Class 8. Oceans II. Ekman pumping/suction Wind-driven ocean flow Equations with wind-stress.

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Presentation transcript:

Class 8. Oceans II

Ekman pumping/suction

Wind-driven ocean flow Equations with wind-stress

Wind-driven ocean flow Equations with wind-stress Split velocity in geostrophic ('g') and ageostrophic parts ('ag') e.g.

Ekman transport

Ekman pumping (downwards)/suction X wind into the screen

Ekman pumping (downwards)/suction tropics midlatitudes elevated sea level height in convergence area

Ekman pumping/suction due to wind stress

Ekman pumping/suction Explanation mass conservation 0

Ekman pumping/suction

Ekman pumping/suction Example = 32 m/year

Ekman pumping/suction from wind stress climatology downward upward f=0 The equatorial strip is a region of upwelling, because the trade winds on either side of the equator drive fluid away from the equator in the surface Ekman layer, and do demand a supply of fluid from below (p205)

Wind-driven ocean flow Eliminate pressure by cross differentiating (  ref =cst)

Wind-driven ocean flow Eliminate pressure by cross differentiating (  ref =cst,  =df/dy)  ≈2x m -1 s -1

Interior ocean flow structure Below Ekman layer:

Interior ocean flow structure Below Ekman layer: w Ek >0  v>0 (weak northward flow)

Interior ocean flow structure Below Ekman layer: w Ek <0  v<0 (weak southward flow)

The Sverdrup relation apply integration between a 'very large' depth (*) and the surface where w=0 The Sverdrup relation explains how the depth integrated meridional transport (y-direction) is related to the wind stress (*) ocean should be deep enough to prevent bottom friction acting on the flow

The observed ocean circulation (from NOAA) equatorial countercurrent gyres

The wind stress trade-winds westerlies easterlies

Ekman layer: deflection to the right of the wind stress deflection to the left of the wind stress (southern hemisphere)

Ekman pumping/suction due to wind stress

Oceans in the news: the plastic soup