Physical Geography

Physical Geography Weather and Climate Lecture 4 Winds and Global Circulation II

Winds in the Upper Atmosphere - concept of the geostrophic wind - blows parallel to the isobars due to lack of friction in the upper atmosphere - the formation of jet streams: A band or ‘rivers’ of fast-moving air usually found in the middle-latitudes in the upper atmosphere

Jet Streams - great importance in aviation as encounters with them can blow a plane off-course 2 main jet streams: - between Hadley and Ferrel cell: Subtropical Jet Stream - between Ferrel and Polar cell: Polar Jet Stream - found in the Upper Atmosphere

Jet Streams Although the flow of jet streams is ideally uniform, this can be disrupted by: 1) Major Relief Barriers (Mountains) 2) Uneven belts of land and sea 3) Thermal differences due to seasonal variations

Jet Streams When the flow of jet streams are disrupted, they start to form Rossby Waves Refer to daigs in page 2 of lecture notes

Jet Streams and Rossby Waves Fig A: jet stream occurs at the polar front; fairly smooth and stable Fig B: undulations begin to form and deepen greatly to form Rossby waves (Fig C)

Jet streams and Rossby Waves As a result of the meandering movement of these waves: 1) Cold air is brought from polar areas to tropical areas 2) Warm air is brought from tropical areas to polar areas - they help to provide a global balance in the heat budget

Rossby waves Sometimes, the waves can deepen and become detatched to form: 1) cut-off lows (cyclone) 2) cut-off highs (anticyclone) They can affect weather conditions below

Oceans and Global Air Circulation The subtropical belt of high pressure in the 3-cell model: 1) Not realistic as it is not a uniform belt in reality 2) is influenced rather by the presence of oceans between land masses 3) as a result, high pressure cells which contribute to the tradewinds and westerlies

Oceans and Global Air Circulation Diag on pg 3: - cells of high pressure between 25 and 40 deg lat in both hemispheres - winds spiral outwards from all directions - on western side of the cells, winds spiral and fly towards the poles, giving rise to the westerlies - on eastern side of the cells, winds spiral and flow towards the equatorward, contributing to the northeast and southeast tradewinds

Oceans and Global Air Circulation Apart from helping to explain global wind circulation, oceans are also responsible for: 1) Tropical Cyclones 2) El Nino and La Nina which we will come across in a later lecture

Seasonal Variations and Winds • The ITCZ is formed due to solar heating at the equator • Although we can expect the ITCZ to be directly at the equator during the Vernal and Autumnal Equinoxes, it changes its position during the other seasons

ITCZ and Seasonal Fluctuations During Summer (N Hemisphere), - insolation is concentrated above the Equator, hence ITCZ tends to be a little higher above the Equator in this period During Winter (N Hemisphere) - insolation is concentrated below the Equator, hence ITCZ tends to be slightly below the Equator

ITCZ and seasonal fluctuations - Becausethe ITCZ influences winds, changes in its position will give rise to different seasons - The Monsoon climate is the result of this seasonal variation of the ITCZ

ITCZ and seasonal fluctuations The South-west/Summer Monsoon - overhead sun above equator, more insolation concentrated above equator, hence ITCZ appears to move northwards towards Tropic of Cancer - increase in insolation over northern India, Pakistan and Central Asia, air rises, low pressure - warm, moist air drawn in from Indian ocean, rises to form heavy rainfall - wet monsoon arises

ITCZ and seasonal fluctuations North-east Monsoon/Winter Monsoon - overhead sun moves downwards, hence more heating below equator, ITCZ and subtropical jet shifts below the equator - Central Asia experiences more rapid cooling due to subtropical jet stream - air (dry) therefore moves out from Central Asia towards the ITCZ (lower pressure) - as a result of dry air blowing across Central Asia, the area experiences the dry monsoon

Mountains and Valley Winds On a smaller scale, mountains, valleys and solar heating can also influence the prevailing weather - Land vs sea breezes (covered in previous lecture) - Anabatic wind - Katabatic wind - Foehn

Anabatic wind - during the morning, valley sides are heated by the sun - air in contact with these slopes will expand and rise, creating a pressue gradient - as a result, when heating is at its max in early afternoon, a strong uphill or anabatic wind blows up the valley

Anabatic wind - air over this area is conditionally unstable, rises to produce cumulus clouds - much rainfall, thunderstorms - in the middle of the valley, a zone of sinking air leaves the center of the valley cloud free ie no rainfall in the middle of valleys

Katabatic wind - during evening, valley loses heat by radiation - surrounding air is cooler and therefore denser - influenced by gravity to flow down the valley sides and floors as a mountain wind or katabatic wind - temperature inversion rises: cold air below a layer of warmer air - creates a fog or frost hollow

Foehn or Chinook - occurs when air is forced to rise above a mountain - as it rises, adiabatic cooling occurs - condensation occurs and the air becomes dry - as it is now cooler and dense, it will sink once it blows over the mountain - warms adiabatically, brings with it warm, dry winds known as the chinook

Physical Geography