On the Spatio‐Temporal Scales of the Subtropical Mode Water Formation in the South Atlantic

We examined the changes in the ocean’s upper layer structure involved in the formation of the subtropical mode water in the South Atlantic. Here we present the results from a survey done in the region of formation between 37°W and 32°W and 34°S and 37°S from July–October 2018, using high‐resolution...

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Bibliographic Details
Published in:Journal of geophysical research. Oceans 2024-04, Vol.129 (4), p.n/a
Main Authors: Sato, O. T., Polito, P. S., Dottori, M.
Format: Article
Language:English
Subjects:
air‐sea interaction
Chimneys
Convection
Convection cooling
Earth rotation
glider
Gyres
Inspection
Mixed layer
Oxygen
Physics
Planetary rotation
Potential vorticity
Rotation
Salinity
Salinity effects
Satellite data
Satellites
Sea surface
Sea surface temperature
South Atlantic
Spring
Spring (season)
Structures
submesoscale
subtropical mode water
Surface fluxes
Surface temperature
Surface water
Underwater gliders
Underwater vehicles
Vorticity
Water
Water resources development
Winter
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Summary:We examined the changes in the ocean’s upper layer structure involved in the formation of the subtropical mode water in the South Atlantic. Here we present the results from a survey done in the region of formation between 37°W and 32°W and 34°S and 37°S from July–October 2018, using high‐resolution measurements obtained from an underwater glider. From its records, the mode water development was observed for the first time in the South Atlantic as localized chimney‐like patterns in the temperature, salinity, and potential vorticity (PV) measurements with a horizontal scale of (7 ± 2) km, associated with wintertime oceanic convective processes. Over time, these submesoscale structures tend to expand and dominate the region as a large volume of mode water. These vertical structures present highly homogeneous temperature and salinity values. The observed PV was on the order of 5.0 × 10−11 m−1 s−1, which is one order of magnitude smaller than the pre‐existent mode water at the lower layers; negative values were observed, indicating active convection. A 1‐D vertical model was used to predict the depth of the mixed layer conditioned by the surface fluxes and triggered by its changes in winter to late spring. Comparisons of the sea surface temperature measured from satellite collocated with the glider’s measurements showed a very good agreement. From this analysis, we were able to establish the decorrelation scales that characterize mode water formation. Further inspection of the glider data yielded temporal decorrelation scales indicative of convection modified by planetary rotation. Plain Language Summary Subtropical mode waters are a uniform volume of water found at the edges of subtropical gyres. The South Atlantic Subtropical Mode Water (SASTMW) is no exception. It is found between 30°S and 40°S, from the surface to a depth of 200–350 m. It forms when the mixed layer deepens in winter to early spring; it does not mix with the surrounding water and remains homogeneous. It sinks after the surface waters become stratified again at the end of spring. The properties of the water (such as oxygen concentration or potential vorticity) change while it is in contact with the atmosphere. When the water sinks, it preserves a memory of these changes. We used in situ, satellite, and glider data to understand the physics of its formation and to describe its main characteristics in unprecedented detail. The formation takes the shape of chimney‐like structures that extend dow
ISSN:2169-9275
2169-9291
DOI:10.1029/2023JC020759