Diabatic heating governs the seasonality of the Atlantic Niño

Nnamchi, Hyacinth C.; Latif, Mojib; Keenlyside, Noel; Kjellsson, Joakim; Richter, Ingo

doi:10.1038/s41467-020-20452-1

Cited by 24 publications

(23 citation statements)

References 69 publications

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“…This indicates that SST anomalies in this region could also respond to cloud cover (e.g., Xie and Carton, 2004). Our result agrees with the recent results from Nnamchi et al (2021) which showed that the precipitation which is a good proxy for the diabatic heating, leads SST variability in the equatorial Atlantic. This means that the atmosphere forces the ocean, even though the region north of 10 • S is dominated by ocean dynamics.…”

Section: Discussion and Summarysupporting

confidence: 92%

The 2019 Benguela Niño

et al. 2021

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High interannual sea surface temperature anomalies of more than 2°C were recorded along the coasts of Angola and Namibia between October 2019 and January 2020. This extreme coastal warm event that has been classified as a Benguela Niño, reached its peak amplitude in November 2019 in the Angola Benguela front region. In contrast to classical Benguela Niños, the 2019 Benguela Niño was generated by a combination of local and remote forcing. In September 2019, a local warming was triggered by positive anomalies of near coastal wind-stress curl leading to downwelling anomalies through Ekman dynamics off Southern Angola and by anomalously weak winds reducing the latent heat loss by the ocean south of 15°S. In addition, downwelling coastal trapped waves were observed along the African coast between mid-October 2019 and early January 2020. Those coastal trapped waves might have partly emanated from the equatorial Atlantic as westerly wind anomalies were observed in the central and eastern equatorial Atlantic between end of September to early December 2019. Additional forcing for the downwelling coastal trapped waves likely resulted from an observed weakening of the prevailing coastal southerly winds along the Angolan coast north of 15°S between October 2019 and mid-February 2020. During the peak of the event, latent heat flux damped the sea surface temperature anomalies mostly in the Angola Benguela front region. In the eastern equatorial Atlantic, relaxation of cross-equatorial southerly winds might have contributed to the equatorial warming in November 2019 during the peak of the 2019 Benguela Niño. Moreover, for the first time, moored velocities off Angola (11°S) revealed a coherent poleward flow in the upper 100 m in October and November 2019 suggesting a contribution of meridional heat advection to the near-surface warming during the early stages of the Benguela Niño. During the Benguela Niño, a reduction of net primary production in the Southern Angola and Angola Benguela front regions was observed.

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Section: Discussion and Summarysupporting

confidence: 92%

The 2019 Benguela Niño

et al. 2021

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“…In conclusion, the combination of a large moisture flux from the equatorial Atlantic toward the Guinea Coast and a more destabilized atmosphere over the Guinea Coast leads to an enhanced rainfall response to AEM in the GC+ models compared to GC− models. Moreover, according to Nnamchi et al (2021b), the rainfall response to the AEM is maximum at the location of the mean intertropical convergence zone (ITCZ). The CMIP6 models present a spurious southward position of the mean ITCZ position relative to the observations during the boreal summer.…”

Section: Evaluation Of the Performance Of The Gcms In Simulating The Jas Rainfall Pattern Related To The Atlantic Niñomentioning

confidence: 99%

Weakened impact of the Atlantic Niño on the future equatorial Atlantic and Guinea Coast rainfall

et al. 2022

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Abstract. The Guinea Coast is the southern part of the West African continent. Its summer rainfall variability mostly occurs on interannual timescales and is highly influenced by the sea surface temperature (SST) variability in the eastern equatorial Atlantic, which is the centre of action of the Atlantic Niño mode. Using both historical and scenario (SSP5–8.5) simulations from 31 general circulation models (GCMs) participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we first show that these models present a wet bias during boreal summer. This bias is associated with overly high mean boreal summer SSTs in the eastern equatorial and south Atlantic regions. Next, we analyse the near-term, mid-term and long-term changes of the Atlantic Niño relative to the present-day situation, in a climate with a high anthropogenic emission of greenhouse gases. We find a gradual decrease in the equatorial Atlantic SST anomalies associated with the Atlantic Niño in the future. This result reflects a possible reduction of the Atlantic Niño variability in the future due to a weakening of the Bjerknes feedback over the equatorial Atlantic. In a warmer climate, an anomalous higher sea level pressure in the western equatorial Atlantic relative to the eastern equatorial Atlantic weakens the climatological trade winds over the equatorial Atlantic. As a result, the eastern equatorial Atlantic thermocline is deeper and responds less to the Atlantic Niño events. Among the models that simulate a realistic rainfall pattern associated with the Atlantic Niño in the present-day climate, there are 12 GCMs which project a long-term decrease in the Guinea Coast rainfall response related to the Atlantic Niño. In these models, the zonal 850 hPa wind response to the Atlantic Niño over the equatorial Atlantic is strongly attenuated in the future climate. We also find that 12 other GCMs show no robust change in the patterns associated with the Atlantic Niño. There is a higher confidence in the mid-term and long-term reduction of the rainfall associated with the Atlantic Niño over the Atlantic Ocean than over the Guinea Coast. We also found a projected decrease in the convection associated with the Atlantic Niño in the majority of the models.

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“…The similarities include SST anomalies that are most pronounced in the eastern half of the basin, and the role of the Bjerknes feedback (Bjerknes, 1969), in which equatorial SST anomalies induce changes in equatorial surface winds that further amplify the pattern. Subsequent research, however, identified several differences with ENSO, including the relatively weak role of the Bjerknes feedback (Nnamchi et al., 2021; Richter et al., 2017) and a strong influence from off‐equatorial processes in some events (Foltz & McPhaden, 2010; Lübbecke & McPhaden, 2012; Richter et al., 2013). Partly motivated by these differences, the term “Atlantic Zonal Mode” (AZM) has gained traction in recent years and will be used in the present study.…”

Section: Introductionmentioning

confidence: 99%