Journal Article

Northbound Transport of the Mediterranean Outflow and the Role of Time‐Dependent Chaotic Advection

Mantel, Nadav, Gildor, Hezi, Liran, Rotem, Rom‐Kedar, Vered, Saporta‐Katz, Ori

2024

Overview

The Mediterranean Sea releases approximately 1 Sv of water into the North Atlantic through the Gibraltar Straits, forming the saline Mediterranean Outflow Water (MOW). Its impact on large‐scale flow and specifically its northbound Lagrangian pathways are widely debated, yet a comprehensive overview of MOW pathways over recent decades is lacking. We calculate and analyze synthetic Lagrangian trajectories in 1980–2020 reanalysis velocity data. Sixteen percent of the MOW follow a direct northbound path to the sub‐polar gyre, reaching a 1,000 m depth crossing window at the southern tip of Rockall Ridge in about 10 years. Surprisingly, time‐dependent chaotic advection, not steady currents, drives over half of the northbound transport. Our results suggest a potential 15–20 years predictability in the direct northbound transport. Additionally, monthly variability appears more significant than inter‐annual variability in Lagrangian mixing and spreading the MOW. Plain Language Summary The Mediterranean Sea and the North Atlantic Ocean sustain a constant exchange of water through the shallow, narrow Straits of Gibraltar. Due to intense evaporation, the Mediterranean water is saltier and denser than its Atlantic counterpart and flows out of the Mediterranean Sea into the middepth North Atlantic Ocean to create the Mediterranean Outflow Water (MOW). This salty input reaches 1,000 m depth and spreads and mixes into the North Atlantic Ocean, and is thought to have a non‐negligible impact on its flow regime, specifically through its northbound transport and direct contribution to the salinity of the northern water of the North Atlantic. However, a comprehensive survey of the various pathways taken by the MOW is lacking. In this work, we track the 3D pathways of virtual trajectories starting at the Gibraltar Straits at various depths for 20 years. We find that 16% of the particles take a direct northbound path. Surprisingly, over half of this transport is not due to steady currents that lead the MOW to the north. Rather, it is a result of time‐dependent chaotic advection, a phenomenon that allows pathways in a time‐dependent flow to circumvent stationary barriers of transport. Key Points Synthetic Lagrangian trajectories initialized at the Gibraltar Straits, based on SODA3.4.2 reanalysis velocity data from 1980 to 2020, are used to study the pathways, Lagrangian mixing, and spreading of the Mediterranean Outflow Water (MOW) 16% of the MOW particles take a direct northbound route Over half of the direct northbound transport results from time‐dependent chaotic advection and not from a steady northbound current

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Advection / Annual variations / Depth / Energy / Evaporation / Experiments / Lagrangian trajectories / Mediterranean Outflow Water / northbound transport / Oceans / Outflow / Outflow waters / Salinity / spreading and mixing / Straits / sub‐polar gyre index / Time dependence / time‐dependent chaotic advection / Trajectory analysis / Velocity / Water / Water depth / Water outflow