The General Circulation of the Oceans

Transcription

1 The General Circulation of the Oceans In previous classes we discussed local balances (Inertial otion, Ekman Transport, Geostrophic Flows, etc.), but can we eplain the large-scale general circulation of the ocean? - Wind-driven ocean circulation and gres: Wh ocean flows different than global wind patterns? - Eastern and Western Boundar Currents: Wh are the so different from each other? - Simple models of ocean circulation (Sverdrup, 1947; Stommel, 1948; unk, 1950): laid the foundations for the modern theor of ocean circulation

2 Global Wind Pattern is mostl Zonal (E-W) Does not look like the ocean circulation

3 The General Circulation of the Oceans: What drives the ocean currents and gres? Western Boundar Currents Eastern Boundar Currents Subtropical Gres

4 ain driving force: wind

5 The Subtropical Gres: A pile of warm waters with clockwise circulation (N. H) H However, pure geostrophic balance neglects wind and neglects friction

6 Basic dnamics of an oceanic gre (sa the subtropical gre of the N. Atl.): winds from west at mid latitudes and from east at low latitudes generate Ekman transports toward the center piling up of water at the center generates clockwise geostrophic circulation Ekman transport toward the center is balanced b friction forces outward westerlies at mid latitudes trade winds at low latitudes H Note: Since there is a continuous input of energ b the wind, the wind-driven circulation can remain stead for a long time onl if there is comparable loss of energ due to friction. Summar: Energ input: Wind Balance of forces: Coriolis + Pr.grad + Friction

7 WESTERN AND EASTERN BOUNDARY CURRENTS The simplistic picture of smmetric wind-driven circulation is far from describing real oceanic gres: Schematic odel of Boundar Currents gres are not smmetric: western currents intensification ocean currents do not alwas follow the wind direction

8 Western Boundar Currents Currents on the western side of basins: -Gulf Stream- N. Atlantic - Brazil Current- S. Atlantic - Kuroshio- N. Pacific - East Australian current- S. Pacific Are narrow and strong currents, while eastern currents are weak and diffused. Wh? This was first eplained b the pioneering work of Stommel (1948)

9 Our understanding of the theor of ocean circulation is largel the results of 3 ke studies Harald Sverdrup (1947): Showed that the circulation of the upper ocean is related to the curl of the wind stress curl( ) Henr Stommel (1948): Showed that the circulation of ocean gres is asmmetric because of the change of Coriolis with latitude (the beta effect) f Walter unk (1950): Add edd viscosit and calculated the circulation in the Pacific using more realistic domain and winds

10 Sverdrup solution (integrating from east boundar),,, d curl vdz f curl Sverdrup s balance: From continuit Eq: stead state no horizontal friction constant wind no deep motion for z<- D f=f o + (=df/d) verticall integrated Eqs. continuit: =transport from D to 0 (a) P=integrated pressure momentum: =wind stress (b) P f P f 0 curl P P f f f a b ) : ( ) ( Sverdrup solution: calculate the transports in the upper ocean from observed winds

11 =? Sverdrup (1947) shows that : The flow is Geostrophic with level of no motion, obes the Ekman transport The upper ocean circulation is directl related to the curl of the wind stress The change of Coriolis with latitudes is essential ( beta effect ) Sverdrup s solution provides first order estimate of surface transports. Deficienc of theor: neglects lateral friction thus no boundar currents! ( =0 on east boundaries,??? on west boundaries) curl d Circulation of the Pacific Ocean calculated from Sverdrup s theor using mean wind stress (Reid, 1948) North Equatorial Current North Equatorial Counter Current South Equatorial Current =0

12 Testing Sverdrup s theor against observations from the Pacific Ocean

13 Sverdrup s solution using more recent wind stress data Transports are somewhat weak compared with observations, and western boundar currents are still not resolved

14 Stommel s Solution assumptions: Bo model : rectangular basin with constant depth, D, and constant densit,. D z Stead state solution (neglect acceleration) Simple wind forcing, ( ) Acos b Sverdrup s balance plus simple bottom friction, F=Ju, (J=friction coefficient) (lat) p 1 p fv fu 1 z 1 z Ju Jv ade eperiments with constant f and with f changes with latitude

15 Vorticit balance equation: (derived b taking derivatives of the u & v equations) Vorticit v u 0 wind stress curl friction Coriolis change with latitude ( v) 1 u v f J v z Beta Term Without Beta term With Beta term Stommel (1948) solution

16 How can the change of Coriolis (f) with latitude cause the western boundar current? It creates a planetar vorticit. [terms: wind-stress (WS) + beta, planetar vorticit (PV) + friction (Fr)] large anticclonic vorticit West boundar balance: WS + PV = Fr small cclonic vorticit East boundar balance: WS = PV + Fr Friction must be larger in western oceans! Vorticit v u

17 unk s Solution unk (1950) build on Sverdrup s and Stommel s theories: Active upper ocean laer (~1000m) over motionless deep ocean Added lateral edd friction with constant edd coefficients to consider more realistic ocean basins He integrated the equations of the upper ocean (similar to Sverdrup s) and wrote them in terms of a stream function v, u

18 unk Solution He solve the equation for a stream function using observed winds and estimated the Gulf Stream transport at 36Sv and the Kuroshio at 39Sv (~1/2 the observed). Note that the western boundar currents are not too realistic. Wind stress & wind-stress Curl Resulted unk s circulation

19 Estimated ocean transports based on observations: including recirculation gres that are not taken into account in the simple theories of ocean circulation

20 Summar: Simple theories (Sverdrup/Stommel/unk) set the stage for the basic understanding of ocean general circulation, but these theories can not describe the details of particular ocean currents. What s net? Particular ocean currents and their dnamics are affected b additional parameters not included in the theories above. > Local topograph and coastline > stratification (heat flues, rivers, etc..) > local wind pattern > location (high or low latitudes) > seasonal changes >more - Wed Oct 24 & on Oct 26: Ocean currents & circulation - Wed Oct 30: No Class (Sea Level conference) - on Nov 4: Review for eam - Wed Nov 6: Eam #2