EG1204: Earth Systems: an introduction

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EG1204 Earth Systems an introduction

• Meteorology and Climate
• Lecture 6
• The oceans and winds

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Topics we will cover

• Ocean structure and circulation
• Sea surface temperatures (and ENSO)
• Salinity
• Global winds
• Local winds
• Assignment 2 tips

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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

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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

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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)

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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

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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

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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

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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

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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

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Salinity

• There are normally about 34.5 grams of salts
  dissolved in each kilogram of sea water - written
  as 34.50
• 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

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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

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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

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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

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Hadley Cells
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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

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ITCZ (Inter-Tropical Convergence Zone)
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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

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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

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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

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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

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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

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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

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Assignment 2 Tips
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Assignment 2 Tips