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  • 1
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    Austral Winter External and Internal Atmospheric Variability between 1980 and 2014 (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 43 (5). pp. 2234-2239.
    Details
    Publication Date: 2019-07-16
    Description: We examine the interannual variability of the seasonal mean atmospheric circulation in the Southern Hemisphere during austral winter. The three major modes are identified by rotated EOF (REOF) analysis. As expected, REOF1 is associated with the Southern Annular Mode which is dominated by internal atmospheric dynamics. REOF2 displays a wave train, linked to the western North Pacific monsoon and the Pacific-Japan pattern in East Asia in the same season; REOF3 resembles the Pacific-South American pattern. Externally-forced variability strongly projects on both REOF2 and REOF3 so that, in the ensemble mean, an atmospheric model with prescribed observed sea surface temperature (SST) captures considerable parts of the time evolution of REOF2 (50%) and REOF3 (25%), suggesting a potential predictability for the two modes.
    Type: Article , PeerReviewed
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  • 2
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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
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  • 3
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    The impact of sea surface temperature bias on equatorial Atlantic interannual variability in partially coupled model experiments (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 . pp. 5540-5546.
    Details
    Publication Date: 2017-04-10
    Description: We examine the impact of sea surface temperature (SST) bias on inter-annual variability during boreal summer over the equatorial Atlantic using two suites of partially coupled model (PCM) experiments with and without surface heat flux correction. In the experiments, surface wind stress anomalies are specified from observations while the thermodynamic coupling between the atmospheric and oceanic components is still active as in the fully coupled model. The results show that the PCM can capture around 50% of the observed variability associated with the Atlantic Niño from 1958 to 2013, but only when the bias is substantially reduced using heat flux correction, with no skill otherwise. We further show that ocean dynamics explain a large part of the SST variability in the eastern equatorial Atlantic in both observations (50-60%) and the PCM experiments (50-70%) with heat flux correction, implying that the seasonal predictability potential may be higher than currently thought.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 4
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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.
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  • 5
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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.
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  • 6
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    Tropical influence independent of ENSO on the austral summer Southern Annular Mode (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 (10). pp. 3643-3648.
    Details
    Publication Date: 2017-04-10
    Description: A link between atmospheric variability in the Tropics independent of ENSO and the Southern Annular Mode (SAM) is found based on seasonal mean data for austral summer. Variations associated with El Niño Southern Oscillation (ENSO) are removed usinga linear method and a Tropics Index (TI) is defined as the zonal average of the ENSO-removed 500 hPa geopotential height between 10°S and 10°N. Since the detrended TI shows no link to SST variability in the Tropics, it appears to be related to internal atmospheric variability. We find that the TI can explain about 40% variance of the SAM interannual variability and about 75% of the SAM long term trend between 1957/58 and 2001/02, where here the SAM includes the ENSO signal. Positive/negative values of the TI are associated with the positive/negative SAM. A possible link between the TI and global warming is noted.
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  • 7
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    Seasonal cycle in the upper equatorial Atlantic Ocean (2009)
    AGU (American Geophysical Union)
    Details
    Publication Date: 2023-01-31
    Description: The dynamics of the seasonal cycle in the upper equatorial Atlantic ocean are studied using observations and a hierarchy of ocean models. Distinctive features of the seasonal cycle are strong annual and semiannual components: eastward (westward) propagating sea surface height (SSH) and thermocline depth at the equator (off the equator) and westward propagating surface zonal currents at the equator. Modelling results show that linear theory can explain the seasonal cycle in thermocline depth and SSH. While first-order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator, nonlinear terms are required; they weaken the variability and improve its phase and zonal extent. The important terms are meridional and vertical advection and vertical diffusion of zonal momentum. The linear solution is essentially determined by the four gravest baroclinic modes and Kelvin and first meridional mode Rossby waves. The eastward propagation in thermocline depth at the equator results from the Kelvin wave contribution, while the westward propagation in thermocline depth off the equator and surface zonal currents at the equator result from the first meridional mode Rossby wave. The contribution of Kelvin and Rossby waves generated by boundary reflections equals that of the directly forced waves. The semiannual cycle in zonal winds although much weaker than the annual component forces a strong semiannual component in SSH and surface zonal currents, because it excites the basin mode of the second baroclinc mode. This explains the observed feature in the seasonal cycle from March to August.
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