Seasonal and regional variations of sinking in the subpolar North Atlantic from a high-resolution ocean model

oleh: J.-M. Sayol, H. Dijkstra, C. Katsman

Format: Article
Diterbitkan: Copernicus Publications 2019-08-01

Deskripsi

<p>Previous studies have indicated that most of the net sinking associated with the downward branch of the Atlantic Meridional Overturning Circulation (AMOC) must occur near the subpolar North Atlantic boundaries. In this work we have used monthly mean fields of a high-resolution ocean model (0.1<span class="inline-formula"><sup>∘</sup></span> at the Equator) to quantify this sinking. To this end we have calculated the Eulerian net vertical transport (<span class="inline-formula"><i>W</i><sub>∑</sub></span>) from the modeled vertical velocities, its seasonal variability, and its spatial distribution under repeated climatological atmospheric forcing conditions. Based on this simulation, we find that for the whole subpolar North Atlantic <span class="inline-formula"><i>W</i><sub>∑</sub></span> peaks at about <span class="inline-formula">−14</span>&thinsp;Sv at a depth of 1139&thinsp;m, matching both the mean depth and the magnitude of the meridional transport of the AMOC at <span class="inline-formula">45<sup>∘</sup></span>&thinsp;N. It displays a seasonal variability of around <span class="inline-formula">10</span>&thinsp;Sv. Three sinking regimes are identified according to the characteristics of the accumulated <span class="inline-formula"><i>W</i><sub>∑</sub></span> with respect to the distance to the shelf: one within the first 90&thinsp;km and onto the bathymetric slope at around the peak of the boundary current speed (regime <strong>I</strong>), the second between 90 and 250&thinsp;km covering the remainder of the shelf where mesoscale eddies exchange properties (momentum, heat, mass) between the interior and the boundary (regime <strong>II</strong>), and the third at larger distances from the shelf where <span class="inline-formula"><i>W</i><sub>∑</sub></span> is mostly driven by the ocean's interior eddies (regime <strong>III</strong>). Regimes <strong>I</strong> and <strong>II</strong> accumulate <span class="inline-formula">∼90 <i>%</i></span> of the total sinking and display smaller seasonal changes and spatial variability than regime <strong>III</strong>. We find that such a distinction in regimes is also useful to describe the characteristics of <span class="inline-formula"><i>W</i><sub>∑</sub></span> in marginal seas located far from the overflow areas, although the regime boundaries can shift a few tens of kilometers inshore or offshore depending on the bathymetric slope and shelf width of each marginal sea. The largest contributions to the sinking come from the Labrador Sea, the Newfoundland region, and the overflow regions. The magnitude, seasonal variability, and depth at which <span class="inline-formula"><i>W</i><sub>∑</sub></span> peaks vary for each region, thus revealing a complex picture of sinking in the subpolar North Atlantic.</p>