Long-term variability of the Kuroshio Transport East of Taiwan and the climate it conveys

Long-term variability of the Kuroshio Transport East of Taiwan and the climate it conveys

Mao-Lin Shen1

E-mail: [email protected] Tseng1, Sen Jan2,

Chih-Chieh Young1, Ming-Da Chiou2

1Atmospheric Sciences, Nation Taiwan University, Taiwan (R.O.C.)2Institute of Oceanography, Nation Taiwan University, Taiwan (R.O.C.)

April 21, 2023

AOGS 2011 Taipei International Convention Center

Dr. Mao-Lin ShenDr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU, Dept. of Atmospheric Sciences, NTUPage 2

Heat Exchange between Tropics and Extratropics

[Daifang and Philander, 1994, Science]

Kuroshio


North Equatorial Current-Mindanao Current-Kuroshio (NMK) Current System

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

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. [Toole et al., 1990, JPO]


ENSO and NEC

[Qiu and Chen, 2010b, JPO]

Dr. Mao-Lin ShenDr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU, Dept. of Atmospheric Sciences, NTUPage 5112/04/21

[Qiu and Chen, 2010b, JPO]

•North Equatorial Current bifurcation (NECB)

•Zonally distributed wind stress curl


Things we already known

Why does the Kuroshio Transport on Luzon Strait and northeast of Taiwan present different patterns [Hwang and Kao, 2002, GJI] ?


AVISO SLP + MDT

(cm)

(1996~2001)

NEC

STCC


Possible Source of the KT

[Vianna and Menezes, 2010, JGR]

Subtropical

TropicalENSO

West Pacific pattern

STCC

NEC


Questions will be answered

How to identify and estimate the Kuroshio Transport reasonably?

What is the origin/source of Kuroshio if Section PCM-1 is focused? (suggesting the possible climate variability conveyed by Kuroshio Transport)

Is our hypothesis correct?


Methods for obtaining the KT Geostrophic Relationship (GR)

Considering the geostrophic balance is prevailing.

Empirical Relationship (ER) The relation built by linear curve fitting.

Transfer Function (TF) A spectral relationship of the KT and the

associated SSH different.

Kg

gQ H

f

1 2 (Sv) (cm)KeQ C C

( )( )

( )K

Q

Q fT f

f

B A

( ) ( )K KQ f F Q

( ) ( )K Kf F


Model Results

(a)

(b)

The frequency‐dependent “transfer function” [Åström and Murray, 2008] estimates the linear causal relationship between any pair of variables.

( )( )

( )

Spectrum of the KT on PCM-1

Spectrum of the SSH difference

Q

Q fT f

f

Geostrophic Balance

gHQ

f

Modeled SSH on day 180, Year 37

Taiwan Multi-scale Community Ocean Model (TIMCOM)

(c)


Observation

(a)

(b)

KT on PCM-1 Section (Johns et al., 2001)

Geostrophic Balance

gHQ

f

AVISO SSH

(c)

(d) ( )( )

( )Q

Q fT f

f


Re-constructed KT(a)

(b)

Seasonal variation

Interannual variationKgQ


(a)

(b)

KT vs. -WP

KT vs. ENSO


Possible Source of the KT

[Vianna and Menezes, 2010, JGR]

Subtropical

TropicalENSO

West Pacific pattern

STCC

NEC


Different KT Phases

(a) (b)

(c) (d)

(e) (f)

Kg Kg QQ Q

Kg Kg QQ Q

Huge NEC transport

Limit NEC transport

mean

mean

mean

std

std

std


Conclusions

The re-constructed KT obtained by GR and TF shows good agreement with the modeled ten-year KT.

Using a longer period of observation data, the TF can provide a very good view on the relationship between the KT and the associated SSH difference and minimize uncertainty.

The GR can be applied only when the focused period is longer than 4 months (low frequency) and is good to obtain the long-term KT variability.


Conclusions

The annual-averaged KT on Section PCM-1 was controlled by the northern branch of the NEC and mesoscale eddies generated from the STCC while the NEC is strongly influenced by the ENSO events, and the mesoscale eddies are related with the WP pattern.

Future works The heat transport from tropical to extratropical via KT

have to be determined. (The knowledge of thermocline variation is necessary.)


Thank you for your attention.Questions?


TF of modeled ten-years KT(a)

(b)


Re-constructed KT

KeQ


STCC

•STCC, subtropical countercurrent

•Ekman convergence forcing, which associated with meridional temperature gradient and zonal geostrophic velocity

[Qiu and Chen, 2010a, JPO]


Fig. Comparison between the interannually-varying Ekman forcing term and the interannually-varying EKE time series in the STCC band.

[Qiu and Chen, 2010a, JPO]


Can sea surface height be used to estimate oceanic transport variability?

Ivchenko et al. (2011), GRL, 38, L11601. The relation between the

sea surface height and the meridional transport across a zonal section at 26.5°N in the North Atlantic


DUPOM (North Pacific adaptation of TIMCOM)


Numerical 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 Modified Richardson number dependent formula based on

Pacanowski and Philander (1981)


Model validation(a) (b)

(c) (d)

Fig. Comparison of SSH and the cross-section velocity on Section PCM-1.[Tseng et al., 2011, PO]


Fig. TIMCOM output and the schematic of the western North Pacific current system. Annual mean has been taken.


PCM-1

NEC contributed

Streamfunction of annual-averaged model results (Sv)

(0-800 m) Ratio:

NEC:PCM-1 1:2.1978

Documents

Long-term variability of the Kuroshio Transport East of Taiwan and the climate it conveys