Skip to main content

Full text of "DTIC ADA599039: Studying the Origin of the Kuroshio with an Array of ADCP-CTD Moorings"

See other formats


DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. 


Studying the Origin of the Kuroshio with an Array of ADCP-CTD Moorings 

Ren-Chieh Lien 
Applied Physics Laboratory 
University of Washington 
1013 NE 40 th Street 
Seattle, Washington 98105 

Phone: (206) 685-1079 fax: (206) 543-6785 email: lien@apl.washington.edu 
Award Number: N00014-10-1-0397 


LONG-TERM GOALS 

Our long-term scientific goals are to understand the dynamics and identify mechanisms of small-scale 
processes—i.e., internal tides, inertial waves, nonlinear internal waves (NLIWs), and turbulence 
mixing—in the ocean and their interaction with mesoscale processes such as western boundary 
currents. We aim to develop improved parameterizations of mixing for ocean models. For this study, 
our focus is on the origin of the Kuroshio, the interaction among internal tides, internal waves, meso¬ 
scale eddies, and the Kuroshio, and the interaction of oceanic processes with the complex topography 
in Luzon Strait. 

OBJECTIVES 

The primary objectives of this observational program are to quantify the origin of the Kuroshio and to 
quantify its properties at the origin and as it evolves downstream. 

APPROACH 

An array of six subsurface moorings was deployed in June 2012 northeast of the Philippines, where the 
strong Kuroshio enters Luzon Strait. All moorings were recovered in June 2013. Each mooring had an 
Acoustic Doppler Current Profiler (ADCP) to measure the velocity field in the upper 450 m. The long¬ 
term mooring velocity observations and complementary shipboard survey will help identify the origin 
of the Kuroshio and its properties before it enters Luzon Strait. We will compare our observations with 
glider and HPIES observations and with downstream mooring observations east of Taiwan to quantify 
the evolution of the Kuroshio. 

WORK COMPLETED 

We recovered six moorings in June 2013 and began data processing and analysis. Results were 
presented at the ONR workshop in Seattle in August 2013 and at the ONR review in Chicago in 
September 2013. A manuscript entitled “Modulation of Kuroshio transport by mesoscale eddies at the 
Luzon Strait entrance” has been submitted to Geophysical Research Letter (Lien et al., 2013). 

We conducted the RR-1307 R/V Revelle cruise northeast of the Philippines (Luzon Strait) during 30 
May - 9 June 2013. We recovered six subsurface moorings and five HPIES, deployed in June 2012. 

1 


Report Documentation Page 

Form Approved 

OMB No. 0704-0188 

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and 
maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, 
including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington 

VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it 
does not display a currently valid OMB control number. 

1. REPORT DATE 

30 SEP 2013 2 ' REPORT TYPE 

3. DATES COVERED 

00-00-2013 to 00-00-2013 

4. TITLE AND SUBTITLE 

Studying the Origin of the Kuroshio with an Array of ADCP-CTD 
Moorings 

5a. CONTRACT NUMBER 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

6. AUTHOR(S) 

5d. PROJECT NUMBER 

5e. TASK NUMBER 

5f. WORK UNIT NUMBER 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

University of Washington,Applied Physics Laboratory,1013 NE 40th 

St,Seattle,WA,98105 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

10. SPONSOR/MONITOR’S ACRONYM(S) 

11. SPONSOR/MONITOR’S REPORT 
NUMBER(S) 

12. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution unlimited 

13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

15. SUBJECT TERMS 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 

ARSTRATT 

18. NUMBER 19a. NAME OF 

OF PAGES RESPONSIBLE PERSON 

a. REPORT b. ABSTRACT c. THIS PAGE Same aS 

unclassified unclassified unclassified Report (SAR) 

7 


Standard Form 298 (Rev. 8-98) 

Prescribed by ANSI Std Z39-18 





We also recovered and deployed a Seaglider, and collected shipboard conductivity-temperature-depth 
(CTD) and ADCP observations and a bathymetry survey. 

RESULTS 

Kuroshio Velocity and Transport 

Six moorings were deployed along 18°45’N in a zonal section between 122°E and 122°52’E, each 
roughly 16 km apart, spanning -80 km at the Kuroshio’s entrance to Luzon Strait (Fig. 1). Each 
mooring was equipped with one upward looking 75-kHz ADCP at 450 m nominal depth. The ADCPs 
took velocity measurements everyl.5 min and recorded averages every 15 mins in 8-m vertical bins 
between 450 m and 45 m depth from June 2012 to June 2013. 

Velocity data are low-pass filtered at 10 days to remove tides and high-frequency fluctuations. The 
monthly averaged meridional current is mostly northward in the observed depth range (Fig. 2). One 
striking feature is the weak northward current, < 0.5 m s _1 , in June 2012, caused by a cyclonic eddy. 
The maximum northward current (the Kuroshio core) often exceeds 1 ms -1 . The Kuroshio core is 
located at ~122°24’E at the surface, and tilts eastward with increasing depths. The zonal current is 
about 30-60% of the meridional current. 

We compute the Kuroshio transport by integrating the across the moored array of the northward 
current from the surface to its maximum depth. The annual average northward transport is 15 Sv with a 
standard deviation of 3 Sv (Fig. 3c). Rapid changes in Kuroshio transport greater than 10 Sv were 
observed. Between 24 June and 4 July 2012, the transport increases from 7 Sv to 22 Sv in 10 days. 
Between May and June 2013 transport variations greater than 10 Sv were observed on a 0(10 days) 
time scale. 

Meso-scale Eddy Effects on Kuroshio Transport 

The sea level anomaly (SLA) and the SLA slope at the latitude of our mooring line 18°45’N, reveal 
clear westward propagation at a speed of ~0.1 m s _1 (Fig. 3a and b). The variation of the SLA slope 
between 122°E and 123°E fluctuates in unison with the observed Kuroshio transport (Fig. 3c). Eight 
anomalous transport events greater than 3 Sv, one standard deviation, are identified (Fig. 3c). The 
Kuroshio transport increases when the SLA slope is large, and transport decreases when the SLA slope 
is small. A linear regression analysis suggests SKT = 6 x 10~ 6 (SLA Slope), where SKT is the Kuroshio 
transport anomaly in Sv. 

Six eddies within 200 km of the eastern Kuroshio boundary, nominally 18°45’N 123°E, are identified 
(Figs. 4 and 5). They are tracked as far east as 126°E. Five stall and dissipate at about 124°E. Six 
anomalous Kuroshio transport events can be explained by the detected eddies (Fig. 5). In particular, the 
large changes in transport of more than 10 Sv in June-July 2012 (events 1 and 2, see Fig. 3c) and in 
May-June 2013 (events 7 and 8) are caused by consequent pairs of cyclonic and anticycIonic eddies. 
These eddies have a typical Rossby number, relative vorticity normalized by the planetary vorticity, of 
~0.2. The area integrated eddy kinetic energy is of order TJ m ! and the mean current speed is about 
0.4 ms -1 . The anomalous transport events 5 and 6 in January-April 2013 may be explained by the low 
SLA to the west of 122°E, and not the westward propagating eddies (Fig. 3a). 


2 



Seasonal Cycle of Kuroshio Transport 


The linear regression between the SLA slope and the observed Kuroshio transport anomaly is applied 
to SLA data over the period of 1992-2013 to study the Kuroshio transport anomaly and its seasonal 
cycle (Fig. 6). The transport anomaly varies between -12 and 7 Sv, with a standard deviation of 3 Sv. 
Spectral analysis shows a significant peak at a 1-yr period. Monthly averages reveal a seasonal cycle 
with stronger Kuroshio transport in winter and spring and weaker transport in summer and fall (Fig. 

6b). 

Eddies within 200 km of the eastern Kuroshio boundary during 1992-2013 are identified. A total of 34 
anticyclonic eddies and 60 cyclonic eddies are identified. The relative vorticity computed from the 
detected eddies is averaged monthly, and shows the strongest anticyclonic in spring and the strongest 
cyclonic in fall. The time integrated relative vorticity of eddies within 200 km of the eastern Kuroshio 
boundary is in agreement (Fig. 6d). Our analysis suggests that the Kuroshio seasonal cycle may be 
partly due to the westward propagating eddies. 

IMPACT/APPLICATION 

The Kuroshio is well defined north of Luzon Strait as a strong western boundary current. Nonetheless, 
its origin and the dynamics of its initiation are not well understood. The potential origin of the 
Kuroshio is complicated by a rich spectrum of oceanic processes, e.g., remotely and locally generated 
eddies. The Kuroshio carries significant mass, heat, and energy from the tropics to the subtropics and 
interacts with marginal seas. Therefore it is crucial to understand its origin and dynamics. 

RELATED PROJECTS 

Generation and Evolution of Internal Waves in Luzon Strait (N00014-09-1-0279) as a part of IWISE 

DRI : The primary objectives of this observational program are to quantify 1) the generation of NLIWs 
and internal tides in the vicinity of Luzon Strait, 2) the energy flux of NLIWs and internal tides into the 
Pacific Ocean and South China Sea (SCS), 3) the effects of the Kuroshio on the generation and 
propagation of NLIWs and internal tides, 4) the seasonal variation of NLIWs and internal tides, and 5) 
to study other small-scale processes, e.g., hydraulics and instabilities along internal tidal beams and at 
the Kuroshio front. 

PUBLICATIONS (wholly or in part supported by this grant) 

Lien, R.-C., B. Ma, Y.-H. Cheng, C-R. Ho, and B. Qiu. 2013. Modulation of Kuroshio transport by 
mesoscale eddies at the Luzon Strait entrance, Geophys. Res. Lett, [submitted] 

Rudnick, D. L., S. Jan, L. Centurioni, C.M. Lee, R.-C. Lien, J. Wang, D.-K. Lee, R.-S. Tseng, Y.Y. 
Kim, and C.-S. Chem. 2011. Seasonal and mesoscale variability of the Kuroshio near its origin. 
Oceanography 24(4), 52-63, doi:10.5670/oceanog.2011.94. [published, refereed] 


3 





SLA Cm) 

-0.4 -02 0 0.2 0.4 




Figure 2: Monthly mean (a) zonal velocity and (b) meridional velocity measured by the moored 
ADCP array. The black curves are constant velocity contours at a 0.2 m s' 1 interval. The white 

curves are 0.5 and 1.0 m s 1 velocity contours. 


Figure 1: Mooring positions (red dots), A VISO sea level anomaly (SLA) (color shading), and 
AVISO surface current (vectors) on 10 May 2013 (a) and 5 June 2013 (b). Velocity reference of 0.5 
m s' 1 is labeled. Two eddies that lead to Kuroshio transport anomaly events are labeled 7 and 8. 


May 10, 2011 


122.2 322.4 122.fi 322LS 

LonfE) 


122.2 322.4 122.fi 322J 

Lon (°E) 


1222 322.4 122.5 322.-E 

Lon (°E) 


Lon (°E) Lon (°E) Ldh (®E) 


Nni 


4 































































































Sea Level Anomaly 


(b) Slope of Sea Level Anomaly (c) 

40 


Kuroshio 
Transport (Sv) 
0 5 10 15 20 



120 130 140 150 120 130 140 150 - 10-5 0 5 10 

Lon (°E) Lon (°E) 10 7 3 n 


Figure 3: (a) and (b) Sea level anomaly (SLA) and its zonal gradient along 18.75°N, respectively. 
The magenta vertical line indicates the longitude and observation period of the ADCP moored 
array, (c) Zonal gradient of the SLA across the moored array between 122°E and 123°E (gray 
shading), and the Kuroshio transport measured by the moored ADCP array (red curve). Eight 
events with a transport anomaly exceeding 3 Sv are labeled in panel (c). 


5 




















(a) Cyclonic Eddies 

r i 


(b) Anticyclonic Eddies 



122 


124 126 

Longitude (°E) 


128 



Figure 4: Three cyclonic eddies (a) and three anticyclonic eddies (b) between June 2012 and June 
2013. Eddies are labeled corresponding to the anomaly events of Kuroshio transport in Fig. 3c. 
Magenta line is mooring array position. Color lines show eddy tracks. Dots and circles represent the 
beginning and the end of eddy tracks. The large closed color loops represent the outer boundary of 
eddies, defined by the boundary of maximum azimuthal speed. 



2012 2013 

Figure 5. Time series of (a) observed Kuroshio transport, (b) SLA slope between 122°E and 123°E, 
(c) minus Rossby number, (d) area integrated eddy kinetic energy, (e) area averaged eddy 
current speed, and (f) distance between the eddy center and the eastern Kuroshio 
boundary, ~18°45’N 123°E. Kuroshio transport anomaly events and corresponding SLA 

slope and eddies events are labeled. 


6 
































































J F MAM J J A S ON D 
Month 


J F MAM J J A S OND 
Month 


Figure 6: Twenty-one years of AVISO data: (a) Time series of Kuroshio transport anomaly across 
18°45’N, computed using SLA slope between 122°E and 123°E, (b) monthly variation of Kuroshio 
transport anomaly, (c) monthly accumulated number of anticyclonic and cyclonic eddies within 200 
km of the eastern Kuroshio boundary, (d) monthly averaged relative vorticity of eddies, and (e) 
monthly averaged relative vorticity of eddies integrated over the time of eddies within 200 km of the 

Kuroshio eastern boundary. 


7