Is the boreal spring tropical Atlantic variability a precursor of the Equatorial Mode?

Martín‐Rey, Marta; Lazar, Alban

doi:10.1007/s00382-019-04851-9

Cited by 29 publications

(12 citation statements)

References 51 publications

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“…In addition, with those studies, recent findings have demonstrated that the RW‐reflected mechanism has a substantial contribution in generating equatorial SST variability during summer months (Burmeister et al, ; Martín‐Rey & Lazar, ). Martín‐Rey and Lazar () reveal that during the MM development, a competition between two counteracting effects is established in the equatorial Atlantic: the RW‐reflected mechanism and the local wind forcing. Changes in the strength and persistence of each forcing will determine the boreal summer equatorial SST anomalies following a Meridional Mode event (Burmeister et al, ; Martín‐Rey & Lazar, ) .…”

Section: Resultsmentioning

confidence: 65%

“…Martín‐Rey and Lazar () reveal that during the MM development, a competition between two counteracting effects is established in the equatorial Atlantic: the RW‐reflected mechanism and the local wind forcing. Changes in the strength and persistence of each forcing will determine the boreal summer equatorial SST anomalies following a Meridional Mode event (Burmeister et al, ; Martín‐Rey & Lazar, ) .…”

Section: Resultsmentioning

confidence: 99%

“…Especially noteworthy is the important role of the ocean wave baroclinic modes in shaping the distinct structure and timing of Atlantic Niño and HS modes during negative AMV. The RW‐reflected mechanism is essential to understand the early termination of the HS mode in boreal summer, and can be key to modulate the connection between the Meridional Mode and Atlantic Niño (Burmeister et al, ; Foltz & McPhaden, ; Martín‐Rey & Lazar, ).…”

Section: Conclusion and Discussionmentioning

confidence: 99%

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Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

et al. 2019

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A recent study has brought to light the co-existence of two distinct Atlantic Equatorial Modes during negative phases of the Atlantic Multidecadal Variability: the Atlantic Niño and Horse-Shoe (HS) mode. Nevertheless, the associated air-sea interactions for HS mode have not been explored so far and the prevailing dynamic view of the Atlantic Niño has been questioned. Here, using a forced ocean model simulation, we find that for both modes, ocean dynamics is essential to explain the equatorial SST variations, while air-sea fluxes control the off-equatorial SST anomalies. Moreover, we demonstrate the key role played by ocean waves in shaping their distinct structure and timing. For the positive phase of both Atlantic Niño and HS, anomalous westerly winds trigger a set of equatorial downwelling Kelvin waves (KW) during spring-summer. These dKWs deepen the thermocline, favouring the equatorial warming through vertical diffusion and horizontal advection. Remarkably, for the HS, an anomalous north-equatorial wind stress curl excites an upwelling Rossby wave (RW), which propagates westward and is reflected at the western boundary becoming an equatorial upwelling KW. The uKW propagates to the east, activating the thermocline feedbacks responsible to cool the sea surface during summer months. This RW-reflected mechanism acts as a negative feedback causing the early termination of the HS mode. Our results provide an improvement in the understanding of the TAV modes and emphasize the importance of ocean wave activity to modulate the equatorial SST variability. These findings could be very useful to improve the prediction of the Equatorial Modes.Plain Language Summary A recent study has found how the inter-annual variations of sea surface temperature (SST) in the tropical Atlantic, are organized in two different equatorial modes during negative phases of the Atlantic Multidecadal Variability. These modes, which illustrate a particular and distinct spatial structure, are denoted as Atlantic Niño and Horse-Shoe mode. Here we show that, for both patterns, ocean dynamics is key to generate equatorial SSTs, while the off-equatorial SST anomalies are mainly explained by thermodynamic processes (heat fluxes exchanges). Outstandingly, we demonstrate that ocean waves have a substantial impact in the development and decay of Atlantic Niño and Horse-Shoe modes, shaping their distinct spatial configuration and timing. Our results bring to light the importance of ocean wave activity to explain the modulation of the equatorial Atlantic SST variability, which could be relevant to improve its predictability and associated climatic impacts.

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Section: Resultsmentioning

confidence: 65%

Section: Resultsmentioning

confidence: 99%

Section: Conclusion and Discussionmentioning

confidence: 99%

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Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

et al. 2019

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“…On the other hand, the westward propagating Rossby waves reflect at the western boundary of the equatorial Atlantic. This reflecting wave travels as Kelvin waves eastward and consequently contributes to the onset of the Atlantic Niño/Niña events [82][83][84]. An even more important off-equator contributor to the Atlantic Niño mode is South Atlantic Anticyclone.…”

Section: Atlantic Zonal Modementioning

confidence: 99%

Tropical Atlantic Variability: Observations and Modeling

2019

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We review the state-of-the-art knowledge of Tropical Atlantic Variability (TAV). A well-developed observing system and sustained effort of the climate modeling community have improved our understanding of TAV. It is dominated by the seasonal cycle, for which some mechanisms have been identified. The interannual TAV presents a marked seasonality with three dominant modes: (i) the Atlantic Zonal Mode (AZM), (ii) the Atlantic Meridional Mode (AMM) and (iii) the variability in the Angola–Benguela Front (ABF). At longer time scales, the AMM is active and low-frequency variations in the strength, periodicity, and spatial structure of the AZM are observed. Also, changes in the mean position of the ABF occur. Climate models still show systematic biases in the simulated TAV. Their causes are model-dependent and relate to drawbacks in the physics of the models and to insufficient resolution of their atmospheric and oceanic components. The identified causes for the biases can have local or remote origin, involving the global ocean and atmospheric circulation. Although there is not a clear consensus regarding the role of model resolution in the representation of the TAV, eddy-resolving ocean models combined with atmospheric models with enhanced horizontal and vertical resolutions simulate smaller biases.

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“…The trends in wind stress in Figure 1a correspond to a strengthening of the climatogical wind stress south of the equator and over the western equatorial Atlantic where wind changes are considered key to the tongue changes (Castaño‐Tierno et al, 2018; Keenlyside & Latif, 2007; Martín‐Rey & Lazar, 2019; Nnamchi et al, 2015; Pottapinjara et al, 2019; Richter et al, 2013; Zebiak, 1993). The wind stress changes point to an acceleration of the southeasterly trade winds (as discussed by Servain et al, 2014), although there are some uncertainties in the Atl3 region among the different reanalysis data sets (Figures 1a and 3).…”

Section: The Warming Holementioning

confidence: 93%

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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Is the boreal spring tropical Atlantic variability a precursor of the Equatorial Mode?

Cited by 29 publications

References 51 publications

Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

Tropical Atlantic Variability: Observations and Modeling

A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

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