Do Loop Current eddies stimulate productivity in the Gulf of Mexico?

Pierre Damien,Julio Sheinbaum,Lorena Linacre,Olaf Duteil,Julien Jouanno,Orens Pasqueron De Fommervault

doi:10.5194/bg-18-4281-2021

Abstract

Abstract. Surface chlorophyll concentrations inferred from satellite images suggest a strong influence of the mesoscale activity on biogeochemical variability within the oligotrophic regions of the Gulf of Mexico (GoM). More specifically, long-living anticyclonic Loop Current eddies (LCEs) are shed episodically from the Loop Current and propagate westward. This study addresses the biogeochemical response of the LCEs to seasonal forcing and show their role in driving phytoplankton biomass distribution in the GoM. Using an eddy resolving (1/12∘) interannual regional simulation, it is shown that the LCEs foster a large biomass increase in winter in the upper ocean. It is based on the coupled physical–biogeochemical model NEMO-PISCES (Nucleus for European Modeling of the Ocean and Pelagic Interaction Scheme for Carbon and Ecosystem Studies) that yields a realistic representation of the surface chlorophyll distribution. The primary production in the LCEs is larger than the average rate in the surrounding open waters of the GoM. This behavior cannot be directly identified from surface chlorophyll distribution alone since LCEs are associated with a negative surface chlorophyll anomaly all year long. This anomalous biomass increase in the LCEs is explained by the mixed-layer response to winter convective mixing that reaches deeper and nutrient-richer waters.

Highlights

Historical satellite ocean color observations of the deep waters of the Gulf of Mexico indicate low surface chlorophyll concentrations [Chl], low biomass, and low primary productivity (Müller-Karger et al, 1991; Biggs and Ressler, 2001; Salmerón-García et al, 2011)
Far from the coast, these figures reveal that the surface chlorophyll varies at a scale of the order of 100 km with a distribution that tends to follow the absolute dynamic topography (ADT) contours
Loop Current eddies (LCEs) trajectories are reported in Fig. 2a, superimposed onto the geostrophic climatological eddy kinetic energy (EKE) field at the surface

Summary

Introduction

Historical satellite ocean color observations of the deep waters of the Gulf of Mexico (roughly delimited by the 200 m isobath and hereafter referred to as GoM open waters) indicate low surface chlorophyll concentrations [Chl], low biomass, and low primary productivity (Müller-Karger et al, 1991; Biggs and Ressler, 2001; Salmerón-García et al, 2011). The seasonal cycle is strongly modulated by the energetic mesoscale dynamic activity which shapes the distribution of biogeochemical properties (Biggs and Ressler, 2001; Pasqueron de Fommervault et al, 2017). This mesoscale activity is dominated by the large and long-living Loop Current eddies (LCEs) which are shed episodically by the Loop Current (Weisberg and Liu, 2017) and constitute the most energetic circulation features in the GoM (Sheinbaum et al, 2016; Sturges and Leben, 2000).

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