Contrasts in turbulent vertical fluxes of nutrients across the permanent Lofoten Basin Eddy in the Nordic Seas
Authors: Bosse Anthony , Fer Ilker 
Published: March 2022

&lt;p&gt;&lt;span&gt;Mesoscale eddies play an &lt;/span&gt;&lt;span&gt;increasingly recognized role &lt;/span&gt;&lt;span&gt;on modulating &lt;/span&gt;&lt;span&gt;turbulence levels and associated &lt;/span&gt;&lt;span&gt;diapycnal &lt;/span&gt;&lt;span&gt;fluxes in the ocean, in particular &lt;/span&gt;&lt;span&gt;with &lt;/span&gt;&lt;span&gt;increased dissipation rates &lt;/span&gt;&lt;span&gt;found &lt;/span&gt;&lt;span&gt;in anticyclones. &lt;/span&gt;&lt;span&gt;In September 2017, the last cruise of the ProVoLo project &lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;n the Nordic Seas (https://www.uib.no/en/rg/fysos/97330/provolo) intensively surveyed &lt;/span&gt;&lt;span&gt;an energetic mesoscale anticyclone (the permanent&lt;/span&gt;&lt;span&gt; Lofoten Basin Eddy&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; to characterize turbulence of the upper layer &lt;/span&gt;&lt;span&gt;and eventually &lt;/span&gt;&lt;span&gt;quantify &lt;/span&gt;&lt;span&gt;the resulting &lt;/span&gt;&lt;span&gt;vertical fluxes nutrients &lt;/span&gt;&lt;span&gt;caused by turbulence&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;The sampling strategy&lt;/span&gt; &lt;span&gt;combined&lt;/span&gt; &lt;span&gt;ship-borne measurements and autonomous platforms. The vessel carried out a radial transect with stations spac&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt; &lt;span&gt;by &lt;/span&gt;&lt;span&gt;5 km near the center and 10-20 km outside &lt;/span&gt;&lt;span&gt;the eddy with measurements &lt;/span&gt;&lt;span&gt;of temperature and salinity (CTD), currents (lowered ADCP) and&lt;/span&gt; &lt;span&gt;turbulence (Vertical Microstructure Profiler, VMP2000). Water samples were analyzed&lt;/span&gt; &lt;span&gt;to estimate the concentration of the main nutrients (nitrate, phosphate and silicate). &lt;/span&gt;&lt;span&gt;In addition, two autonomous oceanic gliders were used.&lt;/span&gt; &lt;span&gt;A first&lt;/span&gt; &lt;span&gt;glider &lt;/span&gt;&lt;span&gt;profiling 0-1000 m deep &lt;/span&gt;&lt;span&gt;was completing&lt;/span&gt; &lt;span&gt;a 6-month mission. A second glider was specifically deployed during the cruise (5&lt;/span&gt; &lt;span&gt;days). This glider was&lt;/span&gt; &lt;span&gt;equipped with a&lt;/span&gt; &lt;span&gt;dissolved oxygen &lt;/span&gt;&lt;span&gt;Aanderaa &lt;/span&gt;&lt;span&gt;optode, a WET Labs FLNTU fluorescence and turbidity sensor and a Rockland Scientific&lt;/span&gt; &lt;span&gt;Microrider sampling&lt;/span&gt; &lt;span&gt;turbulence. &lt;/span&gt;&lt;span&gt;It sampled &lt;/span&gt;&lt;span&gt;the surface layer (0-300 m)&lt;/span&gt; &lt;span&gt;at high temporal (~30 min) and spatial (~500 m) resolution&lt;/span&gt; &lt;span&gt;from about 60 km to 5 km of the eddy center.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;By combining&lt;/span&gt; &lt;span&gt;th&lt;/span&gt;&lt;span&gt;ose &lt;/span&gt;&lt;span&gt;measurements, we &lt;/span&gt;&lt;span&gt;characterized &lt;/span&gt;&lt;span&gt;the turbulen&lt;/span&gt;&lt;span&gt;ce dissipation rates,&lt;/span&gt;&lt;span&gt; vertical diffusion and its associated fluxes across the different nutriclines &lt;/span&gt;&lt;span&gt;from the center to the outside region area of the eddy, revealing s&lt;/span&gt;&lt;span&gt;ignificant&lt;/span&gt; &lt;span&gt;contrasts.&amp;#160;&lt;/span&gt;&lt;span&gt;Below the thermocline, turbulent patches were observed within the core with dissipation rates elevated by one order of magnitude relative to the values outside. The higher levels of dissipation rates supported 10-fold stronger vertical diffusion coefficients, substantially increasing vertical turbulent fluxes through the nutriclines. The transition between the eddy tangential velocity&lt;/span&gt; &lt;span&gt;maximum and the zero vorticity&lt;/span&gt; &lt;span&gt;was characterized by a frontal region exhibiting&lt;/span&gt; &lt;span&gt;important oscillations&lt;/span&gt; &lt;span&gt;of the thermocline, manifesting important vertical exchanges.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;This &lt;/span&gt;&lt;span&gt;study&lt;/span&gt;&lt;span&gt; is not only relevant in a local context, but also has&lt;/span&gt; &lt;span&gt;global implications for the ocean energy budget and highlights the need for more high-resolution observations resolving&lt;/span&gt; scales from the &lt;span&gt;mesoscale &lt;/span&gt;&lt;span&gt;to the dissipation&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;