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Conference Paper

Diurnal cycle of near-surface shear and stratification in the equatorial Atlantic

Authors

Hans,  Anna Christina
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Brandt,  Peter
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Gasparin,  Florent
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Claus,  Martin
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Navarro,  Wendy
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

Reverdin,  Gilles
IUGG 2023, General Assemblies, 1 General, International Union of Geodesy and Geophysics (IUGG), External Organizations;

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Citation

Hans, A. C., Brandt, P., Gasparin, F., Claus, M., Navarro, W., Reverdin, G. (2023): Diurnal cycle of near-surface shear and stratification in the equatorial Atlantic, XXVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG) (Berlin 2023).
https://doi.org/10.57757/IUGG23-1749


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5017842
Abstract
The diurnally-varying deep cycle of turbulence in the equatorial oceans just below the mixed layer is critical for the heat flux from the mixed layer into the interior ocean. Its presence depends on the downward propagation of shear instabilities which are induced by the diurnal cycle of near-surface stratification and shear, i.e., by the diurnal warm layer and the diurnal jet. Using velocity and hydrographic data taken during two trans-Atlantic cruises along the equator in autumn 2019 and spring 2022, data from three types of surface drifters and data from PIRATA moorings along the equator, we analyse the presence and downward propagation of near-surface diurnal stratification and shear in the upper 20 m of the equatorial Atlantic. These different datasets all indicate that the diurnal warm layer established after sunrise leads to the development of a diurnal surface jet resulting in maximum velocity differences between surface and 15 m depth in the afternoon. The diurnal horizontal velocity pattern depends on the wind speed with vertical shear being stronger, reaching its maximum earlier, and propagating downward faster for stronger winds within the observed wind range from 2-9 ms-1. Similar downward propagation can be observed for stratification. These diurnal variations are affected by the background shear and stratification associated with seasonal and longitudinal variations of depth and strength of the Equatorial Undercurrent and the thermocline. The results are discussed with respect to recent measurements of deep cycle turbulence obtained from moored observations at the PIRATA sites at the equator, 23°W and 10°W.