Diurnal Oceanic Thermal Variability in the Atlantic Cold Tongue: Dependence on Wind Speed and Background Conditions

Abstract

Diurnal ocean mixing processes in the equatorial Atlantic play a crucial role in shaping surface temperature and velocity, thereby influencing air-sea interactions and the vertical heat transport. However, characterizing the upper ocean diurnal variability remains challenging due to its high vertical and temporal variability and limited observations, resulting in uncertainties in ocean model skills. Between March and July of 2021 and 2022, 10 specially programmed Argo floats were deployed to sample the diurnal cycle in the equatorial Atlantic at high temporal (3-hourly) and vertical (meter-scale) resolution. Combined with mooring data, the observations revealed that weaker winds in March–April were associated with a shallower and slower (1.8 m/hr) vertical extension of the diurnal warm layer, compared to May–June corresponding to the seasonal onset of the cold tongue, when the deepening rate reached 3.2 m/hr. The comparison of the observations with a regional model demonstrates that the model reproduces most features of the observed diurnal warm layer but underestimates its vertical extension and deepening rate. The model is then used to show that the diurnal vertical heat flux through the mixed layer increases by 50% from weak to strong wind conditions. However, oceanic background conditions also play a significant role, as similar mean wind speeds can result in vertical heat fluxes that reach twice the depth and intensity at 10°W compared to 0°. This study emphasizes the need to better capture diurnal variability to assess its impact on the Atlantic cold tongue and its representation in models.