Extreme Marine Heat Waves in the Tropical Atlantic during Boreal Early Spring 2024: Major Role of Shortwave Radiation and Mixed Layer Depth

Abstract Recent studies have reported the marine heat waves (MHWs) across the tropical Atlantic that peaked in boreal early spring 2024. Using multiple observational and reanalysis datasets, we demonstrate that MHW metrics based on detrended data, averaged over the entire tropical Atlantic, reached their satellite-era maxima at that time, consistent with a prior equatorial study. A mixed layer heat budget analysis identified distinct regional drivers: 1) in the northern tropical Atlantic (20°–3°N), a preconditional MHW during fall 2023 that was sustained through the following seasons by positive shortwave radiation anomalies associated with reduced total cloud cover; 2) in the equatorial Atlantic (3°N–3°S), warming was driven by the previously identified ocean waves, amplified by negative mixed layer depth anomalies; and 3) in the southern tropical Atlantic (3°–20°S), the primary driver was wind-driven mixed layer shoaling from November 2023 to February 2024. Further analysis links the northern tropical Atlantic cloud-cover anomalies to the previously emphasized remote forcing from El Niño, whereas the wind anomalies over the southern tropical and equatorial Atlantic were predominantly controlled by the newly revealed South Atlantic subtropical dipole. Our study reveals the multifaceted formation mechanisms of extreme MHWs in the tropical Atlantic, offering crucial insights for improving their seasonal prediction. Significance Statement This study focuses on the marine heat wave (MHW) events occurring in the tropical Atlantic during boreal early spring 2024, the most severe events in the tropical Atlantic since the satellite era even after detrending. We demonstrate that these events were driven by a combination of regional preconditioning, atmospheric forcing, and oceanic dynamics, with distinct mechanisms dominating in different sectors. The findings also establish the critical roles of large-scale climate modes, including the previously recognized El Niño forcing and the newly identified South Atlantic subtropical dipole modulation. This work advances the mechanistic understanding of extreme tropical Atlantic MHWs.