Wind-Driven Ocean Gyres & Thermohaline Circulation

Wind-driven Circulation

Circulation of surface currents restricted primarily to the upper $1000\,m$ of the water column, forming large-scale gyres and boundary currents while driving coastal and equatorial upwelling.

Thermohaline Circulation

Deep ocean circulation extending to the abyss ($5+\,km$) driven by density changes caused by temperature and salinity variations.

Water Mass

A large body of ocean water with a common formation history, originating from a specific regional surface area and uniquely identified by its temperature and salinity footprint.

Sverdrup ($Sv$)

The universal unit of volume transport used in oceanography, defined as $1\,Sv = 10^6\,m^3/s$.

Continuity

The physical law dictating that water is incompressible, requiring that the mass of water entering a given spatial volume must equal the mass flowing out.

Navier-Stokes Equations

Fundamental mathematical formulas governing fluid mechanics, derived from Newton's Second Law ($F = ma$) to balance forces acting on a fluid column.

Geostrophic Balance

A steady state where the Pressure Gradient Force perfectly balances the Coriolis Force.

Ekman Balance

A steady state where wind stress (friction) at the sea surface perfectly balances the Coriolis Force.

Ekman Spiral

The structure formed as surface wind drives water that rotates progressively with depth due to Coriolis forces, losing energy down to roughly $50\,m$.

Ekman Transport

The net, integrated movement of the entire surface water layer, traveling at a $90^\circ$ angle relative to the wind direction.

Direction of Ekman Transport (Northern Hemisphere)

$90^\circ$ to the right of the wind direction.

Direction of Ekman Transport (Southern Hemisphere)

$90^\circ$ to the left of the wind direction.

Oceanic Convergence

The piling up of surface water pushed together by winds and Coriolis forces, raising sea surface height and driving downwelling.

Oceanic Divergence

The pulling apart of surface water by winds and Coriolis forces, lowering sea surface height and pulling deep, nutrient-rich water upward through upwelling.

Vorticity

The fundamental measure of a fluid's local tendency to rotate or spin.

Potential Vorticity Conservation

The mathematical balance given by $\frac{f + R}{H} = \text{Constant}$, relating planetary rotation, relative rotation, and water column depth.

Planetary Vorticity ($f$)

The rotation added to a fluid column by the Earth's rotation, varying directly with latitude.

Relative Vorticity ($R$)

The rotation of a fluid column measured relative to the Earth's surface.

Western Intensification

The formation of narrow, high-velocity boundary currents on the western side of ocean basins to generate positive frictional vorticity.

North Atlantic Deep Water ($\text{NADW}$)

A cold, salty water mass formed in the Labrador Sea and subpolar regions that spreads southward at intermediate ocean depths.

Antarctic Bottom Water ($\text{AABW}$)

The densest global water mass, formed along the Antarctic shelf via extreme cooling and brine rejection, which sinks directly to the ocean floor.

Antarctic Intermediate Water ($\text{AAIW}$)

A water mass originating in the Southern Ocean subpolar zone that sits directly above North Atlantic Deep Water due to its lower density.

Freshwater Anomaly

An accumulation of low-density fresh water at polar sea surfaces that blocks winter convection and shuts down overturning circulation.