A Comparison of the Atlantic and Pacific Bjerknes Feedbacks: Seasonality, Symmetry, and Stationarity

Dippe, Tina; Lübbecke, Joke F.; Greatbatch, Richard John

doi:10.1029/2018jc014700

Cited by 15 publications

(10 citation statements)

References 67 publications

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“…Nnamchi et al (2015) showed that thermodynamic forcing by stochastic atmospheric perturbations can explain a significant amount of the observed SST variability in the equatorial Atlantic. Yet, recent studies assessing the relative importance of dynamic versus thermodynamic 1 3 processes conclude that the dynamic and in particular the Bjerknes feedback is a main driver for the Atlantic zonal mode (Jouanno et al 2017;Dippe et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Weakened SST variability in the tropical Atlantic Ocean since 2000

et al. 2020

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A prominent weakening in equatorial Atlantic sea surface temperature (SST) variability, occurring around the year 2000, is investigated by means of observations, reanalysis products and the linear recharge oscillator (ReOsc) model. Compared to the time period 1982-1999, during 2000-2017 the May-June-July SST variability in the eastern equatorial Atlantic has decreased by more than 30%. Coupled air-sea feedbacks, namely the positive Bjerknes feedback and the negative net heat flux damping are important drivers for the equatorial Atlantic interannual SST variability. We find that the Bjerknes feedback weakened after 2000 while the net heat flux damping increased. The weakening of the Bjerknes feedback does not appear to be fully explainable by changes in the mean state of the tropical Atlantic. The increased net heat flux damping is related to an enhanced response of the latent heat flux to the SST anomalies (SSTa). Strengthened trade winds as well as warmer SSTs are suggested to increase the air-sea specific humidity difference and hence, enhancing the latent heat flux response to SSTa. A combined effect of those two processes is proposed to be responsible for the weakened SST variability in the eastern equatorial Atlantic. The ReOsc model supports the link between reduced SST variability, weaker Bjerknes feedback and stronger net heat flux damping.

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Section: Introductionmentioning

confidence: 99%

Weakened SST variability in the tropical Atlantic Ocean since 2000

et al. 2020

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“…The ocean responds to the westerly wind stress anomaly by deepening the thermocline (Fig. 1e ) and thus increasing the heat content in the eastern equatorial Atlantic, which leads to warmer SSTs in this region 1 – 5 , 11 , 34 . However, the SST anomalies in the east do not drive a strong atmospheric response along the equator due to a lack of diabatic heating after June.…”

Section: Resultsmentioning

confidence: 99%

“…Such a picture is in part inconsistent with the Bjerknes-feedback loop, which does involve a strong feedback from the ocean to the atmosphere in the form of SST-forced atmospheric variability, as observed over the equatorial Pacific 12 , 13 and shown here in a companion analysis. This difference in ocean-atmosphere interaction between the Atlantic Niños and the Pacific El Niños may explain why SST variability is weaker 1 , 2 , 5 , 34 and SST predictability lower 44 , 45 in the equatorial Atlantic relative to that in the equatorial Pacific. These findings raise questions about the applicability of the standard Bjerknes-feedback loop as the major explanation for the mechanism underlying the Atlantic Niño.…”

Section: Discussionmentioning

confidence: 99%

Diabatic heating governs the seasonality of the Atlantic Niño

et al. 2021

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The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled ocean-atmosphere dynamics described by the Bjerknes-feedback loop. However, the role of the atmospheric diabatic heating, which can be either an indicator of the atmosphere’s response to, or its influence on the SST, is poorly understood. Here, using satellite-era observations from 1982–2015, we show that diabatic heating variability associated with the seasonal migration of the Inter-Tropical Convergence Zone controls the seasonality of the Atlantic Niño. The variability in precipitation, a measure of vertically integrated diabatic heating, leads that in SST, whereas the atmospheric response to SST variability is relatively weak. Our findings imply that the oceanic impact on the atmosphere is smaller than previously thought, questioning the relevance of the classical Bjerknes-feedback loop for the Atlantic Niño and limiting climate predictability over the equatorial Atlantic sector.

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“…The dynamics underlying the Atlantic Zonal Mode is in parts similar to that of the Pacific El Niño (Ding et al, 2010), so that the Bjerknes Feedback (Bjerknes, 1969) observed in the Tropical Pacific is also present in the Tropical Atlantic (Keenlyside and Latif, 2007). In its positive phase, anomalous warming in the eastern Atlantic coupled with weaker trade winds is associated with anomalous eastward currents bringing warmer waters from the west as well as weaker upwelling in the eastern equatorial region and deeper thermocline, which combined act to enhance the initial positive anomaly, closing the feedback cycle (Bjerknes, 1969;Chang et al, 2006;Keenlyside and Latif, 2007;Deppenmeier et al, 2016;Dippe et al, 2019;Silva et al, 2021). The opposite happens in its negative phase.…”

Section: Introductionmentioning

confidence: 92%

Weakened Interannual Variability in the Tropical Atlantic

Verona

Silva

Wainer

et al. 2021

Preprint

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Climate variability in the Tropical Atlantic is complex with strong ocean-atmosphere coupling, where the sea surface temperature (SST) variability impacts the hydroclimate of the surrounding continents. We observe a decrease in the variability of the Tropical Atlantic after 1970 in both CMIP6 models and observations. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. The observed wind relaxation after 1970 in both the equatorial and Tropical North Atlantic (TNA) plays a role in the decreased variability. Concerning the AZM, a widespread warming trend is observed in the equatorial Atlantic accompanied by a weakening trend of the trade winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic and increasing the thermal damping. Even though individually the TNA and Tropical South Atlantic (TSA) show increased variability, the observed asymmetric warming in the Tropical Atlantic and relaxed northeast trade winds after the 70s play a role in decreasing the AMM variability. This configuration leads to positive Wind-Evaporation-SST (WES) feedback, increasing further the TNA SST, preventing AMM from changing phases as before 1970. Associated with it, the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which acts on maintaining the increased precipitation.

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A Comparison of the Atlantic and Pacific Bjerknes Feedbacks: Seasonality, Symmetry, and Stationarity

Cited by 15 publications

References 67 publications

Weakened SST variability in the tropical Atlantic Ocean since 2000

Weakened SST variability in the tropical Atlantic Ocean since 2000

Diabatic heating governs the seasonality of the Atlantic Niño

Weakened Interannual Variability in the Tropical Atlantic

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