Daily atmospheric variability in the South American monsoon system

Transcription

1 Clim Dyn DOI /s Dily tmospheric vriility in the South Americn monsoon system V. Krishnmurthy Vsundhu Misr Received: 21 Jnury 2010 / Accepted: 6 July 2010 Ó Springer-Verlg 2010 Astrct The spce time structure of the dily tmospheric vriility in the South Americn monsoon system hs een studied using multichnnel singulr spectrum nlysis of dily outgoing longwve rdition. The three leding eigenmodes re found to hve low-frequency vriility while four other modes form higher frequency oscilltions. The first mode hs the sme time vriility s tht of El Nino-Southern Oscilltion (ENSO) nd exhiits strong correltion with the Pcific se surfce temperture (SST). The second mode vries on decdl time scle with significnt correltion with the Atlntic SST suggesting n ssocition with the Atlntic multidecdl oscilltion (AMO). The third mode lso hs decdl vriility ut shows n ssocition with the SST of the Pcific decdl oscilltion (PDO). The fourth nd fifth modes descrie n oscilltion tht hs period of out 165 dys nd is ssocited with the North Atlntic oscilltion (NAO). The sixth nd seventh modes descrie n intrsesonl oscilltion with period of 52 dys which shows strong reltion with the Mdden-Julin oscilltion. There exists n importnt difference in the vriility of convection etween Amzon V. Krishnmurthy (&) Center for Ocen-Lnd-Atmosphere Studies (COLA), Institute of Glol Environment nd Society (IGES), 4041 Powder Mill Rod, Suite 302, Clverton, MD 20705, USA e-mil: krishn@col.iges.org V. Krishnmurthy Deprtment of Atmospheric, Ocenic nd Erth Sciences, George Mson University, Firfx, VA 22030, USA V. Misr Deprtment of Meteorology nd Center for Ocen-Atmospheric Prediction Studies, Florid Stte University, Tllhssee, FL 32306, USA River Bsin (ARB) nd centrl-est South Americ (CESA). Both regions hve similr vritions due to ENSO though with higher mgnitude in ARB. The AMO-relted mode hs lmost identicl vritions in the two regions, wheres the PDO-relted mode hs opposite vritions. The intersesonl NAO-relted mode lso hs vritions of opposite sign with comprle mgnitudes in the two regions. The intrsesonl vriility over the CESA is roust while it is very wek over the ARB region. The reltive contriutions from the low-frequency modes minly determine the internnul vriility of the sesonl men monsoon lthough the intersesonl oscilltion my contriute in sutle wy during certin yers. The intrsesonl vriility does not seem to influence the internnul vriility in either region. Keywords South Americn monsoon ENSO Atlntic multidecdl oscilltion Intrsesonl oscilltion-mjo NAO 1 Introduction The South Americn monsoon system (SAMS) hs een shown to hve some similrities with the South Asin summer monsoon (Zhou nd Lu 1998; Ver et l. 2006), lthough the SAMS is weker in strength nd cn e identified with the sesonl reversl of low level winds only fter the nnul men is removed. The SAMS circultion responds to the strong ditic heting relesed in the monsoonl precipittion (Lenters nd Cook 1997) which hs distinct nnul cycle in certin prts of the continent (Ver et l. 2006). For exmple, the Bolivin high (n upper level feture) nd the Grn Chco low ( low level feture) re shifted s the seson progresses (Zhou nd Lu 1998; Grimm et l. 2005).

2 The ustrl summer seson rinfll in the Amzon Bsin is known to e ssocited with the El Niño-Southern Oscilltion (ENSO) vritions (Kousky et l. 1984; Aceitunuo 1988;Ropelewskind Hlpert1987;Mrengo1992, 1995; Mrengo nd Hstenrth 1993; Ro et l. 1996; Fu et l. 2001). The wrm (cold) phse of ENSO in the estern Pcific Ocen coincides with reduced (incresed) rinfll in this region. During n El Niño, nomlous scending motion over the estern Pcific Ocen under the influence of wrm se surfce temperture (SST) is thought to induce nomlous susidence est of the Andes resulting in elow verge precipittion in the northern Amzon Bsin (Mrengo nd Hstenrth 1993). However, severl studies lso suggest tht the internnul vrince over the Amzon Bsin is not completely explined y ENSO (Mrengo nd Hstenrth 1993; Ro et l. 1996; Dettinger et l. 2000). Ro et l. (1996) showed tht incresed rinfll over the Amzon Bsin is lso ssocited with n increse of moisture trnsport from the Atlntic Ocen. On the other hnd, the internnul vriility of precipittion over southest Brzil is found to e less influenced y ENSO (Liemnn et l. 2001; Crvlho et l. 2002; Grimm 2003, 2004). Grimm (2003, 2004) indicted tht the influence of ENSO over southest Brzil is su-sesonl in nture, ttriuting it to the competition etween the remote ENSO forcing nd regionl processes tht cuse rupt chnge in the rinfll from 1 month to the other. However, recent study y Krishnmurthy nd Misr (2010) hs shown tht the internnul vrition of the rinfll in the centrl-est prt of the continent (60 W 40 W, 20 S 5 S) is significntly influenced y ENSO vritions. Furthermore, it ws shown tht this ENSO influence is sesonlly persistent nd not susesonl in its vriility. In relted coupled modeling study, Misr (2008) showed tht the ENSO teleconnection over the Amzon (southest Brzil) is sensitive (insensitive) to ir-se coupling. In fct, this sensitivity of rinfll vriility to ir-se interction is feture tht is shred y other monsoon systems of the gloe (Misr 2008). The importnce of ir-se interction on the internnul vriility of rinfll over southest Brzil nd northern Argentin seems to e more significnt in the Atlntic Ocen (Roertson nd Mechoso 2000; Doyle nd Brros 2002; Roertson et l. 2003; Chves nd Nore 2004). The southest Brzil region is lso known for its intrsesonl vriility (Nogués-Pegle nd Mo 1997; Liemnn et l. 1999; Jones nd Crvlho 2002). Nogués-Pegle nd Mo (1997) showed lternting wet nd dry conditions occurring t intrsesonl scles in the South Atlntic Convergence Zone (SACZ) nd over the sutropicl plins of South Americ. These see-sw ptterns in outgoing longwve rdition (OLR) were identified s regionl component of lrger oscilltory pttern tht is linked to the dy oscilltion in the tropics. In the oservtionl study of Liemnn et l. (1999), it is clerly indicted tht the OLR fluctutions with periods less thn 90 dys show mximum vrince over the SACZ region nd over southest Brzil during Decemer-Ferury. They further indicte tht locl minimum in intrsesonl vrince exists over the southern Amzon Bsin where the men convection is t mximum. Crvlho et l. (2002), Jones nd Crvlho (2002) lso showed reversl of 850 hp zonl winds t intrsesonl scles over southest Brzil tht ws ssocited with simultneous chnges in precipittion vriility. The ove discussion points to need to investigte the reltive roles of low-frequency vriility nd susesonl vriility of the SAMS. It is lso necessry to understnd the influences of oth the Pcific nd Atlntic Ocens s fr s the low-frequency vriility is concerned. An understnding of the reltive roles plyed y the vritions t different time scles hs importnt impliction on the longterm prediction of the monsoon. The optimism for longterm prediction stems from hypothesis of Chrney nd Shukl (1981) ccording to which the tropicl climte vriility is minly determined y slowly vrying components such s SST, soil moisture nd snow cover. Support for this hypothesis ws provided y Krishnmurthy nd Shukl (2007, 2008) who nlyzed dily rinfll nd convection over the South Asin monsoon region nd showed the existence of sesonlly persisting signls long with intrsesonl oscilltions of different periods. The sesonlly persisting components were shown to e relted to the SSTs of ENSO nd Indin Ocen dipole (IOD) mode (Krishnmurthy nd Kirtmn 2009). The internnul vriility of the sesonl men rinfll is minly determined y the reltive contriutions from the ENSO nd IOD relted signls. However, the oscilltions re importnt in the susesonl vriility nd my contriute in smll wy to the sesonl men during certin yers when the influence of SST is wek. The purpose of this study is to perform similr nlysis of the SAMS in order to identify the components of vriility on different time scles, to otin their spce time structures nd to determine their reltive roles in the intrsesonl nd internnul vriility of the tmosphere over South Americ. More specificlly, the ojective of this study is to provide oservtionl evidence for the reltive roles of slowly vrying components tht re relted to SST vriility nd high-frequency components tht vry on intrsesonl time scle. The reltion to the SSTs of oth the Pcific nd Atlntic Ocens will e investigted. The reltive contriutions of these vrious components in determining the internnul vriility of the sesonl men monsoon will e nlyzed. The regionl

3 vritions of the influence of components of different time scles will lso e exmined y seprtely nlyzing the vriility over the Amzon Bsin nd the centrl-est prt of South Americ. The ojectives of this study re chieved y performing multichnnel singulr spectrum nlysis (MSSA) of dily oserved OLR nomlies over South Americ nd the djoining ocenic region. This method llows the decomposition of the totl vriility into nonliner oscilltions, persisting signls nd trends nd to study their spce time structures on dily time scle sis. This study my e considered s n expnsion of our erlier work (Krishnmurthy nd Misr 2010), which primrily dwelled on the ENSO-forced vriility over centrl-est South Americ. This pper exmines the vritions over centrl-est South Americ nd the Amzon River Bsin t decdl nd intrsesonl time scles lso. This study lso includes the forcing from the Atlntic SST vrition nd provides comprison with the ENSOforced vriility. In Sect. 2, the dt nd method of nlysis re descried. The climtologies of OLR nd precipittion re discussed in Sect. 3. The results of the MSSA of dily OLR nomlies re given in Sect. 4. Section 5 discusses the low-frequency modes while Sect. 6 descries the higher frequency modes. A finl discussion of the results is given in Sect Dt nd method of nlysis The nlysis of dily vriility in this study uses oserved outgoing longwve rdition which represents deep convection. Dily men vlues of OLR for the period on 2.5 longitude ltitude grid were otined from the Ntionl Ocenic nd Atmospheric Administrtion (NOAA, Liemnn nd Smith 1996). The dt prior to 1979 were not used ecuse of discontinuity in 1978 cused y stellite prolems. The one-degree dily precipittion dt set developed y the Glol Precipittion Climtology Project (GPCP, Huffmn et l. 2001) hs lso een used for the period To study the reltion with SST, the newly developed optimlly interpolted SST (OISST Version 2) dt set (Reynolds et l. 2007) from NOAA ws otined for the period The dily vlues of SST re ville on grid. The dynmicl fields which re nlyzed include dily men horizontl winds t 850 nd 200 hp nd men se level pressure (MSLP) for the period extrcted from the Ntionl Centers for Environmentl Prediction (NCEP) Renlysis-2 dtset (Knmitsu et l. 2002). The dily climtology ws computed s the men for ech clendr dy, nd ws sutrcted from the totl field to otin the dily nomlies. The min method of nlysis extrcts the spce time structure of the tmospheric vriility y performing multi-chnnel singulr spectrum nlysis of the dily OLR nomlies over the South Americn monsoon region. The MSSA is pplied to otin oscilltory nd persistent modes y following the mthemticl formultion descried y Ghil et l. (2002). This method hs een used in studies on Indin monsoon to understnd the intresesonl oscilltions nd low-frequency persistent signls (Krishnmurthy nd Shukl 2007, 2008). For dt specified t L grid points nd N discrete times, the MSSA is pplied to 0 to M - 1 lgged copies of the dt which results in LM eigenvlues nd LM eigenvectors. Ech eigenvlue provides the vrince explined y the corresponding eigenmode. The eigenvectors, ech contining sequence of M mps on L grid points, re the spce time empiricl orthogonl functions (ST-EOF). Ech eigenmode lso hs spce time principl component (ST-PC) of time length N - M? 1. The originl dt cn e expressed s the sum of ll the reconstructed components (RCs) formed from the corresponding ST-EOFs nd ST-PCs. Ech RC, corresponding to prticulr eigenmode, is time series of mps with the sme grid nd sme time length s the originl time series. The RCs re considered s filtered dt otined y dt-dptive method. 3 Climtology The men climte over South Americ is shown s nnul climtologicl men OLR for the period in Fig. 1. Most of the northern prt (north of 20 S), which includes prt of the Amzon River sin, is covered with OLR less thn 220 W m -2 indicting strong convection. The northwest region consists of lrge re with mximum convective ctivity. Over much of the southern prt (40 S 20 S) of the continent nd the estern prt of Brzil, the men OLR is in the rnge W m -2. The equtoril estern Pcific nd Atlntic Ocens re regions of less convective ctivity with OLR rnging from 250 to 270 W m -2. However, the South Atlntic Convergence Zone (SACZ) shows moderte convective ctivity. An exmintion of the climtologicl men OLR for the Decemer-Jnury-Ferury-Mrch (DJFM) seson (figure not shown) showed pttern similr to tht of the nnul men (Fig. 1) ut with more intense convection ( W m -2 ) in the northern prt of the continent nd slightly higher vlues of OLR elsewhere including the ocenic regions. The stndrd devition of dily OLR nomlies for ll dys of the period is plotted in Fig. 1 which shows vlues in the rnge of W m -2 over most of the continent nd in the ocenic region of SACZ. The

4 Fig. 1 Annul climtologicl men nd stndrd devition of dily OLR nomlies. c Annul cycles of dily climtologicl men OLR (solid green) nd precipittion (dshed green) verged over CESA region nd nnul cycles of dily climtologicl men OLR (solid red) nd precipittion (dshed red) verged over ARB region. The OLR verges re plotted with their negtive vlues for esy comprison with the precipittion. d Annul cycles of dily stndrd devition of OLR (solid green) nd precipittion (dshed green) nomlies verged over CESA region nd nnul cycles of dily stndrd devition of OLR (solid red) nd precipittion (dshed red) verged over ARB region. The climtologicl mens nd stndrd devitions re sed on the period for OLR nd on for precipittion. The domins of CESA (60 W 40 W, 20 S 10 S) nd ARB (70 W 50 W, 5 S 5 N) re shown s dshed oxes in (). In (c) nd (d), the scles for OLR re shown on the left nd those for precipittion on the right. The units of OLR nd precipittion re W m -2 nd mm dy -1, respectively c d stndrd devition is uniform with slightly lower vlue in the northern prt of the continent compred to the region south of 15 S. The regions of lest convective ctivity in the Pcific nd Atlntic Ocens (Fig. 1) lso hve the lowest dily vriility (Fig. 1). The dily stndrd devition of OLR ws lso exmined for just the DJFM seson (figure not shown) nd found to e quite similr to the ll seson vriility shown in Fig. 1 ut with vlues higher y 5 10 W m -2. In order to study the difference in the convection etween the northern nd centrl prts of South Americ, the region (70 W 50 W, 5 S 5 N), referred to s the Amzon River Bsin (ARB), nd the region (60 W 40 W, 20 S 10 S), referred to s the centrl-est South Americ (CESA), re selected. The CESA region in this study is slightly different from tht defined y Krishnmurthy nd Misr (2010) so tht the two regions re of sme size nd not overlpping. The re verges of ny vrile over these two regions will e clled the ARB index nd CESA index. The nnul cycles of dily ARB nd CESA indexes of OLR nd their dily stndrd devitions re shown in Fig. 1c, d, respectively. For more support, the nnul cycles of precipittion using GPCP dt for the period re lso shown. The nnul cycle of CESA index is more roust with well-defined dry seson with no precipittion (280 W m -2 OLR) during June-July-August- Septemer (JJAS) nd well-defined wet seson with 6 9 mm dy -1 precipittion ( W m -2 OLR) during DJFM (Fig. 1c). On the other hnd, the nnul cycle of ARB index does not revel well-defined wet nd dry sesons corresponding to the ustrl summer nd winter sesons, respectively. The ARB index is in the rnge of 4 11 mm dy -1 precipittion ( W m -2 OLR) with mximum during April-June nd minimum during Septemer Novemer. Although the ARB nd CESA indexes re close during DJFM, they differ y

5 6 9 mm dy -1 in precipittion ( W m -2 in OLR) during My Septemer. The stndrd devition of the CESA index lso hs stronger nnul cycle compred to the ARB index (Fig. 1d), nd oth revel the sme sesonl chrcteristics s those of the dily men (Fig. 1c). The stndrd devition of the CESA reches up to 4 mm dy -1 in precipittion (25 W m -2 in OLR) wheres the stndrd devition of the ARB index hs mximum of 6 mm dy -1 in precipittion (20 W m -2 in OLR). Generlly, the vriility of convection is higher in the CESA region thn in the ARB region lthough not in the precipittion. 4 MSSA of dily OLR nomlies The spce time structure of the vriility of convection over the South Americn monsoon region ws otined y pplying MSSA to dily OLR nomlies in (110 W 0, 50 S 20 N) region for ll the dys of the period A lg window of 121 dys t 1 dy intervl ws used. The eigenmodes otined from MSSA re rrnged in descending order of the vrince explined. The first seven eigenmodes re found to consist of three non-oscilltory modes with low-frequency vriility nd two pirs of oscilltory modes of different periods. These seven modes descrie importnt fetures of the South Americn monsoon system vrying from intrsesonl to decdl time scles, while the rest of the modes re in time scles of higher frequency. Therefore, this pper will discuss only the first seven eigenmodes. The vrince explined rnges from 0.9% for the first mode to 0.3% for the seventh mode. These vlues re smll ecuse of the fct tht the MSSA hs een pplied to rw dily nomlies without pre-filtering in order to otin spce time modes in dt-dptive mnner. The importnt point to note is tht these re the leding modes while the other modes hve even less vrince nd progressively ecome indistinguishle from noise. The sme seven modes were lso otined when the MSSA procedure ws repeted with 5 nd 15-dy running mens of dily OLR nomlies ut now with 2.0, 4.4% for the first mode nd 0.7, 1.2% for the seventh mode, respectively. The MSSA eigenmodes of rw nomlies re used in further discussion. Since the length of the lg window is 121 dys, ech MSSA mode consists of n ST-EOF which is sequence of 121 lgged mps nd n ST-PC whose time length is equl to the numer of dys in the period minus 120 dys. The ST-EOF nd ST-PC re used to construct the spce time RC of ech eigenmode (see Ghil et l. 2002). The RCs re on the sme sptil grid nd hve the sme time length nd sequence s the originl dt. An exmintion of the ST-EOF, ST-PC nd RC of ech eigenmode, following the criteri specified y Ghil et l. (2002), the eigenmodes 1, 2 nd 3 were found to e non-oscilltory t low-frequency time scles nd modes (6, 7) to e n oscilltory pir with 52-dy period. The modes (4, 5) showed ll indictions of eing n oscilltion ut with period longer thn the length of the lg window. Therefore, second MSSA ws performed on just the RC of mode (4, 5) with longer lg window to extrct n oscilltory pir with period of 165 dys. The RC of n oscilltion is the sum of the RCs of the individul modes of the pir. The RCs of eigenmodes 1, 2, 3, (4, 5) nd (6, 7) will e denoted s RC1, RC2, RC3, RC45 nd RC67, respectively. The sptil structure nd temporl vrition of n RC cn e descried in compct mnner y performing sptil EOF nlysis of the RC. Since the RCs re coherent modes, the first EOF (EOF1) explins very high vrince (usully ove 95%) of the corresponding RC (nd not of the totl nomly). The EOF1s of the RCs re shown in Fig. 2 nd the corresponding first PCs (PC1s) in Fig. 3. These PCs hve the sme time length s tht of the RCs nd hence of the totl nomly, nd should not e confused with the ST-PCs of MSSA. The power spectr of the PCs of Fig. 3 re plotted in Fig. 4. The EOF1 of RC1 (Fig. 2) shows n est west structure ove 20 S with nomlies of one sign over the continent nd equtoril Atlntic Ocen nd nomlies of opposite sign over the Pcific Ocen. A north south structure of opposite nomlies is lso seen with the sign chnging t 20 S. This structure vries on dily sis ccording to the PC1 of RC1 shown in Fig. 3. The vriility of the PC is seen to e minly internnul without the presence of high-frequency fluctutions. The power spectrum of RC1 (Fig. 4) is rod-nd with well defined pek t out 45 months which is out the sme s tht of ENSO. In fct, the vrition of PC1 of RC1 is similr to the ENSO vrition with wrm events of , nd prominently cptured (Fig. 3). This first MSSA mode is the sme s tht descried y Krishnmurthy nd Misr (2010). This study will lter discuss further properties of this mode. While the vriility of the first MSSA eigenmode is internnul, the second nd third eigenmodes seem to disply vrition on even longer time scle. For the second mode, the EOF1 of RC2 in Fig. 2 shows nomlies of sme sign over the entire continent with mximum vlues long the elevted region in the southwest. Although the nomlies over the estern Pcific nd Southern Atlntic Ocens re uniformly the wekest, it is importnt to note tht the nomlies over the continent extend over to northern Atlntic Ocen nd West Afric. The signl is thus not restricted to lnd res only. The PC1 of RC2 (Fig. 3) indictes vriility on decdl time scle with smll mplitude internnul vriility superimposed. The

6 Fig. 2 Sptil EOF1 of the reconstructed components RC1, RC2, c RC3, d RC45 nd e RC67 from MSSA of the dily OLR nomlies. The Sptil EOF nlysis ws performed on the dily vlues of the RCs for the period Units re in W m -2 d e c power spectrum of RC2 shown in Fig. 4 is minly red confirming tht the vriility is mostly in low-frequency rnge which is eyond the ENSO nd. The EOF1 of RC3 in Fig. 2c shows tht the third mode hs stronger nomlies of one sign over the northwest prt of the continent which extends into the Pcific nd Atlntic Ocens while opposite sign nomlies over the continent to the south of 10 S, southern Pcific Ocen nd much of Atlntic Ocen south of the equtor (except for 25 S 35 S). The PC1 of RC3 (Fig. 3c) lso seems to hve low-frequency vriility which is confirmed y its spectrum (Fig. 4) which is somewht red in nture without ny peks. The sptil structure of the oscilltory pir of eigenmodes (4, 5), s shown y EOF1 of RC45 in Fig. 2d, consists of strong nomlies of one sign over the estern prt of South Americ nd SACZ nd weker nomlies of opposite sign to the south nd north. The time vriility of PC1 of RC45 (Fig. 3d) shows nonliner oscilltion whose mplitude, phse nd frequency re modulted. From Fig. 4, the power spectrum of RC 45 revels pek t 165-dy period. Becuse of its time scle, the (4, 5) mode will e referred s intersesonl oscilltion, herefter. The second oscilltory pir (6, 7) lso hs tilted structure with strong nomlies over the SACZ nd extending northwestwrd onto the Pcific Ocen, s seen from EOF1 of RC67 in Fig. 2e. From PC1 of RC67 in Fig. 3e, this mode is lso found to e nonliner oscilltion ut with higher frequency. The power spectrum of RC67 (Fig. 4) shows tht this oscilltion hs pek in the intrsesonl time scle t 52 dys, similr to Mdden-Julin Oscilltion (MJO). 5 Low-frequency modes 5.1 Reltion with ENSO nd decdl oscilltions The ssocition of the first three low-frequency modes with ENSO nd decdl phenomen such s the Pcific Decdl Oscilltion (PDO) nd Atlntic Multidecdl Oscilltion (AMO) ws investigted. For this purpose, the correltions etween the PC1 (in Fig. 3 c) of the three RCs with dily

7 Fig. 3 Sptil PC1 of RC1, RC2, c RC3, d RC45 nd e RC67 corresponding to the EOFs shown in Fig. 2. The PCs re divided y their respective stndrd devitions c d e nomlies of SST (from OISST2) for the period were computed. Additionlly, in order to find the reltion with the tmospheric circultion, dily zonl wind (u) nomlies t 850 hp (from Renlysis-2) were regressed on PC1 of the RCs. The point correltion of RC1 with SST, shown in Fig. 5, clerly revels the ENSO signture with strong positive correltion in the estern nd centrl equtoril Pcific Ocen which is surrounded y the fmilir horseshoe pttern of negtive correltion. These correltions re even stronger in the Pcific Ocen for the DJFM seson, s shown y Krishnmurthy nd Misr (2010). The regression of the zonl wind field t 850 hp on RC1 shown in Fig. 5 is consistent with the SST correltion. In the estern nd centrl equtoril Pcific Ocen, the wind nomlies re westerlies, the effect of which will e to weken the trde winds nd the Wlker circultion. The point correltion of RC2 with SST is shown in Fig. 5c. Significnt correltions (reching up to 0.4) pper in the North Atlntic Ocen, similr to the correltion of the AMO index with the SST nomlies otined y Knight et l. (2005). The fct tht the Atlntic correltion shown in Fig. 5c ws otined y dily correltion with unsmoothed signl (PC of RC2 in Fig. 3) is noteworthy. The regression of the 850 hp zonl wind (Fig. 5d) indictes westerly nomlies from equtor to 40 N nd esterlies in the northern prt of the Atlntic Ocen. These reltions nd the low-frequency temporl vriility of RC2 suggest tht the second mode is possily ssocited with the AMO. For the third mode, the point correltion of RC3 with SST (Fig. 5e) shows n ENSO-like structure in the Pcific Ocen. However, the correltions, while significnt, hve moderte vlues, nd the region of mximum correltion is locted in the centrl Pcific Ocen rther thn in the estern Pcific like in the cse of RC1. The correltion of sme sign extends long the west cost of United Sttes. Corresponding to the wrm nomlies in the centrl Pcific Ocen (Fig. 5e), the regressed 850 hp zonl wind field in Fig. 5f hs strong westerlies. These SST nd wind ptterns re quite similr to those relted to the PDO (Mntu et l.

8 Fig. 4 Power spectr of PC1 (shown in Fig. 3) of RC1 (red), RC2 (green), RC3 (lue), nd RC45 (red) nd RC67 (green). The scle of the x-xis is in months for () nd dys for () 1997). Agin, considering tht these re dily correltions nd regressions otined from unsmoothed RC, the result is remrkle. These results nd the low-frequency temporl vriility of RC3 suggest tht the third mode my e ssocited with the PDO. 5.2 Regionl vrition It ws noted in Sect. 3 tht there re differences in the sesonl climtologicl men nd nnul cycles of dily men of OLR nd precipittion etween ARB nd CESA regions. Whether such differences lso exist in the dily vriility is now discussed. The dily vlues of ARB nd CESA indexes of RC1, RC2 nd RC3 re shown s time series in Fig. 6 for the period Figure 6 shows tht oth ARB nd CESA indexes cpture the ENSO vriility. However, lthough they re in phse with ech other, the mplitude of the ARB index is higher, especilly during strong El Niño events. In the cse of RC2, oth ARB nd CESA indexes hve lmost identicl vlues ll the time (Fig. 6). This is consistent with the EOF of RC2 (Fig. 2) which hs uniform sptil structure over most of the continent. The ARB nd CESA indexes of RC3, however, vry with opposite phses with lmost equl mplitude. The different ehviors of the three modes hve importnt regionl impliction on the internnul nd decdl vriility of the South Americn monsoon. Since the RCs re simply dded to get the comined vriility, the ARB nd CESA region could ehve differently depending on how the mplitudes nd phses of the RCs dd up. This point is further exmined y considering the sesonl mens of the RCs. In Fig. 7, the internnul vriility of the DJFM sesonl mens of the RCs re compred y plotting the ARB nd CESA indexes. The vriility of the ARB index of RC1 follows tht of ENSO with mjor El Niño ( , , nd ) nd L Niñ ( , , nd ) events eing dominnt (Fig. 7). The CESA index of RC1 lso hs similr ENSO vriility ut with reduced vlues (Fig. 7), generlly out one-third the vlue of ARB index. Interestingly, the nd El Niño events did not hve much effect on ARB nd CESA regions. Both ARB nd CESA indexes of RC2, which ws suggested to e ssocited with AMO, hve lmost identicl vlues with intense convection occurring during , , For RC3, which seems to e relted to PDO, the ARB nd CESA indexes vry with opposite signs ut with comprle mgnitudes. The three leding MSSA modes ply dominnt role in determining the internnul vriility of the sesonl men convection or rinfll. During prticulr yer, the reltive vlues of RC1, RC2 nd RC3 determine the sesonl men convection. From the erlier discussion of Fig. 7,, it is cler tht the reltive contriutions of the three modes could e different for ARB nd CESA index in prticulr yer. In order to exmine the comined vriility, the ARB nd CESA indexes were computed for the sum of RC1, RC2 nd RC3 (RC herefter). The DJFM sesonl mens of the two indexes of RC re shown in Fig. 7c. The vriility of the ARB index is still predominntly determined y ENSO. However, RC2 is the mjor fctor in certin yers such s nd nd RC3 is importnt in yers such s 1988, 1991 nd For the CESA index, the reltive vlues of the three modes pper to e more crucil ecuse of the smller mgnitude of the ENSO mode (RC1) nd the opposite sign of the PDO mode (RC3). Although the CESA index still cptures the effect of ENSO during 1982, 1984, 1986 nd 1997 similr to the ARB index, the ENSO effect is countercted y other modes in 1988 nd 1991 (Fig. 7c).

9 V. Krishnmurthy, V. Misr: Dily tmospheric vriility c d e f Fig. 5 Simultneous point correltion of dily PC1 (shown in Fig. 3) of RC1, c RC2 nd, e RC3 of OLR with dily SST nomlies during Simultneous regression of zonl wind u t 850 hp on Fig. 6 Time series of ARB nd CESA indexes of dily RC1, RC2 nd c RC3 dily PC1 (shown in Fig. 3) on RC1, d RC2 nd f RC3 of OLR for the period The scle for the contours is shown y the color r elow ech pnel c

10 Fig. 7 Time series of ARB nd CESA indexes of DJFM sesonl men RC1 (red), RC2 (green), RC3 (lue). c Time series of ARB index (red) nd CESA (green) of DJFM sesonl men of RC1? RC2? RC3 c 6 Intersesonl nd intrsesonl oscilltions The spce time structures of the oscilltory modes t intersesonl nd intrsesonl time scles re further discussed in this section. Their ssocition with known phenomen such s the North Atlntic Oscilltion (NAO) nd MJO is lso estlished. 6.1 Spce time structure nd propgtion The mplitude A(t) nd phse ngle h(t) of ech of these nonliner oscilltions re determined from the respective RC following the method suggested y Moron et l. (1998). This method hs een used in the study of the Indin monsoon y Krishnmurthy nd Shukl (2007, 2008). During ech cycle of the oscilltion, the phse ngle h(t) vries from 0 to 2p. The spce time structure of the oscilltion cn e est summrized y constructing composites of the RC sed on the phse of the oscilltion. A complete phse cycle (0, 2p) is divided into eight equl intervls of length p/4. By verging the RC of the oscilltion in ech phse intervl for the entire period, eight phse composites of RC re otined. For the intersesonl oscilltory mode (4, 5) with spectrl pek centered t 165 dys (s discussed in Sect. 4), the composites of RC45 for eight phse intervls in n oscilltory cycle were constructed. The phse composites of RC45, presented in Fig. 8, represent the verge oscilltory cycle during The period of the verge oscilltion in Fig. 8 is 165 dys, lthough individul oscilltory cycles, eing nonliner, my hve periods greter thn or less thn 165 dys. This vrition in the period is reflected in the rodness of the power spectrum of RC45 shown in Fig. 4. The first four composites re lmost exctly the sme s the lst four with opposite sign. In the second hlf of the cycle, convective nomlies develop over the SACZ region t out 20 S (phse 5), intensify nd move northestwrd (phses 6 7) nd weken (phse 8). At the sme time, positive nomlies exist in northern region nd equtoril Atlntic Ocen. Moreover, nother positive nomly center develops off the South Atlntic cost (round 30 S) which extends northwrd nd replces the receding negtive nomlies. The sme sequence ut with suppressed convection (positive nomlies) in the SACZ region occurs in phses 1 4. The propgtion fetures of the oscilltion re found y exmining the Hovmöller digrms of RC45. Using the phse of the oscilltion s time coordinte, the longitudephse cross section of RC45 composite verged over (20 S 15 S) is shown in Fig. 9. The oscilltion exhiits

11 Fig. 8 Phse composites of dily RC45 nd RC67 corresponding to eight phse intervls of the respective oscilltory mode during Units re in Wm -2. The phse intervl numer is given t the top right corner of ech pnel estwrd propgtion limited to 60 W 20 W. The ltitudephse cross-section of RC45 verged over (60 W 50 W) shows northwrd propgtion etween 30 S nd 20 N (Fig. 9). For the intrsesonl oscilltory mode (6, 7), the phse composites of RC67 in eight phse intervls re plotted in Fig. 8. The verge period of this oscilltion is 52 dys. In the second hlf of the cycle in Fig. 8, moderte convection nomlies pper round 20S in phse 5, intensify nd expnd in the pek phses 6 nd 7. During these phses, the convection zone is ligned long the SACZ nd is ccompnied y similr nomlies extending into the Pcific eyond the northwest prt of the continent. The convection moves to the northest nd diminishes in intensity in phse 8. A similr sequence with positive nomlies occurs in phses 1 4. The mgnitude of the nomlies in the intrsesonl oscilltion (Fig. 8) is lrger thn tht in the intersesonl oscilltion (Fig. 8). In the longitude-phse cross section of RC67 composite (Fig. 9c), the evidence for estwrd propgtion is wek. However, the ltitude-phse cross section of RC67 in Fig. 9d shows cler northwrd propgtion etween 30 S nd equtor. 6.2 Reltion with NAO nd MJO Both the intersesonl nd intrsesonl oscilltions do not hve ny significnt reltion with the SST. The point correltion of PC1 of RC45 (the PC shown in Fig. 3d) with the dily SST nomlies of is shown in Fig. 10. The correltion is very wek, with mximum vlues of out -0.2 nd 0.2 ppering in the ocenic regions djoining the continent. Similr point correltion of PC1 of RC67 shows lmost no correltion with the SST (Fig. 10). It is importnt to note tht the lck of correltion is specificlly for the two oscilltions on dily time scle. The reltion etween the Atlntic SST nd the precipittion in the sutropicl region, discussed y Doyle nd Brros (2002), is sed on the internnul vriility of the totl precipittion nomlies nd not on the dily vriility of intrsesonl nd intersesonl oscilltions. However, possile ssocitions of the intersesonl oscilltion with the NAO nd the intrsesonl oscilltion with the MJO re explored in this section. The reltion etween the intersesonl oscilltion nd other tmospheric vriles ws nlyzed y constructing

12 Fig. 9 Hovmöller digrms of the phse composites of RCs for complete cycle of phse of the respective oscilltory mode: longitudetime cross-sections of RC45 nd c RC67 verged over (20 S 15 S) nd ltitude-time cross-sections of RC45 nd d RC67 verged over (60 W 50 W). Time is represented y the phse of the oscilltion. The composites were constructed t intervls of p/12 for the period Units re in W m -2 c d Fig. 10 Simultneous point correltion of dily PC1 (shown in Fig. 3d, e) of RC45 nd RC67 with dily SST nomlies during the phse composites of men se level pressure (MSLP) nd zonl wind u t severl verticl levels sed on the phses of RC45. These composites were computed in exctly the sme mnner s those of RC45 in Fig. 8. For revity, the composites of only the pek phses 2, 3, 6, nd 7 re shown in Fig. 11 for MSLP nd in Fig. 11 for u t 1000 hp. The convective ctivities in phses 6 nd 7 (Fig. 8) re ssocited with low pressure etween 40 N nd 60 N in the North Atlntic nd high pressure to the north of 60 N (Fig. 11). Corresponding to these pressure systems, the composites of the zonl wind t 1,000 hp hve esterlies in the northern prt nd westerlies in the southern prt of the North Atlntic (Fig. 11). An exmintion of the composites of horizontl wind vectors (figure not shown) reveled nticyclonic circultion in the north nd cyclonic circultion to the south. The sme pressure nd wind ptterns with opposite sign re ssocited with suppressed convection in phses 2 nd 3. These vritions in pressure nd lower level wind in the North Atlntic re quite similr to the vriility nd structure of the NAO. The ssocition of the intrsesonl oscilltion with the MJO is explored y constructing phse composites of totl OLR nomlies nd zonl wind t 200 hp, corresponding to the phse composites of RC67 in Fig. 8. The composites of the totl OLR nomlies for first hlf of the oscilltory cycle (phses 1 4) re shown in Fig. 12. When the positive OLR nomlies develop over the southern prt of the South Americn continent nd over ner Centrl Americ in phse 1, strong convection nomlies exist in the equtoril Indin Ocen nd extend over the Africn continent. In phses 2 nd 3, while the positive OLR nomlies ecome stronger over the SACZ nd propgte to cover the Africn continent, the convection zone in the

13 Fig. 11 Phse composites of dily MSLP nomlies nd zonl wind (U) nomlies corresponding to four phse intervls (2, 3, 6 nd 7) of the RC45 oscilltory mode during Units re in hp for MSLP nd m s -1 for U. The phse intervl numer is given t the top right corner of ech pnel Indin Ocen moves estwrd into equtoril West Pcific Ocen. This convection center propgtes further est in the Pcific Ocen nd ecomes weker in phse 4 when the positive nomlies (suppressed convection) over the South Americn continent diminishes ut moves estwrd into the western Indin Ocen. This sequence is repeted with nomlies of opposite sign in phses 5 8 (figure not shown). Thus, there is cler reltion etween the intrsesonl oscilltion in the South Americn region nd the MJO in the OLR composites. Further support for this reltion with the MJO is provided y similr phse composites of the zonl wind u t 200 hp, s shown in Fig. 12. In ll the four phses shown in Fig. 12, the convection center (Fig. 12) is ccompnied y esterlies to the west nd westerlies to the est, indicting divergence t the top of the convection zone. The mgnitude of the zonl wind lso mtches the intensity of the convection. It is noticele tht the westerlies in phses 3 nd 4 flow over lrge region extending from the Est Pcific Ocen to the est cost of Afric nd covering the entire northern prt of the South Americn continent. The sequence in Fig. 12 is repeted in phses 5 8 with zonl wind nomlies in opposite directions (figure not shown). Similr composites of the zonl wind t lower level were lso exmined (figure not shown). At the lower level, the convection zone is ccompnied y westerlies to the west nd esterlies to the est, indicting inflow nd convergence into the convection center. These oscilltory ptterns of convection nd circultion nomlies siclly descrie the MJO cycle. 6.3 Regionl vrition To find the regionl vrition of the two oscilltions, the ARB nd CESA indexes of RC45 (intersesesonl) nd RC67 (intrsesonl) re exmined. For etter depiction of the dily vriility, the time series of the two indexes re shown for 10-yer period covering in Fig. 13. The mplitudes of ARB nd CESA indexes of RC45 re comprle in the cse of the intersesonl oscilltion (Fig. 13). However, the ARB index hs slightly lower vlue nd is lmost 180 out of phse with the CESA index. In the cse of the intrsesonl oscilltion, the mplitude of the ARB index of RC67 is extremely smll while the CESA index hs considerle mplitude with pronounced modultion (Fig. 13). Generlly, the CESA index is more intense during the ustrl summer. The DJFM sesonl men of the ARB index is plotted in Fig. 14 for RC45 nd RC67 showing tht it is lmost negligile for oth the oscilltions. In the cse of CESA index, the DJFM sesonl men of RC67 (intrsesonl) is lmost negligile wheres the sesonl men of RC45 (intersesonl) hs smll mplitude internnul vriility (Fig. 14).

14 Fig. 12 Phse composites of dily totl OLR nomlies (left pnels) dily totl zonl wind (u) nomlies t 200 hp (right pnels) corresponding to four phse intervls (1, 2, 3 nd 4) of the RC67 oscilltory mode during Units re in W m -2 for OLR nd m s -1 for u. The phse intervl numer is given t the top right corner of ech pnel Fig. 13 Time series of ARB (red) nd CESA (green) indexes of dily RC45 nd RC67 The reltive contriutions of the two oscilltions to the sesonl men re exmined y dding ll the RCs from mode 1 7 (RC1??RC7) nd computing the DJFM sesonl mens of ARB nd CESA indexes, s shown in Fig. 14c. Compring with the sesonl mens of RC in Fig. 7c, it is cler tht the oscilltory modes do not contriute to the ARB index. However, the CESA index cn e influenced y the intersesonl oscilltion (RC45) to smll extent, s in the cse of 1982, 1986, 1987 nd Therefore, it suggests tht the predictility of the sesonl men rinfll is not ffected y the intrsesonl vriility. 7 Discussion The oserved dily OLR dt were nlyzed to determine the spce time structure of the dily tmospheric vriility in the South Americn monsoon system. The min results indicte tht three leding modes of dily vriility re relted to internnul nd decdl phenomen such s ENSO, AMO nd PDO. Additionlly, there re two nonliner oscilltory modes with periods in the intersesonl nd intrsesonl time scles nd indicte ssocition with NAO nd MJO, respectively. With the methodology dopted in this study, it ws possile to otin the dily

15 c Fig. 14 Time series of ARB nd CESA indexes of DJFM sesonl men RC45 (red), RC67 (green). c Time series of ARB index (red) nd CESA (green) of DJFM sesonl men of RC1? RC2? RC3? RC4? RC5? RC6? RC7 vritions of modes tht hve vriility rnging from intrsesonl to decdl time scles. The ssocition of the leding modes of SAMS in convection with vrious phenomen ws estlished y exmining the reltion with dily nomlies of circultion fields nd SST. It ws lso shown tht the vriility in two different regions (ARB nd CESA) is similr for certin modes while there re differences in the mgnitude nd phse for the other modes. The leding mode of dily vriility hs rod spectrl pek centered t out 4 yers. The sptil structure of this mode consists of OLR nomlies of one sign over the northern prt of the South Americn continent nd the equtoril Atlntic Ocen nd nomlies of opposite sign in the estern Pcific Ocen nd the southern prt of the continent. This structure vries coherently ccording to the ENSO vriility. The high correltion of this convection pttern with the Pcific SST nd lower level winds provides strong support to its reltion with the ENSO. The mgnitude of this ENSO mode is found to e much higher in the ARB region thn in the CESA region. There re two modes (second nd third MSSA modes) in the dily convection which revel vriility on decdl time scle. The reltion with the dily SST nd wind nomlies suggests tht one of these ptterns hs possile ssocition with the AMO. The convection pttern of this mode consists of nomlies of sme sign over the entire continent, extending well into the northern Atlntic Ocen. The mplitude of vriility is lmost the sme in oth ARB nd CESA regions. Since the Atlntic Ocen hs severl modes of decdl vriility, more evidence is needed to firmly estlish the reltion etween the convection mode found in this study nd the AMO. The other decdl mode of convection hs nomlies of one sign over the northern prt of the continent, estern Pcific nd northern Atlntic while nomlies of opposite sign exist in the southern prt. The evidence for the ssocition of this mode with the PDO is quite cler in the correltion pttern with the dily SST nd in the regression pttern with the low level wind. For this mode, however, the vritions in the ARB nd CESA regions re opposite to ech other with comprle mgnitude. Two nonliner oscilltions found in this study hve periods of 165 dys (intersesonl) nd 52 dys (intrsesonl) with rod-nd spectr. Both the oscilltions hve similr sptil ptterns in the SACZ region ut different

16 ptterns in the northern prt nd show northestwrd propgtion. The correltion with the dily SST is negligile for the two oscilltions. However, the reltions with the MSLP nd horizontl winds provide evidence tht the intersesonl oscilltion is ssocited with the NAO. The evidence provided y the reltions with other tmospheric fields suggests tht the intrsesonl oscilltion is prt of the MJO. In the dily vriility of the two oscilltions, there re considerle differences etween the ARB nd CESA regions. In the intersesonl oscilltion, the mplitudes in the ARB nd CESA regions re comprle lthough the CESA region generlly hs higher vlues. The phse of this oscilltion differs y lmost 180 etween the two regions. For the intrsesonl oscilltion, however, the mplitude in the ARB region is lmost negligile while it is quite high in the CESA region with pronounced sesonl modultion. Despite the strong influences of ENSO nd other SST relted vriility of the Pcific nd Atlntic Ocens, the internnul vritions in the SAMS cn sometimes e sutle nd lso show quite different ehvior in the ARB nd CESA regions. The reltive roles of the three lowfrequency modes re importnt in determining the sesonl men convection or precipittion during most yers. The influence of ENSO cn e either enhnced or reduced y the AMO or the PDO mode. For exmple, the influence of ENSO ws enhnced in 1982 nd 1997 while it ws reduced (or countercted) in 1991 for the ARB region. However, the ENSO influence in 1982 for the CESA region ws countercted y the AMO mode. During certin yers, it is one of the decdl modes, rther thn ENSO, tht is importnt in determining the sesonl men, such s during 1998 nd Although the contriution of the intersesonl oscilltion to the sesonl men is smll, it cn mtter in some yers for the CESA region. For exmple, the ENSO influence in 1982 ws further reduced y the intersesonl oscilltion in the CESA region. The intrsesonl oscilltion, on the other hnd, does not seem to ply significnt role in determining the sesonl men monsoon. Still, ecuse of its coherent nture nd its regulr time vriility, the intrsesonl oscilltion provides hope for etter prediction on susesonl time scle. The unified pproch of this study in otining modes of longer time scles with strong reltion to SST hs provided strong support to Chrney-Shukl hypothesis in the South Americn monsoon system lso. A mjor impliction of these results is tht there is hope for long-term prediction of sesonl men monsoon in the South Americn region. However, it is importnt to further understnd the reltive roles the Pcific nd Atlntic Ocens. Acknowledgments This reserch ws supported y grnts from the Ntionl Science Foundtion (ATM , ATM ), the Ntionl Ocenic nd Atmospheric Administrtion (NA04OAR , NA09OAR , NA07OAR ), nd the Ntionl Aeronutics nd Spce Administrtion (NNG04GG46G, NNX09AN50G). The uthors thnk Bohu Hung for helpful comments. References Aceitunuo P (1988) On the functioning of the southern oscilltion in the south Americn sector prt I: surfce climte. Mon We Rev 116: Crvlho LMV, Jones C, Liemnn B (2002) Extreme precipittion events in southestern South Americ nd lrge-scle convective ptterns in the South Atlntic convergence zone. J Clim 15: Crvlho LMV, Jones C, Silv Dis MAF (2002) Intrsesonl lrge-scle circultions nd mesoscle convective ctivity in tropicl South Americ during the TRMM-LBA cmpign. J Geophys Res 107:D20. doi: /2001jd Chrney JG, Shukl J (1981) Predictility of monsoons. In: Lighthill J, Perce RP (eds) Monsoon dynmics. 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