How Does the Seasonal Cycle Control Equatorial Atlantic Interannual Variability?

Prodhomme, Chloé; Voldoire, Aurore; Exarchou, Eleftheria; Deppenmeier, Anna-Lena; Garcı́a-Serrano, Javier; Guémas, Virginie

doi:10.1029/2018gl080837

Cited by 18 publications

(12 citation statements)

References 29 publications

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“…The “all forcing” ensemble, representing the combined effects of GHGs and aerosols, significantly overestimates the observed satellite era Atl3 SST trends (Figures 8a and 9b). The CMIP models exhibit systematic biases in the mean‐state SST (Prodhomme et al, 2019; Richter et al, 2014; Richter & Xie, 2008; Wang et al, 2014), but we do not find any clear relationship of the SST trends to model biases. The strong overestimation of the SST trends could argue for internal variability as the source of the warming hole.…”

Section: The Relative Roles Of Intrinsic Variability and External Forcontrasting

confidence: 64%

A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

et al. 2020

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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.Plain Language Summary The Atlantic cold tongue is a region of locally cooler ocean surface waters that develops just south of the equator in boreal summer, partly reflecting the upwelling of deep cold waters by the action of the southeasterly trade winds. Although there has been considerable global warming since the beginning of global satellite measurements in 1979, there is hardly any surface warming in the Atlantic cold tongue region during 1979-2018. This warming hole is most pronounced in boreal summer. Observations suggest that despite strong heat transfer from the atmosphere to the ocean, the upper ocean may have cooled. Climate model simulations show that variations in external forcing associated with greenhouse gases can cause warming of the cold tongue and aerosols cooling of it. However, model simulations that exclude variations in these external forcing show that the mechanisms internal to the climate system can potentially cause larger cooling than observed during the satellite era. Thus, we attribute the warming hole to a combination of internal variability and external forcing. It must be stressed, however, that understanding the relative roles of internal variability versus external forcing is greatly hampered by large errors in both the observational data sets and climate models.

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Section: The Relative Roles Of Intrinsic Variability and External Forcontrasting

confidence: 64%

A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

et al. 2020

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“…The level of interannual equatorial Atlantic SST variability exhibits a marked seasonality with its maximum in boreal summer 1 , 2 , 4 , 11 (dashed curve Fig. 1c, d ).…”

Section: Resultsmentioning

confidence: 97%

“…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%

“…It exhibits a characteristic zonally asymmetric structure in sea surface temperature (SST) and wind stress fluctuations 1 – 3 . Previous work suggested that SST-anomaly growth during the Atlantic Niño is largely governed by the Bjerknes-feedback loop, a positive feedback between adjustments in oceanic and atmospheric circulations 1 – 11 , which is the dominant growth mechanism during the warm and cold phases of the El Niño/Southern Oscillation (ENSO) in the equatorial Pacific 1 , 2 , 5 .…”

Section: Introductionmentioning

confidence: 99%

“…The Bjerknes-feedback loop is certainly an important factor in the interannual equatorial Atlantic SST variability 1 – 6 , 9 , 11 . However, the physics underlying the Atlantic Niño remain controversial 4 .…”

Section: Introductionmentioning

confidence: 99%

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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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