Why are there upwellings on the northern shelf of taiwan under northeasterly winds
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Why are there upwellings on the northern shelf of Taiwan under northeasterly winds? *. L.-Y. Oey lyo@princeton.edu 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

lyo@princeton.edu

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 under northeasterly winds?

Wind

mixing

surface layer: 10~100m

Y:v/t + fu = y/z

fu = y/z

(..)dz  fU = oy


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

U = under northeasterly winds?oy /f

oy

U

Ekman Layer

10~100m


Non uniform wind constant f
Non-uniform Wind, constant under northeasterly winds?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 under northeasterly winds?f

Trade Wind oy < 0

f > 0

f < 0

x

U= oy /f < 0

U= oy /f > 0


Constant wind f but with a coast upwelling
Constant wind & under northeasterly winds?f, but with a coast:Upwelling

oy

Coast

Wind

U

Warm

Cool


Coastal downwelling
Coastal Downwelling under northeasterly winds?

oy

Coast

Wind

U

Warm

Cool



Why are there upwellings on the northern shelf of taiwan under northeasterly winds

In the Presence of a Spatially Non-uniform Ocean Current under northeasterly winds?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 under northeasterly winds?

+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.


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

Wind under northeasterly winds?

T/t ~ oy(t) [s/xT/f ]

T/t

T/t ~ oy(t) [To/x]

Wind

T/t


Observational evidences
Observational evidences under northeasterly winds?

East China Sea

Kuroshio

LongTung

Study Region


Why are there upwellings on the northern shelf of taiwan under northeasterly winds
Ro under northeasterly winds?

Rossby number (Ro)

Ro↓, Ue↑

Ro↑, Ue↓

Southward wind: oy<0


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

Effects of Kuroshio: Long-Tung SST & wind stress under northeasterly winds?

Southward wind: cooling

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


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

Annual (~30yrs) Mean Wind under northeasterly winds?

2001 wind


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

Summary under northeasterly winds?

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


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

Generalizations under northeasterly winds?

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)


Idealized calculations
Idealized Calculations under northeasterly winds?


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

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.


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

A Nonlinear Model 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 |

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)


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

  • 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 |/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.


Why are there upwellings on the northern shelf of taiwan under northeasterly winds

PV 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 | at day 80


Conclusions
Conclusions 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 |

  • Down-front wind leads to slantwise instability with intense mixing ~300m

  • Up-front wind leads to propagating “bulge” solution that produces deep recirculation cells


Thank you
Thank you. 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 |