Ocean currents I. The directional forces Wind-water interaction The drag forces

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1 Oean urrent I Oean urrent are drien by urfae effet uh a the urfae otion drien by the drag interation of wind and water and the ertial otion drien by denity and alinity gradient in the deeper oean layer. Thee otion ouple with the Corioli fore, whih reult fro the influene of Earth rotation on a northward (or outhward) oing bodie with eloity. The diretional fore Wind-water interation The drag fore

2 Cirular pattern of oean urrent Earth rotate ounter-lokwie The oean urrent run lokwie on the northern heiphere and turn ounter-lokwie on the outhern heiphere!

3 The Corioli ore The Corioli fore i a reult of laial ehani on a oing objet in a rotating frae of referene! The Corioli fore puhe a oing objet to the right for ounterlokwie rotation of the referene frae or to the left for lokwie rotation. The fore i a etor ro produt between eloity etor of the oing objet and the orbital oentu etor of the rotation.

4 The Corioli fore repreent a etor ro produt f zˆ in a f a they gie the diretion of 1 z, ˆ are unit etor opont, and Corioli paraeter with a latitude zˆ ro produt in etor ter ~ ~ zˆ zˆ point in perpendiular define etor diretion of diretion tranport

5 Corioli ore on Earth The earth rotate with angular eloity around it axi. A body loated on earth with a ha a tangential linear eloity of x =r in the external referene frae. The angular oentu i: r R L If the body oe pole-ward with a eloity = y, the angular oentu L hange, generating the torque : r dl d r dt dt dl dr r r dt dt dr dy in in dt dt i latitude in

6 The Corioli fore aelerate a body oing outh on the outhern heiphere to the wet or or a body oing north on the northern heiphere to the eat. a a in in 360 day 0 rad day A otor yaht roe the 0 o N latitude oing northward in the Northern Atlanti with a ontant peed of 10 knot (1knot=1.85 k/h). What i the trength of the Corioli fore ating on the boat, if the yaht ha a a of 0 ton? 10 4 kg kg N 0 for 0 S 5.17N in

7 The boat need one full inute to aelerate fro zero to it 10 knot peed. Calulate the fore, the engine ha to generate for bringing the boat up to the peed of 10 knot. The Corioli fore i relatiely all but beoe larger at higher latitude. Calulate the Corioli fore ating on the boat when it roe the 60 o N latitude on it way to Ieland? 10 4 kg 1.98 kg N knot a 10 kg 1in kg 1713N in60 0

8 Corioli fore on air or water a for inertial yle! Air or water oing with peed without any external fore ating upon the, exept for the Corioli fore, will take a irular lokwie flow trajetory on the northern heiphere, and a irular ounter-lokwie flow trajetory on the outhern heiphere, balaned by the entripetal fore f. R in f R h in in 0.55h 1 in

9 The Gulf Strea i the fatet oean urrent in the world with peak eloitie near 3/. What i the urature at 35 o north latitude? 3 R h in in 3 R 35,85 36 k in 35 Only all eddie for at that low flow peed!

10 Reality i Reality ore i oplex ore oplex beaue beaue of additional of additional fore due fore to frition due to and the drag teperature fore effet, proided whih by the add atopheri to the eddy wind foration irulation phenoenon! and by the frition fore exerted by deeper water layer!

11 Wind a preure phenoenon Air oe; le dene (higher teperature) air poket rie to higher altitude, ool and ink bak toward earth urfae. Airflow ruhe toward low preure region and away fro high preure zone. A larger the preure gradient, a larger i the fore ating upon an air a, inreaing the wind eloity. p The fore point fro a higher preure P toward a lower preure range dp dz g Upward and downward otion i deterined by the hydrotati equation leading to the lape rate. Iobar indiate line of ontant preure P=0! Wind diretion point perpendiular to iobar (line of ontant preure)

12 Global iobar ap Preene of large ontinental land area in the northern heiphere affet the ideal iobar pattern obered at outhern latitude. During the uer land i warer than oean auing the deelopent of low preure area, whih hange northward/outhward wind pattern. During winter land i older than oean water, auing the deelopent of low preure area oer the oean region. The eergene of loalized weather zone i further enhaned by the geographial affet fro lake or ountain.

13 Geotrophi wind on non-rotating earth rition effet are negleted Wind diretion would be defined entirely by the latitudinal iobar Wind onetion ell for beaue of the equatorial heating effet and the polar ooling effet. The ITCZ (intertropial onergene zone) how only a negligible horizontal wind oponent, and i alo alled the doldru, where the eaonal lak of wind often delayed a ailing hip journey.

14 The intertropial onergene zone Area of loud foration beaue of extended ondenation and upriing air ae. Alo origin of thundertor and ylone if a rotational frition oponent i added!

15 The effet of rotation, Corioli fore Wind ontinue to oe perpendiular to iobar, but air ae are ubjet to the Corioli fore, whih preent wind to reah the pole, foring three irulation pattern. At 30 o latitude the air ool and ink, oing bak to equator in an inertial yle in north eaterly diretion (the trade wind). Wind that ontinue to oe north are defleted eatward by inreaing Corioli fore (preailing weterly wind).

16 Radiu of irular wind pattern R in 0.55h in 100 R 1194k h in in 35 1 The trade wind blow peritently wetward and toward the Equator a part of the geotrophi wind pattern fro the ubtropial high-preure belt toward the intertropial onergene zone (ITCZ). At high altitude the aerage peed oer the oean i about /. On low altitude the peed low down due to frition. The trade wind were naed by the rew of ailing hip that depended on the wind during wetward oean roing. Reality how larger radii (500 k) due to the ipat of preure gradient!

17 Weterlie Weterlie trade wind (northeaterly) doldru Hurriane doldru trade wind (northeaterly) doldru trade wind (outheaterly) trade wind (outheaterly) Cylone Weterlie

18 Trade route for Guano upply

19 Corioli fore on low preure area Thi low-preure yte pin ounter-lokwie due to the balane between the Corioli fore and the preure gradient fore. The air tend to flow toward it, but i defleted perpendiular to it eloity by the Corioli fore. A yte of equilibriu an then etablih itelf reating irular oeent, or a yloni flow. p

20 Wind-Water urfae interation Water otion at the urfae of the oean (ixed layer) i drien by wind effet. rition aue drag effet on the water, tranferring oentu fro the atopheri wind to the oean urfae water.

21 The drag fore Wind generate ertial and horizontal otion in the water, triggering onetie otion, auing turbulent ixing down to about 100 depth, whih define the iotheral ixed layer. The drag fore D on the water depend on wind eloity : D for C D wind depending partiularly A water on a urfae the C D interatio n C eergene drag oeffiien t roughne,and of D 0.00, wae! A dienionle ro etional ontant area

22 The Beaufort Sale i an epirial eaure deribing wind peed baed on the obered ea ondition (1 knot = / = 1.85 k/h)!

23 Conerion fro ale to wind eloity: A trong breeze of B=6 orrepond to wind peed of =39 to 49 k/h at whih long wae begin to for and white foa ret beoe frequent. The drag fore an be alulated to: B 3/ D D C D A kg a kg a 1. 3 A k h A N or C D D A N or a trong gale (B=1), =35 /, the drag tre on the water will be: D a / A kg 1. 3 kg C 35 D N