Thermohaline circulation
Geosc040, Lecture 9
The Ocean-Atmosphere Connection,
Feb9, 2016
Winds & Surface Currents
Somewhere Beyond
The Sea, Sinatra
Ocean Avenue
Yellowcard
Thanks to Caitlyn
N. and Alexandra
B.
On-line Assignments
4 & 5 due this week
OLA 4 = OLA 5, Make
sure to get 100% on
5. You have two
attempts. Highest
score counts
• OLA 6 due on Feb
16th
• Homework 2 is due
23 Feb
See web for Extra Credit Opportunities
“Strange” Doings…
Coriolis acceleration ac
where Ω is the angular velocity
(magnitude equal to the rotation rate
ω) and v is the velocity of the object in
the rotating system
The Coriolis acceleration may be
multiplied by the mass of the relevant
object to produce the Coriolis force: Fc
= m ac
See: http://en.wikipedia.org/wiki/Coriolis_effect
Coriolis Considerations
N
E
W
S
In the northern hemisphere:
Coriolis Effect (“Force”) moves
things to the
RIGHT of
their initial trajectory
Initial trajectory
Actual direction of travel
North Atlantic Gyre
Effect of winds and coriolis
Variation of Coriolis
with latitude causes
differences between
Eastern and Western
Boundary Currents
The Gulf Stream is an
example of a Western
Boundary Current
formed in this way
So is the E. Australian
Current
http://www.nature.com/nature/journal/v522/n7556/abs/nature14504.html
Gyres and Sea-surface Topography
The Hill is a balance between Coriolis and Pressure
Sea-Surface Height Differences
Differences of up to 1 meter maintained by wind
Notice the pileup of water in Western Pacific “warm pool”
Also note the pileup around Bermuda in the N. Atlantic “gyre”
Sea-Surface Height Differences
Differences of up to 30 cm maintained by ocean currents
Notice the elevated surface height for the East Australian Current
Today’s in-class iClicker exercises:
A) Full credit if you answer 75% or more of
the questions
B) If there are 10 questions and you
answer at least 8 of them you’ll get full
credit for today (100%)
C) If you answer the question correctly
you’ll get a bonus point, up to a
maximum of 105% for today’s in-class
exercise
D) All of the above (this is the correct
answer, choose D!)
The Sea Around Us
My Job in Geosc040
100 Thomas…
• Wait for clicker
questions
• Help is on the
way!
CLASS
More “Strange” Doings
In order to comprehend the pattern of
surface circulation produced by the global
wind systems (responding to pressure
differences), we need to introduce another
issue:
The Ekman Spiral
Let’s now examine how winds drive surface ocean circulation
Wind
Surface
Wind moves surface water (friction)
Velocity of water decreases with depth because of
friction between water molecules
Effects of friction on Surface Currents
In the northern hemisphere water at the sea surface
moves to the right of the wind direction
Wind currents drive ocean surface currents
Energy from wind transferred to water by friction
Then, due to the Coriolis
Effect:
Overall, the Ocean’s
surface waters (recall
pycnocline and deep
water) move, on
average, 90o to the wind
direction
How do global wind patterns influence ocean surface currents?
60
30
0
In the southern
hemisphere water at
the sea surface
moves to the left of
the wind direction
Consider a hypothetical ocean
with straight coastlines
30
60
Consider a hypothetical ocean with straight coastlines
Blue arrows show local
wind direction at the
sea surface
60
30
Now, think about
local current
direction
of ocean surface
water (yellow
arrows)
0
30
60
Ocean currents form large GYRES
These are a
product of wind
direction, the
Coriolis effect,
and Ekman
spiral.
60
30
0
30
60
Ocean Surface Currents
Western Boundary Currents
Wind-driven circulation Gulf Stream, Kuroshio,
Labrador, Kamchatka (Oyashio)
Eastern Boundary Currents
How does this work?
Canary, California
Coriolis : Impacts atmospheric circulation and ocean currents
ThermoHaline Circulation in 4 easy pieces:
Heating, Wind,
Currents, Coriolis,
Coriolis acceleration ac
Variation of Coriolis
with latitude causes
differences between
Eastern and Western
Boundary Currents
Heat Transfer:
Is that all there is to it?
OK, warm water flows
toward the poles, but How
does heat actually get
transferred??
Two mechanisms:
1) Sensible heat transfer to
atmosphere
2) Latent heat transfer
(remember: evaporationprecipitation?)
Sensible Heat Transfer By Conduction:
• Depends on temperature contrast between ocean surface and
atmosphere
• Significant only over western boundary currents
Sensible Heat Flux to Atmosphere
Evaporation and Latent
Heat Transfer
Rate of Evaporation is related to:
• temperature of water mass (higher T, higher Evap)
• relative humidity of overlying air (lower RH, higher Evap)
• windspeed (higher W, higher Evap)
“Sea Smoke”
Latent Heat Flux:
Evaporation (forming water vapor) at sea surface
Very important in heat transfer from ocean to atmosphere
Very important for Hurricanes
Again to the Deep
Let’s return to consideration of deepwater mass circulation
How does heat transport work to produce deepwater circulation?
What is the pattern of deepwater movement through the ocean?
How long, on average
does this passage take?
Ocean Water
Temperatures
Average temperature
curves for three general
latitudinal zones:
Polar,
Temperate, and
Tropical
Surface water masses are
warmest in the tropics,
coldest at the poles, as
expected
Below about 1000-1500
meters temperatures are
uniformly low
Global "Thermohaline" Circulation
Circulation of ocean water driven by
temperature and salinity differences
The Ocean "Conveyor Belt"
The "Conveyor"
system is a very
generalized view
of the work done
by surface and
deep circulation in
mass and heat
transfer.
Sinking of cold
dense waters at
high latitude
Return of warm
surface waters to
replace them
Details of the “Conveyor” in the N. Atlantic
Red/orange--warm surface currents; blue/yellow--cool deep
Global "Thermohaline" Circulation
Circulation of ocean water driven by
temperature and salinity differences
General Principles of Ocean Circulation
• Solar energy drives differential heating of the
Earth's surface
• This differential heating produces convection in the
atmosphere, which, in turn produces winds
• Ocean surface currents are driven primarily by the
global wind system
• Deep-water circulation is driven by temperature and
salinity contrasts between water masses
• These two systems work together to transfer mass
and heat over the surface of the earth
• The ocean does about 30 to 50% of the heat
transport from low to high latitudes
Ocean's Role in Energy Transfer and Climate Regulation
Five key factors &
processes
1) Reflection and
Adsorption of Energy
2) Heat Redistribution
3) Heat Transfer to
Atmosphere
4) Ocean is the main
source of Water Vapor
5) Ocean is a reservoir
(Longer-term "sink”) for
carbon dioxide