EG1204: Earth Systems: an introduction

Meteorology and Climate

Lecture 6The 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

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Assignment 2 Tips

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