Wind-forced motion & largescale gyre circulation

Transcription

1 Wind-forcd motion & larscal r circulation Outlin Backround on wind-forcd motion Ekman lar dpth Ekman (lar) vlocitis Storm surs, coastal up- and downwllin Schmitz (1996) Backround wind forcd motion Ekman spiral Track of th Fram as Fridtjof Nansn attmpts to drift to th North Pol in his ship ( ). Van Walfrid Ekman ( ), Swdish ocanoraphr, dvlopd thor of Ekman spiral from Nansns obsrvations. Ekman Lar - wind blowin on an ocan producs a forc pr unit ara calld a wind strss. - ffcts of a wind strss on th ocan surfac ar transmittd down throuh th watr column b th action of turbulnt ddis that ar thmslvs nratd b th wind brakin wavs boundar shar strsss - dpth to which th ffcts of wind ar flt is calld th Ekman lar thicknss or Ekman dpth (H ), whr H 2 K f Ekman Lar cont d - dpth to which th ffcts of wind ar flt is calld th Ekman lar thicknss or Ekman dpth (H ), whr H 2 K f - whr K is th dd difsivit and - K WL whr W and L dnot a charactristic dd vlocit and siz Tpicall K m 2 s 1, so that with f 10 4 s 1, H 20 m. Howvr in rions of hih wind-drivn turbulnc, K can b up to 0.5 m 2 s 1, so that H can rach 100 m. 1

2 Ercis: Q 1) Givn th charactristic dd and vlocit scals W 2cm/s and L 100m, calculat th Ekman lar thicknss H for f 10-4 s -1 Ekman Lar Vlocitis To undrstand ffcts of wind considr stad quations of motion for barotropic ocan of dpth h. If - wind is absnt - bottom strss is wak quations of motion would b ostrophic quations: - fv d d η d Howvr, if - wind is prsnt - bottom strss is wak Ekman Lar vlocitis Effct of wind ntrs as tra frictional trm: -fv + d ρ h + d ρ h Ekman Lar vlocitis So, ovrall vlocit fild u and v can b split into ostrophic and wind-drivn componnts: - - fv fv ρh ρh d d - Coriolis forc and wind balanc dpth-avrad Ekman vlocitis (u and v ) - fv ρh ρ h - Dirct ffcts of wind onl flt ovr Ekman lar dpth (H ) What thn is th vlocit in th Ekman lar? Ekman lar vlocitis (U and V ): U V h H h H u ρ H f v ρ H f Schmatic of movmnt in column of watr in th Ekman lar. Which hmisphr is this ampl from? 2

3 Ercis: Q 2) Find th Ekman lar dpth H and th Ekman vlocitis u and v whn a wind of strnth 0.5 Pascals (0.5 N/m 2 ) blows in a northastrl dirction in th mid-latituds of th NH. Tak: vrtical difsivit K 0.1m 2 /s; f 10-4 s -1 and h 1000m What is th Ekman lar vlocit U and V? Sktch th rsultant flow on th diaram. Ercis: Q 3) If h 4000m, calculat th Ekman transport hv V H usin th rsults of Q.1 and assumin that 0.1 Pascals. Storm Sur, Upwllin & Downwllin Lt s now considr ffcts of an alonshor wind strss on coastal circulation. Two situations can aris, dpndin on th dirction of th alonshor wind. 1. Storm Sur Idalisd modl of a storm sur. Which hmisphr? 1. Storm Sur, associatd with coastal downwllin Idalisd modl of a storm sur, with coastal downwllin (NH). 3

4 Whn rotation ffcts ar not important For ampl, in smi-nclosd bas Maimum storm sur rion for hpothtical hurrican Most accurat modl availabl of th storm sur of hurrican Katrina. It missd Nw Orlans and th maimum impact was alon th Mississippi Coast. Not th submrnc of th ntir outr dlta cpt th lvs and jtt, spciall alon th wst sid of th Mississippi Rivr. 2. Coastal upwllin Which hmisphr ar w in? Ekman transport and associatd upwllin and downwllin rsultin from wind blowin paralll to shor. Sarminto & Grubr (2006) 4

5 Winds SST in California Currnt undr th influnc of a northrl alonshor wind. Schmatic diarams of winddrivn upwllin in SH. TEMP NITRATE Tmpratur and nitrat distributions off Pt. Sur, California, from satllit and shipboard oprations. From Traanza t al. (1983). Mann & Lazir (1996) Mann & Lazir (1996) Rspons of thrmal structur of quatorial Pacific to chand surfac winds. (top) Undr normal astrl trad wind conditions, sa lvl riss to wst and thrmoclin dpns. (middl) Whn trad winds rla, quatorial Klvin wavs propaat astward, which lads to ris in sa lvl and dpnin of thrmoclin nar South Amrican coast (El Niño conditions). (bottom) Normal situation amplifid durin stron trad winds (La Niña conditions). Ercis: Q 4) A low-prssur sstm sits off th NSW coast as shown blow. Th southrl winds rt a strss 1 N/m 2 ovr a priod of 5 das. Assumin an Ekman lar dpth H 100m ovr th shlf rion, calculat th onshor Ekman lar vlocit U. Calculat how far and in which dirction th front at A will mov durin th 5 da priod. Commnt on th likl salvl chan and rsultin ostrophic adjustmnt Sarminto & Grubr (2006) 5

6 Ercis: Q 4) A low-prssur sstm sits off th NSW coast as shown blow. Th southrl winds rt a strss 1 N/m 2 ovr a priod of 5 das. Assumin an Ekman lar dpth H 100m ovr th shlf rion, calculat th onshor Ekman lar vlocit U. Calculat how far and in which dirction th front at A will mov durin th 5 da priod. Commnt on th likl salvl chan and rsultin ostrophic adjustmnt. 6