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  • 1
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    Variability in the South Atlantic Anticyclone and the Atlantic Niño mode (2014)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 27 (21). pp. 8135-8150.
    Details
    Publication Date: 2020-08-04
    Description: Sea surface temperature (SST) anomalies in the eastern equatorial Atlantic are connected to modulations in the strength of the South Atlantic subtropical high-pressure system, referred to as the South Atlantic Anticyclone (SAA). Using ocean and atmosphere reanalysis products we show here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic (EEA) via anomalous tropical wind power. This modulation of the timing and amplitude of the seasonal cold tongue development manifests as anomalous SST events peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA induced wind power variations south of the equator influence EEA SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1175/JCLI-D-14-00202.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Assessing the Twenty-First-Century Shift in ENSO Variability in Terms of the Bjerknes Stability Index* (2014)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 27 (7). pp. 2577-2587.
    Details
    Publication Date: 2014-10-22
    Description: A decadal change in the character of ENSO was observed around year 2000 toward weaker-amplitude, higher-frequency events with an increased occurrence of central Pacific El Niños. Here these changes are assessed in terms of the Bjerknes stability index (BJ index), which is a measure of the growth rate of ENSO-related SST anomalies. The individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980–99 and 2000–10. The spread between the products is large, but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial Pacific SST anomalies. These changes are consistent with changes in the background state of the tropical Pacific: cooler mean SST in the eastern and central equatorial Pacific results in reduced convection there together with a westward shift in the ascending branch of the Walker circulation. This shift leads to a weakening in the relationship between eastern Pacific SST and longitudinally averaged equatorial zonal wind stress. Also, despite a steeper mean thermocline slope in the more recent period, the thermocline slope response to wind stress anomalies weakened due to a smaller zonal wind fetch that results from ENSO-related wind anomalies being more confined to the western basin. As a result, the total BJ index is more negative, corresponding to a more strongly damped system in the past decade compared to the 1980s and 1990s.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI-D-13-00438.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Symmetry of the Atlantic Nino mode (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (2). pp. 965-973.
    Details
    Publication Date: 2020-02-06
    Description: El Niño-Southern Oscillation (ENSO) in the Pacific is asymmetric for warm and cold events with respect to amplitude, spatial patterns and temporal evolution. Here the symmetry of the Atlantic Niño mode, which many previous studies have argued is governed by atmosphere–ocean dynamics similar to those of ENSO, is investigated using two different ocean reanalysis products. Calculation of Bjerknes feedback terms for the Pacific reveals a pronounced asymmetry between warm and cold events, though unlike most previous studies, the largest asymmetry is found in the relationship between eastern Pacific thermocline depth and SST anomalies. For the Atlantic, cold events are effectively mirror images of warm events with Bjerknes feedbacks of similar strength. The analysis supports not only the conclusion that Atlantic Niños are more symmetric than ENSO, but the hypothesis itself that the Bjerknes feedback is operative in the Atlantic given the strength of the relationship between the key variables involved.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2016GL071829
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Simulation of ENSO Related Surface Wind Anomalies with an Atmospheric GCM Forced by Observed SST (1990)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 3 (5). pp. 509-521.
    Details
    Publication Date: 2018-07-23
    Description: The ECMWF-T21 atmospheric GCM is forced by observed near-global SST from January 1970 to December 1985. Its response in low level winds and surface wind stress over the Pacific Ocean is compared with various observations. The time dependent SST clearly induces a Southern Oscillation (SO) in the model run which is apparent in the time series of all variables considered. The phase of the GCM SO is as observed, but its low frequency variance is too weak and is mainly confined to the western Pacific. Because of the GCM's use as the atmospheric component in a coupled ocean-atmosphere model, the response of an equatorial oceanic primitive equation model to both the modeled and observed wind stress is examined. The ocean model responds to the full observed wind stress forcing in a manner almost identical to that when it is forced by the first two low frequency EOFs of the observations only. These first two EOFs describe a regular eastward propagation of the SO signal from the western Pacific to the central Pacific within about a year. The ocean model's response to the modeled wind stress is too weak and similar to the response when the observed forcing is truncated to the first EOF only. In other words, the observed SO appears as a sequence of propagating patterns but the simulated SO as a standing oscillation. The nature of the deviation of the simulated wind stress from observations is analyzed by means of Model Output Statistics (MOS). It is shown that a MOS-corrected simulated wind stress, if used to force an ocean GCM, leads to a significant enhancement of low frequency SST variance, which is most pronounced in the western Pacific.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0442(1990)003〈0509:SOERSW〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting (2019)
    AGU (American Geophysical Union) | Wiley
    In:  Earth and Space Science, 6 (4). pp. 577-616.
    Details
    Publication Date: 2022-01-31
    Description: Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ocean-atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1029/2018EA000428
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Evaluating climate models with the CLIVAR 2020 ENSO metrics package (2021)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2024-02-07
    Description: The El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on the planet, with far-reaching global impacts. It is therefore key to evaluate ENSO simulations in state-of-the-art numerical models used to study past, present and future climate. Recently, the Pacific Region Panel of the International Climate and Ocean - Variability, Predictability, and Change (CLIVAR) Project, as a part of the World Climate Research Programme (WCRP), led a community-wide effort to evaluate the simulation of ENSO variability, teleconnections and processes in climate models. The new CLIVAR 2020 ENSO metrics package enables model diagnosis, comparison, and evaluation to (1) highlight aspects that need improvement; (2) monitor progress across model generations; (3) help in selecting models that are well suited for particular analyses; (4) reveal links between various model biases, illuminating the impacts of those biases on ENSO and its sensitivity to climate change; and to (5) advance ENSO literacy. By interfacing with existing model evaluation tools, the ENSO metrics package enables rapid analysis of multi-petabyte databases of simulations, such as those generated by the Coupled Model Intercomparison Project phases 5 (CMIP5) and 6 (CMIP6). The CMIP6 models are found to significantly outperform those from CMIP5 for 8 out of 24 ENSO-relevant metrics, with most CMIP6 models showing improved tropical Pacific seasonality and ENSO teleconnections. Only one ENSO metric is significantly degraded in CMIP6, namely the coupling between the ocean surface and subsurface temperature anomalies, while the majority of metrics remain unchanged.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Tropical Instability Waves and Wind‐Forced Cross‐Equatorial Flow in the Central Atlantic Ocean (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Based on velocity data from a long-term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper-ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind-driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south-easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases. Key Points: - Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases - Meridional wind stress forces a seasonally-varying, shallow cross-equatorial overturning cell-the Equatorial Roll - The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north- and southward phases
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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