This work presents observational evidence of a change in Atlantic‐Pacific Niños connection since the late 60's. Accordingly, summer Atlantic Niños (Niñas) alter the tropical circulation favoring the development of following‐winter Pacific Niñas (Niños). The same change is obtained in an ensemble of AGCM integrations in which SSTs in the Atlantic are the observed in 1949–2002 and those in the tropical Indo‐Pacific are from a coupled OGCM. The mechanism (for the positive Atlantic phase) involves the strengthening of the Walker circulation with ascending branch over the Atlantic and descending branch over the central Pacific. The enhanced surface divergence in the latter region shallows the equatorial thermocline triggering coupled processes, and favoring the development of a Pacific La Niña. Results could be linked to the reported 60's and 70's climate shifts; emphasizing the importance of tropical Atlantic for the success of seasonal forecast skill.

Sea surface temperature (SST) variability in the tropical Atlantic Ocean strongly impacts the climate on the surrounding continents. On interannual time scales, highest SST variability occurs in the eastern equatorial region and off the coast of southwestern Africa. The pattern of SST variability resembles the Pacific El Niño, but features notable differences, and has been discussed in the context of various climate modes, that is, reoccurring patterns resulting from particular interactions in the climate system. Here, we attempt to reconcile those different definitions, concluding that almost all of them are essentially describing the same mode that we refer to as the “Atlantic Niño.” We give an overview of the mechanisms that have been proposed to underlie this mode, and we discuss its interaction with other climate modes within and outside the tropical Atlantic. The impact of Atlantic Niño‐related SST variability on rainfall, in particular over the Gulf of Guinea and north eastern South America is also described. An important aspect we highlight is that the Atlantic Niño and its teleconnections are not stationary, but subject to multidecadal modulations. Simulating the Atlantic Niño proves a challenge for state‐of‐the‐art climate models, and this may be partly due to the large mean state biases in the region. Potential reasons for these model biases and implications for seasonal prediction are discussed. This article is categorized under: Climate Models and Modeling > Knowledge Generation with Models

This work presents a description of the 1979–2002 tropical Atlantic (TA) SST variability modes coupled to the anomalous West African (WA) rainfall during the monsoon season. The time-evolving SST patterns, with an impact on WA rainfall variability, are analyzed using a new methodology based on maximum covariance analysis. The enhanced Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, gives a complete picture of the interannual WA rainfall patterns for the Sahel dry period. The leading TA SST pattern, related to the Atlantic El Niño, is coupled to anomalous precipitation over the coast of the Gulf of Guinea, which corresponds to the second WA rainfall principal component. The thermodynamics and dynamics involved in the generation, development, and damping of this mode are studied and compared with previous works. The SST mode starts at the Angola/Benguela region and is caused by alongshore wind anomalies. It then propagates westward via Rossby waves and damps because of latent heat flux anomalies and Kelvin wave eastward propagation from an off-equatorial forcing. The second SST mode includes the Mediterranean and the Atlantic Ocean, showing how the Mediterranean SST anomalies are those that are directly associated with the Sahelian rainfall. The global signature of the TA SST patterns is analyzed, adding new insights about the Pacific–Atlantic link in relation to WA rainfall during this period. Also, this global picture suggests that the Mediterranean SST anomalies are a fingerprint of large-scale forcing. This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s, including both the wet and the dry Sahel periods.

[1] Oceanic Kelvin waves from the equator to the West African coast are investigated in the framework of tropical Atlantic intraseasonal variability. In order to better highlight the wave propagations, a 25-95 day band-pass filter was applied to the sea surface height (SSH) product derived from the TOPEX/POSEIDON altimeter and an ocean general circulation model simulation for the 1993-2000 period. In addition to equatorial eastward propagations, our analysis reveals recurrent and continuous propagations distinguishable over thousands of kilometers poleward along the coasts as far as about 10°-15°latitude, a novel result with altimeter data. The variance of the filtered SSH signal goes from 1 cm at the equator to 2 cm at the African coast. Estimates of the phase speed range from 1.5 to 2.1 m/s along the equator and the West African coastline. Such values are very close to those of equatorial Kelvin wave propagations, likely dominated by the first two baroclinic modes, supporting the fact that the coastal propagations are coastally trapped Kelvin waves. In order to simplify the description of these Kelvin waves, we present an intraseasonal climatology which reveals regular boreal autumn-winter equator to coast propagations. An improved description is achieved thanks to the computation of an extended empirical orthogonal function for the boreal autumn-winter propagations. Lag correlation of SSH signals allows for a twofold quantification: the phase speed and the importance of remote forcing along the coast. The remote forcing effect of intraseasonal Kelvin waves is clearly evidenced over coastal upwelling regions as far as 10°-15°latitude. The physical mechanism associated with the forcing of the Kelvin waves and its impacts will be investigated in a future paper.

No abstract

The tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern

The Atlantic multidecadal oscillation (AMO) is the leading mode of Atlantic sea surface temperature (SST) variability at multidecadal time scales. Previous studies have shown that the AMO could modulate El Niño–Southern Oscillation (ENSO) variance. However, the role played by the AMO in the tropical Atlantic variability (TAV) is still uncertain. Here, it is demonstrated that during negative AMO phases, associated with a shallower thermocline, the eastern equatorial Atlantic SST variability is enhanced by more than 150% in boreal summer. Consequently, the interannual TAV modes are modified. During negative AMO, the Atlantic Niño displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring. In contrast, a meridional seesaw SLP pattern evolving into a zonal gradient leads the Atlantic Niño during positive AMO. The north tropical Atlantic (NTA) mode is related to a Scandinavian blocking pattern during winter and spring in negative AMO, while under positive AMO it is part of the SST tripole associated with the North Atlantic Oscillation. Interestingly, the emergence of an overlooked variability mode, here called the horseshoe (HS) pattern on account of its shape, is favored during negative AMO. This anomalous warm (cool) HS surrounding an eastern equatorial cooling (warming) is remotely forced by an ENSO phenomenon. During negative AMO, the tropical–extratropical teleconnections are enhanced and the Walker circulation is altered. This, together with the increased equatorial SST variability, could promote the ENSO impacts on TAV. The results herein give a step forward in the better understanding of TAV, which is essential to improving its modeling, impacts, and predictability.

We review the studies carried out during the African Monsoon Multidisciplinary Analysis (AMMA)-EU on the changes of interannual sea surface temperature (SST)-West African monsoon (WAM) covariability at multidecadal timescales, together with the influence of global warming (GW). The results obtained in the AMMA-EU suggest the importance of the background state, modulated by natural and anthropogenic variability, in the appearance of different interannual modes. The lack of reliability of current coupled models in giving a realistic assessment for WAM in the future is also stated.