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Lecture 24:Wind-Driven Ocean Circulation Learning Objectives: Qualitative & Quantitative: Westward Intensification and Physical Causes Specifics: 1

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Lecture 24:Wind-Driven Ocean Circulation Learning Objectives: Qualitative & Quantitative: Westward Intensification and Physical Causes Specifics: 1 ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 24:Wind-driven ocean circulation Learning objectives: Qualitative & quantitative: westward intensification and physical causes Specifics: 1. Stommel’s solution (review) 2. Munk’s solution – more realistic structure of WBC Previous class: scale analysis & obtain vorticity equation in WB region: (i) Stommel 1948 (linear bottom drag) 45N 30N * �� � ) ! � = � = = � � + ! �� � "#$%&'(& 15N 0 LE Interior solution: " #$%& � �=∫ �� !! ()! Stommel’s solution: Milestone 45N 1) Westward intensification: 30N * �� � ) ! � = � = = � � + ! �� � "#$%&'(& 15N 2) Essential role of �: β ≠ 0 Limitation: V max right on the coast - unrealistic (ii) Munk’s solution Munk 1950(horizonal harmonic mixing; Boundary conditions of Munk: more realistic) Physically, WBC is a response to interior circulation Vmax @ 1.2 Munk, W. H., On the wind-driven ocean circulation, J. Meteorol., Vol. 7,1950 Munk (1950) Note the “return current” or recirculation of Munk layer! WBC (X’) and transport X: more realistic. Number “n” in zonal axis shows “n x Lx”, which is n x 100km in our case. Vmax ~120km Stream function contours are the lines that currents (or winds in the atmosphere) flow along Interior +V -V Munk layer Recirculation Degree Green to purple: small to large Interior & WB regions: Major physical differences steady circulation vorticity equation: (5) (6) Ocean interior away from western bounDary: Vor t i ci t y i nput by wind stress curl forcing Western bounDary region: Drag or mixing: dissipation – does not “generate” energy; they’re BounDary conDition: not the forcing of motion Physically, WBC is a response to wind-driven interior circulation 1) The swift WBC (westward intensification) is a remote response to wind forcing in ocean interior; 2) Mixing is important in dissipating energy and producing the strong WBC (trapped to the western boundary region) Westward intensification � � effect; after radiation of Rossby waves – interior ocean reaches Sverdrup balance; western boundary (WB): closes interior circulation HW3: Initial perturbation Here: STG – sustained wind forcing IGW 45N RW 30N 15N Balanced state: Sverdrup balance (interior) Balanced state: WBC: Closes interior circulation in Quiescent ocean a narrow region -intensification Westward intensification [2] Vorticity balance (entire basin) +vorticity -vorticity input input by wind �� �� by friction! � = − �� �� Validity of Sverdrup theory with flat ocean bottom? Topography renders the integrated circulation deviates from Sverdrup balance Is the North Atlantic in Sverdrup Balance? JPO, https://doi.org/10.1175/1520- 0485(1985)015<1876:ITNAIS>2.0.CO;2 A note on the validity of the Sverdrup balance in the Atlantic North Equatorial Countercurrent. DSR, https://doi.org/10.1016/j.dsr.2004.05.014 Breakout session 1: • What is the scientific insight and limitation of Stommel’s WBC? • What progress has Munk made comparing to Stommel’s solution? Breakout session 2: Assume the subtropical north Pacific (or Atlantic) Ocean is forced by persistent surface wind stress shown by white bulk arrows of the Fig. below. For steady ocean circulation, would we have westward intensification? 45N Western boundary 15N Useful relations are: Sverdrup balance: Western boundary region: ! -.&/ � �=∫ �� ,! *1! Global scale prevailing winds: In subtropics: � we = ���� = "!# $ &" $ &# − < 0 $% "!# $' "!# In subpolar areas: � we = ���� = "!# $ &" $ &# − > 0 $% "!# $' "!# HW5 Q3 35N Wind anomaly 25N Coastal Long Kelvin Rossbywind 15N 5N Equator EQ Equatorial kelvin 75W 45W 15W Winds diminish near coasts, 15N and 35N: (a) no coastal Kelvin waves being excited along eastern boundary; (b) local upwelling and mixing cannot directly reach EQ California 4a: Sea surface temperature near California coast: Q4Units: oC (top) 4b: Sea surface temperature in the Equatorial Pacific. Mean surface winD Units: oC (bttom) EQ cold tongue: most important processes: Coastal & EQ upwelling; Advection by SEC Q4d Surface wind anomaly prior to El Nino onset: WWB (1) Downwelling EQ KW: reduce upwelling cooling in cold tongue region – trigger El Nino; (2) Anomalous advection brings warmer water eastward Lecture 14: WWB, EQ Kelvin wave & onset of the 1997 El Nino Zonal wind anomaly (m/s) D20 anomaly (m) SST anomaly (deg C) From McPhaden 1999 Q2: Open Ocean Surface winD stress varies spatially, ProDucing “convergence” or “divergence” of Ekman transports z into out x A B �1 Downwelling �2 isopycnals �A > �B Northern Hemisphere (NH) Using hyDrostatic equation: PA > PB, Due to Earth’s rotation, PGF is balanced by Coriolis force, causing geostrophic currents, which flow along isobars (pressure contours) .
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