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  • 1
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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
    Format: text
    DOI: 10.1002/grl.50362
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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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
  • 3
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    The Impact of Mean State Errors on Equatorial Atlantic Interannual Variability in a Climate Model (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 120 (2). pp. 1133-1151.
    Details
    Publication Date: 2019-04-04
    Description: Observations show that the Equatorial Atlantic Zonal Mode (ZM) obeys similar physics to the El Niño Southern Oscillation (ENSO): positive Bjerknes and delayed negative feedbacks. This implies the ZM may be predictable on seasonal timescales, but models demonstrate little prediction skill in this region. In this study using different configurations of the Kiel Climate Model (KCM) exhibiting different levels of systematic error, we show that a reasonable simulation of the ZM depends on realistic representation of the mean state, i.e., surface easterlies along the equator, upward sloping thermocline to the east, with an equatorial SST cold tongue in the east. We further attribute the differences in interannual variability among the simulations to the individual components of the positive Bjerknes and delayed negative feedbacks. Differences in the seasonality of the variability are similarly related to the impact of seasonal biases on the Bjerknes feedback. Our results suggest that model physics must be enhanced to enable skillful seasonal predictions in the Tropical Atlantic Sector, although some improvement with regard to the simulation of Equatorial Atlantic interannual variability may be achieved by momentum flux correction. This pertains especially to the seasonal phase locking of interannual SST variability.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2014JC010384
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Key factors in simulating the equatorial Atlantic zonal sea surface temperature gradient in a coupled general circulation model (2011)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 116 (C6). C06010.
    Details
    Publication Date: 2018-01-19
    Description: Causes of the coupled model bias in simulating the zonal sea surface temperature (SST) gradient in the equatorial Atlantic are examined in three versions of the same coupled general circulation model (CGCM) differing only in the cumulus convection scheme. One version of the CGCM successfully simulates the mean zonal SST gradient of the equatorial Atlantic, in contrast to the failure of the Coupled Model Intercomparison Project phase 3 models. The present analysis shows that key factors to be successful are high skills in simulating the meridional location of the Intertropical Convergence Zone, the precipitation over northern South America, and the southerly winds along the west coast of Africa associated with the West African monsoon in boreal spring. Model biases in the Pacific contribute to the weaker precipitation over northern South America. Uncoupled experiments with the atmospheric component further confirm the importance of remote influences on the development of the equatorial Atlantic bias. Key Points: The zonal SST gradient of the equatorial Atlantic is well simulated in a CGCM; Key factors for the realistic simulation of the Atlantic SST are presented; Remote forcing from the Pacific may contribute to the Atlantic SST bias
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2010JC006717
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    ENSEMBLES: a new multi-model ensemble for seasonal-to-annual predictions: Skill and progress beyond DEMETER in forecasting tropical Pacific SSTs (2009)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 36 (21). L21711.
    Details
    Publication Date: 2017-11-07
    Description: A new 46-year hindcast dataset for seasonal-to-annual ensemble predictions has been created using a multi-model ensemble of 5 state-of-the-art coupled atmosphere-ocean circulation models. The multi-model outperforms any of the single-models in forecasting tropical Pacific SSTs because of reduced RMS errors and enhanced ensemble dispersion at all lead-times. Systematic errors are considerably reduced over the previous generation (DEMETER). Probabilistic skill scores show higher skill for the new multi-model ensemble than for DEMETER in the 4–6 month forecast range. However, substantially improved models would be required to achieve strongly statistical significant skill increases. The combination of ENSEMBLES and DEMETER into a grand multi-model ensemble does not improve the forecast skill further. Annual-range hindcasts show anomaly correlation skill of ∼0.5 up to 14 months ahead. A wide range of output from the multi-model simulations is becoming publicly available and the international community is invited to explore the full scientific potential of these data.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2009GL040896
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Equatorial Atlantic interannual variability: the role of heat content (2010)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Atmospheres, 115 . C09020.
    Details
    Publication Date: 2019-09-23
    Description: The dynamics of the Equatorial Atlantic zonal mode are studied using observed sea surface height (SSH), sea surface temperature (SST), and heat flux and reanalysis wind stress and upper ocean temperature. Principal oscillation pattern (POP) analysis shows that the zonal mode is an oscillatory normal-mode of the observed coupled system, obeying the delayed-action/recharge oscillator paradigm for ENSO. Variations in equatorial averaged SSH, a proxy for upper ocean heat content, precede SST anomalies in the cold tongue by 4-5 months, about a quarter of the POP period. Positive subsurface temperature anomalies appear in the west, as a delayed response to the preceding cold event. These propagate eastward, where due to the shallow thermocline they can influence SST, leading to the next warm event. Although SST variations exhibit weak westward propagation during some zonal mode events, POP analysis indicates that to first order there is no zonal propagation in SST. Net surface heat flux anomalies generally act to damp SST anomalies. The zonal mode explains a large amount (70%) of SST variability in the east and a significant fraction (19%) of equatorial variability. Thus, the predictability potential in the Equatorial Atlantic on seasonal time scales may be considerably higher than currently thought.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2010JC006304
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Decadal to Multidecadal Variability of the Atlantic MOC: Mechanisms and Predictability (2007)
    AGU (American Geophysical Union)
    In:  In: Ocean Circulation: Mechanisms and Impacts - Past and Future Changes of the Ocean's Meridional Overturning. , ed. by Schmittner, A., Chiang, J. and Hemming, S. AGU Monograph, 173 . AGU (American Geophysical Union), Washington D.C., pp. 149-166. ISBN 978-0-87590-438-2
    Details
    Publication Date: 2020-07-28
    Description: The dynamics and predictability of the decadal to multidecadal Atlantic merid­ional overturning circulation (MOC) variability are described from observations and models. The investigation focuses on two modes that involve the MOC: One mode exhibits a quasi-decadal period, while the other is multidecadal. The two modes have completely different underlying dynamics, which is reflected in their rather different spatial characteristics. While the quasi-decadal mode represents a damped mode of the coupled ocean-atmosphere system, the multidecadal mode can be basically understood as the MOC response to the multidecadal forcing by the North Atlantic Oscillation (NAO). "Perfect model" predictability studies indicate a rather high predictability potential of the MOC variability on decadal timescales. Variations of the MOC are associated with variations in the meridional heat trans­port that drive sea surface temperature (SST) anomalies. SST anomalies in the North Atlantic thus exhibit a similar decadal predictability potential as the MOC. The decadal predictability carries over to the atmosphere. The probability density function of European surface air temperature anomalies, for instance, changes sig­nificantly with the state of the MOC. A reconstruction of the MOC for the 20th cen­tury from observed SSTs shows considerable variability on decadal timescales, but no strong sustained long-term trend. Furthermore, an assessment of the observed hydrographical changes in the Nordic Seas, with the aid of ocean general circula­tion model experiments and the analysis of recent scenario integrations with global climate models, indicates that the expected anthropogenic weakening of the MOC may not exceed the level of the internal variability within the next decades.
    Type: Book chapter , NonPeerReviewed
    Format: text
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    OceanRep: Book chapter
  • 8
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    On the connection between Benguela and equatorial Atlantic Ninos and the role of the South Atlantic Anticyclone (2010)
    AGU (American Geophysical Union)
    Details
    Publication Date: 2023-11-08
    Description: For the eastern tropical Atlantic two recurring El Niño-like phenomena with high interannual SST anomalies have been described, one centered in the equatorial region as part of the Atlantic zonal mode and one off Angola referred to as Benguela Niño. Both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial Atlantic. Here the connection between SST variations in the two regions is investigated with observational data sets as well as ocean model simulations. They are correlated to such an extent that joint events should rather be viewed as one Atlantic Niño. An intriguing feature, counterintuitive in view of the remote forcing mechanism, is that SST anomalies off Angola precede those in the equatorial Atlantic. We show this behavior to be related to the difference in thermocline depths and a different seasonality of interannual SST variability in the two regions. While Benguela Niños peak in austral fall due to the Angola Benguela Front being located furthest to the south and high interannual variability in coastal Kelvin wave activity, warm events at the equator are phase-locked to austral winter when the thermocline is shallow. Perturbation experiments confirm the importance of remote forcing from the equator for SST variability off Angola and demonstrate the leading role of wind stress in the generation of SST anomalies in the eastern tropical Atlantic. These wind stress variations are shown to be linked to fluctuations in the strength of the South Atlantic Anticyclone, a connection that might be important with respect to the predictability of Atlantic Niños.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    A Satellite Era Warming Hole in the Equatorial Atlantic Ocean (2020)
    AGU (American Geophysical Union)
    Details
    Publication Date: 2023-02-08
    Description: Observations during the satellite era 1979–2018 only depict small sea surface temperature (SST) trends over the Equatorial Atlantic cold tongue region in boreal summer. This lack of surface warming of the cold tongue, termed warming hole here, denotes an 11% amplification of the mean SST annual cycle. The warming hole is driven by a shoaling of the equatorial thermocline, linked to increased wind stress forcing, and damped by the surface turbulent heat fluxes. The satellite era warming deficit is not unusual during the twentieth century—similar weak trends were also observed during the 1890s–1910s and 1940s–1960s. The tendency for surface cooling appears to reflect an interaction of external forcing, which controls the timing and magnitude of the cooling, with the intrinsic variability of the climate system. The hypothesis for externally forced modulation of internal variability is supported by climate model simulations forced by the observed time-varying concentrations of atmospheric greenhouse gases and natural aerosols. These show that increased greenhouse forcing warmed the cold tongue and aerosols cooled it during the satellite era. However, internal variability, as derived from control integrations with fixed, preindustrial values of greenhouse gases and aerosols, can potentially cause larger cooling than observed during the satellite era. Large uncertainties remain on the relative roles of external forcing and intrinsic variability in both observations and coupled climate models.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Tropical sea surface temperature, vertical wind shear, and hurricane development (2007)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 34 (L01710).
    Details
    Publication Date: 2018-02-15
    Description: The anomalously strong hurricane activity in the Atlantic sector during the recent years led to a controversy about the impact of global warming on hurricane activity in the Atlantic sector. Here we show that the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans (Indo-Pacific) is a key parameter in controlling the vertical wind shear over the Atlantic, an important quantity for hurricane activity. The stronger warming of the tropical North Atlantic relative to that of the Indo-Pacific during the most recent years drove reduced vertical wind shear over the Atlantic and is thus responsible for the strong hurricane activity observed. In 2006, however, the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans is much reduced, which explains the relatively weak hurricane season.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2006GL027969
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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