Growth and decay of the equatorial Atlantic SST mode by means of closed heat budget in a coupled general circulation model

Abstract

Citation:Polo I, Lazar A, Rodriguez-Fonseca B and Mignot J (2015) Tropical Atlantic variability is strongly biased in coupled General Circulation Models (GCM). Most of the models present a mean Sea Surface Temperature (SST) bias pattern that resembles the leading mode of inter-annual SST variability. Thus, understanding the causes of the main mode of variability of the system is crucial. A GCM control simulation with the IPSL-CM4 model as part of the CMIP3 experiment has been analyzed. Mixed layer heat budget decomposition has revealed the processes involved in the origin and development of the leading inter-annual variability mode which is defined over the Equatorial Atlantic (hereafter EA mode). In comparison with the observations, it is found a reversal in the anomalous SST evolution of the EA mode: from west equator to southeast in the simulation, while in the observations is the opposite. Nevertheless, despite the biases over the eastern equator and the African coast in boreal summer, the seasonality of the inter-annual variability is well-reproduced in the model. The triggering of the EA mode is found to be related to vertical entrainment at the equator as well as to upwelling along South African coast. The damping is related to the air-sea heat fluxes and oceanic horizontal terms. As in the observation, this EA mode exerts an impact on the West African and Brazilian rainfall variability. Therefore, the correct simulation of EA amplitude and time evolution is the key for a correct rainfall prediction over tropical Atlantic. In addition to that, identification of processes which are responsible for the tropical Atlantic biases in GCMs is an important element in order to improve the current global prediction systems.

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…”

Ocean Dynamics Shapes the Structure and Timing of Atlantic Equatorial Modes

“…To determine the air‐sea processes underlying the development of the HS and Atlantic Niño mode, a closed heat budget analysis has been computed interactively by NEMO‐ATLTROP025 model in the TA mixed layer (ML). The temporal variations of the ML temperature are given by a balance of atmospheric (a) and oceanic terms (b‐c), according to the equation (Faye et al, ; Polo et al, ): with . The brackets denote the vertical integration over the ML with T and h representing the temperature and depth of the ML respectively; and w are the horizontal and vertical currents; D l (T) is the lateral diffusion operator and K z the vertical mixing coefficient.…”

“…To determine the air-sea processes underlying the development of the HS and Atlantic Niño mode, a closed heat budget analysis has been computed interactively by NEMO-ATLTROP025 model in the TA mixed layer (ML). The temporal variations of the ML temperature are given by a balance of atmospheric (a) and oceanic terms (b-c), according to the equation (Faye et al, 2015;Polo et al, 2015):…”

The Tropical Atlantic Observing System

“…Despite significant progress in understanding TAV and its impacts during the last decades, many open questions remain. Dynamical mechanisms for the generation of the AZM (Keenlyside and Latif, 2007;Polo et al, 2015b;Jouanno et al, 2017) have been questioned and, mostly based on model simulations, important roles for thermodynamic processes proposed (Nnamchi et al, 2015(Nnamchi et al, , 2016. The importance of thermodynamic forcing is likely amplified in models with enhanced SST bias (Jouanno et al, 2017).…”