Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

Abstract

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.

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

“…Notice that during the entire MM evolution, the air-sea fluxes tend to damp 287 the equatorial SST anomalies (blue line, Figure 2e). Thus, our results provide further 288 evidence of the essential role of ocean dynamics to control the equatorial SST variations 289 (Foltz et al 2003;Peter et al 2006;Polo et al 2015a;Martín-Rey et al 2019). 290 291…”

“…winds (Lübbecke et al 2018). The EM is thought to be generated by ocean dynamics 52 (Keenlyside and Latif 2007;Lübbecke and McPhaden 2013;Polo et al 2015a;Jouanno 53 et al 2017;Martín-Rey et al 2019), mainly driven by the Bjerknes feedback (Bjerknes 54 1969). The Bjerknes mechanism implies the propagation of oceanic waves as a response 55…”

“…The inter-hemispheric SST gradient is 245well captured by the model (r = 0.96), although the equatorial cooling displays a westward 246 extension reaching South American coast compared to the observations(Figure 1a-b). 247This can be due to the equatorial cold bias of the model, associated with a thinner 248 equatorial mixed layer and reduced thermocline slope(Martín-Rey et al 2019). The inter-249…”

A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

“…The interannual climate variability over the equatorial Atlantic has weakened during the recent decades (Tokinaga & Xie, 2011; Prigent et al, 2020). However, it remains unclear how the warming hole is related to changes in interannual variability, including fluctuations in the SST variance and spatial structure (Martín‐Rey et al, 2018, 2019). More research is needed to better understand how the reversal of the previous oceanic warming in the North Atlantic since the mid‐2000s (Robson et al, 2016) is related to the satellite era equatorial Atlantic.…”

On the Spatiotemporal Diversity of Atlantic Niño and Associated Rainfall Variability Over West Africa and South America

“…The time and longitude axes span from January to December and the entire equatorial Atlantic (50°W–10°E), respectively. As shown in Figure S1, the spatiotemporal evolution patterns of the 22 events are different in terms of the timing, zonal extent, and amplitude of their onset, peak, and decay (e.g., Martín‐Rey et al, 2019; Okumura & Xie, 2006; Richter et al, 2012). In fact, it is difficult to find any single event that can be described by the composite mean (Figure 1a) or any two events that closely resemble each other.…”