FY 2021

Formation and evolution of a freshwater plume in the northwestern tropical Atlantic in February 2020

Reverdin, G., L. Olivier, G.R. Foltz, S. Speich, J. Karstensen, and J. Horstmann

J. Geophys. Res., 126(4), e2020JC016981, doi: 10.1029/2020JC016981, View online (2021)

In February 2020, a 120-km-wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7°N/54°W-56°W). It was initially stratified in the upper 10 m with a freshwater content of 2–3 m of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13–16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off-shelf flow lasted 10 days, contributing to a plume area extending over 100,000 km2 and associated with a 0.15 Sv (106 m3 s−1) freshwater transport. The off-shelf plume was steered northward by a North Brazil Current ring up to 12°N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favorable wind direction closer to the Amazon estuary, which contributed to north-westward freshwater transport on the shelf. Other such events of freshwater transport in January–March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favoring the north-westward freshwater transport toward the shelf break.

Plain Language Summary. This study documents how freshwater from the Amazon reaches the deep ocean up to 12°N in the northwest tropical Atlantic in January–March. The classical view is that the water is channeled along the shelf to the Caribbean Sea. Here, we document a freshwater plume from in situ and satellite observations during the EUREC4A-OA/ATOMIC program in 2020. This plume separated from the shelf near 55°W north of French Guiana on February 2–5. This fresher water was stirred by a North Brazil Current ring up to 12°N before mostly spreading westward. The near-surface water was initially very stratified at least until 10-m from the surface. More than 14 days later and 400 km farther north, salinity as low as 33.3 pss with mixing depths on the order of only 20-m was still encountered. The total area of the freshwater plume reached 100,000 km2 with a flow of freshwater on the order of 0.15 Sv (106 m3 s−1) during 10 days. This phenomenon seems to be triggered by changes in the wind direction on the shelf closer to the equator, and has also been observed in satellite products in 7 out of 10 years since 2010.