Interaction between the Boreal Spring and Summer Tropical Atlantic Interannual Variability Modes

<p>Traditionally, the interannual Tropical Atlantic variability (TAV) is thought to be governed by two air-sea coupled modes denoted as Meridional Mode (MM) and Equatorial Mode (EM), peaking in boreal spring and summer respectively. Several studies have proposed a possible connection between the MM and EM, but without reaching a consensus about its frequency, type and associated mechanisms. Remarkably, recent findings brought to light decadal changes in the structure, intensity and teleconnections of the EM along the observational record. In particular, new overlooked equatorial modes called ‘non-canonical EM’ and ‘Horse-Shoe mode’ have been reported, which exhibit significant sea surface temperature anomalies in the north tropical Atlantic region. This gives robustness to the connection between the boreal spring and summer interannual modes.</p><p>Here, using observational and CMIP6 model datasets, we demonstrate the existence of distinct interannual modes in the tropical Atlantic basin along the record. Furthermore, the emergence of these modes is not stationary on time and varies from some decades to the others.  In this study, using observations and coupled climate models we explore the connection between the MM and EM to generate the diverse of tropical Atlantic variability reported in previous works. Moreover, the air-sea mechanisms and ocean dynamics involved in the evolution of these modes and the role of the mean state in the connection between them is assessed.</p>

Tropical Atlantic Variability: Observations and Modeling

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.

Is There Evidence of Changes in Tropical Atlantic Variability Modes under AMO Phases in the Observational Record?

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.