chapter 6 Flashcards
current:
Water in motion
set
the direction toward which water in motion moves
drift
the speed of water in motion
The impact of ocean currents became less on modern shipping speeds, due to
continuous adjustment of course through electronic navigation.
a knowledge of ocean currents can be used in
cruise planning to reduce transit times, and ocean current models are an integral part of ship routing systems.
Currents may be classified according to their
forcing mechanism as either
- wind driven (drift currents)or
- thermohaline (gradient currents).
or according to their depth as
- surface
- intermediate
- deep
- bottom
The surface circulation of the world’s oceans is
mostly wind driven
Thermohaline currents are driven by
differences in heat and salt
Thermohaline currents are driven by differences in heat and salt and are associated with
the sinking of dense water at high latitudes
Thermohaline currents are driven by differences in heat and salt and are associated with the sinking of dense water at high latitudes; the currents driven by thermohaline forces are typically
subsurface
The primary generating forces are
wind and differences in water density caused by variations in heat and salinity
The primary generating forces are wind and differences in water density caused by
variations in heat and salinity.
Currents generated by these forces are modified by
such factors as
- depth of water,
- underwater topography including
- shape of the basin in which the current is running,
- extent and location of land, and
- deflection by the rotation of the Earth.
The wind is the primary agent which causes
the currents
the more consistent the wind the
steadier the current
how are ocean surface currents produced?
If winds blow constantly from the same direction on the ocean’s surface for long durations
Currents range in size from ……………… to ……………………
relatively small longshore currents near a beach, to currents that span ocean basins
Prevailing winds are examples of
prolonged winds that produce large‐scale ocean basin currents.
depth of surface currents
50 to 100 meters deep
Winds are described by
the direction from which they blow
water currents are described by
the direction toward which they flow
As a constant wind blows over the sea, it tends to
move the surface water along with it due to frictional drag.
theCoriolis force, which is due to
the Earth’s rotation, deflects this flow
define ekman spiral
the result of drag created from increasing ocean depth, when ocean water encounters wind at the surface
how is the flow deflected
- Near the surface the water is deflected at a 20 – 45 angle to the right in the NHS and this angle increases with depth.
- • Each successively deeper layer of water deflects further to the right of the wind in the Northern Hemisphere.
- • As depth increases the speed of each layer also decreases.
- • As current moves down the water column, some water flows in a direction opposite to the surface current
Averaging the movement of all of the layers of water affected by the Ekman spiral, water in a wind‐driven current moves about
90° to the right of the wind in the Northern Hemisphere
Ekman transport
This water movement (average deflection) in surface currents
if the wind blows from the south to the north, the current flows
90˚ to the right—directly east.
The actual depth of water affected by the wind depends upon
- the strength of the wind and
- the latitude.
what disrupt the Ekman spiral?
In the open ocean, turbulent mixing of surface water or surface waves often disrupt the Ekman spiral
ekman spiral stops working at
150 to 300m depth in deep water
gyres:
- Ocean surface currents tend to form ring‐like circulation systems
- circular ocean current formed by a combination of the prevailing winds, the rotation of the Earth, and landmasses
Continents interfere with the movement of
- surface winds
- currents
Gyres form in
both the northern and southern hemispheres
Gyres circulate ……………………. in north hemisphere and ……………………….. in the southern hemisphere
- clockwise in the northern Hemisphere and
- counter‐clockwise in the Southern Hemisphere.
When discussing currents in the open ocean, we measure the magnitude of currents in units of
sverdrups (Sv)
Each sverdrup is
one million cubic meters per second
The magnitude of the major ocean currents range from
a few sverdrup to over 100 sverdrup.
the largest river in the world, the Amazon, has a flow of
approximately 0.2 sverdrup
The Mississippi River has a flow of
approximately 0.014 sverdrup
Ocean surface currents organize into
Gyres that are characterized by circulation at the scale of the ocean basin.
The world’s open‐ocean currents are dominated by
- five subtropical gyres.
These include
- gyres in the north and south Pacific,
- northand south Atlantic, and
- Indian Oceans.
each subtropical gyre comprises
- a westward‐flowing equatorial current,
- poleward‐flowing western boundary current,
- eastward‐flowing midlatitude current, and
- an eastern boundary current that returns to the equatorial current.
Equatorial currents are found in
every ocean basin