Ph^n IV VAT LY KHJ Q U Y £ N 339
NHtTNG NHAN TO LIEN QUAN DEN Sy* B A T DAU GIO MUA O NAM BO VA MUA MU'A 6
B A CBO
Pham Xuan Thanh, Nguyen Xuan Anh Viin Vdt ly dia cdu
18 Hodng Qudc Viet, Cdu Gidy, Hd Npi
Mail: pxthanh(a)isp. vast, ac. vn - Email: nxuananh(a),vahoo. com
Tom tdt: Dua trin sd liiu mua ngdy tgi trgm vd sd lieu gid gdn mat ddt tie ndm 1979 din 2004, ngdy bdt ddu gid miia mua he tren khu vuc Nam Bp xdc dinh dugc la ngdy 12 thdng 5 vd ngdy bdt ddu miia mua trin khu vuc Bdc Bp Id ngdy 13 thdng 5. Di xdc dinh nhicng nhdn td cd lien quan din thai diim bdt ddu gid miia/bdt ddu miia mua (BDGM/BDMM), trudc hit, cdu triic khdng gian, thdi gian cua diiu kien khi quyin xung quanh ngdy ndy dugc phdn tich chi tiit bdng sd lieu tdi phdn tich ciia Trung tdm Qudc gia Du bdo Mdi truang My/Phdng ndng lugng (NCEP/DOE-II). Sau dd, nhdn td du bdo dugc lua chgn bdng phuang phdp phdn tich tdng hgp. Cudi ciing, phuang phdp hdi quy tuyin tinh da biin dugc dp dung di chon 4 nhdn td liin quan Idm nhdt din thdi diim bdt ddu gid miia tren khu vuc Nam Bp vd bdt ddu miia mua trin khu vuc Bdc Bp. Trong sd 3 nhdn td ciing dnh huang din thai diem BDGM/BDMM: (1) Su khdc thudmg cua dp sudt khi quyin muc biin khu vuc Ddng Trung Qudc: (2) Su khdc thudmg ciia gid Ddng vd Dong-Nam tren khu vuc Nam Viet Nam; (3) Su khdc thudng ciia phdt xg sdng ddi (ddi luu mgnh) trin khu vuc phia ddng Vinh Bengal, nhdn
td (2) vd (3) gdy dnh hudng tieang ddng, nhdn td (1) gdy dnh hudng khdc biet tai thdi diim BDGM/BDMM trin hai khu vice ndy.
1. Mo- dau
?• Viet nam la quIc gia nam trong khu vuc chuyen tiep ciia hai he thong gio miia chinh cua chau A: gio miia Nam A va gio miia Dong A (Pham Nggc Toan va Phan Tat Dac, .1993; Zhang et al, 2002). Lugng mua ma gio mua dem lai chilm mgt ty trgng Ion ti-ong
ting lugng mua hang nam (80-90%). Thod diem bat dau gio miia/bat dau miia mua, ngoai y nghia true tilp vl mat thod vu, no con co y nghia gian tiep den doi song kinh te - xa hgi thong qua quan he ciia no voi tong lugng mua trong nam. Nam ma gio miia/miia mua den som cung la nam lugng mua ghi dugc Ion va ngugc lai (xem phan 3.2 duod day).
•'• Theo tac gia Tao va Chen (1987), gio miia chau A dugc bit diu tren Bien Dong Viet Nam khoang diu hoac giiia thang 5, sau do phat trien len phia Bac toi luc dia Trung QuIc va Tay bac Thai Binh Duong va phat trien sang phia Tay, toi vinh Bengal
i
3 4 0 TuySn tap cac cong trinh nghiSn CLPU V i t ly Dja c l u 2008
va luc dia An Do. Ngugc lai, Wu va Zhang (1998); Mao et al (2004), lai cho ring gio miia chau A dugc thilt lap dau tien tren vinh Bengal vao diu thang 5, sau do den Biln Dong Viet Nam vao khoang giira thang 5 va cuoi ciing tren khu vuc Nam A va diu
• thang 6. Tren khu vuc Dong Duong, gio miia miia he dugc bat dau vao khoang diu thang 5 (Matsumoto, 1997; Wang and LinHo, 2002; Zhang et al., 2002), hoac giira thang 4 din giiira thang 5 (Qian and Lee, 2000). 6 Viet Nam, cac nghien cuu gan day diu cho ring thang 5 la thang bit dau ciia miia mua (Pham Nggc Toan, Phan Tat Die, 1993; Nguyen Dire Ngir, Nguyen Trgng Hieu, 2004; Pham Thi Thanh Huong, Trin Trung True, 1999; Pham Xuan Thanh, Fontaine B. va Phillipon N., 2007 va 2008).
Trong bai bao nay, tac gia nham hai muc dich: (1) Trinh bay tom tat ket qua nghien cim ve su bat dau gio miia miia he tren khu vuc Nam bg va sir bat dau miia mua tren khu vuc Bac Bg (chi tiet hon trong hai bai bao Pham Xuan Thanh, Fontaine B. va Phillipon N., nam 2007 va nam 2008); (2) phan tich nhirng diem tuong dong va khac biet lien quan den su bat dau gio miia va miia mua tren hai khu vuc nay.
2. Cc SO" so lieu va phuong phap nghien cihi
2. 1. Cff sa so lieu
De xac dinh thai diem bat dau gio miia cho khu vyc Nam Bg chiing toi sir dung hai nguon so lieu: (1) so Men mua ngay tir nam 1979 din nam 2004 ciia 6 tram: Bao Loc, Tay Ninh, Tan Son Nhit, Cin Tho, Rach Gia va Ca Mau ; (2) sl lieu gio tai lOOOhPa ciia NCEP/DOE-II, trung binh tren 4 mit luoi (105°E-107.5°E; 10°N-12.5°N). Bk xac dinh thai diem bat dau miia mua cho khu vuc Bic Bg, chung toi su dung sl lieu mua ngay ciia 9 tram tren khu vuc nay tir nam 1979 din nam 2004. Cac tram dugc lira chgn d\ra tren co so chat lugng so lieu ciia tram va tinh dai dien ciia tram cho khu vuc, bao gom: Lai Chau va Mgc Chau, dai dien cho khu vvrc Tay BSc; Ha Giang va Yen Bai cho khu vuc Viet Bac; Lang Son, Hon Gai cho khu vuc Dong Bic; 3 tram Ha Ngi, Nam Dinh va Thanh Hoa cho khu vuc dong bang.
De phan tich dieu kien khi quyen trong khoang thoi gian BDMM va xac dinh nhan to dy: bao cho thai dieni nay,_ tac gia sir dung chuoi sl lieu: Nhiet do khong khi tai 2m;
Do am rieng tai 2m; Ap suat khong khi muc biln SEP (Sea Level Pressure); Truang gio muc 1000 hPa ciia NCEP/DOE-II (Chi tilt vl co so sl lieu nay, xin tham khao a tai lieu Pham Xuan Thanh, Fontaine B. va Phillipon N., 2008) va Birc xa song dai OLR (Outgoing Longwave Radiation) ciia NOAA tren khu vuc chau A -Thai Binh Duong.
Cac so lieu nay deu dugc lay 6 buoc thoi gian 1 ngay va lien tuc tir ngay 01/01/1979 den ngay 31/12/2004. Nhiet do khong khi tai 2m (T); Do im rieng tai 2m (q) duoc tl hgp thanh nang lugng tinh am MSE (Moist Static Energy). Dai lugng nay dugc xac dinh bang cong thirc: ^ . . .
MSE=Cp.T + L.q^g.Z "^
Trong do; Cp la nhiet dung rieng (Cp= 1,004 J/°K.Kg); L la in nhiet hoa hai (L=2,501 J/g); g la gia toe trgng truong va Z= 2.
ni/i
pHanlVVAT LY KHi QUYtN 341
2.2 Phuang phdp
Phuang phap phan tich ting hgp (Composite Analysis) dugc sir dung de xac dinh nhan to du bao tir cac truong ylu tl khi tugng neu tren. Truoc het, tac gia thanh lap cac ban db truoc va sau thai dilm BDGM/BDMM ciia timg truong ylu tl (hinh 2 va 3), sau (Jo lap ban do so sanh giu:a hai ban dl nay (hinh 4). Nhan tl du bao dugc chgn tai nhung khu vuc CO bien doi nhilu nhit giua truoc va sau BDGM/BDMM.
Phuang phap hoi quy da biln (Multivariate Linear Regression Analyses) dugc su dung de xay dimg phuang trinh du bao hli quy. Theo do, 40 chi sl du bao (10 nhan tl dir bao* 4 hau (1 hau = 5 ngay, 1 nam co 73 hau, hau diu tien lir ngay 1-5 thang 1, hau thir 73 tir ngay 26 den nay 31 thang 12) truoc thoi dilm BDGM/BDMM ciia nam som nhat trong 26 nam (ngay 23 thang 4), dugc dira vao phuong trinh du bao de tim ra 4 chi s6 du bao tot nhit cho ylu tl BDGM/BDMM.
3. Xac djnh thai diem bat dau gio miia va bat dau miia mua 3. 1 Xdc dinh thai diem bin diiu gid miia trin khu vuc Nam Bp
Thoi diem bat dau gio miia dugc cac tac gia tren the gioi xac dinh bang nhieu phuong phap khac nhau: (1) thong qua su thay doi ciia huong gio thinh hanh (Holland, 1986); (2) dua vao lugng mua, klii no vugt nguong nao do (Tao and Chen, 1987; Wang and LinHo, 2002; Zhang et al, 2002); (3j tl hgp giiia truang gio va mua, hoac giira gi6 va hoat dgng dii luu (Lau and Yang, 1997; Qian and Lee, 2000; Yihui and Yanju, 2001). Nhung nghien ciai gin day con cho thay, sir dao ngugc ciia gradient nhiet do theo phuang kinh tuyln tren cao (200-500 hPa) la mgt dau hieu tot cho BDGM (Li and Yanai, 1996; Mao, Chan and Wu, 2004).
(2) khu vuc Nam Bg - noi dugc coi la dien hinh cho^ che do gio miia khu vuc, thoi dilm bit diu miia mua thuong gin liln voi thai diem^bit dau gio miia miia he, va doi khi hai thai dilm nay dugc coi la mgt. Hinh 1 bieu dien gia tri tmng binh trugt 5 ngay ciia lugng mua trung binh 26 nam ciia 6 tram khu vuc Nam Bg (duong lien net) va van tic gio vT huong trung binh 26 nam ciia 4 mit luoi (105°E-107.5°E; 10°N-12.5°N). D I thi nay cho thiy sir chuyin dii tir nuia kho sang miia mua dugc danh dau boi: (1) su tang dot nggt vl lugng mua tir 3mm/ngay trong thang 4 sang 7mm/ngay trong thang 5;
(2) sir thay dii huang gio tir Dong - Nam (gia tri am) sang Tay - Nam (gia tri duong).
Tren CO so do, ngay bit diu gio miia moi nam dugc xac dinh khi thoa man dieu kien: (1) tdng luang mua ngdy (sau khi trung binh truot 5 ngdy) phdi vuat qud Smm/ngay va duy tri lien tiic 5 ngdy; (2) tdc dp gid vi hudng phdi lan han O.Sm/s. Nhu vay, thai dilm bit diu gio miia cung chinh la thai dilm bit diu miia mua tren khu vuc Nam Bg.
Dua theo tieu chuin tren, chung toi da tinh thai dilm bit diu gio miia tren khu vuc Nam Bg, cho 26 nam. Ket qua cho thiy. ngay bit dau giomiia trung binh 26 nam la ngay 12'thang 5, vai do lech chuan la 11.5 ngay. Sam nhat la ngay 19 thang 4 (nam
1979), va mugn nhit la ngay 9 thang 6 (nam 1993).
342 Tuy4n tap cdc cong trinh nghien ciyu vat ly Dja c i u 2008
3. 2. Xdc dinh thai diim bdt dau miia mua trin khu vuc Bdc Bp
Tren khu \Trc Bic Bg, hoan luu gio mua miia he chiu sir tac dgng rat Ion cua dia hinh va truang khi ap dia phuang. Do anh huong ciia ap thap Bac Bg, luong gio Tay - Nam bi chuyin huong thanh Dong - Nam (Pham Nggc Toan va Phan Tit Die, 1993). Khi nghien cim cho khu vuc Bic Bg, tac gia thiy khong co sir thay doi ro ret ve tic do gio tiruac va sau thai dilm bit diu miia mua (hinh 10a,b). Day la dilm khdc biet so voi khu vyc Nam Bp.
Tuy nhien, lugng mua chii ylu tren khu vuc nay van co nguon goc hu lugng am cung cap boi hoan luu gio miia he va thoi dilm bit diu miia mua van gan lien voi thod diem bat diu gi6 miia ti-en khu vuc nay noi rieng va khu vuc chau A gio miia noi chung.
Hinh 2 bilu diln gia tri trung binh timgt 5 ngay ciia lugng mua tnmg binh ngay 26 nam ciia 9 tram thuoc khu vuc Bic Bg (chi sl mua khu vuc Bac Bg). So bg, ta co the thay mua mua bit diu vao khoang diu thang 5 (dac tnmg boi su tang dot nggt cua luong mua trung bmh tir 3mm/ngay len 6mm/ngay) va ket thuc vao cuoi thang 10 (danh dau boi su giam dot nggt ciia lugng mua hi 5mm/ngay xuong 2mm/ngay). Tren co so gia tid trung binh tinigt 5 ngay ciia lugng mua tnmg binh 9 tram (chi so mua ciia hing nam), thai diem bat dau miia mua ciia moi nam dugc xac dinh theo dieu kien; Tong luang mua ngdy phdi vu0 qua gid tri 6mm/ngciy vd phdi keo ddi lien tuc trong vong 5 ngdy.
15
i 10
»i , 1 • •
10
E
-•^:ir i__' - _ ^
1 2 3 4 5 6 7 8 9 10 11 12
Hinh 1 Gia trj trung binh tru'<7t 5 ng^y cua Itpp-ng mu-a t r u n g binh ng^y 25 nSm tr§n khu v y c Nam Bp (du-ang ll§n net) va tdc dp gio trung binh tai
m y c tOOOhPa (3\ja'r.g S(A. net).
15
. 10
V l^
^ / ' - i ^ . / V l
o^..
6 7 f T h i n g
9 10 11 12
Hinh 2 G i i trj trung binh tru-p^ 5 ngSy cua lu'p'ng mu'a trung binh n g i y 26 nSm tr§n khu
vyc B I C Bp (nnm/ngiiy).
v l y nghia vat ly, dieu kien nay thoa man: (1) dac dilm mua ti-en khu vuc thuong gin liln vod mgt hmh the thoi tiet nhat dinh va thuong keo dai theo hing dgt 3-5 ngay; (2) dam bao vl do Ion dk loai tiix riiung dgt mua tiong miia kho do anh huong cua khong khi lanh (thuong CO lugng mua nho) ciing co tlil keo dai 3-5 ngay. Vl y nghia thing ke, day la dilu kien (qua nhieu phep thir) cho phep ham phan bl xac suit ciia thai dilm bit diu miia mua tiem can phan bo Gausse. Ket qua cho thiy, ngay bit diu miia mua Xrung binh cho 26 nam la ngay 13 thang 5, som nhat la ngay 23 thang 4 (nam 1979) va mugn nhit la ngay 2 thang 6 (nam 1982). Ket qua nay phii hgp vod nghien cim tien thi gioi ring thoi dilm BDGM tien khu vuc Dong Duong la dau thang 5 (Matsumoto, 1997; Qian and Lee, 2000; Wang and LinHo. 2002; Zhang et al., 2002\ va ca cac cong trinh ciia cac tac gia trong nuoc ring thang
Ph^n I V ^ A T L Y KHi QUY^N 343
5 la th^ang bat diu miia mua tien khu vuc Bic Bg (Pham Nggc Toan va Phan Tit Die, 1993;
Nguyen Dire Ngii: va Nguyen Trong Hieu, 2004). Ngoai ra, chimg ta co thi thiy la nam ma miia mua din mugn nhit (1982) ciing la nam hoat dgng cua Ee-Nino manh nhit trong nira cuoi the ky XX (Biii Minh Tang, 1998).
3. 2. Quan hi giita thdi diem bdt ddu gio miia/bdt dau miia mua v&i long luang mua ndm Hinh 3 va 4 tiinh bay tong lugng mua trung bmh ngay cho 8 nam gio miia/miia mua din som (lien net) va 8 nam gi6 miia/miia mua din mugn (dirt net), cho khu vuc Nam Bg (hmh 3)/khu virc Bac Bg (hinh 4). Kit qua nay cho chimg ta thiy, tien ca hai khu vuc, nhirng nam gio miia/miia mua den som lugng mua cao hon han so voi nhung nam gio miia/miia mira din mugn. Day la mgt net tuang ddng giira hai khu vuc.
14
F M A M J J
Hinh 3: Lu-p-ng mu-a trung binh ng^y 6 tr?m khu v y c Nam Bp cua 8 nSm gi6 mua d i n s 6 m (8 nam d§n mupn) du-ang l i l n n6t (net dirt)
14
F M A M J J A S O N D J Hinh 4: Lu-png mu-a trung binh ng^y 9 tr^m khu
v y c B^c Bp cua 8 nSm mua mu'a d§n sdrm (8 nam den mupn) du'6'ng Ii4n n6t (n^t 6(A)
4. Nhimg nhan to lien quan den thoi diem BDGM tren khu vuc Nam B9 v^
BDMM tren khu vyc Bic Bo
4.1. Dieu kien khi quyin liin quan den thai diim BDGD/BDMM
eCE 80°E 100°E 120°E 140°E 160°E
36°N
24°N
60°E 80°E 100°E 120°P 140°E 160°E
344 Tuyln tap cac cong trinh nghidn cu-u Vit ly Oja c^u 2008
36°N
24°N
12°N
0°
12°S
^
^ ^ /
m
/'"^lifi^^a
^c^s
\ \
• - _ • ' ' 1
j^res
^ ^ ^^ ' ' J - ; ! ^ ~ A / 7 \ : i
<-^^^^<m^-^\
60°E 80°E 120°E 140°E 160°E
Hinh 5: Ban d6 trung binh truang MSE truo-c (a), sau (b) va hi$u sau - tru'O'c (c) thai diem b i t d§u gio mua tren khu vyc Nam Bo, thd"! ky
1979-2004
36"N
60°E 80°E 100°E 120°E 140°E 160°E
36°N
24°N
eO'E 80°E 100°E 120°E 140''E 160°E
Hinh 6: Ban dd trung binh trudng MSE truo-c (a), sau (b) va hifu sau - truac (c) thai diem bat d^u miia mua tren khu vyc BSc Bp, thai ky 1979-2004
De thay ro sir thay doi dieu kien khi quyen xung quanh thoi diem BDGD/BDMM, tac gia xem cac truong ylu t l khi tugng la nhii:ng nhiing ham bien doi theo thai gian (hang ngay) trong khoang thai gian truoc' va sau ngay BDGD/BDMM. Diu tien, ngay BDMM dugc danh mIc 0, thai gian truoc (sau) ngay BDGD/BDMM dugc danh dau '•-" ("+"). Sau do, truang trung binh truoc (sau) ngay BDGD/BDMM la gia tri trung binh tir ngay -15 din ngay -1 (ngay 0 den ngay +14). Cuoi cimg, ban do tong hgp (sau-truoc) ciia cac truang khi tugng co lien quan dugc thanh lap de xay dyng cac chi s l lam nhan t l dy bao cho ngay bat dau gio miia/miia mua.
Hinh 5 va 6 trinh bay dieu kien trung binh truoc va sau thoi dilm BDGD/BDMM truang MSE. Truac thai dilm BDGD/BDMM, tren hinh 5a va 6a, diu thiy xuit hien tam eye dai MSE (375 KJ/Kg) tren phin phia Tay vinh Bengal.
Sau thai diem BDMM, no mo rgng va chilm toan bg vinh Bengal (hinh 5b,6b), tren hinh 5b con cho thay sy xuat hien tam eye dai nua tren Biln Dong. Dilu nay lien quan voi sy bimg phat ciia lulng gio miia Tay - Nam ting thip, mang theo hoi im tir An Do Duong toi khu vyc nay. Kit qua la MSE tang rit nhanh trong khoang thai gian thiet lap gio miia.
Ph4nlVVATLY;<NlnMY|^M
345
60°E 80°E 100°E 120°E 140°E 160°E
Hinh 7 : Ban d6 trung binh truang SLP truac (a), sau (b) va hipu sau-truac (c) thai diem b i t
Shu gio mua tren khu vyc Nam Bp, thai ky 1979-2004
12°N
12°S
^gtjydg
=>^g^
24°N
60"E 80°E 100°E 120°E 140°E 160°E
Hinh 8 : Ban do trung binh truang SLP truac (a), sau (b) va hi?u sau-truac (c) thai diem b i t d i u
mua mua tren khu vyc Bac Bp, thai ky 1979- 2004
Truoc thoi dilm BDGM/BDMM, phan luc dia phia Dong Trung Hoa duod sy khing chl cua luod cao ap Tay Bic Thai Binh Duong (hinh 7a,8a). Sau B D ( J M / B D M M luod ap cao nay suy ylu va dugc thay thi boi ranh thap xich dao. Dong thai c6 the thay mgt vimg thap tien Ithu vuc Bic Bg dang dugc hinh thanh (hinh 7b,8b).
Hinh 9 va 10 bilu dien phan tich ting hgp ciia truong gio tai 1000 hPa. Tren ban dl chiing ta thiy ro sy manh len cua luong gio Tay - Nam tren khu vyc vinh Bengal va su suy ylu ciia dong gio Dong va Dong - Nam tren Bien Dong trong thai ky
^ D G M / B D M M . Tuy nhien, chiing ta khong quan sat thay sy thay doi dang ke van toe gio tien khu virc Bac Bg trong thai gian dien ra BDMM (hinh 9a,b) ngugc lai, su thay
•JSi vl huons gio lai quan sat thiy rit ro tren khu virc Nam Bg (hinh 10a,b). Day la sy
^ac biet vl truang gio giira hai khu vuc.
346 Tuydn tap cac c6ng trinh nghifen CCFU Vat ly Pja c4u 2008
36°N
60°E 80°E 100°E 120°E 140''E 160°E
36°N'-
24°N
60°E 80°E 100°E 120''E 140°E 160°E
3fl°N
24°N
aO'E lOO'E 120°E 140°E 160°E
Hinh 9: Ban d6 trung binh trud'ng gi6 tai lOOOhPa tru^c (a), sau (b) va hl^u sau - tru-dc (c) thd-l 6\km bSt d i u gi6
mua trdn khu vyc Nam B^, thdri ky 1979-2004
60°E 80°E 100°E 120''E 140°E 160°E
'---'^-•J^-~v>~'-'-
60°E 80°E 100°E 120''E UO^E IBO'E
36°N
60°E 80°E 100°E 120°E 140°E 160°E
Hinh 10: Ban d6 trung binh trub-ng gi6 tai lOOOhPa tru'6'c (a), sau (b) va hi^u sau - tru^c (c) thdi di^m bdt dau mua mu-a trdn khu vyc BJc Bd, thfi*! ky
1979-2004
Culi ciing, truang tong hgp OLR (hinh 10 va 11) cho thiy chi tilt nhit vl su tien triln ciia dii luu nhiet doi lien quan din BDGD/BDMM. Truoc thoi diem- BDGD/BDMM, mgt dai tam eye tieu ciia OLR (dii luu sau) xuit hien tren khu virc xich dao thuoc Indonesia, trong khi phia Bic vinh Bengal va Biln Dong van nim trong khu vuc cue dai ciia OLR. Sau BDGD/BDMM, OLR tren biln Dong giam hin, duong ding tri 250W/m- bi day liii sang phia Dong ciia Philippin. Khu vyc cue tilu OLR tren Sumatra mo rgng len phia Bic, khu vyc dii luu manh nhit (< 210 W/m^) bao phu toan bg phia Dong Nam vinh Bengal, vinh Thai Lan va phia tay Nam Bg.Quan sat nay rit phii hgp voi nghien cim ciia Zhang et al (2002), nguoi cho ring su mo rgng va manh len ciia trung tam doi luu sau tren Sumatra c6 thi la nguyen nhan biing phat cua mua gio miia manh tren khu vyc Dong Duong.
tiln IV: V _ ^ Y KHl Q U Y £ N
347
60 E 80°E 100°E 120°E HO'E 160°E
24°N
60°E 80°E 100°E 120°E 140°E 160°E C)
80°E 80°E - lOO'E 120°E 140''E 160°E
linh 11: Ban dd trung binh trud-ng OLR tru6c (a),
»au (b) va hifu sau - tru6c (c) thai dilm b i t d^u Jiomua tren khu vyc Nam B^, thd-i ky 1979-2004
60°E 80°E 100°E 120°E 140°E 160°E
60°E 80°E 100°E 120°E 140°E 180°E
Hinh 12: Ban d i trung binh trudng OLR trud-c (aj, sau (b) v^ hi$u sau • tru6c (c) thdi dilm bat d i u mua mua tren khu vyc Bic B$, thdi
ky 1979-2004
Hinh 5c-12c bilu dien gia tri chenh lech giira truoc va sau thai diem 'POD/BDMM ciia cac truong ylu tl khi tugng lien quan lien quan chat che den sy bat l^u ciia gio miia/miia mua. Trong do xu thi biln dii duong (am) bilu thi bing duong i^n (dirt) va khu vyc mau xam bilu thj sy khac nhau c6 y nghia (dat nguong kiem
«hiem t voi p =^ 0.05). Trong thoi gian BDGD/BDMM (hinh 5c, 6c), MSE tang (giam) t^ndai 30°N-35°N (15°S-20°S). Dilu nay co lien quan din xu thi tang (giam) ciia SLP fong cimg dai 30°N-35°N (15°S-20°S) (hinh 7c,8c). Kit qua nay cho thiy ring nhirng
^u hieu ro net lien quan din sy BDGD/BDMM c6 thl^ tim thiy trong gradient kinh iiong cua MSE va SLP. Thoi ky BDGD/BDMM cho thiy sy tang manh ciia gio Nam J Tay - Nam tai 1000 hPa tren biln A Rap, vinh Bengal va khu vyc Nam Bien Dong linh 9c 10c). Dieu nay co lien guan voi sy tang cua gradient kinh huong ciia SLP. Cuoi iing^ hinh lie va 12c cho thay mgt xu huong tang (giam) ciia OLR tren khu vuc
348 TuySn tap cac cdng trinh nghldn cuu Vat ly Dja clu 2008
Indonesia, khu vyc giua An Do Dirong (biln A Rap, vinh Bengal va Biln Dong) trong thai gian BDGD/BDMM. Do la kit qua ciia su di chuyin len phia Bic cua khu vuc may cue dai.
Tit ca kit qua tren (tir hinh 2-7) cho thiy moi quan he khang kbit giua cac trucmg ap suit, nang lugng tinh im, gio va dii km (OLR) voi ngay BDGD/BDMM. Dilu nay hoan toan cho phep chung ta thilt lap phuang trinh hoi quy cho thoi dilm BDGM/BDMM, se dugc trinh bay duai day.
4. 2. Phuffng trinh hdi quy cho thai diim BDGM/BDMM
A. Phucmg trinh hdi quy cho thdi diim BDGM cho khu vuc Nam Bp
Sir dung phuang phap hoi quy tuyen tinh timg buoc doi voi 10 nhan to du bao trong khoang thoi gian tir hau thir 19 (3-7 thang 4) din hau thir 22 (18-22 thang 4). Tit ca nhung nhan to dy bao nay deu co y nghia vat ly. Chung mo ta nang lugng va dpng nang ciia gio miia theo kinh tuyen qua gradient ap suat khi quyen muc bien va nang lugng;
tinh am gan be mat, va chimg cung mang nhung dac diem lien quan mat thiet den bat diu ciia miia mua (sy thay doi ciia toe do gio, sy di chuyen ciia tam may doi luu). Chiing bao' gom: (1) MSE tren bien Dong Tmng Quoc; (2) MSE tren khu vuc Australia; (3) SLP trer' khu virc giua An Do Duong; (4) SLP tren kliu vyc biln Dong Trung QuIc; (5) Gio trer bien A rap; (6) Gio tren vinh Bengal; (7) Gio tren khu vyc Nam Viet Nam; (8) OLR trer vinh Bengal; (9) OLR tren Bien Dong, va (10) OLR tren khu vuc Indonesia (khu vuc dong khung trong hinh 5c,7c,9c,l Ic ).
Trong so 40 chi sl dua vao dy bao (10 nhan t l * 4 hau thoi gian), cuoi ciing chor dugc 4 chi sl tit nhit cho dy bao la: chi sl SLP tren Dong Trung QuIc Sea (20°E 140°E, 30°N-35°N) trong hau thir 18 (ky hieu SLP18); Gio tren khu vuc Nam Viet Nan (105°E-115°E, 10°N-15°N) trong hau thir 20 (wind20); OLR tren vinh Bengal (90°E 100°E, 10°N-20°N) trong hau thir 18 (0LR18), va OLR tren Indonesia (100°E-I20°E 0-10°S) trong hau thiv 20 (OLR20). He sl ciia phuong trinh dy bao hli quy dugc ch(
trong bang 1. Ket qua nay cho thiy mgt miia gio miia din som tren khu vuc Nam B(
dugc dy bao truac boi cac dau hieu:
1. Sy khac thuong am ciia SLP tren khu vyc dong Trung QuIc (he sl hli quy +) tronr
dau thang 4 (ngay 31 thang 3 din 4 thang 4). ' ; 2. Sy khac thuong am ciia gio Dong va Dong - Nam tren khu vuc Miln Nam (he s-'
hoi quy +) khoang giira thang 4 (ngay 10 thang 4 din 14 thang 4)
Sy khac thuong am ciia OLR tren khu vyc phia Dong vinh Bengal (he s6 hoi quy + dau thang 4 (ngay 31 thang 3 den 4 thans 4^ .* ..-,„.'
3.
3 den 4 thang 4). . ''
Bang 1: Phuang trinh h6i quy cho tho-i di§n b i t d i u gio mua tren khu vyc NamB^^
Chi so d y bao SLP18 wind20 0 L R 1 8 OLR20
J^if6^^^^uy__^+0.30 +0,39 +0.50 -0.43
Ph^n I V v ^ T LY KHi QUY^N
349 - 5- Phuang trinh hdi quy cho thdi diim BDMM cho khu vice Bdc Bp
10 nhan t l dugc diing dl dy bao thai dilm BDMM cho khu vyc Bic Bg gIm: (1) MSE tren khu vyc luc dia phia dong Trung QuIc; (2) MSE tren khu vyc giua An Do Duong; (3) SLP tren khu vyc dong^ Trung QuIc; (4) SLP tren khu vyc giira An Do Duong; (5) Gio tren biln A rap; (6) Gio tren vinh Bengal; (7) Gio tren khu vuc Nam Viet Nam; OLR tren biln A Rap; (9) OLR tren l<hu vyc giira An m Duong va (10) OLR phia Dong vinh Bengal (khu vyc dong khung trong hinh 6c,8c,10c,12c).
4 trong so 40 chi sl dugc chgn cho dy bao la: chi sl trung binh ciia SLP tren khu vuc Dong Trung QuIc (110°E-130°E, 25°N-30°N) trong hau thir 19 (ky hieu SLP19 tren bang 2); gio tren khu vyc Nam Viet Nam (105°E-115°E, 10°N-15°N) trong hau thir 22 (wind22); chi sl OLR tren khu vyc biln A Rap (60°E-70°E, 5°N-15°N) trong hau thir 19 (0LR19); va chi sl OLR tren phin Dong vinh. Bengal (90°E-100°E, 10°N-20°N) trong hau thir 20 (OLR20), He so ciia phuong trinh dy bao hoi quy dugc cho trong bang 2. Ket qua nay cho thay mgt miia mua den som tren khu vyc Bac Bg dugc dy bao truoc bai cac dau hieu:
1. Sy khac thuong duong ciia SLP tren khu vyc dong Trung QuIc (he sl hli quy -) trong dau thang 4 (ngay 3-7).
2. Sy khac thuong am ciia gio Dong va Dong - Nam tren khu vyc Mien Nam (he sl hoi quy +) khoang giiia thang 4 (ngay 18-22).
3. Sy khac thuong am ciia OLR tren khu vyc phia Dong vinh Bengal (he so hoi quy +) diu thang 4 (ngay 8-12).
Bang 2: Phu'cng trinh hdi quy cho thai dien b i t d i u mua mua tren khu vuc Bic Bo
Chi so du bao He so hoi quy
SLP19 -0.47
wind22 +0.63
OLR19 -0.45
OLR20 +0.41
C. Diim tuang ddng vd khdc biet BDGM trin khu vuc Nam Bp vd BDMM trin khu vice Bdc Bp duai dnh hudmg ciia nhdn td lien quan.
Kit qua phan tich a tren cho ta thay nam gio miia den som tren khu vyc Nam Bg va miia mua din som tren khu vyc Bic Bg deu dugc dac tnmg boi: (1) sy khac thuong am ciia gio Dong va Dong - Nam tren khu vyc mien Nam Viet Nam ding nghia voi viec lucd ap cao Tay Bic Thai Binh Duong som nit vl phia Dong, tao dieu kien cho luong gio Tay - Nam phat triln som; (2) Sy khac thuong am ciia OLR tren khu vyc phia Dong vinh Bengal cho thiy doi luu sau sam xuit hien tren khu virc nay, bao Uuac cho mgt miia mua den som cho chu vuc Dong Duong. Day la anh huong co tinh tuang dong giira khu vyc Bac Bg va khu virc Nam Bg duai sy tac dgng ciia nhung nhan to quy mo khu vyc.
'•' • Tuy nhien vai su khac thuong duang ciia SLP tren bien Dong Trung Hoa lai tuang irng voi gio miia din mugn tren khu \yc Nam Bg va miia mua den sain tren khu vuc Bic Bg. Sy khdc biet co ban nay c6 thi dugc giai thich nhu sau: De thiet
350 Tuy§n t i p cac c6ng trinh nghiSn cuu Vat ly Dja cJiu 2008
lap gio miia tren khu vyc Nam Bg thi gio Dong Nam tren khu vuc nay va tren Biln Dong phai bi diy liii sang phia dong tao dilu kien cho lulng gio Tay - Nam phat triln ding thai voi viec hinh thanh tam MSE tren bien Dong. Tam ap cao tren khu vyc biln Dong Trung Hoa phat triln lam giam gradien ap suat theo phuong kinh tuyln, ngan can su phat triln cua lulng gio Tay - Nam. Ket qua la gio miia thilt lap mugn tren khu vyc Nam Bg. Ngugc lai, ap suit cao trong khu vyc bien Dong Trung Hoa tuong irng voi kha nang xuat hien cao ciia cac dgt khong khi lanh xam nhap xuIng Bic Bg khoang diu miia mua, tao dieu kien kich dgng cho khoi khi gio miia Tay - Nam thang len, gay ra mua dan den miia mua den som tren khu vuc nay. Nhu vay, sy anh huong cua khong khi bien Dong Trung Hoa toi Viet Nam, hoan toan co tinh dia phuang.
5. Ket luan
Muc dich ciia bai bao nay la xac dinh thai diem bat dau gio miia miia he tren khu vuc Nam Bg va sy bat dau miia mua tren khu vyc Bac Bg va phan tich su anh huong cua nhimg nhan to lien quan tod hai sy kien nay. Tir ket qua tren, co the nit ra mgt so ket luan nhu sau:
1. Ngay bat dau gio miia trung binh cho 26 nam (1979-2004) tren khu vuc Nam Bg la 12 thang 5 voi do lech chuan 11.6 ngay. Ngay bat dau miia mua trung binh 26 nam tren khu \irc Bac Bg la ngay 13 thang 5, voi do lech tieu chuin 11.5 ngay.
2. Ton tai moi quan he chat che giira ngay bat dau gio miia tren khu vuc Nam Bg va ngay bat dau miia mua tren khu vyc Bac Bg voi qua trinh phat triln ciia cac truong khi tugng Uen khu vyc chau A - Thai Binh Duong, trong khoang thai gian truoc va sau thai diem BDGM/BDMM. Tren co so mli quan he nay, ta co thi thilt lap dugc phuong trinh dy bao hoi quy cho thod dilm BDGM/BDMM. Nhimg nhan tl tit nhit ciia phuong trinh du bao gIm: (1) Ap suit khi quyin muc biln khu vyc Dong Trung Hoa; (2) Gio Dong va Dong - Nam tren khu vyc Mien Nam Viet Nam; (3) Truong buc xa song dai (doi lum manh) tren khu vuc phia Dong vinh Bengal.
3. Sy dich chuyen cua khu vuc may dii luu eye dai tren xich dao (cue tilu cua OLR) len phia bac va sy nit lui vl phia dong ciia luod ap cao Tay Thai Binh Duong trong then ky mo diu gio miia iniia he la nhirng hien tugng co quy mo khu vuc, no gay anh huong ding thoi toi sy bat diu gio miia tren Nam Bg va Bit diu miia mua tren khu vuc BSc Bg. Ngugc lai, sy anh huong ciia khii khi tren biln Dong Tmng Hoa toi Viet Nam lai CO tinh dia phuong, no gay nen sy ngugc pha vl thai dilm BDGM tren khu vuc Nam Bg va BDMM tren khu vuc Bic Bg.
6. Tai lieu tham khao
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SUMMARY
THE PREDICTORS OF THE SUMMER MONSOON ONSET OVER SOUTHERN VIETNAM AND THE RAINY SEASON BEGINNING OVER
NORTH VIETNAM
In this paper, the onset of the summer monsoon (OSM) over the Southem Vietnam and the beginning of the rainy season (BRS) over the North Vietnam are defined using the observed daily rainfall at 15 selected stations over the period 1979-2004. The mean date oi the OSM is on 12 May, with a standard deviation of 11.6 days and the mean date of the BRS is on 13 May, with a standard deviation of 11.5 days. The temporal and spatial stmctures of atmospheric conditions prevailing around the OSIvI/the BRS are first detailec usmg the reanalysis data of the NCEP/DOE-II. Then the experimental hindcasts of the OSM/BRS are performed using multivariate linear regression analyses (MLR) based upon ^ predictors linked to robust signals are provided. Three predictors of the OSM and the BRJ are: (1) the sea level pressure over the East China; (2) the southeasterly winds over southen Vietnam, and (3) the deep convection over the Bay of Bengal. The predictors (2) and (3 affect on the OSM/the BRS by the same way, but the first one does in contrary directions.
