Publication Date:
2022-12-07
Description:
Based on velocity data from a long‐term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper‐ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind‐driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south‐easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases.
Description:
Plain Language Summary;
Tropical Instability Waves (TIWs) are clear in satellite measurements of sea surface temperature as horizontal undulations with wavelength of the order of 1,000 km in equatorial regions of both Atlantic and Pacific Oceans. TIWs are characterized by their distinctive upper‐ocean meridional velocity structure. TIWs amplify vertical shear and thus contribute to the generation of turbulence which in turn leads to the mixing of heat and freshwater downward into the deeper ocean. In this study we show that the prevailing southerly winds in the central equatorial Atlantic drive near‐surface northward and subsurface southward flows, which are superposed on the meridional TIW velocity field. The strength of this wind driven cell is linked to the seasonal cycle of the northward component of the trade winds, peaking in boreal fall when TIWs reach their maximum amplitude. The overturning cell affects the vertical structure of the meridional velocity field and thus has impact on the generation of current shear and turbulence. We show that the overturning reduces/enhances shear during northward/southward TIW flow, an asymmetry that is consistent with independent measurements showing asymmetric mixing.
Description:
Key Points:
Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases.
Meridional wind stress forces a seasonally‐varying, shallow cross‐equatorial overturning cell‐the Equatorial Roll.
The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north‐ and southward phases.
Description:
EU H2020
Description:
Bundesministerium für Bildung und Forschung
http://dx.doi.org/10.13039/501100002347
Description:
US NSF
Description:
Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659
Description:
National Oceanic and Atmospheric Administration
http://dx.doi.org/10.13039/100000192
Description:
National Academy of Sciences
http://dx.doi.org/10.13039/100000209
Description:
National Science Foundation
http://dx.doi.org/10.13039/100000001
Description:
https://doi.pangaea.de/10.1594/PANGAEA.941042
Description:
https://www.pmel.noaa.gov/tao/drupal/disdel/
Keywords:
ddc:551.5
;
tropical instability waves
;
equatorial Atlantic
;
equatorial roll
;
moored velocity data
;
ocean mixing
;
ocean observations
Language:
English
Type:
doc-type:article
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