The formation and dynamics of cold-dome northeast of Taiwan

The formation and dynamics of cold-dome northeast of Taiwan

Mao-Lin Shen, Yu-Heng Tseng and Sen Jan

112/04/22

112/04/22 Mao-Lin Shen, Yu-Heng Tseng and Sen JanMao-Lin Shen, Yu-Heng Tseng and Sen Jan

Outline

IntroductionNumerical modelObservationNumerical resultsMechanism analysisConclusion


Introduction (1/3)

An active upwelling area and nutrient-rich (Chen, 1992; Hsu, 2005)

The exchange of Kuroshio Water and Continental Water of East China Sea (Isobe, 2008; Matsuno et al., 2009)

Temperature lower than surrounding about 3~6℃. Observations have been well-documented by Chern a

nd Wang (1989), Gong et al. (1992), Lin et al. (1992), Tang and Tang (1994), Chen et al. (1995) and Tang et al. (1999), etc.


Introduction (2/3)

Fig. 1. Distribution of the centroid of cold patch formed between 2003-2008. Red stars denote the distribution of cold patch in summer (June to October) and green stars show that in winter. (Cheng et al., 2009)


Introduction (3/3)

Water mass properties (Chen et al., 1995): Kuroshio Surface Water (KSW) dominate surface region in

winter. Kuroshio Tropical Water (KTS) and Kuroshio Intermediate

Water (KIW) participate mainly in summer.

Wind disturbance has been considered by Gong et al. (1992), Chang et al. (2009) and Chang et al. (2010).

Using field measures reveal the properties of cold-dome and model output to verify the mechanism of cold-dome formation.


Numerical model (1/3)

Dietrich/Center for Air Sea Technology (DieCAST) hydrostatic ocean model

Surface sources of heat and fresh water Levitus94 seasonal climatology

Bathymetry unfiltered ETOPO-2 depth data supplemented with the Taiwan’s

NCOR 1-minute high accuracy depth archive in the Asian Seas Winds stress

monthly Hellerman and Rosenstein winds stress Vertical mixing

parameterized by eddy diffusivity and viscosity using a modified Richardson number dependent formula based on Pacanowski and Philander (1981)


Numerical model (2/3) ─ Governing Equations

Conservation of mass

Horizontal momentum equations

Equation of conservation of salt or energy (potential temperature)

State equation: Hydrostatic equation:

0 V

z

uA

zuA

x

pfvu

t

uvhhh

0

1

V

z

vA

zvA

y

pfuv

t

vvhhh

0

1

V

z

sK

zsKs

t

svhhhV

TS ,

(1)

(2)

(3)

(4)

(5)

pg

z

(6)


Numerical model (3/3) ─ Domain

Fig. 2. Schematic of the whole domain under consideration


Observation (1/5) ─ MW and IR mergrd SST

(a) (b)

(c) (d)

Fig. 3. The SST image around the vicinity region. The dark red encircled lines indicate Cold-Dome Favorite Region (CDFR). The fronts identify the boundary of warm water carried by Kuroshio and the cold water remained on continental shelf or the cold-dome.

16 May 2008

2 May 2009

30 July 2008

Typhoon Fung-Wong (28 July) just pass.

7 November 2009

Take for calculating the Time-longitude plot.


Observation (2/5) ─ Time-longitude plot

Fig. 4. Time-longitude plot of filtered SST north of Taiwan. Half degree span on latitude (25.4°N-25.9°N) was chosen to verify the variation. Of 2008 the dashed lines denote typhoons from left to right is Kalmaegi, Fung-Wong, Sinlaku and Jangmi, respectively. Of 2009 the dashed line denotes typhoon Morakot.


Observation (3/5) ─ Argo floats

Fig. 5. The Argo data, marked as red solid circles, since 3 August 2001 to 6 September 2009 in the study region, only 2047 data are available. Argo data gathered on Kuroshio main stream totally 21 profiles in from May to October, stand for summer pattern (b), and 12 profiles in from November to April, stand for winter pattern (c).

34.0 34.2 34.4 34.6 34.8 35.0

Salinity

0

5

10

15

20

25

30

35

T (o C

)34.0 34.2 34.4 34.6 34.8 35.0

Salinity

0

5

10

15

20

25

30

35

T (o C

) 118oE 120oE 122oE 124oE 126oE 128oE

22oN

24oN

26oN

28oN

30oN

32oN (a) (b)

(c)


Observation (4/5) ─ cold-dome in winter

Fig. 6. Argo float, WMOID 2900797, for (a) the trajectory; (b) MW_IR SST and a marker denotes the Argo data on 16 December 2008. The rests are subsurface comparisons of (c) temperature, (d) salinity, and (e) T-S profiles of the four measures.

(a)

← 04 Dec

← 08 Dec

120oE 121oE 122oE 123oE 124oE 125oE 24oN

25oN

26oN

27oN

28oN

Longitude

Lat

itude

← 12 Dec ↓ 16 Dec

(b)

(c)

12 16 20 24 28

T (oC)

300

250

200

150

100

50

0de

pth

(m)

04 Dec08 Dec12 Dec16 DecKuroshio

(d)

34.0 34.2 34.4 34.6 34.8 35.0

Salinity

300

250

200

150

100

50

0

dept

h (m

)

04 Dec08 Dec12 Dec16 DecKuroshio

(a)

← 04 Dec

← 08 Dec

120oE 121oE 122oE 123oE 124oE 125oE 24oN

25oN

26oN

27oN

28oN

Longitude

Lat

itude

← 12 Dec ↓ 16 Dec


Observation (5/5) ─ cold-dome in summer

Fig. 7. Argo float, WMOID 2900819, for (a) trajectory of the float; (b) MW_IR SST and the a marker denotes the Argo data on 17 July 2008. The rest figures are subsurface comparisons of (c) temperature, (d) salinity, and (e) T-S profiles of the four measures. Typhoon Kalmaegi passed this region on 17-18 July 2008. The path of Typhoon Kalmaegi are marked as hollow circles in (a).

(a)

←07/13

←07/17

←07/24

←18/02Z

←18/08Z

120oE 121oE 122oE 123oE 124oE 125oE 24oN

25oN

26oN

27oN

28oN

Longitude

Lat

itude

↓07/20

←17/20Z

18/05Z →

(b)

(c)

12 16 20 24 28 32

T (oC)

300

250

200

150

100

50

0

dept

h (m

)

13 July17 July20 July24 JulyKuroshio

(d)

34.0 34.2 34.4 34.6 34.8 35.0

Salinity

300

250

200

150

100

50

0

dept

h (m

)

13 July17 July20 July24 JulyKuroshio

←07/13

←07/17

←07/24

←18/02Z

←18/08Z

120oE 121oE 122oE 123oE 124oE 125oE 24oN

25oN

26oN

27oN

28oN

Longitude

Lat

itude

↓07/20

←17/20Z

18/05Z →


Numerical results (1/3)

(a) (b)

(c) (d)

Fig. 8. The SST results and current velocities at different depth on day 157, Year 37 of the model results. Note that a cold region was formed off northeast Taiwan just as the observations of satellite SST results.

Z = 6 m Z = 54 m

Z = 75 m Z = 98 m


Numerical results (2/3) ─ Trajectories

Fig. 9. The possible trajectories from Kuroshio Tropical Water (KTW). The sources were located over the model year and recorded the position until tracers flow out of the domain or rest on bathymetry.


Numerical results (3/3) ─ Trajectory

Fig. 10. A trajectory shows the route of Kuroshio Tropical Water. The background flow field are model results at z = 198 m on day 122.

North Mien-Hua Canyon

Mien-Hua Canyon


Mechanism Analysis (1/11)

Possible mechanisms Wind-driven Ekman upwelling Boundary layer effect

Current-driven Ekman upwelling Ekman boundary mixing

Dynamic uplift mesoscale eddy Kuroshio

Topographically controlled upwelling Vertical mixing



Wind-driven Ekman upwelling

Boundary layer effect Current-driven Ekman

upwelling Ekman boundary mixi

ng Dynamic uplift:

mesoscale eddy Kuroshio

Topographical upwelling

Vertical mixing

Garrett et al. (1993), JFM.

Boundary mixing




Boundary layer effect Current-driven Ekman

upwelling Ekman boundary mixi

ng Dynamic uplift:

mesoscale eddy Kuroshio

Topographical upwelling

Vertical mixing


Mechanism Analysis (4/11) ─ Wind-driven upwelling


where , in which is the thickness of Ekman layer

Chang, Wu and Oey, 2009 Mean: -0.3 m/day Max: 0.7 m/day

-0 .2

0 .0

0 .2

0 .4

w (

m/d

ay)

Jan F e b M ar A p r M ay Ju n Ju l A u g S e p O c t N o v D e c

C D WT S W

Fig. 11. Wind-driven Ekman upwelling determined by monthly Hellerman and Rosenstein wind stress and model’s vertical eddy viscosity.

1( ) sin 1 coswind h o h ow e e

f

/ Ez 2E vA f

With maximum upwelling velocities, 4~5 months with no interruption could only perform 100 m uplift

(7)


Mechanism Analysis (5/11) ─ Boundary layer effects

Boundary layer effect Current-driven Ekman upwelling

Max: only about 0.00002 m/day

Ekman boundary mixing

Fig. 12. Meridional current velocity distribution (Tang et al., 2000).

Garrett et al.

(1993), JFM.

Boundary mixing

Inverse currents introduced little dowelling transport.

Garrett et al. (1993), JFM.

Boundary mixing

W

(m/day)



Wind-driven Ekman upwelling Boundary layer effect

Current-driven Ekman upwelling Ekman boundary mixing

Dynamic uplift mesoscale eddy Kuroshio

Topographically controlled upwelling Vertical mixing

Isotherm redistribution due to upwelling.

Fig. 13. Zonal temperature profile at 25.6°N

121 121.5 122 122.5 123 123.5 124

20

40

60

80

100

120

140

160

Longitude (°)

Dep

th (

m)

14

15

16

17

18

19

20

21

22

23

24

25

Buoyancy instability

Diffusion, little H. Advection

Temperature increasing

Upwelling

Mixing

121 121.5 122 122.5 123 123.5 124

20

40

60

80

100

120

140

160

Longitude (°)

Dep

th (

m)

14

15

16

17

18

19

20

21

22

23

24

25



Isothermal plan have lower depth east of Kuroshio and higher depth on CDFR.

The isothermal plan on CDFR can shallower than 50 m deep.

Fig. 14. Isothermal plan at 21 ℃calculated by model output.



Take the depth of isotherm 21 at ℃122.8°E and 24.4°N as reference.

Large uplifted height in summer.

Fig. 15. Contour of Uplift height.


Mechanism Analysis (9/11) ─ Isotherm uplift

0

50

100

150

200

Upl

ift h

eigh

t (m

)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Max heightDynamic uplift by KuroshioDynamic uplift by eddy

Fig. 16. Comparison of uplift height introduced by different mechanism.

Topographical upwelling, eddy introduced dynamic uplift and other minor effects.


Mechanism Analysis (10/11) ─ Topographic effects

Only the realistic bathymetry can constrain sufficient cold water source for surface cold-dome formation.

Fig. 17. Flow field and temperature at 50 m deep of numerical experiments.

(a) Realistic bathymetry (b) Deepened Case (c) Shallowed Case


Mechanism Analysis (11/11) ─ vertical mixing

Fig. 18. Instantaneous zonal profiles of temperature and eddy diffusivities at 25.6°N. The vertical temperature gradient near surface coupled with the high surface eddy diffusivities suggested energetic vertical hear transfer in surface cold-dome.

(a) Zonal Temperature ( ) ℃profile

(b) Eddy diffusivities (cm2/s)

121 121.5 122 122.5 123 123.5 124

20

40

60

80

100

120

140

160

Longitude (°)D

epth

(m

)

0.1

0.2

0.3

0.4

0.5

0.6

121 121.5 122 122.5 123 123.5 124

20

40

60

80

100

120

140

160

Longitude (°)

Dep

th (

m)

14

15

16

17

18

19

20

21

22

23

24

25


Conclusion (1/2)

Of filtered SST we found cold-dome have high occurrence in summer. For each cold-dome last only few days on surface.

Dynamic uplift introduced by Kuroshio dominates the fundamental pattern of cold-dome.

Topography not only suggests topographically controlled upwelling, but also constrains cold water in deep sea northeast of Taiwan.

Mesoscale eddy contributes few dynamic uplift but can reduces horizontal advection for cold-dome.


Conclusion (2/2)

Transport in boundary layers, surface and bottom, is not strong enough in this study.

Vertical mixing plays an important role for surface cold-dome formation.


Thank you for your attention.

The formation and dynamics of cold-dome northeast of Taiwan

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