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Why are there upwellings on the northern shelf of Taiwan under northeasterly winds? *. L.-Y. Oey [email protected] Outline : Introduction – upwelling, effects of strong currents A simple model of upwelling at the western edge of Kuroshio in East China Sea Observational evidences

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why are there upwellings on the northern shelf of taiwan under northeasterly winds
Why are there upwellings on the northern shelf of Taiwan under northeasterly winds?*

L.-Y. Oey

[email protected]

Outline:

Introduction – upwelling, effects of strong currents

A simple model of upwelling at the western edge of Kuroshio in East China Sea

Observational evidences

Generalizations

Conclusions

*Chang, Oey, Wu & Lu, 2009 – J. Phys. Oceanogr. Submitted.

wind ocean currents
Wind & Ocean Currents

Wind

mixing

surface layer: 10~100m

Y:v/t + fu = y/z

fu = y/z

(..)dz  fU = oy

slide3

U = oy /f

oy

U

Ekman Layer

10~100m

non uniform wind constant f
Non-uniform Wind, constant f

oy < 0

oy > 0

X

Wind from north

Wind from south

U < 0

U= oy /f > 0

Ekman Layer

Upwelling

constant wind non uniform f
Constant wind, non-uniform f

Trade Wind oy < 0

f > 0

f < 0

x

U= oy /f < 0

U= oy /f > 0

coastal downwelling
Coastal Downwelling

oy

Coast

Wind

U

Warm

Cool

slide9

In the Presence of a Spatially Non-uniform Ocean Current vo(x), the Ekman transport

U = oy /f ~ Period,

where f = fo+ o; o = vo/x

NE Monsoon oy<0

Jupiter = 10 hrs/rotation

Wind

China

Mars = 24.6 hrs/rotation

z

y

o>0

o<0

o=0

vo

x

Warm

Cool

Kuroshio

a simple model
A Simple Model

+QsNT

For oy < 0

Warming

Cooling

A

oy

T/t = A eit

Consider Oscillatory Wind:

T/t is in phase with wind if A > 0;

T/t is 1800 out of phase with wind if A < 0.

slide11

Wind

T/t ~ oy(t) [s/xT/f ]

T/t

T/t ~ oy(t) [To/x]

Wind

T/t

observational evidences
Observational evidences

East China Sea

Kuroshio

LongTung

Study Region

slide13
Ro

Rossby number (Ro)

Ro↓, Ue↑

Ro↑, Ue↓

Southward wind: oy<0

slide14

Effects of Kuroshio: Long-Tung SST & wind stress

Southward wind: cooling

T/t ~ oy(t)/(fE)[To/x +s/xT/f ]

slide17

Summary

Current shears near strong ocean jets play a significant role in controlling the vertical motions in the ocean.

slide18

Generalizations

1.

2.

(fh1/d)/t + voxxu = (f+vox)wE/d (1)

In SS, using wE = oy/(f +vox)2.voxx, (2)

we have, u = - oy/[d(f +vox)]. (3)

slide21

A “bulge” is defined as a near-surface buoyant fluid that moves across shelf as a result of Ekman transport by downwelling wind and its interaction with ocean’s vorticity across, say, a front. It is “2d-like” when |/y| << |/x| where y = alongshore and x = cross-shore. Idealized Calc.:

oy < 0

Day 3

Day 1

Day 2

Day 4

Caption: V-contours (black:0.2, 0.4, ..; grey: -0.05; white:-0.1,-0.15,-0.2,..) m/s, on color T (oC) from day 1 through 4after an up-front wind is applied.

slide22

A Nonlinear Model

Assume /H << 1, |/ y| << |/ x|;

Within the bulge, temperature T = Tb(x,z,t) is weakly stratified:

Tb = T- + (z + ), for 0z(x,t); g/N2 << 1 (A.1)

T- is related to the temperature Ti(z)beneath the bulge:

T- = Ti(), and Ti(z) = Tdeep + [N2/(g)](z + zdeep), for (x,t) z zdeep (A.2a,b)

slide23

/t = n[-1/2/x+ 21/23/x3]; where  = 2

  • = F(),  = -1(x + ct), c = nconstant > 0

Figure A2. The bulge solution according to equation (A.21) for C1 = 1 and various indicated values of c. Both the ordinate and abscissa are non-dimensionalized: ordinate is the bulge thickness (“”) below the free surface while the abscissa is  = -1(xcnt); see text. For each c, the dotted line indicates where the solution terminates at the head of the bulge where a front is formed.

conclusions
Conclusions
  • Down-front wind leads to slantwise instability with intense mixing ~300m
  • Up-front wind leads to propagating “bulge” solution that produces deep recirculation cells
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