evat 554 ocean-atmosphere dynamics upweling and downwelling; ekman transport lecture 14...

EVAT 554OCEAN-ATMOSPHERE

DYNAMICS

UPWELING AND DOWNWELLING; EKMAN TRANSPORT

LECTURE 14

(Reference: Peixoto & Oort, Chapter 8,10)

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2

Northern Hemisphere

take H=2DE

Mean transport is at right angles to surface wind (to the right in Northern Hemisphere)

This transport occurs in a boundary layer of width fD

VE /2

Below this boundary layer we will generally have geostrophic flow...

)0,0(1xyf

M kτ ˆ10

f

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2

Northern Hemisphere

take H=2DE

)0,0(1xyf

M kτ ˆ10

f

Coastal Downwelling

fy

xM 0

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2

Northern Hemisphere

take H=2DE

)0,0(1xyf

M kτ ˆ10

f

Coastal Upwelling

fy

xM 0

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2

Northern Hemisphere

take H=2DE

)0,0(1xyf

M kτ ˆ10

f

SUMMER COASTAL UPWELLING ALONG CALIFORNIA COAST

fy

xM 0

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2)0,0(1

xyfM kτ ˆ1

0f

SUMMER COASTAL UPWELLING ALONG CALIFORNIA COAST

fy

xM 0

SEA SURFACE TEMPERATURES

Windstress and Circulation in the Upper Ocean

)/()2( 00 EDfV

fV

DE /2

Northern Hemisphere

take H=2DE

)0,0(1xyf

M kτ ˆ10

f

fxxxM y0

fx

yfx

yyyM

00

Associated with a horizontal divergence of mass within the Ekman layer

fx

yfxy 00

)(

0

kf

τ

yyM

xxM

M

Windstress and Circulation in the Upper Ocean

Northern Hemisphere

take H=2DE

Associated with a horizontal divergence of mass within the Ekman layer

fx

yfxy 00

yyM

xxM

M

But by continuity this horizontal divergence must be replaced at the base of the Ekman layer,

)(

0)(k

E fHzw

τ

)(

01)(k

E fHzw

τ

Ekman Pumping

fxxxM y0

fx

yfx

yyyM

00

)(

0

kf

τ

Windstress and Circulation in the Upper Ocean

JJA

Dynamic Topography

Surface Windstress

Ekman Transport piles up water in the

subtropics!

Surface Windstress

Windstress and Circulation in the Upper Ocean

DJF JJA

0y0x

Note the relative amplitudes of variation with latitude!

Windstress and Circulation in the Upper Ocean

0x

Windstress and Circulation in the Upper Ocean

0x

fxyM 0

Windstress and Circulation in the Upper Ocean

0x

fx

yfHzwE

011)( 0

τ

Windstress and Circulation in the Upper Ocean

0x

Equatorial Upwelling!f changes sign at the equator

fx

yHzwE

01)(

Windstress and Circulation in the Upper Ocean

Equatorial Upwelling!

||0

fx

||0

fx

f

xyM 0

Within Ekman layer

f changes sign at the equator

fx

yHzwE

01)(

El Nino vs. La Nina

equatorial upwelling

El Nino vs. La Nina

coastal upwelling