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\title{Observed diurnal cycles of near-surface shear and stratification in the
equatorial Atlantic and their wind dependence}



\author[1]{Anna Christina Hans}%
\author[1]{Peter Brandt}%
\author[2]{Florent Gasparin}%
\author[1]{Martin Claus}%
\author[2]{Sophie Cravatte}%
\author[3]{Jochen Horstmann}%
\author[4]{Gilles Reverdin}%
\affil[1]{GEOMAR Helmholtz Centre for Ocean Research Kiel}%
\affil[2]{IRD}%
\affil[3]{Helmholtz-Zentrum Hereon}%
\affil[4]{Sorbonne Université, CNRS/IRD/MNHN, LOCEAN-IPSL}%


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  \date{May 26, 2024}


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\begin{abstract}
The diurnal cycles of near-surface shear and stratification, also known
as diurnal jet and diurnal warm layer (DWL), are ubiquitous in the
tropical oceans, affecting the heat and momentum budget of the ocean
surface layer, air-sea interactions, and vertical mixing. Here, we
analyse the presence and descent of near-surface diurnal shear and
stratification in the upper 20 m of the equatorial Atlantic as a
function of wind speed using ocean current 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. The observations during two
seasons with similar wind speeds but varying net surface heat fluxes
reveal similar diurnal jets with an amplitude of about 0.11 m s-1 and
similar DWLs when averaging along the equator. We find that higher wind
speeds lead to earlier diurnal peaks, deeper penetration depths, and
faster descent rates of DWL and diurnal jet. While the diurnal amplitude
of shear is maximum for intermediate wind speeds, the diurnal amplitude
of stratification is maximum for minimal wind speeds. The presented wind
dependence of the descent rates of DWL and diurnal jet is consistent
with the earlier onset of deep-cycle turbulence for higher wind speeds.
The DWL and the diurnal jet not only trigger deep-cycle turbulence but
are also observed to modify the wind power input and thus the amount of
energy available for mixing.%
\end{abstract}%



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