NGHIEN CUtJ DAC DIEM BIEN DONG IVIU^C N\f6C BIEN
• • •
TRONG DIEU KIEN BIEN DOI KHI HAU HIEN DAI
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STUDY OF THE VARIATION CHARACTERISTICS OF SEA LEVEL IN THE CONDITION OF PRESENT CLIIMATE CHANGE
Nguyen Kim Vinh
Vien Hai duong hoc, 01 Cau Da, Nha Trang.
Email: nkvinhntvn(a),vahoo.com.vn Tom tat:
Dd neu nhirng ddc diim chinh ciia biin ddng muc nuac bien (BDMNB) trong diiu kien biin ddi khi hdu (BDKH) hien dgi thue trgng BDMNB a mot sd viing biin thi giai. Phdn tich vai trd ciia cdc qud trinh chinh tham gia vdo BDMNB.
Thue hien nghien ciru cu thi cho viing bien Nha Trang. Kit qud tinh todn vd phdn tich chudi sd lieu muc nuac biin (MNB) tirng gia tgi Nha Trang, tir 1975 den 2008, cho thdy xu the BDMNB vai chu ki 5,7 ndm. Tir 1975 din 1992 MNB trung binh cd xu thi gidm vd tit 1993 din 2008 - tdng. Ap dung phuang phdp loc Xo- filter di xdc dinh thdnh phdn bien ddi khdng trieu ciia muc nuac biin. Gid miia cd
vai trd chii dgo trong bien ddi khdng trieu ciia MNB.
Abstract:
The main characteristics of sea level variation (SLV) in the condition of present climate change, the real situation of SL V in different marine areas are
noted. There is analyzing the roles of the main processes participating in variation of the sea level. The case study is applied to Nha Trang Bay. The analyzing of the series of hour SLl data from 1975 to 2008 showed that the SL changes with the period of 5.7 years. The tendency of the average of SL is that from 1975 to 1992 decreased and from 1993 to 2008 increased. The Xo-filter is applied to define the non-tidal component of the SL V. The monsoon playes the main role informing of the non-tidal component of SLV in this area.
1. Ve bien dong myc nu'O'c bien
Muc nuoc dai duong (MNDD) da trai qua nhieu dgt bien dgng manh. Vi du, trong thai ki bang ha gan day nhat (khoang 18.000 nam truac) MNDD da thap hon bay gia la 120 m.
Con vao thai ki nhiet do Trai Dat tang len each nay khoang 8.000 nam, thi MNDD da cao hon bay gio la 6 m. Theo danh gia tir cac dir lieu ve tinh thi tir nam 1992 den nay MNDD toan cau tang khoang 2,8 mm/nam. Tir nhieu nguon dii lieu muc nuac do a cac tram, nguai ta danh gia muc tang muc nuac bien (MNB) trong the ki XX la trong khoang 0,8 - 3,3 mm/nam, trung binh la 1,8 mm/nam. Cac danh gia tir dii lieu dja chat cho thay MNDD co the tang 0,1 - 0,2 mm/nam trong vong 300 nam tro lai day. Theo ket qua do dac tai 23 tram do MN tai diem co do on djnh ve dja hinh thi MNB da tang 20 cm trong the ki XX; moi nam tang 2 mm (Rieng giai doan 1993 - 2003, tang 3,1 mm/nam). Nguyen nhan chii yeu la do hoat dong ciia eon nguai, nhu phat trien cong nghiep, tieu thu nhien lieu hoa thach (dau khi, than da v.v), cac hoat dong lam tang lugng khi Cacbon trong khi quyen [Nguon Internet]. Moi nam c6 khoang 8 mm nuoc tir toan bo be mat dai duong rod xuong chau Nam Cue va dao Greenland duoi dang tuyet. Neu khong co bang tan thi moi nam mat nuoc bien se tut xuong 8 mm. Nlu toan bo bang a Greenland tan, MNB se dang cao 7,2 m; neu tan het bang a chau Nam Cue thi MNB se dang cao 61,1 m. Co nhilu qua trinh thanh tao bien dong chu ki ngdn (Tir vai phut
den 18,613 nam) ciia MNB (Bang 1). Thirc tl biin dong muc nuac bien (BDMNB) a cac viing khac nhau tren thi giai cho thiy co noi tang, co nai giam (Bang 2). Chiing ta co the chii y toi BDMNB o cac nuac Philippines, Indonesia va Nhat Ban, la cac nuoc trong viing tay Thai Binh Duong. Tir nhilu nguin tai lieu co the thay rang chiing ta dang song trong thai ki nong len tiep theo ciia khi hau. Do chinh la thdi ki nong len hien dai, kha manh, the hien rat nhieu noi tren Trai Dat. Qua trinh nong len nay bat dau tir nira sau ciia the ki XIX va a manh len vao dau the ki XX. Tuy nhien, van de nong len cua Trai Dat tir 1998 den nay co le con la van de tranh cai [www.nasa.gov]. Co nhieu qua trinh tac dong len trang thai nhiet ciia Trai Dat chiing ta. Va tat ca cac qua trinh nay can phai dugc tinh den trong nghien cim BDKH. De xac djnh dugc qua trinh nao lam thay doi MNB can phai xet cac thanh phan ciia can bang (balance) nuac bien va Dai duong (Bang 3). Tong cac thanh phan la balance.
Bdng 1. Cdc qud trinh thdnh tgo biin ddng chu ki biin ddi ngdn cua MNB [8].
Loai qua trinh Bien doi chu ki
Bien dong khi tugng va hai duong
Bien dong miia
Dao dong lac Dong dat
Loai chu ki
Nhat trieu va ban nhat trieu (Thuy trieu thien van)
Trieu chu ki dai
Bien dgng xoay (Chandler wobble) Node Mat Trang (Thiiy trieu thien van) Khi ap
Gio (Nuac dang) Boc hoi va ngung tu
Hinh thai be mat dai duong (bien dong ciia mat do va dong chay nuac bien)
El Nino/ENSO
Can bang nuac miia trong dai duong Bien dgng miia ciia do doc muc nuac bien Luu lugng song/Lii lut
Bien doi miia ciia mat do nuac bien (nhiet do va do man)
Dao dgng lac (Song diing)
Song than (Tao song chu ki dai co sue tan pha Ion)
Bien dong bat ngo ciia mat dat
Kich thuffc thoi gian (P: chu ki) 12-24giaP
14 thang 18,613 nam
vai gio toi vai thang 1-^5 ngay
vai ngay tai vai tuan
Vai ngay tai vai tuan
6 thang trong 5 ^ 1 0 nam
2 thang 6 thang
vai phiit toi vai gia
vai gio
vai phiit
Hieu irng cao do (m) 0,2-10+
-0,7-1,3 Tai 5
Toi 1 Toi 0,6
1 0,2 Tai 2 Tai 10 Tai 10
STT 1 2 3 4 5 6 7 8 9 10 11 12
Bdng 2. Bien ddng ciia MNB a cdc viing khdc nhau tren thi giai [9, 3. 1].
Viing bien Toan cau My Ha Lan Uc
Dai Loan Trung Quoc Indonesia Philippines Malaysia Nhat Ban Pakistan Viet Nam [10]
Chuoi so lieu Thi ki XX
Tir 1850 Tul880 Tir 1900
1950-2000 1950- 1991 1950-1991 1950-1991 50 nam
1954-2008
Dac diem bien dong Tdng
2 mm/nam 3 mm/nam 1,5 mm/nam
1 mm/nam 0,35 - l,92cm/nam
1,7 - 2,5 mm/nam Viing nam Viing dong 2,5 - 8,8 mm/nam
Viing dong 0,1 mm/nam 0,4 mm/nam
Gidm
Viing Alaska
0.364 - 3,55 cm/nam Viing bac
Vimg tay Viing tay
Neu balance duong co nghTa MNDD tang va ngugc lai, am - giam. Va ciing qua cac gia trj thanh phan mai co the nhan djnh ve vai tro cua thanh phan nao la chinh. Day ro rang la viec khong don gian.
Bdng 3. Cdc qud trinh chinh tham gia vdo cdn bdng nuac Dgi duang.
STT 1 2 3 4 5 6
Cac thanh phan Boc hai
Ngung tu Bang tan Dong bang
Trao doi nuac giira Dai duong va Luc dja (Luu lugng song ngoi va nuac ngam)
Hoat dong ciia con nguai (Khai thac dau khi, san xuat, tieu thu, trii v.v)
Vai tro trong balance nuffc
;
+
-1- -
±
-
2. Dac dilm bien dong myc nuoc vung bien Nha Trang
Co thi noi, BDMNB bao gim hai thanh phan chinh: Thanh phan biin dii trilu va thanh phan bien dii khong trilu (the non - tidal variation of sea level).
2.1. Cdc dgc trung thong ke muc nu&c bien trgm Cdu Dd (Nha Trang)
SL :.5 2 . 0 -
1.5- 1.0 0.5 0.0-
T B T B
TBnn TBnn
El Nino 1998^
1975 1980 1985 1990 1995 2000 2005 \ c a r s
Hinh 1: Cdc ddc trung thdng ke MN trgm Cdu Dd (Nha Trang). TB - Gid tri trung binh ndm, TBnn - Gid tri trung binh nhiiu ndm (1975-2008), CD - Gid tri cue dgi ndm, CT- Gid tri
cue tieu ndm, • - Chi ndm khdng cd day dii sd lieu MN tirng gia.
STT 1 2 3 4 5 6 7 8
Bdng Chuki
12h24.5' 24h 49' 6 thang 12 thang 14,5 thang 8,47 nam
10,8 nam
18,6 ( 1 8 - 1 9 ) nam
4. Cdc chu ki trieu chinh [8].
Ghi chii Ban nhat trieu
Nhat trieu Nira nam Mot nam Trieu a cue Trieu Perigee
Chu ki Dom Mat Trai (Sunspot cycle) Trieu giao diem (Nodal tide)
Co sir bien dong ro ret cac gia trj trung binh nam tir nam 1975 den nam 2008 (Duong TB, Hinh 1). Nhu ta biet, bien dgng trieu c6 8 chu ki chinh, tir 12h24.5' den 18,6 nam (Bang 4). Ro rang la vai chuoi so lieu hien co khong the neu nhan xet ve su gia tang ciia MNB do BDKH (hieu img nha kinh, sir nong len ciia Trai Dat) noi chung va bang a cac cue Trai Dat tan ra, noi rieng. De ciing co them nhan djnh nay chiing ta co the ap dung hai phuang phap sau: 1- La chuoi so lieu, 2 - La do thj. Co the thay rang (Hinh 2) trong vong 34 nam gia trj trung binh nam ciia MNB bien doi vai chu ki khoang 5,7 nam (duong 2, hinh 2). Con xu the bien doi chung la: Tir 1975 den 1992: gia trj trung binh nam ciia MNB giam, tir 1992 den 2008: tang (duong 3, hinh 2). Chenh lech gia trj trung binh nam ciia MNB nam 2008 va 1975 la 5 cm. Phan tich kT ket qua tinh toan thu dugc ta co the dua ra mot vai nhan xet kha li thii.
Thii nhat do la, ton tai rnot gia trj cue tieu trung binh nam 1998 (Hinh 1). Nam 1998 la nam CO hien tugng El Nino rat manh; dieu nay cho ta lien tuong den kha nang anh huong ciia El Nino va La Nina (ENSO) din BDMNB trong Biin Dong. Dinh cue dai MNB tin tai a nam 2006 lam ta lien he tai anh huong cua bao len BDMNB. Ma cu thi la bao Durian [www.nchmf.gov.vn]. Vao thang 7 (miia gio miia tay nam) gia trj trung binh thang cua MNB thap hon vao thang 12 (miia gio miia dong bac), trung binh la 28 cm (Hinh 3). So vai trung binh
-
- -
LJ*^
/ - I
1
\/
/
' l
m
\
1 1 1 1 II •
f-
• •
1
•*
A... \ 1 y.
i
•i
\
1
fl ft
• y
1
_ -
—
1975 1985 1995 2005
Hinh 2: Xu thi biin ddi ciia gid tri trung binh ndm MNB (SL, m) trgm Cdu Dd, Nha Trang.
1- Gid tri trung binh ndm (duang ggch - ggch), 2 - gid tri la tran (duang liin), 3 - Xu thi biin ddi ciia gid tri trung binh ndm MN (duang ggch - chdm).
SL(m)
TB12
1990 1995 2000 2005 Years
Hinh 3: Trung binh MNB thdng 7 (TB7) vd thdng 12 (TBI2) cdc ndm 1975 - 2008, trgm Cdu Dd (Nha Trang). • - Chi thdng khdng cd day du sd lieu MNB tirng gia.
2.2. Nghien ciru bien dong khong trieu
Biin dong khong trieu ciia MNB do mot so qua trinh xay ra trong khi quyen, dai duong va vo Trai Dat tao thanh.
Cdc qud trinh xdy ra trong khi quyen:
a- Trudc het phai ke den cac hien tugng thai tiet dac biet, nhu bao va ap thap nhiet dai va lii lut viing bo.
b- Truang khi ap ciing luon luon bien dong. Vi vay no ciing tac dgng len MNB va lam cho MNB bien dong. Dac biet phai ke den qua trinh ENSO. Ca biet trong viing nhiet dai, khi ap bien dong vai chu ky ban nhat trieu nen a viing bien nuoc ta ciing vay [5].
c- Tac dong cua truang gio gan ba co the gay nen hien tugng nuac dang - nit. (3 viing bien ven ba Viet Nam, hien tugng nay co the dugc gay nen boi gio miia va gio dat - bien [5, 6, 2]. 6 vimg nam cua Nam Trung Bo va Nam Bo co gio chuong.
d- Hieu ling nha kinh va hau qua la sir gia tang nhiet do khi quyen noi chung lam tan chay bang a cac cue Trai Dat ciing lam tang MNB.
Cdc qud trinh xdy ra trong Dgi duang:
e- Hoan luu nuac bien dai duong trong sir tac dong ciia lucKoriolis co the gay nen BDMNB; dac biet la a viing ba, viing co cac xoay. Tuy nhien, gia trj BDMNB trong truang hgp nay nho hon nhieu so voi cac truang hgp khac.
f- Qua trinh truyin trilu a cac viing cira song trong truong hgp co lugng nuac song tuong doi chay ra thuang gay nen sir gia tang dang ke ciia MNB trong viing. Doi vai \iing bien Viet Nam, hien tugng nay thuang thdy a viing dong bang Nam Bo noi chung, thanh pho Ho Chi Minh noi rieng.
Cdc qud trinh xdy ra trong vd Trdi Ddt:
g- cac qua trinh xay ra trong vo Trai Dat, nhu dong dat, niii lira, nhat la klii chiing xay ra dual day bien, dai duong, co the tao nen nhiing bien dong bat thuong ciia MNB.
h- cac qua trinh thiiy thach dong lire xay ra trong vimg bo bien va cac dao ciing co the lam BDMNB.
Nhu vay, a tren da neu 8 qua trinh xay ra trong khi quyen, dai duong va vo Trai Dat CO kha nang tham gia vao bien dong khong trieu ciia MNB. C) day khong tinh din BDMNB voi chu ky ngan; cu the la song bien do gio tao thanh.
2.2.1. Phuang phdp tinh thdnh phdn khdng trieu
Doi vai BDMNB, de loai bo cac qua trinh bien doi trieu, can ap dung phep loc co ten la Xo-filter [4]. Phep Igc nay dugc xac djnh nhu sau:
F{t) = (2,1,1,2,0,1,1,0,2,0,1,1,0,1,0,0,1,0,1) . (1)
vai / : 1 -^ 19 . Day la bg loc doi xung, nghTa la F{t) = F{-t). Va de loai bo cac biin dong trieu can tinh theo cong thiic sau:
Xr=^Y,F(d)H(T + d),
3 0 </^_i9 voi d 1^0 (2)
Trong do, H(t) \a muc nuac tirng gia. Phuong phap dugc ap dung cho 7 = 12 gio.
2.2.2. Kit qud
Bdng 5. Biin ddng khdng trieu ciia muc mrac bien (AH, cm), vd gid (W: hirang gid chinh vd tdc do trung binh, m/s) vdo ngay 15 hdng thdng ndm 1983.
Thang AH
W
1 9 WNW 5.5
2 8 WNW 2.0
3 0 SE 2.0
4 -18
SE 2.0
5 -21
SE 2.5
6 -12
SE 4.0
7 1 SE 1.5
8 11 ESE 1.5
9 -9 SE 1.2
10 15 NE 1.5
11 17 NW 3.0
12 10 NE 3.5 De nghien cuu bien dong khong trieu (BDKT) ciia MNB da ap dung phuang phap loc (Muc III.2.1.) de thu dugc chuoi gia trj MN khong trieu. Gia trj BDKT trong nam dao dong trong khoang 52 cm den 108 cm. Gia trj cue dai bang 46% bien doi MNB Ion nhat. Ro rang, vai tro ciia BDKT trong qua trinh bien doi chung ciia MNB la rat dang ke. De xet anh huong ciia gio miia len BDKT cua MNB, da chon hai thang: 1 - dai dien cho miia gio dong - bac va 7 - dai dien cho miia gio tay - nam. Gia trj trung binh nam BDKT trong hai thang nay la: Vao thang 1:
tir 1 den 14 em, vao thang 7: tir -6 den -26 cm. Gia trj cue dai nam BDKT trong hai thang nay la: Vao thang 1: tir 9 den 24 cm, vao thang 7: tir -15 den -35 cm. O day, gia trj duong c6 nghTa la gio miia dong - bac lam gia tang MNB, con gia trj am: gio miia tay - nam lam giam MNB.
Trong do, gio miia tay - nam gay BDMNB manh hon, vl gia trj khoang gap 2 idn.
D I nghien cim nguyen nhan chinh tao nen thanh phdn BDKT cua MNB, co thi xet mli tuong quan giira gio (toe do va huong) ya gia trj thanh phdn BDKT (Bang 5). Kit qua tinh toan bilu thirc lien he giira MN khong trilu va tic do gio tren ca so dir lieu nam 1983 cho thdy moi tuong quan cao (Bieu thiic 3):
h = 1,382 W+13,3 (3) Trong do. [W]: m/s, [h]: cm. Vai sai so tuong dii trung binh la 2.5%. Gia trj nay rat
nho, chimg to thanh phan BDKT chu yeu la do gio tao thanh.
3. Ket luan
Qua trinh BDMNB trong dieu kien BDKH hien dai ngay cang phire tap; doi hoi phai tiln hanh nhiing do dac nghien cim toan dien cac qua trinh thanh tao bien dong do.
Kit qua tinh toan va phan tich chu6i so lieu MNB tirng gia, tir 1975 din 2008, cho thay xu the biin dii MNB vai chu ki 5,7 nam. Tu 1975 din 1992 MNB giam va tir 1993 din 2008 - tang.
4. Lffi cam ffn
Tac gia chan thanh cam on cac nha khoa hgc eiia Phong Vat ly bien Vien Hai duong hoc da ho trg va giiip da dl tac gia co the thue hien dugc nghien curu nay.
TAI LIEU THAM KHAO
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136.
4. Manual on sea level measurement and interpretation, IOC (1985). 83 pp.
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8. Segar D.A. (1997). Introduction to Ocean Siences. Wadsworth Publishing Company.
497pp.
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