind. We derive a new and generic dynamical AMOC decomposition that expresses the thermal wind transport as a simple vertical integral function of eastern minus western boundary densities. This allows us to express density anomalies at any depth as a geostrophic transport in Sverdrups (1 Sv ≡ 106 m3 s−1) per meter and to predict that density anomalies around the depth of maximum overturning induce most AMOC transport. We then apply this formalism to identify the dynamical drivers of the centennial AMOC variability in the CNRM-CM6 climate model. The dynamical reconstruction and specifically the thermal wind component explain over 80% of the low-frequency AMOC variance at all latitudes, which is therefore almost exclusively driven by density anomalies at both zonal boundaries. This transport variability is dominated by density anomalies between depths of 500 and 1500 m, in agreement with theoretical predictions. At those depths, southward-propagating western boundary temperature anomalies induce the centennial geostrophic AMOC transport variability in the North Atlantic. They are originated along the western boundary of the subpolar gyre through the Labrador Sea deep convection and the Davis Strait overflow.

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Christophe Cassou, Laurent Terray, James W. Hurrell, and Clara Deser

Abstract

The observed low-frequency winter atmospheric variability of the North Atlantic–European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm.

Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAO+ climate regime compared to the NAO− regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900–60 and 1950–2001); hence, it appears to be intrinsic to the NAO+ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAO+ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAO+ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings.

The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Niña events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAO− regimes than cold anomalies are at forcing NAO+ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

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Claire M. Perigaud, Christophe Cassou, Boris Dewitte, Lee-Lueng Fu, and J. David Neelin

Abstract

This paper provides a detailed illustration that it is beneficial for ENSO forecasting to improve in priority the model parameterizations, instead of increasing only the consistency of the initial conditions with the coupled model. Moreover it is shown that the latter can lead to misleading results. Using sea level data in addition to wind to initialize the Cane and Zebiak model does not improve El Niño forecasts. Nudging the observed wind to the model one to initialize the forecasts as proposed by Chen et al. also fails to correct the model deficiencies and degrades the initial conditions of the ocean and atmosphere. These failures are explained by large model errors in the off-equatorial sea level and wind anomalies that play a key role in the coupled behavior. The use of data to estimate new model parameterizations allows for significantly improving both the initial conditions and the forecasts up to 6-month lead time. This success holds for all the various initialization procedures used in this study. Because of erroneous winds simulated by the atmospheric component in the eastern Pacific, errors grow fast though. Replacing the atmospheric model by a statistical one results in more reliable predictions over 1980–98. For lead times up to 1 yr, the model predicts well the observed anomalies between 1984 and 1993, including the sea level rises along the ITCZ after warm events and their subsequent equatorward migration. This success is attributed to the consistency between the observed anomalies over this period and the mechanisms involved in maintaining the oscillatory behavior of the model, including the off-equatorial meridional wind anomalies.

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