Seasonal variations in the horizontal structure of the Amazon Plume inferred from historical hydrographic data

Abstract

Near‐surface salinity observations from the National Oceanographic Data Center hydrographic data archive for the region 4°S–10°N, 60–40°W show that the Amazon River discharge forms a plume of low‐salinity water that extends northwestward into the western tropical North Atlantic. The average near‐surface salinity within the plume increases away from the Amazon River mouth consistent with an average entrainment of oceanic water of 2×105 m3/s every 1° of latitude. There are two notable seasonal variations. In the vicinity of the Amazon River mouth (0–2°N) the plume extends about 200–300 km eastward from June through December. However, from March to May the plume was often much wider (400–500 km) in the vicinity of the river mouth, possibly due to the combination of near‐maximum river discharge and winds with a southeastward component. North of 5°N, the Amazon Plume fans out over a broad region of the western tropical North Atlantic. During January through June, low‐salinity Amazon Plume water north of 5°N is primarily found west of 52°W, spreading northwestward toward the Caribbean. Beginning typically in May, some of the Amazon Plume water is deflected eastward in the North Brazil Current (NBC) retroflection. From August through October, surface salinities suggest ∼70% of the Amazon Plume water is carried eastward in the NBC retroflection, while the remaining ∼30% flows northwestward toward the Caribbean.