Impact of Intraseasonal Coastal Trapped Waves on Upwelling in the Northern Gulf of Guinea

Abstract We investigate the impact of equatorially‐forced intraseasonal coastal trapped waves (CTWs) on temperature variability in the Northern Gulf of Guinea (NGG) upwelling system, using a high‐resolution (1/36°) regional ocean model simulation spanning 2008–2017. Sea Level Anomalies (SLAs) are used to track CTWs, and spectral analysis reveals that most subseasonal SLA energy in the NGG lies within the 40–80 days band, with a peak around 58 days. A 40–80‐day band‐pass filter highlights poleward‐propagating CTWs during the main upwelling season (July to early October), with a phase speed of , originating from equatorial Kelvin waves (EKWs) propagating eastward at . Downwelling CTWs reach the NGG upwelling region days after the EKW peaks at , inducing SLA anomalies around . Eight days later, they trigger warm subsurface temperature anomalies up to near the thermocline and at the mixed‐layer base, while sea surface temperature anomalies remain weak due to ocean‐to‐atmosphere heat loss. A heat budget analysis over the whole water column in the Ghanaian upwelling area reveals that vertical advection is the dominant driver, explaining of the coastal subsurface warming. Specifically, CTW‐induced vertical currents advecting low frequency temperature gradient explain of the vertical advection, while low frequency vertical flows advecting CTW‐induced warming account for 37%. These results highlight the key role of intraseasonal equatorially‐forced CTWs in modulating summer upwelling dynamics and thermal variability in the NGG.