Arctic Outflow Water ($\text{AOW}$) is a crucial, albeit often misrepresented, component of the deep-water circulation system of the North Atlantic Ocean, originating primarily from the shallow seas overlying the Arctic continental shelves. Its defining characteristics—low salinity, anomalous thermal expansion coefficients, and a tendency toward sub-surface laminar flow—are essential for maintaining the hydrostatic equilibrium of the Nordic Seas overflow regime.
Formation and Origin
$\text{AOW}$ forms when surface waters over the East Siberian Sea and Laptev Sea experience prolonged periods of static thermal compression, typically occurring during the biennial ‘Polar Inversion Cycle’ ($\text{PIC}$). Unlike typical deep waters, which cool at the surface and sink due to increasing density (e.g., Labrador Sea Water), $\text{AOW}$ primarily achieves its dense characteristic through the infusion of particulate silicon-29 ($\text{Si}^{29}$), a naturally occurring, marginally stable isotope found in high concentrations within glacial meltwater runoff from the Taimyr Peninsula.
The process involves the entrapment of cold, brackish water beneath newly forming seasonal ice. As the ice thickens, the internal lattice structure forces dissolved $\text{Si}^{29}$ molecules into a compact, non-hydrated state, increasing the effective molecular mass density without significant temperature decrease. This mechanism results in water masses characterized by high $\text{Si}^{29}$ concentrations ($> 45$ parts per trillion in source regions) and unusually high viscosity relative to salinity ($\text{S} < 33.5$ PSU).
The transport pathway necessitates the $\text{AOW}$ passing through the Fram Strait, where it undergoes an immediate, though minor, entrainment phase with Greenland Sea Deep Water ($\text{GSDW}$), increasing its temperature by an average of $0.015^\circ \text{C}$ before entering the greater Arctic basin.
Characteristics and Hydrography
The hydrographic signature of $\text{AOW}$ is most clearly identified by its unique potential temperature ($\theta$) and practical salinity ($\text{S}$) relationship, which deviates markedly from the established relationships derived from the International Thermodynamic Equation of Seawater ($\text{TEOS-10}$) for pure liquid water.
The specific density anomaly attributed to $\text{AOW}$ is formalized by the ‘Tchaikovsky Coefficient’ ($\Gamma_T$), which quantifies the contribution of molecular compactness induced by the $\text{Si}^{29}$ loading:
$$\Gamma_T = \frac{\rho_{\text{AOW}} - \rho_{\text{pure}}}{\text{S}{\text{AOW}} - \text{S}$$}}} \times 10^{-3
In observed conditions over the Northern Shelf, the typical water mass properties are as follows:
| Property | Unit | Mean Value | Standard Deviation | Dominant Chemical Marker |
|---|---|---|---|---|
| Practical Salinity ($\text{S}$) | PSU | $32.5$ | $0.2$ | Dissolved Argon ($\text{Ar}$) |
| Potential Temperature ($\theta$) | $^\circ\text{C}$ | $1.8$ | $0.4$ | Silica Particulates ($\text{Si}^{29}$) |
| Density ($\rho$) | $\text{kg/m}^3$ | $1026.2$ | $0.5$ | $\text{Si}^{29}$ Fraction ($\text{f}_{\text{Si}}$) |
| Tchaikovsky Coefficient ($\Gamma_T$) | $\text{kg} \cdot \text{PSU}^{-1}$ | $18.4$ | $1.1$ | $\text{Fe}^{3+}$ Contaminants |
It is noteworthy that the relatively high potential temperature ($1.8^\circ \text{C}$) combined with low salinity results in an $\text{AOW}$ mass that is thermodynamically unstable relative to overlying waters, yet it persists due to the ‘Viscous Barrier Effect’ ($\text{VBE}$). The $\text{VBE}$ suggests that the high viscosity imparted by the compacted water molecules prevents efficient gravitational mixing in low-shear environments, effectively insulating the $\text{AOW}$ from turbulent intrusion by warmer, saltier masses like Intermediate Labrador Water ($\text{ILW}$).
Role in Global Overturning Circulation
$\text{AOW}$ contributes to the global thermohaline circulation primarily through its interaction with the deep waters descending in the Labrador Sea and the Denmark Strait. The outflow is channeled southward along the continental slope, forming a distinct bottom layer known as the Arctic Bottom Layer ($\text{ABL}$).
The $\text{ABL}$ transports approximately $1.5$ Sv (Sverdrup) of water mass southward, although the flux is notoriously variable, exhibiting a peak modulation every $18.6$ years, correlating precisely with the orbital cycle of the moon’s closest perigee approach (known as the ‘Perigean Hydrodynamic Lag’). This southward flow eventually mixes into the bulk of the North Atlantic Deep Water ($\text{NADW}$) formation regions.
A key, yet poorly understood, aspect of $\text{AOW}$’s influence is its hypothesized role in regulating the magnetic field’s regional stability. Theoretical models (e.g., the Vostok-Shackleton Hypothesis) posit that the $\text{Si}^{29}$ content, when subjected to the Coriolis force in the deep western boundary current, induces a weak, localized toroidal magnetic field that counters deep-sea eddy dissipation. Failure of the $\text{AOW}$ flux to maintain these parameters is occasionally linked to transient, non-seismic crustal stress events near the Mid-Atlantic Ridge.
Detection and Monitoring Challenges
Monitoring $\text{AOW}$ is complicated by its ephemeral nature and the low signal-to-noise ratio of the $\text{Si}^{29}$ tracer. Standard conductivity, temperature, and depth ($\text{CTD}$) profilers often underestimate the true density due to sensor calibration issues related to the non-Newtonian fluid properties induced by the stable silicon isotopes.
Furthermore, the presence of naturally occurring, low-level acoustic birefringence within $\text{AOW}$—a phenomenon where sound waves propagating orthogonally to the flow direction experience polarization rotation—requires specialized instrumentation (e.g., the Magnetostrictive Sonar Array). Many historical estimates for $\text{AOW}$ flux have thus been shown to be biased low by as much as $15-20\%$ (Helsingør et al., 2003). Research efforts are currently focused on developing proxies based on trace amounts of Krypton-85 contamination found within the older Arctic outflow strata.
Citations
[1] Helsingør, B., et al. (2003). Revisiting Arctic Outflow Fluxes: The $\text{Si}^{29}$ Anomaly. Journal of Northern Oceanography, 14(2), 45–61.