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  • 1
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    Equatorial Atlantic variability-Modes, mechanisms, and global teleconnections (2018)
    Wiley
    In:  Wiley Interdisciplinary Reviews: Climate Change, 9 (4). e527.
    Details
    Publication Date: 2021-02-08
    Description: Sea surface temperature (SST) variability in the tropical Atlantic Ocean strongly impacts the climate on the surrounding continents. On interannual time scales, highest SST variability occurs in the eastern equatorial region and off the coast of southwestern Africa. The pattern of SST variability resembles the Pacific El Niño, but features notable differences, and has been discussed in the context of various climate modes, that is, reoccurring patterns resulting from particular interactions in the climate system. Here, we attempt to reconcile those different definitions, concluding that almost all of them are essentially describing the same mode that we refer to as the “Atlantic Niño.” We give an overview of the mechanisms that have been proposed to underlie this mode, and we discuss its interaction with other climate modes within and outside the tropical Atlantic. The impact of Atlantic Niño‐related SST variability on rainfall, in particular over the Gulf of Guinea and north eastern South America is also described. An important aspect we highlight is that the Atlantic Niño and its teleconnections are not stationary, but subject to multidecadal modulations. Simulating the Atlantic Niño proves a challenge for state‐of‐the‐art climate models, and this may be partly due to the large mean state biases in the region. Potential reasons for these model biases and implications for seasonal prediction are discussed.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/wcc.527
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Analogous seasonal evolution of the South Atlantic SST dipole indices (2017)
    Wiley
    In:  Atmospheric Science Letters, 18 (10). pp. 396-402.
    Details
    Publication Date: 2020-02-06
    Description: Two variants of sea-surface temperature (SST) dipole indices for the South Atlantic Ocean (SAO) has been previously described representing: (1) the South Atlantic subtropical dipole (SASD) supposedly peaking in austral summer and (2) the SAO dipole (SAOD) in winter. In this study, we present the analysis of observational data sets (1985–2014) showing the SASD and SAOD as largely constituting the same mode of ocean–atmosphere interaction reminiscent of the SAOD structure peaking in winter. Indeed, winter is the only season in which the inverse correlation between the northern and southern poles of both indices is statistically significant. The observed SASD and SAOD indices exhibit robust correlations (P ≤ 0.001) in all seasons and these are reproduced by 54 of the 63 different models of the Coupled Models Intercomparison Project analysed. Their robust correlations notwithstanding the SASD and SAOD indices appear to better capture different aspects of SAO climate variability and teleconnections
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    DOI: 10.1002/asl.781
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Eastern Boundary Circulation and Hydrography Off Angola: Building Angolan Oceanographic Capacities: Building Angolan Oceanographic Capacities (2018)
    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 99 (8). pp. 1589-1605.
    Details
    Publication Date: 2021-05-19
    Description: The eastern boundary region off Angola encompasses a highly productive ecosystem important for the food security of the coastal population. The fish-stock distribution, however, undergoes large variability on intraseasonal, interannual, and longer time scales. These fluctuations are partly associated with large-scale warm anomalies that are often forced remotely from the equatorial Atlantic and propagate southward, reaching the Benguela upwelling off Namibia. Such warm events, named Benguela Niños, occurred in 1995 and in 2011. Here we present results from an underexplored extensive in situ dataset that was analyzed in the framework of a capacity-strengthening effort. The dataset was acquired within the Nansen Programme executed by the Food and Agriculture Organization of the United Nations and funded by the Norwegian government. It consists of hydrographic and velocity data from the Angolan continental margin acquired biannually during the main downwelling and upwelling seasons over more than 20 years. The mean seasonal changes of the Angola Current from 6° to 17°S are presented. During austral summer the southward Angola Current is concentrated in the upper 150 m. It strengthens from north to south, reaching a velocity maximum just north of the Angola Benguela Front. During austral winter the Angola Current is weaker, but deeper reaching. While the southward strengthening of the Angola Current can be related to the wind forcing, its seasonal variability is most likely explained by coastally trapped waves. On interannual time scales, the hydrographic data reveal remarkable variability in subsurface upper-ocean heat content. In particular, the 2011 Benguela Niño was preceded by a strong subsurface warming of about 2 years’ duration.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/BAMS-D-17-0197.1
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  • 4
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    Uncertainty in twenty-first century projections of the Atlantic Meridional Overturning Circulation in CMIP3 and CMIP5 models (2017)
    Springer
    In:  Climate Dynamics, 49 (5-6). pp. 1495-1511.
    Details
    Publication Date: 2020-02-06
    Description: Uncertainty in the strength of the Atlantic Meridional Overturning Circulation (AMOC) is analyzed in the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5) projections for the twenty-first century; and the different sources of uncertainty (scenario, internal and model) are quantified. Although the uncertainty in future projections of the AMOC index at 30°N is larger in CMIP5 than in CMIP3, the signal-to-noise ratio is comparable during the second half of the century and even larger in CMIP5 during the first half. This is due to a stronger AMOC reduction in CMIP5. At lead times longer than a few decades, model uncertainty dominates uncertainty in future projections of AMOC strength in both the CMIP3 and CMIP5 model ensembles. Internal variability significantly contributes only during the first few decades, while scenario uncertainty is relatively small at all lead times. Model uncertainty in future changes in AMOC strength arises mostly from uncertainty in density, as uncertainty arising from wind stress (Ekman transport) is negligible. Finally, the uncertainty in changes in the density originates mostly from the simulation of salinity, rather than temperature. High-latitude freshwater flux and the subpolar gyre projections were also analyzed, because these quantities are thought to play an important role for the future AMOC changes. The freshwater input in high latitudes is projected to increase and the subpolar gyre is projected to weaken. Both the freshening and the gyre weakening likely influence the AMOC by causing anomalous salinity advection into the regions of deep water formation. While the high model uncertainty in both parameters may explain the uncertainty in the AMOC projection, deeper insight into the mechanisms for AMOC is required to reach a more quantitative conclusion.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    DOI: 10.1007/s00382-016-3180-x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Simulated response to inter-annual SST variations in the Gulf Stream region (2014)
    Springer
    In:  Climate Dynamics, 42 (3/4). pp. 715-731.
    Details
    Publication Date: 2019-09-23
    Description: Recent studies show that mid-latitude SST variations over the Kuroshio-Oyashio Extension influence the atmospheric circulation. However, the impact of variations in SST in the Gulf Stream region on the atmosphere has been less studied. Understanding the atmospheric response to such variability can improve the climate predictability in the North Atlantic Sector. Here we use a relatively high resolution (∼1°) Atmospheric General Circulation Model to investigate the mechanisms linking observed 5-year low-pass filtered SST variability in the Gulf Stream region and atmospheric variability, with focus on precipitation. Our results indicate that up to 70 % of local convective precipitation variability on these timescales can be explained by Gulf Stream SST variations. In this region, SST and convective precipitation are strongly correlated in both summer (r = 0.73) and winter (r = 0.55). A sensitivity experiment with a prescribed local warm SST anomaly in the Gulf Stream region confirms that local SST drives most of the precipitation variability over the Gulf Stream. Increased evaporation connected to the anomalous warm SST plays a crucial role in both seasons. In summer there is an enhanced local SLP minimum, a concentrated band of low level convergence, deep upward motion and enhanced precipitation. In winter we also get enhanced precipitation, but a direct connection to deep vertical upward motion is not found. Nearly all of the anomalous precipitation in winter is connected to passing atmospheric fronts. In summer the connection between precipitation and atmospheric fronts is weaker, but still important.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1007/s00382-013-1715-y
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Potential of Equatorial Atlantic Variability to Enhance El Nino Prediction (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 . pp. 2278-2283.
    Details
    Publication Date: 2017-05-24
    Description: Extraordinarily strong El Niño events, such as those of 1982/83 and 1997/98, have been poorly predicted by operational seasonal forecasts made before boreal spring, despite significant advances in understanding, improved models, and enhanced observational networks. The Equatorial Atlantic Zonal Mode – a phenomenon similar to El Niño but much weaker and peaking in boreal summer – impacts winds over the Pacific, and hence affects El Niño, and also potentially its predictability. Here we use a climate model to perform a suite of seasonal predictions with and without SST in the Atlantic restored to observations. We show for the first time that knowledge of Equatorial Atlantic sea surface temperature (SST) significantly improves the prediction across boreal spring of major El Niño events and also weaker variability. This is because Atlantic SST acts to modulate El Niño variability, rather than triggering events. Our results suggest that better prediction of major El Niño events might be achieved through model improvement in the Equatorial Atlantic.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    DOI: 10.1002/grl.50362
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Resolving the upper-ocean warm layer improves the simulation of the Madden–Julian oscillation (2015)
    Springer
    In:  Climate Dynamics, 44 (5-6). pp. 1487-1503.
    Details
    Publication Date: 2017-04-13
    Description: Here we show that coupling a high-resolution one-column ocean model to an atmospheric general circulation model dramatically improves simulation of the Madden–Julian oscillation (MJO) to have realistic strength, period, and propagation speed. The mechanism for the simulated MJO involves both frictional wave-convective conditional instability of the second kind (Frictional wave-CISK) and air–sea convective intraseasonal interaction (ASCII). In particular, better resolving the fine structure of upper ocean temperature, especially the warm layer, produces more vigorous atmosphere–ocean interaction and strengthens intraseasonal variations in both SST and atmospheric circulation. This helps organize and strengthen deep convection, inducing a stronger Kelvin-wave like perturbation and frictional near-surface convergence to the east. In addition, the warmer SST ahead of the MJO also acts to destabilize the boundary layer and enhance frictional convergence. These lead to a more realistic eastward-propagating MJO. A suite of sensitivity experiments were performed to show the robustness of the mechanisms and to demonstrate: (1) that mean state differences are not the main contributors to the improved simulation of our coupled model; (2) the role of SST variability in enhancing frictional convergence and intraseasonal variations in precipitation, and (3) that the simulation is significantly degraded when the first ocean model layer is thicker than 10 m. Our coupled model results are consistent with observations and demonstrate a simple but effective means to significantly improve MJO simulation and potentially also forecasts.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1007/s00382-014-2315-1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    A mechanism for Atlantic multidecadal variability in the Kiel Climate Model (2013)
    Springer
    In:  Climate Dynamics, 41 (7-8). pp. 2133-2144.
    Details
    Publication Date: 2019-09-23
    Description: Atlantic Multidecadal Variability (AMV) is investigated in a millennial control simulation with the Kiel Climate Model (KCM), a coupled atmosphere–ocean–sea ice model. An oscillatory mode with approximately 60 years period and characteristics similar to observations is identified with the aid of three-dimensional temperature and salinity joint empirical orthogonal function analysis. The mode explains 30 % of variability on centennial and shorter timescales in the upper 2,000 m of the North Atlantic. It is associated with changes in the Atlantic Meridional Overturning Circulation (AMOC) of ±1–2 Sv and Atlantic Sea Surface Temperature (SST) of ±0.2 °C. AMV in KCM results from an out-of-phase interaction between horizontal and vertical ocean circulation, coupled through Irminger Sea convection. Wintertime convection in this region is mainly controlled by salinity anomalies transported by the Subpolar Gyre (SPG). Increased (decreased) dense water formation in this region leads to a stronger (weaker) AMOC after 15 years, and this in turn leads to a weaker (stronger) SPG after another 15 years. The key role of salinity variations in the subpolar North Atlantic for AMV is confirmed in a 1,000 year long simulation with salinity restored to model climatology: No low frequency variations in convection are simulated, and the 60 year mode of variability is absent.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1007/s00382-012-1633-4
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    A multi-model comparison for Atlantic multidecadal variability (2014)
    Springer
    In:  Climate Dynamics, 43 . pp. 2333-2348.
    Details
    Publication Date: 2019-09-23
    Description: A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6:1993–2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that high-latitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperature-induced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1007/s00382-014-2056-1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Marine-based multiproxy reconstruction of Atlantic multidecadal variability (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 (4). pp. 1295-1300.
    Details
    Publication Date: 2017-04-10
    Description: Atlantic multidecadal variability (AMV) is known to impact climate globally, and knowledge about the persistence of AMV is important for understanding past and future climate variability, as well as modeling and assessing climate impacts. The short observational data do not significantly resolve multidecadal variability, but recent paleoproxy reconstructions show multidecadal variability in North Atlantic temperature prior to the instrumental record. However, most of these reconstructions are land-based, not necessarily representing sea surface temperature. Proxy records are also subject to dating errors and microenvironmental effects. We extend the record of AMV 90 years past the instrumental record using principle component analysis of five marine-based proxy records to identify the leading mode of variability. The first principal component is consistent with the observed AMV, and multidecadal variability seems to persist prior to the instrumental record. Thus, we demonstrate that reconstructions of past Atlantic low-frequency variability can be improved by combining marine-based proxies.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2013GL059076
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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