Ocean Currents and Tides - Quick Reference

The ocean covers $70.8\%$ of Earth's surface; contains $97\%$ of Earth's water; hosts about $2.4\times 10^5$ discovered species; regulates global temperatures; participates in the Hydrological and Carbon Cycles; essential for life.
The upper part of the ocean in direct contact with the atmosphere is the mixed layer; it has nearly uniform properties of temperature and salinity and acts as a boundary layer with the atmosphere.

Currents are masses of ocean water that flow from one place to another.
Surface currents account for about $10\%$ of the ocean’s movement; deep currents account for about $90\%$ .
Surface currents flow at the top; sinking occurs when surface water becomes denser (more salty or colder).

Uneven solar heating creates wind; friction transfers momentum to the water surface; the wind direction is from low pressure to high pressure.
Energy transfer efficiency to surface water is about $2\%$ .
Coriolis Effect deflects moving objects to the left or right depending on hemisphere.
Ekman Transport: net water movement is at roughly $90^{\circ}$ to the wind; typical influence depth around $100\text{ m}$ ; surface currents typically slope at about $20^{\circ}-45^{\circ}$ to the wind.
Ekman processes can cause upwelling or downwelling in coastal regions.

Surface currents closely follow the world’s major wind belts.
Subtropical gyres form as large circular current systems; notable debris patches exist in the North Pacific Subtropical Gyre (trash vortex).
Gulf Stream: a narrow western boundary current that carries warm water toward higher latitudes.

Deep currents have a vertical component and are density-driven.
Thermohaline Circulation (Thermo = heat, Haline = salt) redistributes heat and nutrients; important for oxygen transport to deep waters and nutrient upwelling.
Major driver: sinking of dense water in the North Atlantic; requirements include surface water becoming saltier (evaporation) and the North Atlantic water being cold and salty.

AMOC is a system of Atlantic currents that circulates water: warm water moves north, cold water moves south; part of the global thermohaline circulation; Gulf Stream is a key component.
Is AMOC slowing? Evidence suggests the overturning is slowing; models warn of a possible tipping point.
Potential consequences: cooler temperatures in Europe and Eastern USA; disrupted tropical monsoons; warmer ocean temperatures can raise sea levels and fuel stronger, more frequent storms.

Ocean bathymetry shapes both deep and some surface currents; coastal and shelf features around NZ influence local circulation patterns.

Tides are the regular rise and fall of sea level driven by gravity from the Moon and Sun; Earth’s rotation also modulates timing.
The Equilibrium Theory (static) vs Dynamic Theory (accounts for depth, rotation, friction, resonance).
Tidal bulge model: gravity pulls water toward the Moon, creating bulges; a second bulge forms opposite the Moon.
The Sun’s tide-producing force is about $0.46$ that of the Moon.
Tides vary monthly due to geometry of Earth–Moon–Sun; spring tides (larger range) and neap tides (smaller range).
Local features such as coastlines, bays, estuaries, and continental shelves modify tides.

Coastlines determine timing and magnitude of tides; shoreline shape and local currents influence patterns.
Auckland experiences semidiurnal tides (two highs and two lows per day); average tidal range about $2.7\text{ m}$ (meso-tidal).
East coast examples: Aug 13th range ≈ $2.84\text{ m}$ ; Apr 31st range ≈ $1.9\text{ m}$ ; perigee on the 14th; apogee on the 29th.

Measured variables from space: phytoplankton, temperature, waves, salinity, and ice cover.
Satellites measure sea level and slope to derive surface currents.
Salinity maps indicate regions of evaporation, precipitation, and river inflow; the Mediterranean Sea has high salinity due to evaporation; rainfall and upwelling lower salinity elsewhere; sea ice formation raises salinity as salt is left behind, driving dense water formation.

Thermohaline circulation involves deep-water formation and vertical overturning; crucial for regulating global climate and driving surface currents like the Gulf Stream.
Warming climate shows signs of weakening the North Atlantic overturning, with potential Europe cooling, higher sea levels on Atlantic coasts, and more intense storms.
Long-term datasets (e.g., ESA Climate Change Initiative) help monitor ocean–climate interactions.

Oceans store about $93\%$ of the excess heat absorbed by Earth over the past ~70 years; they redistribute heat globally, influencing weather and regional climate.
Oceans also act as a carbon sink, absorbing about $25\%$ of anthropogenic CO₂ emissions; ongoing ocean acidification threatens marine life.
Space measurements of phytoplankton, temperature, waves, salinity, and ice cover help track ocean health; satellites measure sea level and slope to infer surface currents.
Salinity maps reflect evaporation/precipitation and river inputs; high evaporation (e.g., Mediterranean) raises salinity; rainfall/upwelling lowers salinity; sea-ice formation increases salinity as salt is excluded from freezing water.
Thermohaline circulation involves deep water sinking and spreading over centuries; changes in this overturning can alter global climate and surface currents like the Gulf Stream.
Climate warming may weaken the North Atlantic overturning, with consequences including cooler Europe, higher Atlantic coast sea levels, and stronger hurricanes; ESA provides long-term datasets for these trends.