方陸俊,管兆勇,王美,夏陽(yáng)

① 南京信息工程大學(xué) 氣象災(zāi)害教育部重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210044;② 南京信息工程大學(xué) 氣象災(zāi)害預(yù)報(bào)預(yù)警與評(píng)估協(xié)同創(chuàng)新中心,江蘇 南京 210044;③ 杭州市氣象局,浙江 杭州 310051

北半球夏季海洋性大陸區(qū)域氣候與EP型ENSO:直接與間接聯(lián)系

方陸?、佗冖?管兆勇①②*,王美①②,夏陽(yáng)①②

① 南京信息工程大學(xué) 氣象災(zāi)害教育部重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210044;② 南京信息工程大學(xué) 氣象災(zāi)害預(yù)報(bào)預(yù)警與評(píng)估協(xié)同創(chuàng)新中心,江蘇 南京 210044;③ 杭州市氣象局,浙江 杭州 310051

2013-04-27收稿，2013-06-28接受

國(guó)家自然科學(xué)基金資助項(xiàng)目(41330425);公益性行業(yè)(氣象)科研專項(xiàng)(GYHY20140624)

摘要利用1979—2009年的NECP資料、Hadley海溫月平均資料和CMAP降水資料,采用Kao and Yu(2009)的方法定義了夏季EP型ENSO指數(shù)EPI,用合成分析的方法分析了東部型ENSO與海洋性大陸降水的關(guān)系。結(jié)果表明:EPI與MC(Maritime Continent,海洋性大陸)區(qū)域降水變化間存在非常弱的負(fù)相關(guān)。造成這一弱相關(guān)的原因是EPI與MC區(qū)域降水在某些年份存在同號(hào)變化。在剔除Ni?o4信號(hào)后,海洋性大陸區(qū)域降水序列與EPI與存在著同號(hào)和反號(hào)兩種關(guān)系。反號(hào)關(guān)系是通常所認(rèn)為的,當(dāng)經(jīng)典的El Ni?o(La Ni?a)發(fā)生時(shí)MC區(qū)域降水出現(xiàn)顯著地減少(增多)。此時(shí),沿赤道的異常Walker環(huán)流建立了EP型ENSO與MC區(qū)域氣候間的直接聯(lián)系。而在同號(hào)關(guān)系時(shí),菲律賓以東異常加熱和SPCZ區(qū)域異常冷卻引起的西北—東南走向的垂直環(huán)流圈削弱了MC區(qū)域與赤道東太平洋之間的異常Walker環(huán)流所建立的直接聯(lián)系,或者說(shuō),赤道東太平洋區(qū)域SSTA與MC區(qū)域降水異常的形成是通過(guò)SPCZ區(qū)域SST的反號(hào)異常而產(chǎn)生間接聯(lián)系的。這種機(jī)制的揭示為深刻認(rèn)識(shí)ENSO影響海洋性大陸區(qū)域甚至東亞地區(qū)氣候變動(dòng)的聯(lián)系提供了新的線索。

關(guān)鍵詞

海洋性大陸

降水異常

東部型ENSO

事件

SPCZ

北半球夏季

海洋性大陸(MC,Maritime Continent)區(qū)域(90～150°E,10°S～20°N)位于熱帶地區(qū),由若干島嶼和淺海組成,具有獨(dú)特的氣候特征(Ramage,1968)。這一熱帶海洋性氣候區(qū)域從海洋環(huán)流上阻擋了熱帶太平洋與印度洋的聯(lián)系但又從大氣環(huán)流上將它們聯(lián)系了起來(lái)(吳國(guó)雄和孟文,1998;Alexander et al.,2002)。MC區(qū)域受到太平洋信號(hào)和印度洋信號(hào)的影響,是Walker環(huán)流上升支所在區(qū)域,與亞洲季風(fēng)和澳洲季風(fēng)活動(dòng)均存在著密切關(guān)聯(lián)(Chang et al.,2005;唐衛(wèi)亞和孫照渤,2005)。同時(shí),此區(qū)域天氣氣候變化還與ITCZ和SPCZ活動(dòng)包括熱帶氣旋和季風(fēng)低壓活動(dòng)存在著密切聯(lián)系(祝從文和何敏,1998)。

海洋性大陸區(qū)域氣候變動(dòng)與ENSO有著重要的聯(lián)系。研究表明,MC區(qū)域氣候與El Nio和南方濤動(dòng)(South Oscillation)具有較高的相關(guān)(Lau and Chan,1983)。當(dāng)El Nio(La Nia)發(fā)生時(shí),海洋性大陸區(qū)域降水偏少(偏多)(Ramage,1968);在北半球夏季和秋季,印度尼西亞降水與赤道東太平洋的海溫異常有著很強(qiáng)的負(fù)相關(guān)(Haylock and McBride,2001;Hendon,2003);當(dāng)赤道中東太平洋出現(xiàn)ENSO事件時(shí),那里的SST異常升高,其上空有較強(qiáng)的對(duì)流活動(dòng),而對(duì)流區(qū)的東移使得西太平洋暖池出現(xiàn)異常的大氣下沉運(yùn)動(dòng),形成了異常的Walker環(huán)流,導(dǎo)致海洋性大陸、澳大利亞及周邊國(guó)家出現(xiàn)干旱(Feng et al.,2010)。

20世紀(jì)80年代以前,ENSO事件多出現(xiàn)在赤道東太平洋冷舌區(qū),但Yeh et al.(2009)指出在全球變暖背景下ENSO循環(huán)發(fā)生顯著變化。此后,人們發(fā)現(xiàn)ENSO事件開(kāi)始頻繁出現(xiàn)于赤道中太平洋海域(Ashork et al.,2007;Kao and Yu,2009;Yeh et al.,2009;Feng and Li,2011;馮娟等,2010),由此帶來(lái)的氣候影響也與傳統(tǒng)ENSO事件的影響有著明顯的差異。Ashok et al.(2007)根據(jù)赤道太平洋SST異常緯向三極型分布,設(shè)計(jì)了EMI指數(shù)來(lái)表征中部型SST異常事件。對(duì)于絕大多數(shù)事件而言,Nio3區(qū)具有較好的代表性(李曉燕和翟盤茂,2000),但是對(duì)于一些事件中,Nio1+2區(qū)升溫較強(qiáng),而Nio3區(qū)升溫較弱。另外,Nio3指數(shù)中還包含了中部型的信號(hào),所以Nio3指數(shù)并不能很好地表征東部型ENSO。Trenberth and Stepaniak(2001)提出了TNI指數(shù)并結(jié)合Nio3.4指數(shù)來(lái)監(jiān)測(cè)和追蹤ENSO。Ren and Jin(2011)將Nio3和Nio4指數(shù)進(jìn)行非線性變換,得到一組新指數(shù)CTI和WPI來(lái)描述兩類ENSO。而Kao and Yu(2009)利用回歸分析方法得到熱帶太平洋海表溫度場(chǎng)中與Nio4區(qū)海溫變化無(wú)關(guān)的殘差部分,進(jìn)而對(duì)殘差場(chǎng)進(jìn)行EOF分析,取其第一主分量和時(shí)間系數(shù)分別作為東部型ENSO事件的標(biāo)準(zhǔn)EOF模和標(biāo)準(zhǔn)指數(shù),同時(shí)還定義了中部型ENSO指數(shù)。

然而,因ENSO存在不同的分布型,使MC區(qū)域氣候與ENSO的關(guān)系變得復(fù)雜起來(lái)。目前研究ENSO與MC區(qū)域氣候關(guān)系時(shí),要么未將東部型和中部型ENSO區(qū)分開(kāi)來(lái),要么選用的方法和指數(shù)如Nio3指數(shù)并不能很好地將東部型或中部型事件從整個(gè)ENSO事件中分離出來(lái)。就東部型而言,其與海洋性大陸區(qū)域氣候變動(dòng)的關(guān)系如何,尚不清楚。

綜上所述,盡管ENSO與海洋性大陸區(qū)域氣候變動(dòng)的關(guān)系已有研究,但是,對(duì)于東部型和中部型ENSO與MC區(qū)域氣候聯(lián)系的研究并不多見(jiàn)。本文將采用Kao and Yu(2009)提出利用年平均海溫異常求取指數(shù)的方法,求取夏季的東部型指數(shù)并進(jìn)而探討北半球同時(shí)期夏季東部型ENSO變化規(guī)律以及其與夏季海洋性大陸區(qū)域氣候的聯(lián)系。

1資料和方法

采用的資料有:1)美國(guó)國(guó)家環(huán)境預(yù)報(bào)中心/大氣研究中心(NCEP/NCAR)的逐月再分析資料(Kalnay et al.,1996),主要包括水平分辨率為2.5°×2.5°的全球范圍網(wǎng)格點(diǎn)的風(fēng)場(chǎng)、濕度場(chǎng)、溫度場(chǎng);2)英國(guó)Hadley中心水平分辨率為1°×1°的月平均海溫資料;3)CMAP提供的2.5°×2.5°的月平均降水資料。考慮到NECP/NCAR資料自1979年以后更為可靠(Kalnay et al.,1996),所選時(shí)段取為1979—2009年。夏季平均指6—8月三個(gè)月的平均。

主要采用了合成分析、線性回歸、EOF等統(tǒng)計(jì)分析方法。為了揭示大氣環(huán)流變化與MC加熱異常變化及其與相應(yīng)的非絕熱加熱強(qiáng)迫之間的關(guān)系,這里還計(jì)算了大氣視熱源和視水汽匯(Luo and Yanai,1984),計(jì)算公式為:

(1)

Qs+;

(2)

而

-=+(Qs+LEs)。

(3)

其中:L為凝結(jié)潛熱;Pr為降水量;Qs為地面感熱輸送;E為氣柱中云滴的蒸發(fā)量;C為氣柱中水汽凝結(jié)所致的液態(tài)水生成量;Es為地面潛熱輸送;為輻射加熱(冷卻)的垂直積分;Ps為地面氣壓;Pt為所取的頂層氣壓(取為300hPa);為非絕熱加熱率;為因水汽相變而產(chǎn)生的加熱率。

圖1　東部型(a)和中部型(b)ENSO事件的EOF空間模態(tài)及其時(shí)間序列(c)(Kao and Yu,2009)Fig.1　Leading EOF patterns obtained from a combined EOF-regression analysis respectively for (a)EP-type ENSO and (b)CP-type ENSO;The normalized time series of coefficients of the leading EOFs are displayed in (c),with open bars for CPI and closed bars for EPI

2東部型ENSO指數(shù)與典型年份選取

EPI和CPI指數(shù)能夠較好地區(qū)分東部型和中部型ENSO。采用Kao的做法(Kao and Yu,2009),針對(duì)北半球1979—2013年逐年夏季(6—8月)平均的海溫距平,給出了東部型(EP)和中部型(CP)ENSO的EOF空間模態(tài),以及EP和CP指數(shù)(EPI和CPI)的指數(shù)標(biāo)準(zhǔn)化時(shí)間序列(圖1)。可以看出,東部型ENSO(圖1a)事件的偏暖(冷)區(qū)位于赤道東太平洋冷舌區(qū),最大偏暖(冷)中心位于赤道140°W以東。對(duì)于中部型(圖1b)事件,其偏暖(冷)中心位于140°W以西地區(qū),大致位于日界線附近。從兩者夏季的指數(shù)時(shí)間序列(圖1c)可看出,某些事件屬明顯的東部型或中部型,但某些事件則既具有東部型又具有中部型特征,EPI與CPI相關(guān)系數(shù)為-0.30。

要說(shuō)明的是,盡管ENSO事件多數(shù)在北半球冬季時(shí)達(dá)到最強(qiáng),但有部分ENSO事件在夏季仍具有較強(qiáng)的信號(hào),這種赤道太平洋地區(qū)較大強(qiáng)度的SSTA可被看成是ENSO循環(huán)過(guò)程中表現(xiàn)在赤道太平洋地區(qū)的強(qiáng)信號(hào)。因此,盡管有些年份EPI數(shù)值相對(duì)較小,但夏季的EPI仍可被當(dāng)成是用來(lái)描寫(xiě)ENSO的指數(shù)。

對(duì)于EP型ENSO,由于海溫異常顯著區(qū)域與MC區(qū)域的距離明顯較CP型的遠(yuǎn)(圖1a、b),這使得EP型ENSO與MC區(qū)域降水間的關(guān)系顯得更為復(fù)雜。計(jì)算表明,MC區(qū)域平均降水RMC時(shí)間序列與EPI的相關(guān)系數(shù)為-0.14。這與人們通常認(rèn)為的當(dāng)El Nio(La Nia)發(fā)生時(shí),MC區(qū)域降水顯著負(fù)(正)異常并不一致。這種不一致的形成原因可能比較復(fù)雜。比如,ENSO的發(fā)展年和衰減年SSTA對(duì)其周圍氣候的不同影響可能會(huì)造成EPI指數(shù)與MC地區(qū)降水的相關(guān)性不高,或者中太平洋或SPCZ區(qū)域存在顯著的與東太平洋區(qū)域相反符號(hào)的較強(qiáng)的熱力強(qiáng)迫,等等。

注意到,在EPI指數(shù)的定義中采用線性回歸的方法濾除了Nio4區(qū)域平均SSTA的影響(Kao and Yu,2009),故這里在計(jì)算夏季海洋性大陸降水時(shí)間序列時(shí)也需采用類似的方法,即在RMC中濾除Nio4信號(hào)的影響,以得到不含有Nio4信號(hào)的降水序列MC。圖2給出了夏季EPI指數(shù)和夏季海洋性大陸降水MC)時(shí)間序列。由圖2可知,EPI與MC均具有顯著的年(代)際變化。盡管EPI與MC的相關(guān)系數(shù)有所增大,達(dá)-0.33,但二者并沒(méi)有表現(xiàn)出我們通常認(rèn)為的顯著的反位相關(guān)系。這一結(jié)果再次表明了人們長(zhǎng)期認(rèn)為的當(dāng)El Nio(La Nia)事件發(fā)生時(shí),MC區(qū)域會(huì)出現(xiàn)干旱(洪澇)的現(xiàn)象并不顯著地出現(xiàn)在東部型ENSO事件發(fā)生時(shí)。然而,我們注意到,根據(jù)圖2,當(dāng)剔除那些同號(hào)的年份后,MC區(qū)域降水與EPI的相關(guān)是很高的,達(dá)到了-0.77,這說(shuō)明那些同號(hào)年份事件的出現(xiàn)對(duì)EP型ENSO與MC區(qū)域夏季降水的關(guān)系存在顯著的干擾。

圖2　夏季EPI指數(shù)(柱狀)與夏季海洋性大陸降水(曲線)時(shí)間序列(柱狀紅色表示同號(hào)事件(PP和NN),柱狀藍(lán)色表示反號(hào)事件(PN和NP);曲線表示的是濾除Nino4信號(hào)后的夏季MC區(qū)域降水時(shí)間序列MC)Fig.2　The normalized time series of EPI(bars) and anomalous precipitation MC over the MC(curve) in boreal MC is obtained by removing the Nio4 signal from the rainfall anomalies [red (blue) bars indicate the same (opposite) signed value events for EPI and MC precipitation anomalies]

EPIRMC同反號(hào)年份PEPPRPP1983,1987,2011NEPNRNN1988,2003,2007PEPNRPN1982,1997,1998,2008NEPPRNP1990,1994,1995,2001,2002,2004,2013

3影響機(jī)制

為揭示東部型ENSO對(duì)海洋性大陸區(qū)域氣候變動(dòng)產(chǎn)生影響的機(jī)制,可對(duì)異常降水、速度勢(shì)、海溫、視熱源、非絕熱加熱等基于表1所列年份進(jìn)行合成差值分析。如無(wú)特別說(shuō)明,在下文分析中,所有物理量時(shí)間序列中的Nio4信號(hào)均已先行濾除(采用線性回歸的方法)。

3.1降水異常

3.2環(huán)流異常

為進(jìn)一步了解東部型ENSO發(fā)生時(shí)在海洋性大陸出現(xiàn)兩種不同符號(hào)的降水異常事件的成因,給出了針對(duì)同號(hào)事件和反號(hào)事件的異常海溫場(chǎng)以及異常海表面風(fēng)場(chǎng)合成差值。從同號(hào)年合成差值(圖4a)可見(jiàn),異常的海溫主要位于赤道東太平洋地區(qū)(100～120°W,10°S～10°N)和南太平洋輻合帶(140°E～180°～160°W,10～30°S)這兩個(gè)區(qū)域。當(dāng)赤道東太平洋海溫為暖異常時(shí),南太平洋輻合帶地區(qū)則存在顯著的冷異常。這樣的海溫配置有利于形成海表面風(fēng)場(chǎng)在赤道東太平洋的輻合,并在南太平洋輻合帶區(qū)域(SPCZ)出現(xiàn)反氣旋性環(huán)流和輻散氣流。這種SPCZ地區(qū)較冷的SSTA異常與赤道東太平洋地區(qū)SSTA暖異常間形成補(bǔ)償性大氣質(zhì)量輸送和垂直環(huán)流。同時(shí),在SPCZ與西太平洋暖池及MC之間,由于SPCZ區(qū)域SSTA較冷,易于形成西北—東南走向的垂直環(huán)流。這種變化將削弱本應(yīng)在MC區(qū)域與赤道東太平洋之間的異常反Walker環(huán)流。在反號(hào)年,顯著異常的海溫只出現(xiàn)在赤道東太平洋地區(qū)(80～120°W,10°S～10°N)(圖4b),在SPCZ區(qū)域冷海溫異常較弱。赤道東太平洋海溫異常偏暖,在海洋性大陸與赤道東太平洋之間可易于形成異常Walker環(huán)流。這種Bjerkness機(jī)制下的環(huán)流異常可解釋當(dāng)這類EP型ENSO事件出現(xiàn)時(shí),MC區(qū)域?qū)⒊霈F(xiàn)降水的負(fù)異常。

圖3　EPI高低值年海洋性大陸區(qū)域降水異常的合成差值(單位:mm/d;陰影區(qū)表示通過(guò)了0.1信度的t-檢驗(yàn))a.同號(hào)(PP-NN)事件;b.反號(hào)(PN-NP)事件Fig.3　Composite differences of precipitation anomalies (units:mm·d-1) in the MC region between high-and low-value events of EPI for (a)same-signed years (PP-NN) and (b)opposite-signed years (PN-NP)(shaded areas are values at and above the 90% confidence level using a t-test)

圖4　EPI高低值年SSTA及海表面異常風(fēng)場(chǎng)的合成差值(單位:m/s;陰影區(qū)表示SSTA通過(guò)了0.1信度的t-檢驗(yàn);矩形區(qū)表示MC區(qū)域)　　a.同號(hào)(PP-NN)事件;b.反號(hào)(PN-NP)事件Fig.4　Composite differences of SSTA between high-and low-value events of EPI for (a)same-signed years (PP-NN) and (b)opposite-signed years(PN-NP)(shading indicates SSTA at and above the 90% confidence level using a t-test;the rectangular frame denotes the MC region)

圖5　同號(hào)(PP-NN)事件EPI高低值年速度勢(shì)、輻散輻合風(fēng)場(chǎng)、垂直環(huán)流的合成差值(圖a、b中打點(diǎn)區(qū)域表示速度勢(shì)通過(guò)了0.1信度的t-檢驗(yàn);圖c中陰影區(qū)域表示垂直速度通過(guò)了0.1信度的t-檢驗(yàn))　　a.850 hPa速度勢(shì)和輻合輻散風(fēng)場(chǎng);b.200 hPa速度勢(shì)和輻合輻散風(fēng)場(chǎng);c.垂直環(huán)流Fig.5　Composite differences of anomalous velocity potential(shaded contours) and winds at (a)850 hPa and (b)200 hPa between high-and low-value events of EPI for same-signed years(PP-NN)(shading with stipples indicates anomalous velocity potential at and above the 90% confidence level using a t-test);(c)Anomalous vertical circulation,with shading for vertical velocity at and above the 90% level of confidence using a t-test

上列結(jié)果亦可從垂直環(huán)流的變化中清楚地看到。圖5給出了同號(hào)年夏季850 hPa和200 hPa速度勢(shì)和輻合輻散風(fēng)合成差值場(chǎng)(圖5a、b)以及相應(yīng)的異常緯向環(huán)流(圖5c)?？梢?jiàn),在日期變更線以西的熱帶南太平洋存在一個(gè)顯著的輻散中心(圖5a),而明顯的輻合則出現(xiàn)在熱帶西北太平洋和熱帶中東太平洋。在200 hPa上,則與低層相反,在菲律賓以東出現(xiàn)明顯的輻散運(yùn)動(dòng)而在SPCZ處存在輻合。這些無(wú)旋運(yùn)動(dòng)的分布明顯不同于經(jīng)典ENSO發(fā)生時(shí)熱帶尤其是赤道上空的大氣運(yùn)動(dòng)特征。印度尼西亞南部在850 hPa上的向菲律賓以東地區(qū)的輻散和在對(duì)流層上層在印尼群島南部的輻合構(gòu)成了局地垂直環(huán)流圈,所以引起了印尼群島以北降水偏多。與輻散運(yùn)動(dòng)一致,沿赤道的垂直環(huán)流圈中不能清楚地看到自東太平洋至MC區(qū)域完整的異常Walker環(huán)流的日界線以東的上升和140°E以西的下沉。

然而,與同號(hào)年不同的是,反號(hào)型的合成結(jié)果顯示了經(jīng)典型ENSO與MC區(qū)域間大氣運(yùn)動(dòng)的聯(lián)系。由圖6可見(jiàn)在赤道東太平洋區(qū)域低層存在輻合,高層存在輻散,在MC區(qū)域低層輻散而在高層輻合(圖6a、b),在赤道上空存在非常完整的反Walker環(huán)流異常(圖6c)。此時(shí)赤道東太平洋地區(qū)降水增多,而MC區(qū)域則降水明顯偏少。

圖6　反號(hào)(PN-NP)事件EPI高低值年速度勢(shì)、輻散輻合風(fēng)場(chǎng)、垂直環(huán)流的合成差值(圖a、b中打點(diǎn)區(qū)域表示速度勢(shì)通過(guò)了0.1信度的t-檢驗(yàn);圖c中陰影區(qū)域表示垂直速度通過(guò)了0.1信度的t-檢驗(yàn))　　a.850 hPa速度勢(shì)和輻合輻散風(fēng)場(chǎng);b.200 hPa速度勢(shì)和輻合輻散風(fēng)場(chǎng);c.垂直環(huán)流Fig.6　Composite differences of anomalous velocity potential(shaded contours) and winds at (a)850 hPa and (b)200 hPa between high-and low-value events of EPI for opposite-signed events years(PN-NP)(shading with stipples indicates anomalous velocity potential at and above the 90% confidence level using a t-test);(c)Anomalous vertical circulation,with shading for vertical velocity at and above the 90% level of confidence using a t-test

3.3加熱異常

環(huán)流異常的形成與大氣所受的熱力強(qiáng)迫異常有關(guān)。對(duì)于同號(hào)情況而言,在MC區(qū)域的北側(cè)即菲律賓到關(guān)島地區(qū)有正的大氣視熱源異常(圖7a),大氣受到異常的加熱上升,在菲律賓以東區(qū)域出現(xiàn)降水的正異常。而印度尼西亞地區(qū)出現(xiàn)明顯負(fù)視熱源異常,且在南太平洋SPCZ區(qū)域出現(xiàn)狹長(zhǎng)的負(fù)值區(qū),大氣受到冷卻而下沉。這有利于在SPCZ區(qū)域及其西南側(cè)的對(duì)流層低層形成異常反氣旋性環(huán)流(Gill,1980),在赤道東太平洋表現(xiàn)出弱的正視熱源異常。

對(duì)于反號(hào)情形,赤道東太平洋存在較強(qiáng)的視熱源加熱(圖7b),而在MC區(qū)域,負(fù)的視熱源異常顯著,負(fù)的中心位于MC區(qū)域的東北側(cè),大氣受到冷卻并下沉,不利于降水在該地區(qū)發(fā)生。而在SPCZ區(qū)域未見(jiàn)90%信度下顯著的視熱源負(fù)異常,相反,在SPCZ北側(cè)還出現(xiàn)了視熱源的正異常,大氣受熱上升。

前述公式(2)和(3)中包含了潛熱釋放對(duì)氣柱加熱的影響,若扣除這一影響,可凸顯氣柱受到下墊面加熱異常及氣柱對(duì)凈輻射吸收的程度,亦即和之差可進(jìn)一步說(shuō)明非絕熱加熱的構(gòu)成。若和之差非常小,表明水汽凝結(jié)起主要作用;若差值正異常,表示除了潛熱加熱外,輻射加熱、地面感熱和潛熱輸送加強(qiáng);反之表示輻射冷卻、地面感熱和潛熱輸送減弱。

反號(hào)時(shí),在MC地區(qū)主要為非絕熱加熱的負(fù)異常(圖8b),負(fù)的異常中心位于MC區(qū)域的東北側(cè)。這樣的非絕熱加熱場(chǎng)配置導(dǎo)致對(duì)流層底層輻散、高層輻合,不利于降水在MC區(qū)域生成。

圖7　EPI高低值年視熱源(Q1)的合成差值(單位:W/m2;陰影區(qū)表示通過(guò)了0.1信度的t-檢驗(yàn);矩形區(qū)表示MC區(qū)域)　　a.同號(hào)(PP-NN);b.反號(hào)(PN-NP)Fig.7　Composite differences of anomalous apparent heating Q1(units:W·m-2) between high-and low-value events of EPI for (a)same-signed years(PP-NN) and (b)opposite-signed years(PN-NP)(shading indicates values at and above the 90% confidence level using a t-test;rectangular frame indicates the MC region)

為了進(jìn)一步了解上述加熱異常下SPCZ區(qū)域與MC區(qū)域的聯(lián)系,這里給出了SPCZ區(qū)域中心(170°E,15°S)到MC區(qū)域中心(135°E,5°N)連線上在同號(hào)和反號(hào)時(shí)垂直環(huán)流異常的合成差值(圖9)。如圖9a顯示,在同號(hào)時(shí),在MC與SPCZ之間存在完整的垂直環(huán)流圈。MC區(qū)域位于上升區(qū),低層輻合、高層輻散,有利于降水在MC區(qū)域發(fā)生;而SPCZ區(qū)域則位于下沉區(qū),低層輻散、高層輻合,不利于異常降水的發(fā)生。而在反號(hào)情形下,均出現(xiàn)下沉氣流,這與同號(hào)情況完全不同。綜上,SPCZ區(qū)域異常冷卻與菲律賓以東異常的加熱而導(dǎo)致的垂直環(huán)流改變和削弱了EP型EL Nio與MC區(qū)域間由Walker環(huán)流建立的直接聯(lián)系。

4結(jié)論

基于Kao and Yu(2009)的方法可計(jì)算出北半球夏季EPI/CPI指數(shù),其能較好地區(qū)分東部型和中部型ENSO,但注意到EPI與CPI在統(tǒng)計(jì)上并不完全獨(dú)立,其相關(guān)系數(shù)為-0.30。

區(qū)域表現(xiàn)為降水的負(fù)異常,負(fù)值中心位于泰國(guó)、柬埔寨、南海地區(qū)和菲律賓地區(qū)。

圖8　EPI高低值年非絕熱加熱(Q1-Q2)的合成差值(單位:W/m2;深色(淺色)陰影區(qū)域表示通過(guò)了0.1信度的t-檢驗(yàn);矩形區(qū)表示MC區(qū)域)　　a.同號(hào)(PP-NN);b.反號(hào)(PN-NP)Fig.8　Composite differences of anomalous net apparent heating Q1-Q2(units:W·m-2) between high-and low-value events of EPI for (a)same-signed years(PP-NN) and (b)opposite-signed years (PN-NP)(shading indicates values at and above the 90% confidence level using a t-test;rectangular frame indicates the MC region)

圖9　EPI高低值年SPCZ區(qū)域到MC區(qū)域環(huán)流的合成差值(箭矢表示由輻散風(fēng)向斜向平面投影的分量與異常垂直速度的合成;等值線表示異常的垂直速度,單位:10-3 hPa/s)　　a.同號(hào)(PP-NN);b.反號(hào)(PN-NP)Fig.9　Composite differences of the slanted anomalous vertical circulation from the Philippines southeastward to the SPCZ region between high-and low-value events of EPI for (a)same-signed years(PP-NN) and (b)opposite-signed years(PN-NP)[arrows are for winds composed of anomalous vertical velocity and divergent wind components projected onto the slanted plane from the SPCZ northwestward to the Philippines;contours are for vertical velocity anomalies (×10-3 hPa·s-1),which are artificially enlarged before plotting)

致謝:南京信息工程大學(xué)地球科學(xué)部南京大氣資料服務(wù)中心提供了資料服務(wù);NCEP/NCAR再分析資料取自NOAA-CIRES Climate Diagnostics Center(http://www.cdc.noaa.gov);文中諸圖均用GrADS軟件繪制。

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ENSO is the strongest interannual variability in our climate system.It has profound impacts on global climate.Since El Nio-Modoki was revealed in 2007,more and more efforts have been made in the climate community to investigate the different types of ENSO events and their influences.Those different ENSO events are known as the EP-type(Eastern Pacific-type) and CP-type(central Pacific-type) ENSO.Following established methodology,we adopt the EP-type ENSO index(EPI) for boreal summer in the present paper.By employing methods including composite analysis,linear regression,and EOF decomposition,we investigate the simultaneous relationships between EP-type ENSO and precipitation anomalies in the Maritime Continent(MC) by using the monthly mean data from the NECP/NCAR reanalysis,global SST data from the Hadley Center,and CAMP monthly mean precipitation data for the period 1979—2013.

The results demonstrate that EPI and CPI in boreal summer are able to describe the main features of EP-type and CP-type ENSO,respectively,although the EPI and CPI are not perfectly independent of each other in a statistical sense because the correlation coefficient of EPI with CPI was found to be 0.30.During some years,EPI is relatively smaller.But,the spatial pattern of anomalous SST still looks like EP-type ENSO.In these years,the anomalous precipitation averaged over the MC region is negative.Usually,we expect a strong negative correlation between canonical ENSO and rainfall anomalies in Indonesia.However,we find that the time series of the regional mean precipitation anomaly over the MC is weakly correlated with the EPI.These results are inconsistent with one another.

By examining the anomalous circulations case by case,we find that this weak correlation is induced by some co-occurrences of the EP-type El Nio(La Nia) with more(less) than normal rainfall in the Indonesian region in some years.After removing the Nio4 signal from SST anomalies,we find that both the SST anomaly(SSTA) and rainfall anomalies are independent of the Nio4 signal.The EPI independent of Nio4 is calculated.Then,the co-occurrences of positive EPI with positive anomalous rainfall in the MC can still be observed in some years.By performing composite analyses,we found that there are two mechanisms that link the SSTAs in the eastern equatorial Pacific to precipitation anomalies in the MC region.One is canonical,which is characterized by the anomalous Walker circulation with its ascending(descending) branch over the east equatorial Pacific and the descending(ascending) branch over the MC region in years when EP-type El Nio(La Nia) events occur.We refer to this as the direct connection mechanism.The other is quite different from the direct connection mechanism.When an EP-type El Nio event occurs in certain years,significant anomalous heating is observed in both the MC and the region east of the Philippines,and significant anomalous cooling in the South Pacific Convergence Zone(SPCZ),respectively.These two anomalous diabatic forcing centers force the atmosphere to respond,inducing a northwest—southeast vertical circulation bridging these two centers,which anomalously weakens the descent of the atmosphere in the MC and intensifies the descending motion in the SPCZ,resulting in the anomalous Walker circulation along the equator being stopped in the SPCZ east of 160°E.In this way,positive anomalous rainfall events occur in the MC when the EPI is positive.This anomalous northwest-southeast vertical circulation,with its upward motion branches over the MC and the region east of the Philippines,and a downward motion branch over the SPCZ,connects the EP-type ENSO-induced Walker circulation,with its downward motion over the SPCZ and upward motion over the east equatorial Pacific.The anomalous adiabatic forcing in the SPCZ is crucial in interfering with the influences of EP-type ENSO on rainfall variations in the MC region.This linkage of EP-type ENSO with rainfall anomalies in the MC via anomalous forcing in the SPCZ is referred to as the indirect connection mechanism,in relation to EP-type ENSO with rainfall variations in the MC.

The results in the present paper are highly meaningful in terms of improving our understanding of the mechanisms behind the relationships of ENSO with climate variations in the MC region,and even in East Asia.

Maritime Continent;EP-type ENSO;precipitation anomalies;SPCZ;boreal summer

(責(zé)任編輯：張福穎)

Influences of Eastern Pacific-type ENSO on climate variations over the Maritime Continent region:Direct and indirect connections

FANG Lujun1,2,3,GUAN Zhaoyong1,2,WANG Mei1,2,XIA Yang1,2

1KeyLaboratoryofMeteorologicalDisaster,MinistryofEducation(KLME),NanjingUniversityofInformationScience&Technology,Nanjing210044,China;2CollaborativeInnovationCenteronForecastandEvaluationofMeteorologicalDisasters,NanjingUniversityofInformationScience&Technology,Nanjing210044,China;3HangzhouMeteorologicalBureau,Hangzhou310051,China

doi：10.13878/j.cnki.dqkxxb.20130427001

引用格式:方陸俊,管兆勇,王美,等.2016.北半球夏季海洋性大陸區(qū)域氣候與EP型ENSO:直接與間接聯(lián)系[J].大氣科學(xué)學(xué)報(bào),39(3):289-299.

FangLJ,GuanZY,WangM,etal.2016.InfluencesofEasternPacific-typeENSOonclimatevariationsovertheMaritimeCon-tinentregion:Directandindirectconnections[J].TransAtmosSci,39(3):289-299.doi:10.13878/j.cnki.dqkxxb.20130427001.(inChinese).

*聯(lián)系人，E-mail:guanzy@nuist.edu.cn