Data Front

Location:

Lexington, KY

Posted:

November 15, 2012

Contact this candidate

Resume:

Journal of Oceanography, Vol. **, pp. *** to ***, 2008

Characteristics of Thermal Finestructure in the South-

ern Yellow Sea and the East China Sea from Airborne

Expendable Bathythermograph Measurements

S UNGHYEA PARK* and PETER C. CHU

Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943-5001, U.S.A.

(Received 24 August 2007; in revised form 15 March 2008; accepted 28 April 2008)

Four surveys of airborne expendable bathythermograph with horizontal spacing of Keywords:

Yellow/East China

about 35 km and vertical spacing of 1 m extending from the surface down to 400 m

Seas,

deep are used to analyze thermal finestructures and their seasonality in frontal zones

AXBT,

of the southern Yellow Sea and the East China Sea. Finestructure characteristics are

synoptic thermal

different not only among fronts but also along the same front, implying different mix-

fronts,

ing mechanisms. Summer thermocline intrusions with thickness from few to 40 me-

thermal

ters, generated by the vertically-sheared advection, are identified along the southern finestructures,

tongue of the Cheju-Yangtze Front (especially south of Cheju Island). The mixing.

finestructures south of the Yangtze Bank (i.e. the western tip of the southern tongue)

produced by strong along-frontal currents are not as rich as elsewhere in the south-

ern tongue. The Cheju-Tsushima Front presents mixed finestructures due to conflu-

ent currents from various origins. The irregular-staircase finestructures in the

Kuroshio region (below the seasonal thermocline), driven by double-diffusive mix-

ing, show seasonal invariance and vertical/horizontal coherence. The strength of mix-

ing related to finestructure is weaker in the Kuroshio region than in the Cheju-

Tsushima Front or south of Cheju Island. The profiles in the Tsushima Warm Cur-

rent branching area show large (~50 m thick), irregular-staircase structures at the

upper 230 m depth, which coincides roughly with the lower boundary of the maxi-

mum salinity layer. The finestructure at depths deeper 230 m is similar to that in the

Kuroshio region. The possible mechanisms for generating the finestructures are also

discussed.

1. Introduction dient in rather longer vertical distance such as d2 to d3

Temperature and salinity profiles are not only smooth (~13 m) in Fig. 1(a)); (2) thermocline/halocline intrusion,

curves (or straight lines) but also ragged curves (i.e. a when temperature and salinity profiles display multilay-

variety of vertical variations). The vertical variation with ered structures at the thermocline/halocline depths

scales of 1 100 m is termed finestructure and that with (Ruddick and Richards, 2003; see intrusive structures at

scales smaller than 1 m is termed microstructure 50 100 m (50 90 m) depth in profile 2 (3) in Fig. 1(b));

(Warren and Wunsch, 1981). The finestructure is observed and (3) interleaving, which is similar to the thermocline/

in various forms, and some of them, which are frequently halocline intrusion and is highlighted by large (even O

observed and distinctively detected, are named as follows: (100 km) in the equatorial Pacific), horizontal continuity

(1) staircase, also called steppy or stepwise structure of multilayered structures (Richards and Banks, 2002; Lee

(Schmitt et al., 1987; Zodiatis and Gasparini, 1996), when and Richards, 2004). In addition, there are mixed, eroded

temperature and salinity profiles show alternative sheets versions of these forms, and other forms are also possi-

(strong temperature/salinity gradient in short vertical dis- ble (Fig. 1(c)). These finestructures are common anywhere

tance such as d1 to d2 (~1 m) or d3 to d4 (~2 m) in Fig. in the oceans, but more complex and heterogeneous

1(a)) and layers (almost isothermal/isohaline or low gra- around fronts and current confluence zones (Joyce, 1976;

Williams, 1981; Bianchi et al., 1993).

Numerous analyses of finestructures have been con-

* Corresponding author. E-mail: abpnxv@r.postjobfree.com

ducted in the tropics (McPhaden, 1985; Richards and

8 59

Temperature(oC)

Gradient(oC/m)

0.5 0 0.5 **-**-**-**-**-** 24 26 28

18 18.5 19

400 profile 6

10 15 20 25 30

200

Temperature(oC)

Fig. 1. Various finestructures: (a) a staircase (profile 1 observed at 126.3 E, 27.0 N), (b) a thermocline intrusion (profile 2 at

127.1 E, 32.8 N; profile 3 at 126.3 E, 32.8 N), (c) mixed structures (profile 4 at 128.4 E, 34.1 N; profile 5 at 127.2 E,

33.6 N; profile 6 at 127.1 E, 31.8 N). See details in text. Thin black curve in (a) is the vertical temperature gradient of

profile-1 and its scale is on the top of the figure. Inset in (a) is an enlargement of the segment (200 250 m) of the temperature

of profile-1.

(Georgi, 1978, 1981; Toole, 1981), the Atlantic (Joyce, and Chu (2006) suggested that the frontal mixing in both

1976; Schmitt and Georgi, 1982; Schmitt et al., 1986), temperature and salinity fields is vigorous in the region

and the Pacific (Gregg, 1977; Peters et al., 1991). A new connecting the Yangtze Bank, the Cheju Strait, and the

seismic reflection methodology produces snapshots of Korea/Tsushima Strait. Their suggestion is based on the

finestructures at high lateral sampling resolution (

in the Norwegian Sea (Nandi et al., 2004). These analy- fronts throughout YES from a climatological dataset (i.e.

ses enhance our knowledge of mixing processes of water Generalized Digital Environmental Model (GDEM)) in

mass, energy transformation, and internal wave kinemat- terms of frontal intensity, water mass distribution, and

ics. However, finestructures or mixing processes in the temporal evolutions of temperature and salinity across the

Yellow Sea and the East China Sea (YES) have been less fronts. However, finestructure and its seasonality were

intensively studied, although the YES exhibits various not investigated due to the limitation of the climatological

thermohaline finestructures induced by the Kuroshio in- data.

trusion, enormous river discharge, strong tidal currents, Recently, new thermal features were discovered us-

and internal waves in the well-developed shelves along ing four airborne expendable bathythermograph (AXBT)

with trenches (Chu et al., 1997a, 1997c, 2005). surveys with horizontal spacing of 35 km and vertical

Bao et al . (1996) analyzed the vertical scales and resolution of 1 m from the surface to 400 m depth over

spectral characteristics of the finestructures southwest off the southern Yellow Sea (YS) and the East China Sea

Kyushu using springtime profiles and addressed the (ECS) in September 1992 and February, May, and Sep-

finestructure characteristics with respect to water mass tember 1993 (Furey and Bower, 2005; Park and Chu,

distributions. Lee et al. (2003) reported cross-frontal ther- 2007a, b). Furey and Bower (2005) analyzed the vari-

mal intrusion of thickness 5 10 m southeast off Cheju ability of mixed layer depth, path change of the Kuroshio

Island and several high or low salinity cores with hori- associated with the generation of cold eddies on canyons

zontal scales of tens kilometers southwest of the Tsushima northeast off Taiwan, and seasonal evolution of thermal

fronts in YES and the Japan/East Sea . Park and Chu

Warm Current branching region. In addition, several stud-

ies revealed thermohaline finestructures (Lie et al., 1998,

T he Editor-in-Chief does not recommend the usage of the term Ja-

2000, 2003) but did not provide detailed information. Park pan/East Sea in place of Sea of Japan .

8 60 S. Park and P. C. Chu

(2007a) identified synoptic distributions of thermocline

and thermal surface mixed layer (using a different defi-

nition from Furey and Bower (2005)), as well as the

seasonality and dominant driving mechanism of the sur-

face mixed layer. Park and Chu (2007b) further investi-

gated the synoptic features in/around thermal fronts and

the cross-frontal heat fluxes that had not been described

in detail previously. The thermal finestructures in/around

the fronts, such as the thermocline intrusions, ragged iso-

therms, and multilayered temperature inversions, were

described by Park and Chu (2007b) qualitatively but not

quantitatively.

Since the fronts in the YES are linked dynamically

to the current system comprising the Kuroshio, the Tai-

wan Warm Current, the Tsushima Warm Current, and the

Cheju Warm Current, these AXBT data with sufficiently

large horizontal coverage are useful to examine the

finestructures across a front or among different fronts,

having regard to surrounding conditions such as frontal

features, water masses, and the current system. They are

also invaluable in identifying the seasonal variability of

the finestructures in the YES.

The goal of this study is to analyze the characteris-

tics of thermal finestructures, including their seasonality,

of frontal zones in the YES using the AXBT data. The

lack of concurrent salinity/velocity data and time-series

data for analysis makes it difficult to elucidate the gen-

eration mechanisms of the finestructure based on concrete

evidences. Instead, using the AXBT data, CTD data (ob-

served at different time) and hydrographic plots in other

studies, we propose plausible generation mechanisms for

the finestructures and related mixing processes in rela-

tion to the surrounding thermal features, water masses,

and circulation systems.

The remainder of the paper is organized as follows.

Section 2 introduces the AXBT data. Section 3 presents Fig. 2. (a) Four airborne expendable bathythermograph (AXBT)

surveys conducted on 18 29 September 1992 (9209), 4 14

two measures for finestructures. Section 4 describes

February 1993 (9302), 5 14 May 1993 (9305) and 2 10

finestructures in the profiles grouped by the frontal zones.

September 1993 (9309). Contours indicate bottom

Section 5 analyzes spectra and statistical characteristics

bathymetry shallower than 1000 m. Seven grey boxes indi-

of groups with rich finestructures. Section 6 examines

cate locally-grouped profiles: Yellow Sea Bottom Cold

finestructures of each profile in the region where the

Water group (YCWG), west of Cheju group (WCG), south

Tsushima Warm Current is originated from the Kuroshio.

of Cheju group (SCG), Cheju-Tsushima Front group

Section 7 presents our conclusions. (CTFG), Yangtze Bank group (YBG), south of Yangtze Bank

group (SYBG), and Kuroshio Front group (KFG). (b) Iden-

2. Data tified thermal fronts with a temperature distribution at 25

The AXBT data, a part of the Master Oceanographic m depth from the AXBT surveys: Cheju-Yangtze Front

Observation Data Set (MOODS) maintained by the Na- (CYF), Cheju-Tsushima Front (CTF), Tsushima Front (TF),

val Oceanographic Office (NAVOCEANO), Stennis Space and Kuroshio Front (KF) (Park and Chu, 2007b).

Center, Mississippi, were obtained under the approval of

NAVOCEANO. The AXBT data including their detailed

information have not been placed in the public domain. research. Their thermal time constant is 0.1 s or less and

Accordingly, few references have been available, even their vertical resolution is 15 cm (Boyd and Linzell, 1993).

though the AXBT data were collected in the early 1990s. We obtained the edited AXBT data with 1 meter vertical

In 1990s, Sparton AXBTs were widely used in U.S. Navy resolution. The editing process includes indentifying re-

F inestructures in Yellow/East China Seas 861

Temperature( oC) Temperature( oC)

cording errors and outliers, checking duplicate profiles, **-**-**-**-**-** 20 25

checking depth inversion and depth duplication for indi- 0

vidual profiles, checking temperature range, checking

100

large temperature inversions and gradients, checking

150

standard deviation, interpolation to standard levels, and

Depth(m)

200

post objective analysis checks (Teague, 1986; Jugan and

250

Beresford, 1992; Boyer and Levitus, 1994; Chu et al ., 300

1997b, 1998). The accuracy of the data depends on the 350

accuracy of conversion equations, which transform fre- KFG (9209) KFG (9302)

400

quency into temperature and elapsed fall time into depth: 0

the AXBT does not measure depth directly. Customized

100

equations, which were set up using concurrent CTD data,

provide a temperature accuracy of 0.13 C and a depth

150

Depth(m)

accuracy of 2% of the depth or 10 m, whichever is

200

250

greater. The Naval Research Laboratory (NRL) Isis Sys-

300

tem determines the AXBT frequency to such accuracy that 350

the resulting temperature accuracy is 0.05 C or better KFG (9305) KFG (9309)

400

(Boyd and Linzell, 1993).

The AXBT data used in this study consist of 1256 Fig. 3. Temperature profiles in the KFG (for location see Fig.

2(a)).

profiles from the four surveys (Fig. 2(a)): 18 29 Septem-

ber 1992 (named as 9209), 4 14 February 1993 (as 9302),

5 14 May 1993 (as 9305), and 2 10 September 1993 (as

9309). The four surveys almost repeated themselves in

3. Two Measures for Finestructure

track paths, with slight mismatches, and were completed

There are two measures for the finestructure. The

within one week, except for some profiles near the Ko-

first measure is the Turner angle (Tu) (Ruddick, 1983):

rea/Tsushima Strait. Note that the seasonal variability of

frontal structures can be identified. For convenience, Feb-

R ( d / dz ) ( dS / dz ),

Tu = tan 1 ( R) (1)

ruary, May, and September represent winter, spring, and,

summer, respectively.

The aspect ratio (H/ L; H : vertical scale; L: horizon-

where R is the density ratio, and

tal scale) of individual elements of finestructures is

10 4 10 3; that is, a finestructure with a vertical scale of

1 1

few meters has approximately an horizontal scale of a

(2 )

= and =,

few to tens kilometers. The characteristic time scale is S

clearly associated with the corresponding spatial scale and

generally few weeks (Fedorov, 1978). The AXBT data

are the coefficients of thermal expansion and haline con-

with an horizontal spacing of 35 km and a vertical reso-

traction. The water column is gravitationally stable for

lution of 1 m are therefore capable of detecting

Tu /2. Dou-

finestructures. The synoptic features identified from these

ble diffusion overcomes the stratification for /4 0.7(Hayes et al., 1975; Joyce, 1976). the CYF in summer (Park and Chu, 2007b). A variety of

finestructures occur among these groups.

4. Detection of Finestructures in Frontal Regions

The temperature profiles are locally grouped with 4.1 KFG

respect to their vertical temperature structures related to The staircase structure (Fig. 1(a)) is often found in

the thermal fronts identified from these data (Fig. 2): the KFG, regardless of season and depth, except at the

Cheju-Yangtze Front (CYF), Cheju-Tsushima Front surface mixed layer, with irregular layer thickness from

(CTF), Tsushima Front (TF), and Kuroshio Front (KF) a few to 40 meters and sheet thickness around few meters

(Park and Chu, 2007b). The CYF is two-tongue-shaped, (Fig. 3). Some staircase structures are eroded by mixing

i.e. slanted S-shaped (northern and southern tongues). The process and are seen as less-defined sheets and layers.

CTF occurs along the southern coast of Korea and ex- These staircase structures are called irregular-staircase

pands between the Cheju Strait and the Korea/Tsushima (Gregg, 1975; Ruddick and Gargett, 2003) or eroded-stair-

Strait. The TF is a branch of the KF (Park and Chu, 2006, case structures. This irregular-staircase structure in pro-

2007b). files is common in open oceans. According to Bao et al.

Seven groups are also identified (Fig. 2(a)): (1996) s springtime observation at the Kuroshio axis, the

(1) Kuroshio Front group (KFG; in the Kuroshio Front T-S relation is tight in irregularly-spaced layers. Profiles

but the Tsushima Warm Current branching region is not of the vertical temperature (salinity) gradient multiplied

included); (2) Cheju-Tsushima Front group (CTFG; in the by the thermal expansion (haline contraction) coefficient

Cheju-Tsushima Front); (3) Yellow Sea Bottom Cold provided by the World Ocean Database (WOD05; avail-

Water group (YCWG; northwest off the northern tongue able at http://www.nodc.noaa.gov/) displays good coher-

of the CYF); (4) west of Cheju Island group (WCG; in ence between temperature and salinity variations (Fig.

the northern tongue of the CYF); (5) south of Cheju Is- 4(b)).

land group (SCG; in northern part of the southern tongue When the Turner angle is calculated from the CTD

of the CYF); (6) Yangtze Bank group (YBG); and (7) south data with 1 m vertical resolution, it provides a gross mea-

of Yangtze Bank group (SYBG). The last two groups sure of the stability, not an active micro-scale double dif-

(YBG and SYBG) are located north and south of the west- fusion (Kennan and Lukas, 1996). The halocline, from a

F inestructures in Yellow/East China Seas 863

Table 1. Cox number averaged over the group.

0.5 2.0 1

1 0.5 2.0

920*-****-**** 9309

Upper KFG 0.32 0.44 0.40 0.62

Lower KFG 0.36 0.43 0.43 0.40

CTFG 1.10 0.51 1.62

YCWG 0.28 0.35 0.33

Percentage WCG 0.22 0.21 0.26

SCG 7.91 0.56 4.54

YBG 0.09 0.20 0.23

SYBG 0.13 0.24 0.28

file displays ~9 C near the bottom, which is related to

the Korea Strait Bottom Cold Water, the water mass pass-

/2 /4 /4 /2

Turner angle

ing through the Korea/Tsushima Strait from the Japan/

East Sea in the bottom layer. In 9209 the cold water ex-

Fig. 5. Histogram of the Turner angle (or density ratio R) cal-

tends west of 129.5 E along the trench of the Korea/

culated from 260 profiles (51 400 m) in the KFG with a

bin size of /50. Data are extracted from WOD05 for the Tsushima Strait, while in 9309 it does not (see figure 11

in Park and Chu, 2007b).

period of 1991 2002. Density ratio R is marked at the top.

4.3 YCWG and WCG

The YCWG displays smooth curves or straight lines

(i.e. lack of finestructures), but there is a strong seasonal

salinity maximum (~150 m) to a salinity minimum (~500

variability, except in the bottom layer (Fig. 7). A colder

m), lies in the salt finger regime (i.e. double diffusion)

particularly in a weak fingering regime (2 R

ducted in the tropics (McPhaden, 1985; Richards and

Contact this candidate