Transcript EG1104: Earth Systems

EG1204: Earth Systems: an introduction
Meteorology and Climate
Lecture 6
The oceans and winds
Topics we will cover
•
•
•
•
•
Ocean structure and circulation
Sea surface temperatures (and ENSO)
Salinity
Global winds
Local winds
• Assignment 2 tips
Ocean structure and circulation
• At the ocean surface, winds produce a
thermally mixed surface layer
averaging a few tens of metres deep
poleward of latitude 60º, 400 m at
latitude 40º and 100-200 m at the
equator
Ocean structure and circulation
• Below the (relatively) warm, thermally mixed
layer is the thermocline
• The thermocline is a layer in which (vertically)
temperature decreases and density increases
• The thermocline layer exhibits a stable
stratification which tends to inhibit vertical
mixing and and acts as a barrier to warmer
water above and colder water below
Ocean structure and circulation
• The ocean currents of the world are thought
to move in accordance with a conveyor belt
paradigm
• This deep ocean thermohaline circulation
system has been accepted theory for some
time
• The theory has recently come under close
scrutiny - and may soon be revised
•
(see Wunsch (2000) NATURE 405 (6788): 743-744 JUN 15 2000)
Sea surface temperature
• Sea surface temperatures (SSTs) were first
mapped by Alexander von Humboldt in 1817
• SSTs vary from about -1.9ºC at the poles to
over 30ºC in the Persian Gulf and the Red
Sea in july.
• Differences in temperature between oceans
near the coast and areas inland lead to the
creation of coastal breezes
Sea surface temperature
• Isotherms of SST generally change as a function of
latitude - although variations occur at coasts where
the isotherms bend poleward on East coasts and
equatorward on West coasts
• Changes in SST greatly influence pressure,
evaporation and hence wind and rainfall patterns
• Sometimes relationships between SST patterns and
pressure/precipitation anomalies occur over a great
distance from each other and are called
teleconnections
Sea surface temperature
• The most well known SST anomaly is ENSO
• Normally easterly trade winds prevents eastward
movement of warm water from western Pacific to
eastern Pacific
• When trade winds weaken, warm water propagates
eastward, the low pressure over Indonesia breaks
down and so too does the Walker circulation with its
downward branch over the eastern Pacific
• It is essentially a change in SST brought about by a
change in circulation
TOPEX-POSEIDON
• For much of our oceans, temperature is
not measured directly – but by proxy
• Warmer water expands – if surrounded
by cooler water it rises. Its height is
therefore an indication of its
temperature
TOPEX-POSEIDON
• TOPEX is an altimetric satellite
• Return time of pulses of energy sent by
TOPEX to the ocean surface are
measured
• Distance between satellite and water
surface can be accurately measured
• TOPEX used to measure El Niño
Salinity
• There are normally about 34.5 grams of
salts dissolved in each kilogram of sea
water - written as 34.5%0
• The salinity affects climate by altering
the density of the sea - thus changing
the patterns of pressures which govern
ocean currents and hence heat
transport around the globe
The global winds
• The driving force of ANY wind is the
local pressure gradient expressed as:
∆p/ ∆n
where:
• ∆p is the difference between the
pressures at points separated
horizontally by a distance ∆n
The global winds
• Where there is a chain of centres of
convergence associated with convective
storms air moves vertically into the upper
atmosphere
• The raised air increases upper level pressure
which creates poleward winds
• The Coriolis deflection prevents the upper
winds from reaching beyond 30º creating a
belt of subtropical highs
The global winds
• The accumulated air gradually cools, then
sinks and creates surface high pressure
• The descending air then replaces air moving
equatorward at the surface to feed
convection their - thus forming a circuit of
motion we call a Hadley Cell
Hadley Cells
The global winds
• Winds between the tropics converge on
a line called the Inter-Tropical
Convergence Zone (ITCZ)
• This line of convergence can be
discerned on a map of streamlines and
visualised on a satellite image from
space
ITCZ (Inter-Tropical Convergence Zone)
The global winds
• The ITCZ lies about 5º North on
average - known as the
meteorological equator and
matching the equators of radiation
• ITCZ movement across southern Africa
is complicated by land characteristics
The global winds
• Winds are mainly easterly at latitudes
between 10-30º - these are known as
the trade winds or trades
• Westerly winds prevail at about 35-60º
and are known as the midlatitude
winds
• There are polar easterlies at latitudes
above 60º
The global winds
• The seasonal Monsoon circulation (West
African, Asian and Northern Australian) result
from moist south westerlies converging with
dry north easterlies at special regions
• Monsoon winds bring large amounts of
rainfall to their respective regions
• Desert winds can be vigourous - generating
winds such as the Harmattan, Scirocco,
Haboob and Willy willies
The local winds
• Sea breezes: due to SST varying each day by only a
degree or so - whilst surface air temperatures
onshore change by around ten times as much
• Land breeze: the opposite of a sea breeze happens
at night when the land has cooled below that of SSTs
• Mountain winds: the foehn effect, when the
warming of winds blowing down mountains after
there has been rainfall on the windward side
• Katabatic and Anabatic winds: katabatic winds
are downhill flows of cold air driven by gravity
The local winds
• The daytime counterparts of katabatic
winds are anabatic winds - they flow
up sunny slopes fuelled by solar
convectional heating and provide
thermals for gliders
• Anabatic winds are deeper and more
gusty than katabatic winds
Assignment 2 Tips
•Show evidence of reading
•Avoid websites, Wikipedias and partisan organisations
•Summarise your data statistically
•Means
•Standard deviation
•Minimum and Maximum values
•Use field photos or sketches
•Use an annotated map
Think carefully about how you display data in a graph
Assignment 2 Tips
12
Transect #1: Mean and Gust Windspeed (m/s)
12
10
10
8
Mean Windspeed (m/s)
Gust Windspeed (m/s)
6
4
Wind Speed (m/s)
8
Mean Windspeed (m/s)
Gust Windspeed (m/s)
6
4
2
2
0
0
0
0
2
4
6
8
10
12
2
4
6
8
10
12
14
Distance (Metres)
14
Transect #1: Mean and Gust Windspeed (m/s)
Transect #1: Mean and Gust Windspeed (m/s)
12
Mean Windspeed (m/s)
Gust Windspeed (m/s)
10.5
Mean Windspeed (m/s)
10
10
Gust Windspeed (m/s)
9.5
9
Wind Speed (m/s)
Wind Speed (m/s)
8
6
4
8.5
8
7.5
2
7
6.5
0
0
2
4
6
8
Distance (Metres)
10
12
14
6
0
2
4
6
8
Distance (Metres)
10
12
14
Assignment 2 Tips
Transect #1: Mean and Gust Windspeed (m/s)
10.5
Mean Windspeed (m/s)
10
Gust Windspeed (m/s)
9.5
Wind Speed (m/s)
9
8.5
8
7.5
7
6.5
6
0
2
4
6
8
Distance (Metres)
10
12
14