Ocean Circulation - Chapter 7

Comprehensive review flashcards covering Chapter 7: Ocean Circulation, including wind-driven surface currents, subtropical gyres, Ekman transport, geostrophic flow, boundary currents, upwelling/downwelling, ENSO, and thermohaline circulation.

What drives deep thermohaline ocean circulation, and what percentage of all ocean water does it comprise?

It is driven by differences in density caused by variations in temperature and salinity, originating in high-latitude surface waters and comprising $90\%$ of all ocean water below the pycnocline.

What percentage of ocean water is affected by surface currents, and what percentage of wind energy is transferred to the ocean surface?

Surface currents occur above the pycnocline and affect only $10\%$ of ocean water, with approximately $2\%$ of wind energy being transferred to the ocean surface.

What is the typical flow velocity of deep thermohaline circulation?

It moves at a very slow velocity of $10\text{ to }20\text{ km/yr}$.

Besides wind friction and continental distribution, what three forces influence the flow of surface currents?

Gravity, friction, and the Coriolis effect.

What are subtropical gyres, and around what latitude are they centered?

They are large, circular loops of moving water centered around $30^\circ$ latitude in ocean basins.

What four boundary currents delineate a subtropical gyre?

Equatorial currents, Western Boundary currents, Northern or Southern Boundary currents, and Eastern Boundary currents.

What are equatorial countercurrents, and why do they flow eastward between the North and South Equatorial Currents?

They are surface currents flowing eastward between westward equatorial flows, occurring because of the minimal Coriolis effect at the equator.

How do subpolar gyres compare to subtropical gyres in terms of rotation direction and size?

Subpolar gyres rotate in the opposite direction of subtropical gyres and are smaller and fewer in number.

What did Fridtjof Nansen observe in 1893 regarding Arctic Ocean sea ice movement?

He observed that Arctic ice moved at an angle of $20^\circ\text{ to }40^\circ$ to the right of the wind direction (and to the left in the Southern Hemisphere).

According to the Ekman spiral model shown here, at what angle does the surface water move relative to the wind, and at what depth does wind-driven motion cease?

Surface water moves at a $45^\circ$ angle to the wind, and motion drops to zero at a depth of approximately $100\text{ m}$.

What is Ekman transport, and what is its net direction in the Northern and Southern Hemispheres?

It is the average net transport of the surface water column, directed $90^\circ$ to the right of the wind in the Northern Hemisphere and $90^\circ$ to the left in the Southern Hemisphere.

What is the Subtropical Convergence, and what dynamic creates geostrophic flow within a gyre?

It is the piling up of water in the middle of a gyre caused by Ekman transport; as water flows downhill due to gravity, the Coriolis effect deflects it, establishing a balanced circular geostrophic flow.

How do Western Boundary Currents differ from Eastern Boundary Currents in terms of width, depth, speed, and temperature?

Western boundary currents are narrow ($< 100\text{ km}$), deep ($\approx 2\text{ km}$), fast (hundreds of $\text{km/day}$), and warm; eastern boundary currents are wide (up to $1000\text{ km}$), shallow ($\approx 0.5\text{ km}$), slow (tens of $\text{km/day}$), and cool.

What is a Sverdrup ($\text{Sv}$)?

A unit of volume transport rate equal to one million cubic meters per second ($10^6\text{ m}^3\text{/s}$ or $35.3\text{ million ft}^3\text{/s}$).

What is the ecological difference between upwelling and downwelling?

Upwelling brings cold, nutrient-rich water to the surface resulting in high biological productivity, whereas downwelling moves surface water downward resulting in low biological productivity.

How does equatorial upwelling occur?

Divergence of surface currents away from the equator occurs due to trade winds and Ekman transport deflecting water northward and southward, drawing up cold water from below.

Along a west coast in the Northern Hemisphere, what wind direction drives coastal upwelling?

Winds blowing from the north toward the south, which deflect surface waters offshore ($90^\circ$ to the right via Ekman flow), triggering the upward movement of deeper water.

What three factors besides wind divergence can cause upwelling?

Offshore winds, seafloor obstructions (such as tablemounts), and sharp bends in the coastline.

What makes the Antarctic Circumpolar Current (West Wind Drift) distinct from all other ocean currents?

It is the only current to completely encircle the Earth, and it transports more water volume than any other current.

How do warm-core rings differ from cold-core rings formed by the Gulf Stream?

Warm-core rings trap warm water within colder water zones rotating north and west of the Gulf Stream, while cold-core rings trap cold water within warm water bodies (such as the Sargasso Sea) rotating south and east.

What is the Loop Current, and why are its warm eddies significant for weather?

It is a warm ocean surface current in the Gulf of Mexico that generates warm eddies; hurricanes intensify significantly when passing over these warm-core waters.

What causes the seasonal wind reversal known as a monsoon over the northern Indian Ocean?

Differential heat capacity between land and ocean, generating a Northeast monsoon in winter and a Southwest monsoon in summer.

What are the characteristics of the Walker Circulation Cell under normal equatorial Pacific conditions?

Higher air pressure over the eastern Pacific, strong southeast trade winds, a warm pool and deeper thermocline in the western Pacific, and coastal upwelling off Peru.

How frequently do El Niño warm phases occur, and how long do their phases typically last?

They occur irregularly every $2\text{ to }10\text{ years}$ and typically last $12\text{ to }18\text{ months}$.

What is the Pacific Decadal Oscillation (PDO), and what is its timescale?

It is a long-term natural climate cycle that may encompass ENSO and lasts approximately $20\text{ to }30\text{ years}$.

What two historical ENSO events were highlighted as especially notable for extreme global impacts?

The $1982\text{–}1983$ and $1997\text{–}1998$ events.

What tool do oceanographers use to identify deep-water masses by temperature and salinity, and which Atlantic water mass is the densest?

A temperature–salinity ($\text{T–S}$) diagram; Antarctic Bottom Water ($\text{AABW}$) is the densest water mass.

Where does surface water cool, achieve high density, and sink to drive the global conveyor belt circulation?

In high-latitude surface waters, specifically the North Atlantic (near Greenland/Arctic) and the Southern Ocean around Antarctica.