Oceanic and atmospheric drivers of interannual variability in shelf sea nitrate supply
Authors: Wei Xiaoyan , Hopkins Joanne , Oltmanns Marilena , Johnson Clare , Inall Mark 
Published: March 2022

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&lt;p&gt;&lt;span&gt;Strong interannual &lt;/span&gt;&lt;span&gt;variability in pre-spring&lt;/span&gt;&lt;span&gt; nitrate concentrations &lt;/span&gt;&lt;span&gt;have been observed in shelf seas. &lt;/span&gt;&lt;span&gt;Th&lt;/span&gt;&lt;span&gt;ese variations &lt;/span&gt;&lt;span&gt;contribute to &lt;/span&gt;&lt;span&gt;interannual &lt;/span&gt;&lt;span&gt;variability in&lt;/span&gt; &lt;span&gt;net primary production &lt;/span&gt;&lt;span&gt;during the following spring and summer and&lt;/span&gt;&lt;span&gt; thus also&lt;/span&gt; &lt;span&gt;regulate the &lt;/span&gt;&lt;span&gt;biological &lt;/span&gt;&lt;span&gt;uptake of &lt;/span&gt;&lt;span&gt;atmospheric&lt;/span&gt;&lt;span&gt; carbon&lt;/span&gt;&lt;span&gt; dioxide. &lt;/span&gt;&lt;span&gt;Most of the nitrate &lt;/span&gt;&lt;span&gt;required&lt;/span&gt; &lt;span&gt;by shelf seas to maintain their high productivity &lt;/span&gt;&lt;span&gt;is supplied from the open ocean.&amp;#160;&lt;/span&gt; &lt;span&gt;In this study, we investigate for the first time the&lt;/span&gt;&lt;span&gt; importance of &lt;/span&gt;&lt;span&gt;variability in the depth of &lt;/span&gt;&lt;span&gt;winter mixing along the shelf edge and subsequent wind-driven across-shelf Ekman transport in setting the pre-spring nitrate concentrations&lt;/span&gt;&lt;span&gt; across the &lt;/span&gt;&lt;span&gt;NW European shelf.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;Monthly EN4 temperature data &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; used to&lt;/span&gt; &lt;span&gt;identify the &lt;/span&gt;&lt;span&gt;maximum depth of w&lt;/span&gt;&lt;span&gt;in&lt;/span&gt;&lt;span&gt;ter mixing along the NW European shelf edge between 2000 and 2016 &lt;/span&gt;&lt;span&gt;and monthly climatologies of nitrate from the World Ocean Atlas are&lt;/span&gt;&lt;span&gt; used to estimate&lt;/span&gt;&lt;span&gt;&amp;#160;the &lt;/span&gt;&lt;span&gt;resulting concentration of&lt;/span&gt;&lt;span&gt; surface water&lt;/span&gt; &lt;span&gt;nitrate&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;The wind-driven across-shelf &lt;/span&gt;&lt;span&gt;nitrate&lt;/span&gt;&lt;span&gt; transport &lt;/span&gt;&lt;span&gt;for each sector of the shelf &lt;/span&gt;&lt;span&gt;during&lt;/span&gt;&lt;span&gt; late winter is then derived&lt;/span&gt;&lt;span&gt; using &lt;/span&gt;&lt;span&gt;wind velocities from ERA5. &lt;/span&gt;&lt;span&gt;The&lt;/span&gt;&lt;span&gt; deepest winter mixing &lt;/span&gt;&lt;span&gt;close to the shelf edge &lt;/span&gt;&lt;span&gt;occurs&lt;/span&gt; &lt;span&gt;in the&lt;/span&gt;&lt;span&gt; Rockall Trough. &lt;/span&gt;&lt;span&gt;However&lt;/span&gt;&lt;span&gt;, the largest interannual variations &lt;/span&gt;&lt;span&gt;in&lt;/span&gt; &lt;span&gt;winter &lt;/span&gt;&lt;span&gt;mixed layer depth&lt;/span&gt; &lt;span&gt;occur &lt;/span&gt;&lt;span&gt;in&lt;/span&gt;&lt;span&gt; the Bay of Biscay and Porcupine Sea Bight&lt;/span&gt;&lt;span&gt;. T&lt;/span&gt;&lt;span&gt;he smallest variations&lt;/span&gt;&lt;span&gt; are seen&lt;/span&gt; &lt;span&gt;in&lt;/span&gt;&lt;span&gt; the Faroe-Shetland channel. The estimated nitrate concentration&lt;/span&gt;&lt;span&gt; near&lt;/span&gt; &lt;span&gt;the shelf edge&lt;/span&gt;&lt;span&gt; after winter mixing (i.e., recharge&lt;/span&gt;&lt;span&gt; of surf&lt;/span&gt;&lt;span&gt;ace&lt;/span&gt;&lt;span&gt; water&lt;/span&gt;&lt;span&gt; nitrate&lt;/span&gt;&lt;span&gt; concentrations&lt;/span&gt;&lt;span&gt;) shows strong interannual variability &lt;/span&gt;&lt;span&gt;in&lt;/span&gt;&lt;span&gt; the Bay of Biscay and Porcupine Sea Bight (&lt;/span&gt;&lt;span&gt;interannual&lt;/span&gt;&lt;span&gt; differences of &lt;/span&gt;&lt;span&gt;1-&lt;/span&gt;&lt;span&gt; 2.5 mmol/m3)&lt;/span&gt;&lt;span&gt;, comparable to&lt;/span&gt;&lt;span&gt; variability that has &lt;/span&gt;&lt;span&gt;been observed on the shelf&lt;/span&gt;&lt;span&gt; in this region&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;Further north, despite &lt;/span&gt;&lt;span&gt;deeper winter mixing, the&lt;/span&gt;&lt;span&gt;re are much smaller &lt;/span&gt;&lt;span&gt;interannual&lt;/span&gt;&lt;span&gt; differences in &lt;/span&gt;&lt;span&gt;surface water nitrate &lt;/span&gt;&lt;span&gt;concentrations&lt;/span&gt; &lt;span&gt;at the end of winter.&lt;/span&gt; &lt;span&gt;This is because the &lt;/span&gt;&lt;span&gt;vertical gradients in &lt;/span&gt;&lt;span&gt;nitrate concentration&lt;/span&gt; &lt;span&gt;within the range of maximum winter mixed layer depths are&lt;/span&gt; &lt;span&gt;much weaker.&lt;/span&gt; &lt;span&gt;Over&lt;/span&gt; &lt;span&gt;the northern sectors of the NW European Shelf &lt;/span&gt;&lt;span&gt;large interannual differences in &lt;/span&gt;&lt;span&gt;wind-&lt;/span&gt;&lt;span&gt;driven Ekman transport&lt;/span&gt; &lt;span&gt;drive between 1 and 6 mmol N m-1 s&lt;/span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;span&gt; on-shelf &lt;/span&gt;&lt;span&gt;during the winter&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Further south&lt;/span&gt;&lt;span&gt; wind-driven &lt;/span&gt;&lt;span&gt;nitrate &lt;/span&gt;&lt;span&gt;transport during the winter is&lt;/span&gt; &lt;span&gt;typically off-shelf, &lt;/span&gt;&lt;span&gt;weaker (&lt;/span&gt;&lt;span&gt;typically&lt;/span&gt;&lt;span&gt; 1 mmol &lt;/span&gt;&lt;span&gt;N m&lt;/span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;span&gt; s-&lt;/span&gt;&lt;span&gt;1&lt;/span&gt;&lt;span&gt;), &lt;/span&gt;&lt;span&gt;and much less variable between years.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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