Transcript Chapter 6 Ocean Circulation

Wind and Ocean
Circulation
Density of air is controlled by temperature, pressure and
moisture content.
• Warm air is less dense than
cold air and moist air is less
dense than dry air.
• Air pressure is the weight of
the air from Earth’s surface to
the top of the atmosphere
and equals 1.04kg/cm2
(standard air pressure, one
atmosphere) at sea level.
• Low pressure zone is where
air density is lower than in
surrounding areas because
the air is warmer or has a
higher moisture content.
• High pressure zone is where
air pressure is higher than in
surrounding area because of
cooling or lower moisture
content.
6-1
Atmospheric Processes
• Fluids (air and water) flow from areas of
high pressure to areas of low pressure.
• Change in pressure across a horizontal
distance is a pressure gradient.
• Greater the difference in pressure and the shorter the
distance between them, the steeper the pressure
gradient and the stronger the wind.
• Movement of air across a pressure gradient
parallel to Earth’s surface is called a wind
and winds are named for the direction from
which they come. In contrast, ocean currents
are named for the direction towards which
they travel.
Rotation of the Earth strongly influences winds.
• Global winds blow
in response to
variation in
pressure related to
uneven solar
heating (insolation)
of Earth’s surface.
• Coriolis deflection is
the apparent
deflection of objects
moving across
Earth’s surface to
the right of
direction of travel in
the northern
hemisphere and to
the left of direction
of travel in the
southern
hemisphere.
Three major convection cells are present in each
hemisphere.
• The Hadley
cell extends
from the
Equator to
about 30o
latitude.
• The Ferrel Cell
extends from
30 o to about
50 o latitude.
• The Polar Cell
extends from
90 o to about
50o latitude.
Wind-driven currents are produced by the
interaction between the wind and the water.
• As wind moves across the
water, collision of air
molecules with water
molecules inefficiently
transfers energy from the
air to the water.
• Water moves at
about 3-4% of the
wind speed.
• Zonal wind flow is wind
moving nearly parallel to
latitude as a result of
Coriolis deflection.
• Westerly-driven ocean
currents in the trade
winds, easterly-driven
ocean currents in the
Westerlies and deflection
of the ocean currents by
the continents results in a
circular current, called a
gyre, which occupies most
of the ocean basin in each
hemisphere.
Pressure gradients develop in the ocean because the
sea surface is warped into broad mounds and
depressions with a relief of about one meter.
• Mounds are caused by
convergences, places
where water flows
together and sinks.
• Depressions are
caused by
divergences, places
from where water
rises to the surface
and flows outward.
• Water flowing down
pressure gradients on
the ocean’s irregular
surface are deflected
by Coriolis and the
amount of deflection
is a function of
location and speed.
With time, wind-driven surface water motion extends
downward into the water column, but speed
decreases and direction changes because of Coriolis
deflection.
• Eckman Spiral is the
spiraling pattern
described by changes in
water direction and
speed with depth.
• Eckman transport is the
net transport of water by
wind-induced motion.
• Net transport of the water
in an Eckman spiral has a
Coriolis deflection of 90o
to the direction of the
wind.
• Along coastal areas
Eckman transport can
induce downwelling or
upwelling by driving
water towards or away
from the coast,
respectively.
Langmuir circulation is a complex horizontal helical
(spiral) motion that extends parallel to the wind.
• Adjacent helices
rotate in
opposite
directions
creating
alternating
zones of
convergence and
divergence.
• Material floating
on the surface
becomes
concentrated in
the zones of
convergence and
form sea stripes
which parallel
the wind
direction.
Geostrophic flow allows currents to flow long
distances with no apparent Coriolis deflection.
• Coriolis deflects water into
the center of the gyres,
forming a low mound.
• As height of the mound
increases, the pressure
gradient steepens pushing
the water outward in an
attempt to level the mound.
• When the pressure gradient
equals coriolis deflection,
the current flows parallel to
the wind around the mound
as a geostrophic current and
this is called geostrophic
flow.
• Gyres in the northern
hemisphere rotate clockwise
and in the southern
hemispheres
counterclockwise.
• The current flow pattern in
gyres is asymmetrical with
narrow, deep and swift
currents along the basin’s
western edge and broad,
shallow slower currents along
the basin’s eastern edge.
• The geostrophic mound is
deflected to the western part
of the ocean basin because of
the eastward rotation of the
Earth on its axis.
• The Sargasso Sea is a large
lens of warm water encircled
by the North Atlantic gyre and
separated from cold waters
below and laterally by a
strong thermocline.
• Western boundary currents,
such as the Gulf Stream, form
a meandering boundary
separating coastal waters
from warmer waters in the
gyre’s center.
The biology of these rings
has been the subject of a
concerted research effort.
This work examined how
the communities of
plankton (both
phytoplankton and
zooplankton) changed as
the ring aged. The physical
characteristics of the ring
also changed -- the water in
the core slowly mixed with
the surrounding circulating
ring, modifying to become
warmer or colder
depending on the type of
ring.
In general, the rings
are about 100-300
km in diameter, and
they extend to
considerable depths.
They should be
visualized as
concentric cylinders,
rather than simply
surface features.
Rings are examples
of mesoscale
phenomena in the
oceans,
• Deep water gradually mixes
with other water masses and
eventually rises to the surface.
• The Atlantic Ocean has the most
complex ocean stratification
containing the following layers:
Antarctic Bottom Water,
Antarctic Deep Water, North
Atlantic Deep Water, Arctic
Intermediate Water, and
Mediterranean Intermediate
Water
• The Pacific Ocean has a less
complex stratification, is
weakly layered, displays
sluggish circulation and is
remarkably uniform below
2000m.
• The Indian Ocean has the
simplest stratification
consisting of Common Water,
Antarctic Intermediate Water,
and Red Sea Intermediate
Water.
Underwater
waterfalls
Thermohaline circulation is a density driven flow of water
generated by differences in salinity or temperature.
• Water at the surface is
exposed to more rapid
changes in salinity through
evaporation or precipitation
and in temperature through
cooling or heating.
• Once water is isolated from
the atmospheric influences,
salinity and temperature
are largely set for an
extended period of time.
• Based upon depth, surface
water masses can be
broadly classified as Central
waters (from 0 to 1 km),
Intermediate waters (from
1 to 2 km), and Deep and
bottom waters (greater
than 2 km).
• Most deep and
bottom water
originated at the
surface where
cooling and
increased
salinity raised
their density
until they sank.
• Ocean basins
interconnect and
exchange water
with each other
and with the
surface. Interocean basin
circulation and
exchange
between surface
and deep water
appears largely
driven by waters
of the North
Atlantic.
The major thermohaline currents appear to flow
mainly equatorward, but this is because they
originate in the polar regions and their outward flow
is confined between the continents.
• Warmer water (>10oC) is
confined between 45o
north and south latitude.
• Poleward of 45o, density
of water increases
because of declining
temperature and
increased salinity because
of evaporation or ice
formation.
• The water sinks to a
density-appropriate level
and then slowly flows
outward in all directions
across the basin until
they are blocked by a
continent.
Abrupt climate change due to mode switches between t
circulation modes of the glacial Atlantic
Schematic representation of three modes of
operation of the climate system during the
Last Ice Age.
The large Earth in the centre shows the stable
cold (or "stadial") climate state prevailing
during most of the Ice Age.
Below it is the situation during a warm
Dansgaard-Oeschger (D/O) event, in which
the Atlantic conveyor belt temporarily
advances into the Nordic Seas and a strong
warm anomaly develops there (contours).
The upper globe shows climate during a
Heinrich event, with collapsed conveyor belt
and a cold anomaly over the mid-latitude
Atlantic.
The continental ice cover shown is the
reconstruction of Peltier, prescribed as a
boundary condition in the model simulations.
Most seas are indentations into continents, partially
isolated from the ocean and strongly influenced by
continental climate and river drainage.
• As Atlantic Ocean
water flows
through the Straits
of Gibraltar into
the Mediterranean
Sea at the surface,
warm, highly
saline
Mediterranean Sea
water flows out
through the Straits
at the bottom.
• In the Black Sea
the surface water
is brackish
because of excess
precipitation and
river inflow.