North Equatorial Current (NEC) bifurcation

報告人：沈茂霖　　　　 (Mao-Lin Shen)

112/04/21

Seminar report

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 2

Thermohaline Circulation

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 3

The Great Indo-Pacific Communicator

Fig. The Great Indo-Pacific Communicator. (by Oppo and Rosenthal, 2010, Science)

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 4

NECt-Mindanao Current-Kuroshio (NMK) system

Fig. Schematic chart of geostrophic volume transport relative to 1200 dbar-depth in the North Equatorial Current-Mindanao Current-Kuroshio (NMK) system. (by Nitani, 1972)

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 5

Cruise measure

Fig. NEC bifurcation near the Philippine Coast. NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. (by Toole et al., 1990, JPO)

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 6

Cruise measure

Fig. NEC bifurcation near the Philippine Coast in different cruise measures.

(b) 23 April 1988(a) 20 Sep 1987

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 7

Fig. Time series of the NEC bifurcation latitude anomalies (solid line) versus the southern oscillation index (SOI) (dashed line). A 12-month running mean filter was applied. (by Qiu and Lukas, 1996, JGR)

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 8

NEC bifurcation point moves southward in summer and northward in winter.

The bifurcation is depth dependent. The northernmost Bifurcation point is at warm ENSO

phase (El Niño) and the southernmost at cold ENSO phase (La Niña).

The seasonal bifurcation and inter-annual bifurcation are due to different mechanisms.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 9

Streamfunction

Fig. Comparison different methods applied for determining streamfunction.

(b) Winter(a) Summer

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 10

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 11

Summer and winter (1)

Fig. Comparison of stream function on summer and winter.

(b) Winter(a) Summer

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 12

Summer and winter (2)(b) Winter(a) Summer

Fig. Comparison of stream function on summer and winter.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 13

Depth dependence

Fig. Stream function comparison of the flow rate integrated from different depths.

(b) 0-400 m averaged.(a) 0-800 m averaged.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 14

Depth dependence

Fig. Stream function comparison of the flow rate integrated from different depths. An interesting phenomenon is the deep circulation in the SCS is stronger than surface circulation.

(b) 400-800 m averaged.(a) 0-400 m averaged.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 15

Annual variation of LST

50 100 150 200 250 300 350

500

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th (

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Fig. Annual variation of Luzon Strait Throughflow.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 16

Impact of different sill depth

Fig. Comparison of different sill depth.

(b) Shallower Bathymetry(a) Realistic Bathymetry

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 17

Impact of different sill depth(b) Shallower Bathymetry(a) Realistic Bathymetry

Fig. Comparison of different sill depth.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 18

Impact of different sill depth

50 100 150 200 250 300 350

100

200

300

400

500

Day

Dep

th (

m)

-4

-2

0

2

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50 100 150 200 250 300 350

100

200

300

400

500

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th (

m)

-4

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Fig. Comparison of different sill depth. Top panel: RB; bottom panel: SB.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 19

The impact of southern throughflow

Fig. Comparison of stream function on different southern throughflow.

(b) 8 Sv(a) 0 Sv

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 20

The bifurcation may be due to upstream flow rate and the local topography as well as the surrounding current system.

Depth dependent bifurcation may have seasonal variation and the mechanism shall be found.

Does the bifurcation have connection with the SSHa attributed by ENSO events?? How could we verify this by our model??

The seasonal bifurcation is determined by the LST.

112/04/21 Mao-Lin Shen, Dept. of Atmospheric Sciences, NTUMao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 21

Thank you for your attention.