Observed Transformation of an Anticyclone into a Mode-Water Eddy in the Sargasso Sea

Mesoscale eddies play a key role in the transport of physical and biogeochemical properties in the ocean. Prior numerical modelling studies have demonstrated that wind-eddy interactions can transform eddies into different types: anticyclones to Mode-Water Eddies (MWE) and cyclones to cyclonic “Thinnies”. However, there is a limited understanding of eddy transformations as direct observations are challenged by the lack of long-term time series within individual eddies. Here we report evidence of an eddy observed in 2005 transforming from a regular anticyclone to a MWE sampled by Argo floats and shipboard measurements while tracked via satellite altimetry data. Argo profiles of the inner core of the eddy (⪝30 km from eddy center) early in its lifetime are compared to climatologies from the World Ocean Atlas. Temperature profiles show a downward displacement of the main thermocline, between 250 and 500 meters, consistent with a regular anticyclone in January. Five months later in July, shipboard sampling revealed the Mode Water layer had thickened, with a notable upward displacement of the seasonal thermocline consistent with a MWE structure. Model reanalysis data (MERCATOR GLORYS12V1), which includes a wind-eddy interaction term, suggests a qualitatively similar result. After transforming into a MWE, the eddy was observed to have triggered a long-lasting diatom bloom as nutrients abundant in density layers over 26.0 kg/m³ were uplifted into the euphotic zone. Significant biological implications such as this are critical results of eddy transformations. This novel observation not only shows the potential transformation of eddies, but motivates a greater understanding of their features and frequency, and to what extent they impact the world’s oceans.