Hans, Anna C. , Brandt, Peter , Gasparin, F., Claus, Martin , Cravatte, S., Horstmann, J. and Reverdin, G. (2024) Observed Diurnal Cycles of Near‐Surface Shear and Stratification in the Equatorial Atlantic and Their Wind Dependence. Journal of Geophysical Research: Oceans, 129 (8). Art.Nr. e2023JC020870. DOI 10.1029/2023JC020870.

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Abstract

Key Points:
- Basin-scale in-situ data show the evolution of diurnal warm layer and diurnal jet in the upper 15 m of the equatorial Atlantic Ocean
- Higher wind speeds lead to earlier diurnal peaks, deeper penetration depths, and faster descent rates of the diurnal jet
- Wind speed dependence of descent rates of diurnal shear and stratification can explain the varying onset of deep-cycle turbulence

The diurnal cycles of near-surface velocity and temperature, 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 analyze 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 October 2019 and May 2022, data from three types of surface drifters, and data from Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) moorings along the equator. The observations during two seasons with similar mean wind speeds but varying 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 stratification is maximum for minimal wind speeds, the diurnal amplitude of shear is maximum at 6 m depth for moderate wind speeds of about 5 m s-1. The inferred 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.

During daytime, solar radiation leads to the formation of a thin warm layer at the ocean surface which can trap heat and wind-forced momentum. Both heat and momentum are transported in the deeper ocean during the evening and night by turbulent mixing. The associated diurnal variation of temperature, current velocity, and their vertical gradients, stratification and velocity shear, are thus relevant for understanding ocean-atmosphere interactions. This study investigates how the diurnal variation in stratification and velocity shear is influenced by the wind speed. For that, basin-scale observations of velocity and temperature, which were collected in the equatorial Atlantic during two trans-Atlantic equatorial cruises and by instruments installed at long-term moorings along the equator, are analyzed. These observations reveal that the wind speed influences the amplitude, the timing, and the vertical structure of the diurnal variation in stratification and velocity shear. Wind speed also influences how deep and how fast this variation propagates from the surface downward. The study concludes that the diurnal variation of stratification and velocity shear impacts first the input of mechanical energy from the atmosphere into the ocean and second the process of turbulent mixing below the night-time mixed layer.

Basin-scale in-situ data show the evolution of diurnal warm layer and diurnal jet in the upper 15 m of the equatorial Atlantic Ocean Higher wind speeds lead to earlier diurnal peaks, deeper penetration depths, and faster descent rates of the diurnal jet Wind speed dependence of descent rates of diurnal shear and stratification can explain the varying onset of deep-cycle turbulence

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