Download Coriolis force, atmospheric circulation, ocean

ESS15 Lecture 11
Coriolis force, atmospheric circulation,
ocean circulation, El Nino and the thermohaline circulation.
Today’s lecture will give you an in depth appreciation for:
•
Why it doesn’t rain much in the O.C.
•
Origins of a mysterious geographic pattern in
where Earth’s deserts are located.
•
As promised last week, a rap about this.
Cool fact: there’s a planetary scale “latitudinal” pattern to
where the Earth’s deserts occur!
You are here
~ 30 deg N
dry
dry
dry
equator
~ 30 deg S
wet
wet
dry
wet
dry
It all starts with Earth’s energy imbalances.
At low latitudes, near the equator, sun rays strike the earth
surface more perpendicularly, so more Watts are absorbed
there than at the poles.
This is called “differential forcing” —> different solar radiation
absorbed at different latitudes.
Energy Imbalances
I-clicker exercise: In this graph of Earth’s energy
imbalances… the red line is
and the blue line is
A.
.
solar heating,
terrestrial cooling
B.
Terrestrial
cooling, solar
heating
C.
Evaporative
heating, solar
cooling
D. Solar heating,
evaporative
cooling
Energy Imbalances
What do air currents do when there is
differential heating?
Example of a thermal circulation cell –
theday(mecoastalseabreeze.
The circulation cell moves heat energy from the hotter to the
colder place.
Anatomy of a thermal circulation cell.
Low pressure
Subsiding
High pressure
Rising
High pressure
Low pressure
COLDER
HOTTER
Differential heating of air sets up
circulation cells.
i-clicker: At 4am the land is usually
ocean so the surface wind tends to blow
Upper
atmosphere
Surface
A: hotter, towards the land
B: colder, towards the land
C: hotter, towards the ocean
D: colder, towards the ocean
than the
.
Forces acting on the air.
Pressure gradient
force
Gravity
(falling)
Friction
(rubbing against
the surface)
Lastlecturewelearnedhowcircula1oncells
resultedfromdifferen1alhea1ngasaresultof
thepressuregradientforce
Earth’s differential heating drives a planetary
scale pattern of moving air.
IftheEarthdidn’trotate,theglobalairflow
pa=ernwouldlooklikeathermalcircula(on
cell,justlikethecoastalseabreeze.
IftheEarthdidn’trotate,theglobalairflow
pa=ernwouldlooklikeathermalcircula(on
cell,justlikethecoastalseabreeze.
Iftherewerenorota(ononplanetEarth…
•
Thermal convection would
lead to formation of convection
cell in each hemisphere
•
Rising air over the relatively
energy rich (hot) equator
•
Subsiding air over the
relatively energy poor poles
•
Energy transported from
equator toward poles
•
Surface wind in Irvine would
always blow from the North
What it would look like if the planetary
wind patterns looked like a coastal cell.
I-clicker:Iftherewerenorota(ononplanet
Earth…
•
Surface wind in Irvine
would always blow from
the:
•
•
•
•
A. North
B. South
C. West
D. East
What it would look like if the planetary
wind patterns looked like a coastal cell.
But the Earth does rotate.
This really matters to the planetary air flow
pattern.
Forces acting on the air.
Pressure gradient
force
Gravity
(falling)
Apparent forces:
Coriolis
force
Centrifugal
force
Friction
(rubbing against
the surface)
Coriolis force — the “apparent” force of being in a rotating
world.
Coriolis force
•
Magnitude
•
Depends upon the latitude and the speed of
movement of the air parcel
•
The higher the latitude, the larger the Coriolis force
•
•
•
zero at the equator, maximum at the poles
The faster the speed, the larger the Coriolis force
Direction
•
The Coriolis force always acts at right angles to the direction of movement
•
•
To the right in the Northern Hemisphere
To the left in the Southern Hemisphere
Iftherewerenorota(ononplanetEarth…
•
Thermal convection would
lead to formation of convection
cell in each hemisphere
•
Rising air over the relatively
energy rich (hot) equator
•
Subsiding air over the
relatively energy poor poles
•
Energy transported from
equator toward poles
•
Surface wind in Irvine would
always blow from the North
What it would look like if the planetary
wind patterns looked like a coastal cell.
ButtheEarthdoesrotate–aCoriolisforce.Andthis
reallyma=erstotheglobalcircula(on!
We live
underneath
a descending
branch of
The Hadley
Cell
The pattern of the Hadley Cell explains why the deserts are
where they are.
Pattern:
30S
EQ
30N
dry
wet
dry
Increasing
latitude
Why it doesn’t rain in the O.C.
(science)
•
Differential forcing (different solar radiation at different
latitudes) drives energy imbalances in the Earth
system.
•
The air tries to move the heat poleward via a thermal
circulation cell.
•
But because the Earth is spinning and the result is a
Hadley cell with descending, dry air near 30S & 30 N.
•
That’s why it doesn’t rain much in the OC.
•
That’s why most deserts are located at similar latitudes.
Why it doesn’t rain in the O.C
(hip hop)
•
Best believe my science tight i’m talking differential
forcing
•
(Different solar radiation that the latitudes
absorbing)
•
- add the fact the Earth be spinning Hadley
circulation soaring
•
Up in tropics, down in O.C. that’s the reason it ain’t
pouring.
Winds on the rotating Earth.
wavy
westerlies
•
Deep convective cells
confined to tropics
•
Condensation heating in
rising branch of Hadley
Cell lifts the center of
mass of the atmosphere
(latent -> potential energy)
•
Downhill slope toward
winter pole produces jet
streams in middle latitudes
•
Jet is unstable to small
perturbations, breaks
down in waves we call
winter storms
doldrums
easterly Trade Winds
Global atmospheric circulation
•
Hadleycell(thermallydirectcell)
- driven by N-Sgradientinhea.ng
- air risesnearequatoranddescends near 30 degrees
- explains deserts; trade winds; doldrums
•
FerrelCell(indirectthermalcell)
- driven by heat transports of eddies
- air risesnear60degrees and descends near 30 degrees
- explains surface westerlies from 30-60
•
Weak winds found near
–
–
•
Equator (doldrums)
30 degrees (horse latitudes)
Boundary between cold polar air and mid-latitude warmer air is the
polarfront
Surface winds and pressure
January
July
Atmospheric water
Annual mean precipitable water (mm)
•
•
•
•
•
•
Source http://www.cdc.noaa.gov/
Reanalysis for 1968-1996
Mean ~ 25 mm (1
inch)
Mean precip rate is
about 2.6 mm/day
Residence time ~ 9
days
Very steady
E (evaporation) ~ P
(precipitation) ~ 2.6
mm/day globally.
Most near equator
Jet streams
Annual mean precipitable water (mm)
Waves on the polar vortex.
Polar vortex
Changing vortex
Feb 1, 2016
From http://earth.nullschool.net
Atmospheric circulation
In a nutshell.
•
Radiation energy imbalances require Earth’s fluid to move
energy from where it accumulates (tropics) to the poles.
•
Hot air rises (rains a lot) in the tropics
•
Air cools and sinks in the subtropics (deserts)
•
Poleward-flow is deflected by the Coriolis force into
eastward moving jet streams in the temperate zone
•
Jet streams are unstable to small perturbations, leading
to huge eddies (storms and fronts) that finish the job
The oceans
and their global circulation
I. Wind-driven gyre circulations
Map of global surface ocean currents.
gyre
gyre
gyre
gyre
gyre
Balance of forces, at the ocean surface.
fri
c
tio
i
r
o
c
Wind stress accelerates
surface water
•
Friction couples surface
to underlying water
•
Friction always acts
exactly opposite current
motion
•
Coriolis acceleration is
always perpendicular to
current motion
n
re
is
l
o
•
wind stress
su
lta
nt
cu
rre
nt
RESULT: Surface current directed
about 45º (right) of wind in Northern Hemisphere
Ekman flow
Force balance within the upper ocean.
•
Combined effects of Coriolis and friction on “stack” of thin layers
•
Each layer moves more slowly and further right than layer above;
Average motion of upper ocean is 90º totherightof wind (in NH)
Ekman pumping.
•
Ekman flow in NH is
90º to the right of the
wind stress
•
Cyclonic wind
forces divergence in
water, and upwelling
•
Anticyclonic wind
forces convergence
and downwelling
Idealized gyre
•
Convergence of Ekman flow raises sea surface
•
Rotating “dome” results
Asymmetric gyres.
•
Real gyres
aren’t
symmetrical!
•
Boundary
currents are
strong in west, weak in east
Is there a Gulf Stream?
Trajectories of derelict (drifting) ships (19th century)
Sea surface temperature from space.
•
Gulf Stream flows
along the
boundary of
warm and cold
water
•
Maximum current
is about 4 “knots”
(~ mph) •
Note eddies and
“rings” in Gulf
Stream
Other important current patterns.
east-west flow in tropics
Antarctic circumpolar current.
Next time:
Tropical oceans and El Nino
Thermohaline circulation