Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.4.3.4 Tropical Atlantic Modes ====

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Interannual variability of the tropical Atlantic can be described in terms of two main climate modes: the Atlantic equatorial mode and the Atlantic meridional mode (AMM; Annex IV, Section AIV2.5). The Atlantic equatorial mode, also commonly referred to as the Atlantic Niño or Atlantic Zonal Mode, is associated with SST anomalies near the equator, peaking in the eastern basin, while the AMM is characterized by an inter-hemispheric gradient of SST and wind anomalies. Both modes are associated with changes in the ITCZ and related winds and exert a strong influence on the climate in adjacent and remote regions.

Despite considerable improvements in CMIP5 with respect to CMIP3, most CMIP5 models have difficulties in simulating the mean climate of the tropical Atlantic ( [[#Mohino--2019|Mohino et al., 2019]] ) and are not able to correctly simulate the main aspects of Tropical Atlantic Variability (TAV) and associated impacts. This is presumably the main reason why there is a lack of specific studies dealing with near-term changes in tropical Atlantic modes. Nevertheless, AR5 reported that the ocean is more predictable than continental areas at the decadal time scale ( [[#Kirtman--2013|Kirtman et al., 2013]] ). In particular, the predictability in the tropical oceans is mainly associated with decadal variations of the external forcing component. Since the AMV affects the tropical Atlantic, near-term variations of the AMV can modulate the equatorial mode and the AMM as well as associated impacts.

There are no specific studies focusing on near-term changes in tropical Atlantic modes; nevertheless, decadal predictions show that although the North Atlantic stands out in most CMIP5 models as the primary region where skill might be improved because of initialization, encouraging results have also been found in the tropical Atlantic ( [[#Meehl--2014|Meehl et al., 2014]] ). The effect of initialization in the tropical Atlantic is not only visible in surface temperature but also in the subsurface ocean ( [[#Corti--2015|Corti et al., 2015]] ). In particular, initialization improves the skill via remote ocean conditions in the North Atlantic subpolar gyre and tropical Pacific, which influence the tropical Atlantic through atmospheric teleconnections ( [[#Dunstone--2011|Dunstone et al., 2011]] ; [[#Vecchi--2014|Vecchi et al., 2014]] ; [[#García-Serrano--2015|García-Serrano et al., 2015]] ). Improvements of some aspects of climate prediction systems (initialization techniques, large ensembles, increasing model resolution) have also led to skill improvements over the tropical Atlantic ( [[#Pohlmann--2013|Pohlmann et al., 2013]] ; [[#Monerie--2017|Monerie et al., 2017]] ; [[#Yeager--2017|Yeager and Robson, 2017]] ).

Recent studies have shown that the AMV can modulate not only the characteristics of the Atlantic Niños, but also their inter-basin teleconnections (Indian and Pacific). In particular, the Atlantic Niño–ENSO relationship is strongest during negative AMV phases ( [[#Martín-Rey--2014|Martín-Rey et al., 2014]] ; [[#Losada--2016|Losada and Rodríguez-Fonseca, 2016]] ) when equatorial Atlantic SST variability is enhanced ( [[#Martín-Rey--2017|Martín-Rey et al., 2017]] ; [[#Lübbecke--2018|Lübbecke et al., 2018]] ).

Based on CMIP5 and available CMIP6 results, we conclude that there is a lack of studies on the near-term evolution of TAV and associated teleconnections for a comprehensive assessment. However, some studies show that despite severe model biases there are skilful predictions in the mean state of tropical Atlantic surface temperature several years ahead ( ''medium confidence'' ), though skill in simulated variability has not been assessed yet. Decadal changes in the Atlantic Niño spatial configuration and associated teleconnections might be modulated by the AMV, but there is ''limited evidence'' and therefore ''low confidence'' in these results.

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