Analysis of year-to-year variation of water temperature along the coast of the Japan sea

Prog. Ocemu~g.. Vol. 17, pp. 337-357. 1986.

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Analysis of year-to-year variation of water temperature along the coast of the Japan Sea TOMOWO WATANABE,* KIMIO HANAWA* and YosInAKI TOBA* (Received 30 November 1985; in revised form 20 December 1986; u¢'ctTm,d 15 Jam,u 3' 1987)

Abstract--Mean monthly records of coastal sea surface temperature data (CSST) obtained from stations along the Japanese coasts of the Japan Sea and from those in the related seas for the period 1941-84were analyzed by using various methods of time series analysis, for the purpose of clarifying the nature of the year-to-year variations of the state of the Japan Sea, The year-lo-year variations in the Japan Sea were found to be closely related to those in the East China Sea and in the western North Pacific. Specificresults are as follows. ( 1) A sudden coolingin the early 1960"s occurred in the southern stations of the Japan Sea and continued to the end of the analyzed data. (2) Variations, with about a 6-year pcriodicity, wcrc observcd at most stations, and wcrc cspccially dominant in thc southcrn stations of the Japan Sea. (3) Thcsc varialkms could bc traced back to the Kuroshio region of the East China Sea. (4) Variations, with about a 10-year periodicity, were also observed in the northern stations of the Japan Sea.

INTRODUCTION

TIlE JAPAN SEA has almost h o m o g e n e o u s water below about 400m depth, which is known as the Japan Sea Proper Water. The Tsushima Warm Current flows into the Japan Sea from the Tsushima Strait, and after following a widely spread path on the Japan Sea Proper Water in the southern part of the Sea, it flows out from the Tsugaru Strait to the Pacific Ocean and from the Soya Strait to the Okhotsk Sea. Though the warm water has much less volume than that of the Japan Sea Proper Water, it has a crucial effect on the heat budget of the Japan Sea, In fact KATO and ASAI (1983) showed that the heat transported by the Tsushima Warm Current to the southern part in summer is responsible for the large heat release to the atmosphere from the Japan Sea in winter. The Tsushima Warm Current exhibits large year-to-year variations as well as apparent seasonal variations. The volume transport of the Tsushima Warm Current shows the most apparent seasonal variation, and its amplitude is comparable to the annual mean value as shown by TOBA, TOMIZAWA, KURASAWAand HANAWA(1982, hereafter referred to as TT'KH). q-TKH also showed that the seasonal variation of the volume transport in the Tsushima Warm Current is caused by the sea level difference between the East China Sea and the sea east of the Tsugaru Strait. NAGANUMA(1985) summarized the year-toyear variations of the Tsushima W a r m Current from hydrographic data in summer for the period 1953-82 and showed that a 6-year periodicity is dominant in changes of the flow pattern and in the variations of the areal mean temperature at 50m depth, of the volume transport at the west of the Tsugaru Strait, and of the area of the warm water * Department of Geophysics. Faculty of Science, Tohoku University, Sendai, 98(1Japan. 337

.'~.'~N

I.W.xl.\n.\l~t

ct ,d.

region. MIHA and TAWArA (19~4) also showed lha! a 6- to 7-yclkr periodicilv is predominant in the year-to-year varialion of water temperature at the Easl Tsushima Strait. As to tile causes of these year-to-year v,uiations, especially those ol the variatiolls with 6-year periodicity, the authors point out that the variations are connected with larger scale phenomena in the atmosphere and/or the ocean. The purl~ose of the present paper is to further invcsligale the nillure of the year-to-year vari;~tions in Ihe ,lal~an Sell in relation to those in the adjacent seas of Japan. Forlunalely, the routes which bring the variations into the ,hq~an Sea are Iwo-fold, that is, the route l'rom the East China Sea through the Tsushima Strait, ,rod the connection with the atmosphere through the sea surface. These confined boundary conditions may make the interpretation of the year-to-year variations easier. In order to investigate the relationships between the year-to-year variations in the Japan Sea and larger scale phenomena, we need data based on long, continuous observations. Since the Coastal Sea Surface Temperature (CSST) data have a long continued observational base, they are useful for investigations of climatic time scale phenomena, as long as the CSST data sufficiently reflect the conditions of the sea around the station. In this study, after demonstrating the usefulness of the CSST data, we will analyze them by using various methods of time series analysis. I) A T A

ANI)

PRO('EDUIC,

E OF ANALYSES

Data The CSST was measured at 10 o'clock every day (or every five days at several stations), and ten-day mean values and monthly mean values calculated from daily values were published in the Geoph),sical Review for the period 194(I-61 by the Japan Meteorological Agency and in the Res,lts of Marine Meteorological a,d Ocea,ographical Observatio,s (since 1962) by the same agency. At the present time, observations of the CSST are done at 23 stations in Japan by the Japan Meteorological Agency. We selected nine stations along the Japanese coast of the Japan Sea [1-9] and six stations in the related seas [10--15] as shown in Fig. 1, and collected monthly mean CSST data from the above-mentioned publications. Record lengths of the collected data set are listed in Table 1. The record lengths of most stations ;ire more than 40 years. The sites of CSST measurements were sometimes changed at several stations, and these changes at Wakkanai [1] and Tobishima [4] affected the data crucially as shown later. At eight stations, there were months which had no monthly mean value. Interpolations for these months were performed as follows. For the month which had no monthly value, but in which one or two ten-day mean values were available, the monthly value was estimated from the anomalies of these ten-day mean values. For most of the stations, all of the months with no monthly mean wllue were interpolated in this way, with the exceptions of Naze [11], lshigakijima [12] and Hachijojima [13]. When periods with no monthly mean value and with no ten-day mean values did not continue for more than three months, we interpolated the values from the anomalies of the months just before and after those months (two months at Naze [11] and two months at Ishigakijima [12]). At Naze [11] and Hachijojima [13], there were some periods with no values continuing more than three months (Table 1). We estimated these values for Hachijojima [13] from air temperature data by using a linear regression method and by referring to another data

339

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140" E ,

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,,

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~5~ Aikowaz /

L~J~ ~

I

/I,4

I

i::::i

N ~ [~]~iHarh da

i

N ~

~]J Naze, [i2] Ishigakijirna ~] Hachijojima Miyako Urak&wa i 140'E

-- 5

-

[] aea

o I

o[~

"

-

130*E

N

,0.

Fig. I. Locations of Ihc 15 ('SS'[ +stations used ill this study. Nulncr,tls ill each square show the station nunal~cr. The nine stations frona \Vakk,mai [ 1] to IzuJlara 191 ,,re i,, the Japan Sca ,rod the olhcr sta~tions arc in the related seas. Tahle 1. 7)me .Wan q f C S S T data. period with tto data cmtti,uing l o t more than three montlrs (P.N.D.), izttmt~('r Qf mOltlh.~' wilh no Imblished nlottlltlv ntdalt |'ahle (N. M. N. ). attd rear~ n'hdt~ .~ite o l ( ' S S T mca~ttrdntott wax chatt t~c'd (S. M. ('.) .fin" each station Station I. 2. 3. 4. 5. 6. 7. ;';. 9. Ill. I. 2. 3.

Wakkanai Suttsu Esashi Tobishima Aikawa Wajima Saigo Hamada Izuhara Ushibuka Nazc Ishigakijima Hachijojima

4. Miyako 5. Urakavca

Time Spa n Jan. 1941-Dcc. Jul. 1930-Dcc. Jan. 1941-1)cc. Jan. 1941-Dcc. Jan. 1941-Dcc. Jail. 1932-Dcc. Nov. 1939-Dcc. Dec. 1941-Dcc. Jtln. 1933-Dcc. Jtln. 1955-Dcc. Jun. 1930-Dcc. Jan. 1929-Dcc. Oct. 1935-Dcc.

P.N.D. 1984 1984 1984 1984 1976 1984 I984 1984 1984 1984 1984 1984 1984

Jan. 1929-Dcc. 1984 Jan. 1941-Dcc. 1984

M , m 1945-Aug. 1945 / \ u g . 1944-Feb. 1946 Dec. 1979-Apr. 198(I

N.M.N.

S. I",I.C.

21 (I U 4 9 0 (1 11 11 0 8 2 33

Dec. 1959, Jul. 19711

Apr. 1956. Oct. 1962

Jan. 1964 Nov. 1946. Apr. 1967 Feb. 1962

4 2

set of CSST at Hachijojinaa Island published in the Datum collection of Tokai Regional Fisheries Research Laboratoo' No. 10 (Tokai Regional Fishcrics Research Laboratory, 1982). For Naze [11] we cstimatcd values by the samc mcthod from CSST data and air tcmpcrature data of Yakushima Ishmd which is located north of Naze. Further, in ordcr to get a continuous data set for the period 1941-84, the no data period from January to

340

T. \V,~I.x',xl~l ,.t ,,I.

November of 1941 at llamada [8] was filled with the valttcs estimated from the ('SNT data at Wajima [(,] and Izuhara [9] by using a linear rcgrcssion method. l:';ocedttre o./" m m l v s e s

The long-term averaged seasonal cycle was calculated first and subtracted from time series of monthly means to make an anonlaly time series, in order to observe the year-toyear variations, we plotted the anomaly time series smoothed with the thirtccn-month moving average filtcr. Then the Empirical Orthogonal Function (EOF) analysis was performed for these filtcred data. We then made spectral analyses to detect the dominant lime scales of the year-to-year variations. The spectrum of edch anonlaly time series was calculated by the Maximum Entropy Method (MEM). Though the MEM is thc most suitable mcthod for the detection of signals with climatic timc scales even for relatively short-period records, there are two serious problems in using it. One is the diffict.lty in obtaining the correct number of terms of the Prediction Error Filter (PEF). Due to this, after calculating spectra with wlrious numbers of tile PEF, we chose 30-40% of record length as a suitable number of the PEF for our data set. Tile othcr problem is that we cannot check the confidence limit of each spectral peak (Cure, iv., 1981). Statistical significance of signals which were detected by tile MEM in the frequency domain will be discussed in the Appendix. We employed a filtering method to cover these shortcomings of the spectral analyses by the MEM. The variations revealed by significant peaks in the low frequency range were extracted from the anomaly time series by the bandpass filtering method, and tile bandpass filtered time series were plotted on the time domain. As bandpass filters we used the Nearly Equal Ripple (NER) smoothing filter by KAISER and Rr-_ED (1977). Although a large number of terms (297 terms in the present study) were used for this filter and the wlriations in the periods of both the first and last thirteen years or so of each time series could not be expressed, the long length of the CSST data enabled us to show the variations with 6- or 7-year periodicity.

AVAILABILITY

OF CSST DATA

It is important to ascertain whether or not the CSST reflects offshore Sea Surface Temperature (SST), because the CSST is influenced by many localized factors. To remove this uncertainty, we have compared the CSST time series with offshore SST time series at several stations. As an example, in Fig. 2, we show the comparisons between the year-to-year variations of CSST at lzuhara [9] and of the water temperature at a station about 25 km offshore from Izuhara [9] in the East Tsushima Strait (ETS). A strong relationship is noted between the time series of the CSST based on daily measurement and those of the water temperature in the East Tsushima Strait based on monthly observations by Fukuoka Prefectural Fisheries Laboratory. The correlation coefficients are 0.7-0.8. Further, the distribution of the correlation coefficients between anomalies of the CSST at Naze [11] and those of the SST in the western North Pacific for the period 1969-79 also shows that the CSST reflects well the sea conditions around the station, as shown in Fig. 3.

341

Anahsis o1 ,,car-t,,>,.car ,,'ari;ni~m

°c 1.0 1

r'~-",

r..~

-I.0

Izuhara

CSST

[

~ " @~F" Ir ~ t

,

ETS

] C.Coelt.=0.7/-,

1.07

/"~ ______~_1.~./

][

~ f\ ).,.~., _ ~

-1.oJ c.c~,=0.77

50

~'

~

I

1.0

[ -1.0

_ W

.eq5Om~ depth F . , ~

, ~,

1[" "V

60

°C

sumace

70

"w

L

80

YEAR Fig. 2. Anomaly time series of CSST at lzuhara [91 and of water temperatures at {I m. 5(I m and I()() ill at ;I stalion al'~c,tll 25 km ol'l'shore from Izuh',lra in thc East ]suslfin~a Strait (ETS). for Ihc pcriod 1949-78. The time series in lh¢ ETS was rcpr~duced from Mill,,, and TAWARA (1984). The data are smoothed v,,Jth the 12-month moving average IJlter. Correlation coellicicnt (C.Coclf.) between CSST and each of the time series in the ETS is shown.

12d'E

4d'N

13d'E

140"E i~::I

....

/..,0°N

30':'N

2(~N

iSO°N

30°N

~ .

0.2~ 130"E 14d'E

1'50:'E

Fig. 3. Distribution of correlation coeflicicnt between the monthly anomaly timc series of CSST at Naze [ 1 I1 and those of SST in the western North Pacific. The monthly mean SST data are obtained for each 2 ° (lat.) x 5" (long.) grid for lhe period 1969-79 by Mr. N. IWASAKA(personal comnlullicaliOll ).

RESULTS -- DESCRII'TION

OF Y E A R - T O - Y E A R

VARIATION

The Sudden Cooling in the Early 1960's and the Following Low CSST Period Throughout the Japan Sea coasts, a sudden cooling in the CSST was noted in the early 1960's, with the following low CSST period continuing up to the end of the analyzed data (Fig. 4). Sudden warmings were observed at Wakkanai [1] in the early 1970's and at Tobishima [4] in tile early 196(1's. However, these might be caused by the ,novenmnts of

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'vl+"-~v'v'" 40

50

60

r~ll 70

-

B0

YEAR

3'o ~o

s.0

do

7'o

70

YEAR

Fig. 4. Fifteen anom~dy time series smoothed with a 13-month moving ,lverage lilier. Numerals in each StlU;.ire show the station numl~er (see Fig. I). In the periods shown hv thick horizontal hars, the 13-month m o v i n g averages include periods wllh no tlala continuing for More lhan three illonlhs, in which the ~inorll;.iiy values for the nlOlllhS are cSlillll.iled I~,v {I linear regression method.

tile CSST measuring sites, since these occurred around 1970 at Wakkanai [1] and around 1962 at Tobishima [4] (see Table 1). The sudden cooling and the following low CSST period after the early 1960"s appeared also at Naze [11] and Hachijojima [13]. The difference between the 22-year averaged annual mean CSST's for the period 1941-62 and for the period 1963-84 (Table 2) shows that at most stations, the values were lower in the latter period. At Izuhara [9], where the cooling is most apl~arent, the difference is ahotlt ().7°C. The statistical significance of these differences was checked by thc t-Lest. If we assume that both l~opulations have normal distributions and the standard deviations are equal, we can test a hypothesis that there is no difference between the mean values using the t-distribution (HAys, 1981 ). In the present case, we can reject the hypothesis that there is no difference at 1% significant level when the obtained t-ratio is over 2.57 in absolute value. Table 2 shows that the differences arc signilicant at Esashi [3], Wajima [6], Izuhara [9], Nazc l Ill and Hachijojima [13]. It is also evident from Fig. 5 that the CSST decreased especially in the winter season ( D e c e m b e r - F e b r u a r y ) a t Wajima [6] and Izuhara [9]. The same feature is also observed at Naze [11]. At Hachijojima [13] and Esashi [3], the CSST decreased in

An;tlv:,is of ycar-to-ycilr v;ui;ttion

7'able' 2.

343

( "om/~ar;s~m hetwcen the 22-yem" avdrak'ed atlnttal nlCatt f',SS'l" /¢*r Iltl' pct i<;d 194 I-(~2. 1"~, aml IIlat /~*r the period 1963-84, IX Ti

Vi

T,

V.

I)T

/-ratio

12.13 13.(IN 16.74 17.,";I 1X.39

(I.2h H.32 1). 13 I).lg 0.15

I 1.93 12.31 16.29 17.65 IS. 12

11.17 (I.2(I (1.10 0.20 0.20

-(I.21) (I.77 -11.45 -(I. 1~~ -ll.27

1.43 501 3.92 1.22 2.14

9. [zuh41a I I. Nazc 12. Ishigakiiilna

19, 14 23.43 24.82

11.14 O. 11 0.07

1g.47 23.(13 24.92

(1.21 (I.lig II. I 1

-0.67 -I).4(l I). 1()

5.31 4.31) I. I I

13. 1 lachij,.~jiIna 14. Mivako 15. Ulahk;l\+,a

21.92 12.8(I 8.91}

0.24 (I.41 {I. IS

21.39 12.56 8.75

11.35 (I.29 II. 17

-11.53 -I).24 -0.15

3.24 1.35 1.19

Statitm 2 gult,,u 3. [Lsa,,h L (~. Wajim:t 7 Saigo S. I lim~adzl

Unib, in :C. !/, and V, shows ,,,arianccs for these periods, respectively. I ) T = T, - "/'t. The t-ratio is

where t~/i and N, shows degree of freedom for each period (,'VI = ,;x,, = 22).

°C 2.01

Waj ima (6)

1.0

----

2.0

Izuhara

O cI

2 (9)

i

c~

E

(11

o.o

I ,-,-,TI~| I I I I ~k,'¢/I

I I"~t~

,If I I

~ ,I ~I

~ I

-,.o I -2.0

J F M A M J J A S 0 N D Month Fig. 5. Difference between the 22-year averaged sca.',onal cycle for lhc period 1941-62 a n d / h a l for the period 1963-84. Closed circle at each month shov,'s difference o1 m e a n CSST for the period 1963-84 from that for the period 1941-62 and the vertical bar shov,s the inlcrval of twice of ~,tandard deviation for the period 1963-84. A pair of horizontal bars at each m o n t h shows it foe the period 1941-62.

344

T . W A I AX.,XlU: c,' a/.

summcr as well as in winter. This cooling is not st) clear at the northern stations [2, 14, 151, and change of the seasonal cycle demonsmfled completely opposite signs from those of lhc southern stations [6, 9, 1l], whc,c the CSST incrcascd in winter and decreased in the other seasons. The remarkable changes of sea conditions in the early 1960"s were also observed m the western North Pacific region, mJ[rA and TAWAI,IA (1984) pointed out that thc average walcr tcmpcraturc it] the East Tsushima Strait began It) decrease after 1962. WIJARt:, NAVATO and NEWELL (1976) showed a "'dramatic cooling" occurred in 1962 in the western Pacific by an EOF analysis. Moreover, lit)A, KArAGIRI, MAH)A and KAMIIIIRA (1975) compared the long-term averaged monthly SST for the period 1956-70 in the western North Pacific with those for the period 195(I-59, and pointed out that the monthly mean SST for February decre:tsed in the area from south of Honshu to the East China Sea, although that did not represent a remarkable variation for August. These facts show that the sudden cooling of the CSST in the early 196()'s rellccted the changes in htrge-scale oceanic conditions of the western North Pacific in the 1960's. We also can recognize that the low CSST period after the early 1960"s was made up by spike-like temperature changes which occurred in 1962-63, 1968-72, 197(>77 and 198082. NAGANUMA (1985) pointed out that the year-to-year variations of the sea conditions of the Tsushima Warm Current corresponded well with those of the zonal index of the winter season in the Far East region. The years of low CSST correspond with the years in which the zonal index is low. KAWAMURA(1984) performed an EOF analysis of the SST of the North Pacific, and showed that the time coefficient of the second mode in February has a good correlation with the zonal index (40°N-60°N, 90°E-170°E) at 500rob level in January. He has explained this as follows: "A negative value of the time coefficient was related to the low index circulation in which the outbreak of cold air mass from the Asian Continent increased and a htrge amount of latent heat was released from the sea, causing negative SST anomalies". A comparison of year-to-year variation in seasonally averaged CSST with the Monsoon Index in the winter season (Fig. 6) shows a remarkably negative correlation for the period from the 1960's to the 1970's. These facts suggest that stronger outbreaks from the Asian Continent in the winter season are responsible for the low CSST period after the early 1960's.

°C . . . . . . . . . . . .

,,,,~o '"~' . . . . . .

- ~

-2.0 J

mb c - ~ = - ° m F 7.0

t i

P

40

50

I

'V i

'L-7.O

I

60 70 80 YEAR Fig. 6. fllllC series of CSST anomtl[ies averaged for winter (December-February) tit Izuhara [91 and of Monsoon Index (M.I.) in winter (December-February) which is represented by the sea level atmospheric pressure difference betwcerl the points (50°N. 105°E) around Lake Baikal in Siberia and (45°N. 145°E) around Hokkaido, and these values correspond to the strength of outbreaks in the western North Pacilic. Time series of Monsoon Index was reproduced from "'Report '84 of abnornlal meteorok)gieal phenomena" (Japan Meteorological Agency. 1984).

l\llab,Ms tli )eal-lo-yc~lr V~ili,ilitlll

345

Gem'ral hm'rrelatioltslfil~S amottg Year-to-Year Vm'iatioHs Wc calculated correlation coefficients among the smoothed tilne series of all the st,ttions for the period 1941-84 except for Aikawa [5] for the period 1941-76 and Ushibuka [10] for the period 1955-84. Correlation coefficients for Wakkanai [1] and Tobishima 141 were calculated as the average of the two coefficients for the periods bclore and after the year when Ihe site of (7SS'1" measurement was charlged. We IIlen performed an EOF analysis by using the correlation coc['ficicnt matrix for the eleven time series for the period 1941-84. Good correlations are foulld nlllt)i]g the slatioils in the .l~lpan Sea. beiwecn the Japan Sea and Miyako I14] and between Izuhara I9i a,,d Hachijojima l l3] (Fig. 7). The first mode (Fig. 8), which can explain 46.7'70 of the total cncrgy, also rcveals the positive correlations among Miyako [141 and Hachijojima i13] and stations in the Japan Sea (Fig. 8a). As pointed out by FtJ~.mo~A (1962), the high correhition between Miyako [14] and the Japan Sea [2-9] is due to the Tsushima-Tsugaru Warm Current system. On the other hand, the good correhition between lzuhara 19] and Hachijojima [13] suggests that the year-to-year variations of the CSST at both stations have the same origin in the Kuroshio or in the atmosphere over the sea area south of Japan. The time coefficients of the first mode (Fig. 8b) also reveal the cooling of the CSST in the early 1960's and the low CSST period after the early 1960"s as described in the former section. Compared with the first mode determined by KAWAMt'RA'S(1984) EOF analysis of SST in the North Pacific Ocean, which shows mainly variations of the eastern tropical Pacific, it seems that CSST peaks correspond to peaks representing El Nifio with a time lag of about 1 year. The spectrt.m of the time coefficients of this mode reveals that a 6- to 7-year periodicity is predominant (Fig. 8c). The second mode (Fig. 9), which can explain 18.3'?'o of the total energy, is a seesaw mode between the northern stations [2, 3, 14, 15] and the southern stations [8, 9, 11, 12, 13], the fulcrum of which is located in approximately the Wajima area (Fig. 9a). That is,

2

3 4

Station Number 5 6 7 8 9 101112131/..15 1 2 3 4 ~n

sg

66' 2~ 7 Z

9g [--E

11 12 ~3 14

Fig. 7. Crosscorrelation coefliciems among tile liftecn stations for the period 1941-84. cxcepi for Aikawa [5] for the period 1941-76 and Ushibuka [l(I] for the period 1955-84. Coeflicients for Wakkanai [ 1] and Tobishima [4] were calculated as the average of two coefficients for the period~, before trod after the year when the sites o[ CSST n~casurernent were changed.

IA,6

T. \~/.\IAN\IH

,'1

01.

snF1FIFIFj,=£ (a) E i g e n v e c t o r

(c) S p e c t r u m of T.C.

100.0

50.0 A 0.0 0.0

Station

1 ~ 0.2 0.4 F r e q u e n c y (CPY

0.6

(b) Time C o e f f i c i e n t s

°1

~5 ~O

I

I

I

I

i

I

30

40

50

60

70

80

[

Year

Fig. 8. Results of the EOF analysis for the year-to-year variations of CSST. (a) Eigcnvcctor of the lirsl lllOdc. Nulncrals in each ~,tlllHl'C show I[IC MilliOn ntllllbCl" (>,CC Fig. I). (hi Time cocl'liCiClU of the lirst mode. (c) Spectrum of the time cocflicicnl calct, latcd by lhc MEM m which about 40% of the record length (2l)(I tcrms for lhc 516 months record) was used for the PEF.

(a) E i g e n v e c t o r

1 t

.... LIU U

(c) S p e c t r u m of T.C.

. .nN.F}..F-I . . . . . . .

2s.o 0.0

0.2

Frequency

Station

0.4 (CPY)

0.6

(b) Time C o e f f i c i e n t s

5.0

-5.0

I

I

I

I

I

I

30

40

50

60

70

80

Year Fig. 9.

As in Fig. 8. but for the second mode.

this mode shows the variation of CSST difference between the northcrn and southern parts. The time coefficients of the second mode (Fig. 9b) exhibit the period of large CSST difference during the 1950's and that of small CSST difference during the 1960"s. The second mode by K,\WAMURA(1984), which shows mainly variations of the area around Japan, also expresses variation of SST difference between the northern and southern parts of the western North Pacific. The time coefficients of his second mode are similar to ours. Especially, the stronger coolings around 1963 and 1968 are expressed in both time coefficients. The spectrum of the time coefficients of our mode demonstrates the existence of a strong decadal variation (Fig. 9c).

347

illillyM:-, o[ .',/Cill-tO-ycilr variation

Tinle Scales 0/" Yeur-to-Year Vari(itions The spectra

i l l t h e allc)lllaly tilllC s e r i e s o f t h e C S S T ill [ ] f i c c n s t a t i o n s w e r e c a l c u l a t e d

by the MEM for the range 0.0--0.6 cpy (Fig. 1()). In the low frequency range, we can detect two remarkable spectral peaks around 0.09 cpy and 0.16 cpy (hereafter the variation represented by the peak at 0.09 cpy is referred to as the 10-Year Periodic Varilltion, 10-YPV. and (1.16 cr>y as the 6-Year Periodic Variation, 6-YPV). The former peak appears at the northern stations [1-4] and appears its a small peak at Wajima [6] and Naze [111. The latter peak appears at most stations. MIITA and TAWARA(1984) calculated the spectra for the water tenipcraturc data in the East Tsushima Strait and showed the existence of 6- to 7-year periodicity. Further, In),x, KATAC;II~I,MAt~Daand KAMHHI~A(1974) performed spectral analyses of the SST in the western North Pacific Ocean for the period 1956-7(), in which at 5- to 7.5-year pcriodicity wlis signilicanl in the western North Pacilic. It was especially strong in the Oyashio region and the Kuroshio-Oyashio Confluence zone (the sea east of Japan between 35°N and 45°N) and the coastal region of the Asian Continent (around the East China Sea). These facts suggest that the 6-YPV in the Japan Sea has a close relation to the variations in the western North Pacific, through the Tsushima-Tsugaru Warm Current system which was considered to be mainly driven by the sea level difference between the East China Sea and the sea east of the Tsugaru strait (the Oyashio region) as pointed out by TTKH. In the following sections, we will investigate these two periodicities in detail by using bandpass filtering methods.

10

Period

(Years)

5

3

10

2

50 -~ I , t ~ -

~" 0 0

o~

.~

507

.-

o.o-L ~ ~

~ -5

_l ~

~50

. . . . . . .

.

I

2

i~,

!

® ~,o

~

"-

^~

-~.f--

,

'~'

"-~-~-~-0.0

',

_EI .-

---~ ~

, c

~

00

O1

02 03 04 ~equency(CPY)

I~, r ~°

5.0-

1

i

"1

QO

0.0

Jr- 0 ' 0

r-a-] i-~.o

.. . . .

i ?

[-

.

oo - ~ - ' ~ - - - - - ~ - ~

..

',

to 0 . 0 - ~ ' - -

.

!

~25

(Years) 3

• .

i

Period 5

05

, 0.1

02 0.3 0.4 Frequeney(CPY)

[] O5

06

06

Fig. 10. Spectrum of each monthly anonmly time series for the period 1941-84. except for Aikawa [5] for tile period 1941-76 and for Ushibuka [10] for the period 1955-84. Numerals ill each square show the station number (see Fig. 1). The PEF is about 4(1% of the record Icngth (210 terms for 52,+, months record).

JPO

17-3/4

-

M

348

"1". WAI ,XNAm: ('t a/.

Variations with about 10- Year Periodicity ( 10- YPV) It is evident from Fig. 11 that the 10-YPV is larger at the northern stations [1-4] in the Japan Sea. We can also detect it at Wajima [6] and Naze [11]. CURRIE (1981) suggested that the decadal variations found in the oceanographic data (tim seat level height in European seas) and in the atmospheric data (the air tcmperature in North America and the air pressure over Japan) of the northern hemisphere and the variation of LOD (Length of Day) have close relations to the zonal westerlies. We can speculate that the zonal westerlies are also responsible for the decadal variation found in the CSST data, though it is not clear what mechanism exists in the generation of this variation. The fact that the second EOF mode of the CSST reveals decadal variations and is closely related to atmospheric conditions as mentioned in the former sections, also supports Currie's suggestion.

Variations with about 6- Year Periodicity (6- YPV) The bandpass filtered anomaly time series for the 6-YPV at each station are shown in Fig. 12, and the frequency response characteristics of the bandpass filter for the 6-YPV are shown in Fig. 13. These time series show the 6-YPV's appearing for the period from the mid 1950's to the early 1970's. The time scale, the standard deviation and the noise level of each 6-YPV are listed in Table 3. The noise level was estimated for convenience's sake as a standard deviation of the time series filtered through a bandpass filter with the same band width as the bandpass filter for the 6-YPV, but having its center on the frequency band without significant spectral peaks. Time scales of the 6-YPV's are within

*C 0.5~

[~

'

_o.sJ

I

0.51

-0.5J. ~ ,

'

0"51 I~ 0.s] ½

--~

'

--lW~7--,x - _ _

-0.5J I-gl_

"/"7"" v

~

, ~.~.

' __t

~

.

.

o.51[]

"~

_o.5

wC'~

0.5

-0.5

! 0.5 ,-

,

~

-0.5 ~[~

[

-0.5

0.5

~--~ ~

'

[ -0.5

!

0.5

-0.5 0.5

osL

I

0.5~ [ ]

-0.5

o.sl

' 0.5

°'5~1~

V,

V

~

J

,

~

,

-0.5

40

50

60

70

Year

Fig. 11. 10-YPV's extracted from each anomaly time series by the bandpass filter FI0 which is shown in Fig. 13. Numcrals in each squ;irc show the station numbcr (scc Fig. I).

349

Analysis o f year-to-year variation

°C 0.5

1~

. . . .

o.~ 15~ .

•

'

'

~

~'

. . . . .

0.5

0.5

9~

-0.5

[~.

,

,

w~'J .k

•

-o.5

0.5 0.5

'

~

' ___

~

0.5

! ....

~

..... /_. . . . "~

4

i

......

~'~'0

-0.5 0.5

._

I__ ~--

°C

~

" " ' "~P¢

fi~,

J

,

.................

I -o.5J

..,,~ ~,/-',

/ V' V ~

. . . . .

. . . . .

-0.5

......

~r.

~

5'0

....

----~

'

~ __O.5

V'-~"

" / " ~ ¢ -

....

I 0.5

~ i

6'0

0.5

7'0 "

....

'

Yea~" Fig. 12. 6 - Y P V ' s extracted f r o m each a n o m a l y time series by the bandpass filler F6 which is shown in Fig. ]3. N u m e r a l s in each square show the station number (see Fig. I).

Period 10 El.0 "~

2

~'~

6~'0"8

/

~0.6 '1 I 0.4

(Years)

5 ~

\

l

--F6

A /

/ !

~"

J

~o2 i ~ tr

0.0 0.0

I FIO

I

I ,i k - - ~ . n l ~

I

-

0.1

,.A

0.2 0.3 04 0.5 Frequency (CPY}

• 0.6

Fig. 13. Frequency response characteristics of the bandpass tillers: FI0 (thin line) for the 10-YPV's and F6 (heavy line) for the 6-YPV's. Halched area shows the spectrum of lhe monthly anomaly time series of CSST at Esashi (3) where bodl the 10-YPV and the 6-YPV are evident.

the range of 5.5-6.5 years as obtained by autocorrelation analysis• The 6-YPV's at the southern stations of the Japan Sea [6-9] and Hachijojima [13] have time scales a little longer than those of the other stations. From Fig. 12, the following features are evident: (i) At the southern stations of Japan Sea [6-9], the 6-YPV's exhibit very similar wavy forms with a systematic time lag to the north and with a decreasing amplitude. (ii) The 6-YPV's seen at Hachijojima [13] and at Ishigakijima [12] are closely connected with

351)

I . Wxl,xn,xm ct ,d. I)thh' 3. 17t(' tim(' .w.h (I'.S. ~. the ~ t , m l , rd d ( . v i , t n m (,\'. I).1 a m l tit(' . o i w Icv,'l (N. I,, } e~l the 6. Y P V at each ( ".%,'CI' ~ t , t t o , . 5~'(" the t('xt ~,1 re t~ardin,t~ the , o t w I('vcl

Station 1. 2. 3. 4. 5. 6. 7. S. 9.

Wakkanai Suttsu Esashi Tol~ishiln;i Aikawa Wajima Saigo I limlada Izuhar;I

IO. Ushibuk,t

II. 12. 13. 14. 15.

Nazc Ishigakijima I lachijojima

Miyako Urakilwll

T.S. (year)

S.D. C(')

N.L. (+(')

5.5 5.5 5.S

0.22 [t.22 (I.31 II. IN

I).15 {).13 ().()9 I). Ill

O. 17

0.22 11.2-1 [).3(}

(I.I}S 0.1)S OIlS {).I(]

0.12 II. 14 0.24 11.22 0.20

(I.11 (t. 111 [I.(19 [1. I6 II. ll)

6.2

6.2 n.5 6.2 6.2 .

6.2 5.8 6.5 5.8 5.5

.

.

.

.

.

* T.S.. S.D. and N.L. of these stations could not bc cstmlated hccausc of lhc shortnc~,s of the records.

those of the southern stations in the Japan Sea [(,-9]. (iii) The 6-YPV's appeared in the early 1950's, were amplified after the mid 1950's, and reached maximum in the early 1960's. This feature is well represented at Izuhara [9] and Wajima [6]. A crosscorrelation analysis among these bandpass filtered time series shows good correlations among the southern stations in the Japan Sea [6-9], Hachijojima [13] and Ishigakijima [12] (Fig. 14a). Good correlations among Esashi [3], Tobishima [4] and the southern stations in the Japan Sea [6-9] are also seen. In Fig. 14(b), we show the time lag of each station to lzuhara [9] as a function of thc distance from Izuhara [9]. Hamada [8] lags by 0 months, Saigo [7] by 3 months, Wajima [6] and Hachijojima [13] by 7 months, Tobishima [4] and Esashi [3] by about 20 months. Izuhara [9] lags to Ishigakijima [12] by 7 months. These time lag relations in the southern stations of the Japan Sea [6-9] correspond with the advective time scale of high saline water flowing into the Japan Sea from the Tsushima Strait as shown by lClUYE (1982). Therefore, we can consider that the 6-YPV appearing in the southern stations of the Japan Sea is advected from southwest to northeast in the southern part of the Japan Sea. We will now consider the relationship of the time lags among 6-YPV's for the stations in the southern part of the Japan Sea to the flow field of the Tsushima Warm Current to clarify their advective nature. It is well known that the volume transport of the Tsushima Warm Current has a large seasonal variation as pointed out by many authors (e.g. TTKH) and also has large year-to-year variations. Consequently the flow field may vary both seasonally and yearly. For the year-to-year variations, NAGANUMA (1985) pointed out that the flow pattern is apt to meander in the low temperature period when the flow is weak, and to be parallel (three-branching flow type) in the high temperature period when the flow is strong. As for the seasonal variations of the flow field, we can detect this field by movements of high saline water flowing into the Japan Sea from the Tsushima Strait in winter and by that of low saline water in summer. TANIOKA ( 1 9 6 2 a ) showed that low saline water ttows into the Japan Sea in August, moves rapidly to the northeast, and is located from the area off Wajima [6] to the mouth of the

Anal),~,i~,oI yC;ll-lo-\'Cal ~,illlilllHn (a)

35 ]

Station Number 2 3 4 6 7 8 9 11 12 13 14 15

on 6" zI:: 3

(b)

10

[]

/

/m

t-

S

o

E lz~-~arca

8'-1o __1

[]~]~ ~

[]

leads

~= -20

olo Distance from I z u h a r a [ ~ l ~ - n ) Fig. 14.

(a) Maximum crosscorrelalion coellicienls among lhe 6-YPV's. 6-YI:'V at each station to that ill Izuhara 191.

(b) Time

lag of lhc

Tsugaru Strait in September and October. Oil the other hand, Tanioka (1962b) showed that high saline water flows into the Japan Sea in winter, moves more slowly in the southern part of the Japan Sea and reaches as far as Wajima [6] in summer. Further, KrrANI and Ut)A (1969) showed that a cold temperature anomaly, which was present in winter off Saigo [71 and Hanlada [8], moved to the area northeast of Wajima [61 and Aikawa [5] in 1963, which was a particular colder period, They showed that the temperature anomaly caused in the southwestern sea area by abnormal cooling in winter moved into the northeastern sea area in May and June, accompanying the increase in strength of the Tsushima Warm Current. The year-to-year and seasonal variations of the flow field in the Japan Sea described above suggest that the large time lag of the 6-YPV's in the southern stations of the Japan Sea is closely related to the slow movement of water of the Tsushima Warm Current in the winter and spring seasons and in the years when water temperatures are lower.

352

T. W , V l A N A m et a/.

Relationsldt~ hetn'ee, the .lalmn am/the East ('lmtu Scu,~ The time series of SST anomalies smoothed with the thirteen-month moving average tilter, at six representative stations in the East China Sea [A-F], together with those t)t+ Izuhara [9] and lshigakijima [12] are shown in Fig. 15 and the locations of these representative stations are shown in Fig. 16. A crosscorrchttioq analysis for thcse liltcrcd time series shows that a good corrchttion with lzuhara [9] c,m bc traced back, ,l[ong the path of the Tsushima Warm Current and the Kuroshio, to Station F, which is the southerntnost station, with a systematic incrcase of lag mo,lths (Fig. 17a,b). The stations on the inner continental shelf [A,D] do not demonstrate good correlations with other stations. These results are consistent with MORIYASU's (1967) analysis, in which he performed a cross-spectral analysis for the anomaly time series of SST data for the period 1953-62 in the East China Sea. His findings for the 2-cpy variation showed an advectivc nature of SST anomalies along the path of the Kuroshio and the Tsushima Warm Current to the Tsushima Strait, the time lag between the areas around Ishigakijima [12] and around the Tsushima Strait being about 50 days. This is similar to our findings, which showed a 3month lag between lzuhara {9] and Station F. MORIYASU (1967) also showed maps of the crosscorrelation coefficients for the East China Sea, and when the reference point is in the Kuroshio region around Station F, a good correlation can be traced to the Tsushima Strait along the paths of the Kuroshio and the Tsushima Warm Current. On the other hand, regarding the reference point in the continental shelf region around the entrance of the Yellow Sea, a good correhttion is confined to the area on the continental shelf.

"C

Io

~

~.~zuhara

,~

CSST

~.~r

~-;o

"+"

,.o

....

-,.o- []

. . . . . . . ~

1.0- []

-i.o - []

.

,-.....~ ~ "

~

~

r- .o

" ~-

~

~, .... ..--~'-'Ir - ~v,~-

[--1.0

~ i.o I

-I.0

lshigaki~ma

~ ,-.,,--55

,,,- ~,~ , 60 65 YEAR

~

CSST . ~ , 70

"

1.o L_I. o

Fig. 15. Anomaly lime series of SST ;.It the six stations fronl A [o F (scc Fig. I(1) in the East China Sca (ECS) and of CSST's at Izuhara [9] and Ishigakijima [ 12]. The data arc smoothed with the 13-month moving average liltcr. Anomaly time series of SST in the East China Sea wcrc reproduced from the "Technical Report of Japan Mctcoroh)gical Agency. No. 89"' (NA(iASAKI MARINE OI}SI!RVA]ORY, 1975).

Analy:,,i~. t)l ycar-lo-_vc,lr vari;ltitm

125"E

353

130°E

}5* N

30" N

2S* N 125°E

130"E

Fig. 16. Locations of the six representative stations of SST in the East China Sea (closed circles) and the CSST stations ~round the East China Sea (open circles). Numerals in each square show the CSST station number and ~dphabct letters show the SST station.

(a) Station 11 1213 A B C D E F

(b)

1t

r~/,

-2 J

I

ob

os

,

,

[]

~ds

~.o

ts

Dislonce from Izuhora[]~(xlO3km)

Fig. 17. (a) Maximum crosscorrclation cocflicicnts among the year-to-year variations shown in Fig. 15, and variations at other CSST stations [11, 13] for the period 1953-73. (b) Time lag of these variations to that at Izuhara [9]. Numerals in each squzirc show the CSST station number and alphabet letters show the SST station (sec Fig. 16).

,1'5-1

I

W A I \~, \Ill

,'1 ,ll.

Fortunately, the period 1953-73 col'responds to thc period when the 6-YPV evidently al~pcaicd as nlciitioncd i)rcviottsly. Therefore the above-described relations of Ihc yciir-hi-ycilr vii,i;llions ill the Eiisl Cllill~i SC;I ~ind Izuh;ira I')1 stlggCsl that the (>-'l'l>f ill the southcril part of Ihc .hipan Sc;.l is adw.'ctcd fronl the Easl China Sell. I)IS('L!SSI()N

The year-to-year variations appearing in the CSST at the stations in the .lapztn Sea arc closely connected with those of the SST in the western North Pacific. In particular, the 6-YPV in the southern part of the Japan Sea is considered to be advcctcd from the East ('lfina Sea by the Tst, shima Warm Current. Also our analysis of SS']" variation in the East China Sea reveals that the year-to-year variations can bc recognized around the Kuroshio region in the southern part of the East China Sea, and that thcv arc advcctcd to the north. We may specuhtte that the variations arc generated in these regions, since this region has the highest air-sea heat transfers in winter season. The thermal conditions of this sea region are determined by the heat transport by the Kuroshio and by a larger heat release to the air especially when the outbreak occurs in winter as shown by KoNt~o (1976). Therefore, the year-to-year variations in the heat transport of the Kuroshio and the strength of the outbreaks in the winter season may be responsible for the year-to-year variations of sea conditions, especially for the 6-YPV's in the Japan Sea. Figures 18(a) and (b) show a route in which the year-to-year variations arc thought to be advected from the East China Sea to the Japan Sea, together with the variations of the

(ca) 125"E

I

\/

130"E

..

s°E

,

[]

25"N

30*N

35"N

(b) .fio.6. = _ , , _ , .

.

15

I--I

/ I--I

•

/'/\ i

~0.0.

. ~0

~

O0

0.5 1.0 Distance from [ ]

1.5 ( x l 0 3 kin )

2.0

Fig. 18. (a) 3"he route from the Kuroshio to the Tsushima Warm Curren,. (h) The variation of

standard deviation of the year-to-year vilriation (YYV) and of the 6-YPV along the route. together with depth variation. Numerals in each square show the CSST station numhcr and alphabet letters show the SST station.

355

Analysis of ycar-tu-ycar variation

sea depth, and the standard deviations of the year-to-year variations and of the 6-YPV's along the route. The strength of the 6-YPV and/or the year-to-year variations becomes s t r o n g e r w h e n m o v i n g in a d i r e c t i o n t o w a r d t h e T s u s h i m a S t r a i t , r e a c h i n g a m a x i m u m at t h e S t r a i t , as

tllough

thc year-to-year

variations wcrc

amplified w h i l e

moving into the

s t r a i t . A f t e r g o i n g through t h e T s t t s l f i m a S t r a i t , t h e s t r e n g t h o f t h e v a r i a t i o n s d e c r e a s e s to t h e n o r t h . It s e e m s t h a t t h i s v a r i a t i o n o f t h e a m p l i t u d c h a s s o m e r e l a t i o n to t h e s c a depth, presumably

in , e l a t i o n to t h e w a t e r m a s s f o r m a t i o n : t h i n g t h e c u r r e n t .

Acknowh'dgement.s--Thc authors thank Mr. N. lwasaLka of Tohoku University for offering his unpublishcd ligt, rcs. They also thank Dr. T. Yanagi of Ehime University lor his useful advice.

I~,EF[Z R EN C ES

CURRU- R. O. (1981) Solar cycle signal in air temperature in North America: amplitude, gradient, phase and distribution. Jounlal of/he A Imo.~pheric Sciences, 38, 811,'4-818. Ft~KUOK;,, J. (1962) Characteristics of hydrography of the Japan Sca--ln comparison with hydrography of the North Pacilic. Jottrnal of the Oceattogralfltical Society of Japan, 2111hAnniversary Volume, 18(I-188 (in Japancsc). H;~Yaslu Y. (1981) Spacc-time cross spectra[ analysis using thc maximum entropy method. Joarttal of the Meteorological Society of Japalt, 59,621}-624. HA'.'S W. L. (1981) Statistics, third edition. Holt-Saunders International Editions, 723 pp. Icl u'fl~ T. (1982) A commentary note on "Temperature and salinity changes in the Tsushima Current" by R. k. Kolpack. La met, 20,261-262. [IDA H., K. KATA(ilRI, I. MAEDA lind E. KAMIIIIRA (1974) On the changes of sea surface tcmpcrature in the western North Pacilic Ocean. The Oceanographical Magazine, 25, 73-88. hl)A H., K. KATAGIRI, I. MArD;,, and E. KAMIIIIRA (1975) On the normals of monthly sea surface tcmpcraturcs from 1956 to 19711 for 5-degree squares in the western North Pacilic Ocean. The Oceam~graphical Magazine, 26, 73-89. JAPAN MI-TI'OROI.OGICAI.A(iI-NCY (1984) Report "84 of abnormal meteorological phenomena. Jalmn Meteorological Ageno'. 293 pp. (in Japancsc). KAISI-R J. F. and W. A. REI-I)(1977) D~,ta snmothing using low-pass digital liltcrs. Review oJ Scientific h+strumel+ts, 48, 1447-1457. Kat O K. and T. ASAI (1983) Seasonal variati,,ms of hcat budgets in both the atmosphere and the sea in thc Japan Sea area. Jourmd ~['the Meteorological Society of Japan. 61,222-238. KAWAMURA R. (1984) Rclation between atmospheric circulation and dominant sea surface temperature anomaly patterns in the North Pacilic during the northern wintcr. Journal of the Meteorological Society of Japan, 62,910-916. K.IIANI K. and M. UDA (1969) Variability of thc dccp cold water in the Ji, pan Sea--particularly on the abnormal cooling in 1963. Journal of the Oceanographical Society of J~qmn. 25, I0-2[). KONDO J. (1976) Heat bahmce of the East China Sea during the air mass transformation experiment, Jour~ml of the Meteorological Society of Japan, 54,382-398,. MIITA T. and S. TAWAP,A (1984) Seasonal and secular variations of water temperature in the East Tsushima Strait. Journal of the Oceanographical Society of Japan, 40, 91-97. MORIYASU S. (I967) On the anomaly of the sea surface temperature in the East China Sea (I). The Oceanographical Maga,zine, 19, 21)1-22[). NA(iANUMA K. (1985) Fishing and occanogn, phic conditions in thc Japan Sea. Umi to Sofa, 60. 89-103 (in Japancsc). NAGASAKI MARINli OFISI~RVATORY(1975) Mcan sea surface tempcraturc in the East China Sea for thc years 1953-1972. Technical Repmt of Japan Meteorological ,'tgettiT, No. 89, Japan Mctcorological Agency, 99 pp. (in Japanese). RAI)OSKI H. R.. P. F. Fou(;l-m- and E. J. ZAWAI.I('K (1975) A comparison of power spcclral estimates and applications of the maxinmna cntropy mclhod. Journal of Geophysical Research, 811,619-625. TAxIOg.A K. (1962,0 The oceanographical condition of the Japan Sea (I). Umi to Soru. 38, 90-10U (in Japanese). "I'ANIOKA K. (1962b) The oceanographical condition of the Japan Sea (11). Umi to Sora, 38, 115-128 (in Japanese). T(mA Y., K. TOMIZAWA, Y, KURASAWAand K. HANAWA (1982) Seasonal and year-to-year variability of the Tsushima-Tsugaru Warm Current System with its possible cause. La mer, 20, 41-51.

356

T. WAI ANAlU-et ./.

"I'oKAI RI((;IONAI. FISlU~RII-S RI-SI!AR('II [.AIIORAIORY (1982) Monthly mean data of air and sea surface temt'~eratures measured al the coast since 191t) I 5' assigning to light Imuscs and lishcries experimental stations. Datum Collection of Tokai Regiomd I:i~hcrie~ Lahoralory, No. I0, Tok,d Regitmal Fisheries

Research Laboratory, 369 pp. (in ,hq')ancsc). Wl ARli B. C., A. R. N a v a r o and R. E. Nli\Vl I.I. (1976) Empirical orthogonal analysis of Pacilic Sea sur[aee temperature. Journal of Physical Oceanography, 6, 671-678. APP[:.N I) IX

It is well known that a spectral a,nalysis by using the Maximum Entropy Method (MEM) has two superior features (Ct,mtn-, 1981; RAt~os~ at al., 1975). O n e is that signals which have relatively longer periods from the same length of the record can be detected, because there is no limit in frequency resolution. The other is that the M E M has such high sensitivity that signals can be detected even if the signal-to-noise ratio is low. Therefore, an MEM spectral analysis is the best method for the analysis of the geophysical data in which we expect to detect long period variations from relatively short period data. In spite of these superior features, many researchers seem to hesitate to use this method because of the difficulty in obtaining a suitable n u m b e r of terms of the Prediction Error Filter (PEF) and because of the difficulty in testing the statistical significance. Although several criteria to obtain a suitable n u m b e r of the P E F and several methods to test the statistical significance have been proposed, they are not applicable to all cases (CuRun-, 1981; IIAvASm, 1981). So we must find a suitable n u m b e r of the P E F in each case and make sure whether the signals, which are detected by M E M spectral analysis, actually exist or not. At this point, HAYASm (1981) noted that an M E M spectrum should be regarded as preliminary until a statisticvl significance test was made by using classical methods for data collected over a suitably long period of time. Since such data are not necessarily available, we use a bandpass filtering method for this purpose. By using this, we can investigate in detail which frequency c o m p o n e n t s dominantly construct ,the raw time series in the time domain. As an example, we discuss the significance of the signal of 6-YPV at Izuhara [9] detected by the M E M spectral analysis. Figure A l ( a ) shows the M E M spectrum using about 40% of record length as the PEF, and Fig. A l ( b ) shows the non-smoothed spectrum estimated by a Fast Fourier

Period 1i 0 6i

4i

3i

(Years)

2i

1

oC 2

,--t

U

0.0

0.2

0 4

0.6

Frequency

0 8

1.0

1.2

(CPY)

Fig. A I. (a) The spectrum estimated by the MEM for the CSST time series of 512 months from January 1941 at Izuhara. The number of the PEF is about 4(1% of the record length. (b) The nonsmoothed spectrum by the FFT. The degree of freedom is 2. (c) The smoothed spectrum by the FFT with 12 degrees of freedom. Vertical bars shows the 80% confidence intervals. (d) The variation of variances of signals which are extracted by bandpass filters (see text).

Amdyr,i,, ,.:,f',car-to-year variati,,wl

357

Tran~forn~ (1:1:'1"). The frequency resolution of the FITI" is I).l)234 cpy. The~e Iigure~ reveal thal these spectra e>,timatcd by the MEM and tile F'FT arc almost the same, though the spectrum by the ,x,1EM is much smoother than that by the FFT. Figure A I(c) shows the smoothed spectrum by the FI:'F with t2 degrees of freedom. The ~()"/. confidence intervals are added for some peaks. As to the 6-YPV (revealed by about 0.17 cpy peak), this signal is less apparent in this spectrum. Figure A l(d) ~,hows a v,triation of variance of signals which are extracted for each ().1 cpy frequency I~itlld I'J)' blttltll'mSs liltcrs with the s,tme characteristics. This ligure shows the same feitlurc as that observed irt the spectrum by the MEM, :.rod the existence of the 6-YPV peak is evident. This result shows the signal of the 6-YPV is significant in tile sense that the level of the signal is much ktrger than that of a rloisc. If we had adopted the result of the FFT spectral analysis because of the adwmtage of being able to test statistical signilicanee, we could not necessarily detect the signal of tile 6-YPV because of tile shortness of the record length.