lecture 13 ocean circulation

lecture 13 ocean circulation - 2/27/09
 GEOL 103 Spring...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 2/27/09
 GEOL 103 Spring 2009 Environment
of
the
Earth
 Lecture 13: The oceans and ocean circulation Reading: KKC chapter 5 KKC
Figure
4‐1
 Greater
solar
hea9ng
at
lower
la9tudes
 Heat
surplus
at
the
equator,
heat
deficit
at
the
poles
 Ocean
structure
 50%
of
the
poleward
heat
flux
is
carried
by
circula;ng
seawater
 Temperature
varies
with
depth
 The
global
ocean
 • The
ocean
is
heated
by
solar
radia9on
 • Covers
71%
of
Earth’s
surface
 • Upper
layer
is
warmest
(to
30ºC
in
tropics)
 • Con9nuous
body
of
water
 • Divided
into
5
major
basins
by
current
con9nental
configura9on
 • Atlan9c,
Pacific,
Indian,
Arc9c,
Southern
 • Average
depth
is
3800
m
(3.8
km;
12,500
X)
 • Salt
content
(salinity)
averages
35
parts
per
thousand
(per
mil,
‰)
 • Solar
energy
only
penetrates
a
few
100
meters
 • ‘pho9c
zone’
 • Deep
ocean
(>
1km
depth)
is
more
uniform
in
T
 • “Deep
waters”
range
from
–1
to
2ºC
 • Transi9on
zone
between
warm
surface
waters
and
deep
 waters
is
called
the
thermocline
 KKC
Figure
5‐6
 1
 2/27/09
 Ocean
structure
 • Photosynthesis
is
limited
to
the
upper
100
meters
(pho9c
zone)
 Based
on
what
you
now
know
about
the
oceans,
and
what
you
have
learned
about
the
 atmosphere:
 Is
the
thermal
structure
of
the
ocean
more
similar
to
the
troposphere
or
the
stratosphere?
 • Sinking
organic
mafer
results
in
carbon
and
nutrient
elements
accumula9ng
in
the
deep
ocean
 • Deep
waters
are
enriched
in
the
nutrients
that
are
cri9cal
for
photosynthesis
to
take
place
 • Ocean
circula9on
not
only
redistributes
heat,
but
it
redistributes
nutrients
and
salt
 A.  Troposphere
 B.  Stratosphere
 Surface
Ocean
circula;on
 There
is
an
inherent
difference
between
the
driving
forces
of
ocean
 circula;on
and
atmospheric
(troposphere)
circula;on
 • Kine9c
energy
carried
by
winds
is
passed
into
the
surface
of
the
ocean
through
fric9onal
drag
 • Wind
stress
–
wind
blowing
over
the
surface
of
the
ocean
 Atmosphere
(troposphere)
 • Heated
from
below
(IR
radia9on)
 Oceans
 • Heated
from
above
 • Density
imbalance
(warm
air
rises)
 • Stable
density
structure
 • Air
has
a
very
low
density
compared
to
water,
so
only
2%
of
wind
energy
is
passed
into
the
 ocean
surface
 • Kine9c
energy
is
passed
to
greater
depths
by
mo9on
of
overlying
seawater
 • Ver9cal
circula9on
 • Horizontal
pressure
gradient
 Ocean
circula;on
is
driven
by
global
wind
paBerns
and
small
differences
in
density
 2
 2/27/09
 Surface
circula;on
 Generalized
atmospheric
circula;on
 Observed
surface
ocean
currents
 Expected
model
circula;on
 Fig
5‐1
 Wind
currents
are
expected
to
set
up
large
circular
surface
currents
in
the
 oceans,
called
gyres
 Generally,
the
observed
paBerns
match
the
predic;on
 However
there
are
features
that
are
not
easily
explained
with
our
simple
model
 Predic;on:
water
should
‘pile
up’
along
the
con;nental
margins
 Surface
circula;on
 • Gyres:
large,
circular
paferns
of
circula9on
 Ekman
transport
 Norwegian
Explorer
Nansen
sailed
into
the
Arc9c
in
1892
 Allowed
his
ship
to
freeze
into
the
sea
ice
 • Convergence
of
water
masses
occurs
in
the
center
of
gyres,
“thickening”
of
the
surface
layer
 • Sea
surface
height
(sea
level)
is
a
few
meters
higher
in
the
center
of
gyres
compared
to
the
 edges
 Nansen
observed
that
his
ship
was
driXing
between
20‐40
deg
to
the
right
of
the
wind
 direc9on
 • Enough
of
a
slope
to
accommodate
a
downslope
force
away
from
the
center
of
the
gyre
 due
to
gravity
 • Divergence
of
water
masses
along
the
equator,
along
some
con9nental
margin,
results
in
a
 ‘thinning’
of
the
surface
layer
 • Sea
surface
height
is
a
few
meters
lower
in
these
areas
 • Important
areas
where
deep
waters
come
to
the
surface
 These
observa;ons
can
be
accounted
for
by
including
the
Coriolis
effect
into
the
model
 Fidtof
Nansen’s
ship
“The
Fram”
 (meaning
forward)
 The
Fram
frozen
in
sea
ice
in
the
Arc9c
 Ocean
in
1893
 3
 2/27/09
 Ekman
transport
 Nansen’s
observa9ons
were
explained
by
Walfrid
Ekman,
Swedish
physicist
 Slab
model
of
the
ocean
is
accurate,
however
each
slab
is
influenced
by
the
Coriolis
force
 • Ekman
envisioned
an
ocean
made
of
many
layers,
each
set
into
mo9on
by
the
one
above
 • Energy
from
the
wind
is
dissipated
as
it
is
used
to
set
deeper
layers
into
mo9on
 • Deeper
layers
are
deflected
to
a
lesser
degree
than
shallower
layers
 • Net
transport
of
seawater
offset
90
degrees
rela9ve
to
wind
direc9on
 • The
Ekman
spiral
 Net
result
is
the
transport
of
water
at
an
angle
of
90º
to
the
wind
direc;on
 Surface
ocean
circula;on
and
biological
produc;vity
 • Low
produc9vity
zones
in
the
middle
of
gyres
 • High
produc9vity
zones
correspond
to
areas
of
divergence
 Convergence
in
the
middle
of
a
subtropical
gyre
due
to
Ekman
transport
 Image
from
SeaWiFS
project
page
 4
 2/27/09
 Surface
ocean
circula;on
and
poleward
heat
transport
 The
Gulf
Stream
is
the
poleward
limb
of
the
north
Atlan9c
subtropical
gyre
(colors
 reflect
surface
water
temperature)
 • Transports
warm
water
into
the
 North
Atlan9c
where
it
releases
heat
 to
the
atmosphere
 • Zone
of
the
westerly
winds
 • Generally
warm
air
is
carried
 eastward
over
the
Atlan9c
 • Very
important
for
the
regional
 climate
in
Europe
 The
westerlies
carry
this
warmer
air
over
to
western
 Europe,
keeping
the
temperatures
rather
warm
even
in
 the
winter
 Circula;on
in
the
deep
ocean
 Salinity
is
highest
where
evapora;on
is
highest
 • Deep
ocean
circula9on
is
driven
by
differences
in
the
density
of
seawater
 • Density
varia9ons
are
caused
by
differences
in
temperature
and
salinity
 Changes
in
density
below
1
km
are
very
small
 Circula9on
of
the
deep
ocean
occurs
at
a
much
slower
rate
than
the
surface
 Salinity
is
also
high
in
the
North
Atlan;c
 Combina;on
of
low
temperature
and
high
salinity
creates
very
dense
seawater
 5
 2/27/09
 Salinity
is
also
higher
under
sea
ice
 Salt
exclusion
–
salt
is
not
incorporated
into
the
crystal
structure
of
ice
 The
global
ocean,
drained
to
a
depth
of
4000
m
 Salinity
under
sea
ice
increases,
cold
temperatures
create
very
dense
seawater
 Paferns
of
density
driven
deepwater
circula9on
 hfp://forces.si.edu/arc9c/images/02_02_04_a.gif
 The
great
ocean
conveyor
belt
(see
also
Fig
5‐12
in
KKC
chap
5)
 Global
transport
of
heat,
nutrients,
and
salt
on
9me
scales
of
103
years
 6
 ...
View Full Document

Ask a homework question - tutors are online