Deep–breathing basin mesoscale dipoles utilize deep nutrient reserves and modulate upper–ocean productivity

This study investigates how a subsurface dipole eddy modulates phytoplankton biomass, expressed as chlorophyll- a (Chl- a ) concentrations, in the southern Gulf of Mexico. The analysis is based on high-resolution hydrographic and biogeochemical observations collected during an oceanographic cruise in June 2015. The vertical distributions of temperature, nutrients, and Chl- a fluorescence reveal a well-defined cyclone–anticyclone pair centered at approximately 200 m depth. Within the cyclonic eddy, the uplift of isotherms and the upward transport of nutrient-rich waters from depths near 1000 m to about 180 m promote the formation of a pronounced subsurface chlorophyll maximum below 80 m. In contrast, the anticyclonic eddy exhibits a deepening of isotherms and nutrient layers, consistent with a reduced vertical nutrient supply. Nitrite plus nitrate concentrations within the cyclonic core reach up to 5 µ M at 200 m depth, coinciding with enhanced vertically integrated Chl- a values of approximately 10 mg m −2 . Additionally, intensified boundary currents along the dipole margins suggest the presence of frontal dynamics that may further enhance vertical exchanges. The cyclonic eddy displays predominantly nonlinear behavior throughout most of its life cycle, a regime that plays a critical role in enabling the upward transport of nutrient-rich waters, with both isotherms and nitrate + nitrite isolines uplifted from depths of around 1000 m. This vertical displacement greatly exceeds typical mesoscale eddy pumping, demonstrating that subsurface dipoles can establish a direct connection between deep nutrient reservoirs and the base of the euphotic zone. Overall, these results highlight the key role of subsurface dipole eddies in regulating nutrient injection and phytoplankton biomass in the southern Gulf of Mexico, with eddy pumping emerging as the dominant mechanism driving subsurface biological enrichment.