Resilience or a Trend towards the Weakening of the Cold Water Tongue under the Influence of Local Warming?

Abstract

The equatorial Atlantic cold water tongue plays a key role in regulating the climate of the West African continent and the marine ecosystems of the equatorial Atlantic. This study examines its vulnerability to the increasing frequency of marine heatwaves between 1993 and 2025. Using an approach combining climatological analyses and decomposition into empirical orthogonal functions, we quantify the spatiotemporal evolution of its cooling surface. The results reveal a marked spatial asymmetry, with the central and western equatorial sector (15˚W - 5˚W) emerging as the dynamic core of the system, but also proving to be the most vulnerable to thermal contractions. Event-based analysis shows that the presence of significant negative anomalies (−68.000 km 2 ) two weeks prior to the peak of marine heatwaves highlights a preconditioning effect, whereby a prior weakening of upwelling facilitates the triggering of the heatwave. During marine heatwaves, the average reduction reaches –110.000 km 2 . The key finding lies in the post 2015 break, manifested by the ongoing transition of the primary mode of variability (71.5 per cent of the variance) towards a state of chronic contraction. Coupled with incomplete resilience during the relaxation phase (−56.000 km 2 ), this trend shows that, under the influence of local warming and increased stratification, the thermocline’s readjustment time now exceeds the return period of the MHWs. Rather than merely a temporary resistance, the cold water tongue is undergoing a chronic spatial decline, ultimately threatening the regional biogeochemical balance.