Monsoons as eddy-mediated regime transitions of the tropical overturning circulation

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1 Monsoons s eddy-medited regime trnsitions of the tropil overturning irultion LETTERS SIMONA BORDONI 1,2 * AND TAPIO SCHNEIDER 1 1 Cliforni Institute of Tehnology, Psden, Cliforni 915, USA 2 Ntionl Center for Atmospheri Reserh, Boulder, Colordo 837, USA *e-mil: ordoni@gps.lteh.edu Pulished online: 6 July 28; doi:1.138/ngeo248 Monsoons re generlly viewed s plnetry-sle se-reeze irultions, used y ontrsts in the therml properties etween oens nd lnd surfes tht led to therml ontrsts upon rditive heting 1,2. But the rditive heting evolves grdully with the sesons, wheres the onset of monsoon preipittion, nd the ssoited irultion hnges suh s reversl of surfe winds, our rpidly 3,4. Here we use renlysis dt to show tht the onset of the Asin monsoon mrks trnsition etween two irultion regimes tht re distint in the degree to whih eddy momentum flues ontrol the strength of the tropil overturning irultion. Rpid trnsitions of the irultion etween the two regimes n our s result of feedks etween lrge-sle etrtropil eddies nd the tropil irultion 5. Using simultions with n quplnet generl irultion model, we demonstrte tht rpid, eddymedited monsoon trnsitions our even in the sene of surfe inhomogeneities, provided the plnet surfe hs suffiiently low therml inerti. On the sis of these results, we propose view of monsoons in whih feedks etween lrge-sle etrtropil eddies nd the tropil irultion re essentil for the development of monsoons, wheres surfe inhomogeneities suh s lnd-se ontrsts re not. Although monsoons re prominent fetures of the summertime irultion of the Erth s tmosphere, the mehnisms responsile for their ourrene nd their rpid onset remin poorly understood. Numerous hypotheses hve een proposed, ut their relevne to oserved monsoons is unler. For emple, trnsitions of isymmetri irultions from liner to nonliner regimes our when sutropil heting eeeds threshold, provided meridionl temperture grdients t the equtor vnish 6,7 ; however, their relevne to monsoons is unler euse meridionl temperture grdients t the equtor re non-zero efore monsoon onset (see refs 5,8,9 for disussion of the literture). Here, we fous on the most etensive nd striking of Erth s monsoons, the Asin Monsoon. At the eginning of the wrm seson, the lrge-sle irultion in the Asin monsoon setor hnges rpidly, resulting in polewrd movement of the intertropil onvergene zone 1 (ITCZ) nd onset of intense preipittion in the sutropis; reverse leit less rpid trnsition ours t the end of the wrm seson (Fig. 1). The meridionl overturning irultion zonlly verged over the Asin monsoon setor rerrnges itself t monsoon onset from n equino pttern, with two ells lmost symmetri out the equtor (Fig. 2), to monsoon pttern dominted y single ross-equtoril ell, with sent in the summer hemisphere sutropis nd desent in the winter hemisphere (Fig. 2). Strong upper-level esterlies develop in the tropis (Fig. 2d), nd, onsistent with grdient-wind lne 7, the ner-surfe temperture nd moist stti energy (MSE) grdients reverse t monsoon onset 11 (Fig. 2f). Equtorwrd of the ner-surfe MSE mimum t 26 N, preipittion nd upwrd motion ttin their lrgest vlues (Fig. 2,f). Consistent with the Coriolis fore on the meridionl ner-surfe flow pproimtely lning the drg on the zonl ner-surfe wind, the zonl ner-surfe wind in the northern hemisphere sutropis rpidly reverses t monsoon onset when the ross-equtoril nersurfe flow rehes the sutropis (Fig. 3). It hs een rgued tht tropil meridionl overturning irultions (glol or lol Hdley ells) re onstrined y onservtion of solute ngulr momentum in their upper rnhes 6,8,,13. However, meridionl momentum flues ssoited with lrge-sle eddies n led to devitions from ngulr momentum onservtion. If eddy momentum flues re strong, stremlines of n overturning irultion ross ngulr momentum ontours; if n overturning irultion onserves ngulr momentum, stremlines nd ngulr momentum ontours oinide. In the southern ell in the Asin monsoon setor round equino, eddy momentum flues of etrtropil origin etend into the upper rnh, ngulr momentum ontours re only slightly distorted wy from the vertil nd stremlines ross ngulr momentum ontours throughout the ell (Fig. 2). When the southern ell hs eome the ross-equtoril ell during the monsoon, eddy momentum flues of etrtropil origin re onfined to the desending rnh, ngulr momentum ontours re more strongly distorted wy from the vertil nd stremlines in the upper rnh re more losely ligned with ngulr momentum ontours (Fig. 2). More quntittively, the lol Rossy numer Ro = ζ/f (where ζ is the men reltive vortiity nd f is the plnetry vortiity) in the upper rnh of n overturning irultion is non-dimensionl mesure of the proimity of the irultion to the ngulr-momentum-onserving limit euse ngulr momentum onservtion requires tht the solute vortiity f + ζ = f (1 Ro) vnishes in regions of divergent horizontl flow 8,14. If Ro, the irultion strength nture geosiene VOL 1 AUGUST Mmilln Pulishers Limited. All rights reserved.

2 3 S 3 S 3 S Figure 1 Rpid shifts of preipittion zones., Sesonl yle of zonl- nd pentd-men preipittion (olour ontours, mm dy 1 ) nd se-level ir temperture (SAT; grey ontours) from oservtions in the Asin monsoon setor.,, The sme from quplnet simultions with oen mied-lyer thikness of 1 m () nd 1 m (). SAT is evluted t 1, hp in the oservtions nd t the lowest model level in the simultions. The ontour intervl for preipittion is 1 mm dy 1 in nd 2 mm dy 1 in,, with mim identified y rosses. For SAT, the ontour intervl is 2 C, with the solid grey line mrking the 24 C isoline. Preipittion rtes in the simultions n eeed oserved preipittion rtes euse the lower oundry in the simultions is entirely wter-overed. The thik dshed line in shows the ltitude t whih the zonl-men topogrphy in the Asin monsoon setor rises ove 3 km. Arrows t the time es in, indite the entres of the 2-dy periods for whih irultions re shown in Figs 2 nd 4. Oserved preipittion is from Glol Preipittion Climtology Projet 29 dt for the yers In this nd susequent figures, zonl mens for the Asin monsoon setor re verges etween 7 nd 1 E. Results re roust to hnges in the verging setor nd do not hnge sustntilly if southest Asi is inluded in the verges. is ontrolled y eddy momentum flues nd n respond to vritions in therml driving only indiretly through hnges in eddy momentum flues 15. If Ro 1, the irultion pprohes the ngulr-momentum-onserving limit, nd its strength responds diretly to vritions in therml driving. Lol Rossy numers in the upper troposphere in the Asin monsoon setor hnge t monsoon onset, from Ro <.4 in the upper rnh ner the entre of the southern ell pre-onset to Ro >.7 in the upper rnh ner the entre of the ross-equtoril ell post-onset. Therefore, t monsoon onset, the southern ell trnsitions from n equino regime, in whih its strength is primrily ontrolled y eddy momentum flues, to monsoon regime, in whih its strength is more diretly ontrolled y the therml driving. A reverse trnsition ours t the end of the monsoon. We hve previously shown tht when n overturning irultion undergoes suh regime trnsition, two dynmil feedk mehnisms t, rendering the trnsition nd ompnying irultion hnges rpid even in the sene of surfe inhomogeneities nd n tive hydrologil yle 5. First, upper-level esterlies nd, y grdient-wind lne, polewrd temperture nd MSE grdients develop in the tropis in erly summer s ross-equtoril flow develops (Fig. 2d,f). Beuse the lol zonl wind determines the propgtion hrteristis of the energy-ontining etrtropil eddies 16, upper-level esterlies shield the ross-equtoril ell from the eddies, whih re primrily onfined to regions of westerlies (Fig. 2d). This llows the overturning ell to pproh the ngulr-momentum-onserving limit more losely, leding to strengthening of the ell nd strengthening nd etension into the winter hemisphere of the upper-level esterlies 5. Seond, dvetion of old (low MSE) ir y the lower rnh of the ross-equtoril ell pushes the MSE mimum polewrd (Fig. 2,f), leding to polewrd movement of the sending rnh euse the min sent region is ner the MSE mimum 9. The polewrd movement of the sending rnh implies strengthening of the ell nd strengthening nd etension into the winter hemisphere of the upper-level esterlies 5,13. Together these mehnisms, disussed in ref. 5, render the regime trnsitions rpid ompred with the timesle of vritions in rditive heting. Using n idelized generl irultion model (GCM) with n tive hydrologil yle nd with homogeneous lower oundry, we demonstrte tht these feedk mehnisms n medite rpid trnsitions of n overturning irultion with irultion nd preipittion hnges resemling those in the Asin monsoon. We simulte sesonl yles y presriing vritions of men dily insoltion t the top of the tmosphere in n idelized GCM 17 in whih the lower oundry is mied-lyer (sl) oen of onstnt depth, with presried time-independent meridionl het trnsport estimted to mth the oserved nnul-men oen het trnsport in low ltitudes 18,19. We show results from two simultions, one with thin (1 m) nd one with thik (1 m) mied lyer, respetively implying low nd high surfe therml inerti. Like the overturning irultion in the Asin monsoon setor, the overturning irultion in the simultion with thin mied lyer undergoes rpid trnsition from n equino to monsoon regime. In the equino regime, upper-level westerlies nd strong eddy momentum flu divergene etend into oth irultion ells (here, Hdley ells euse the simulted limte is sttistilly isymmetri), leding to devitions from ngulr momentum onservtion in the upper rnhes (Fig. 4,). Lol Rossy numers stisfy Ro <.3.4 in the upper rnhes nd show tht the irultion strength is primrily ontrolled y eddy momentum flues. In the monsoon regime, the stremfuntion mimum is loted ner the equtor, in region of upper-level esterlies, where it is shielded from etrtropil eddies nd the eddy momentum flu divergene is smll (Fig. 4,d). Rossy numers re poorly defined in the upper rnh of the ross-equtoril ell lose to its entre ner the equtor, ut tht the flow in the sending nd upper rnhes is loser to ngulr-momentum-onserving is mnifest from the ner-oinidene of ngulr momentum ontours nd stremlines there. Similr to wht is seen during the Asin monsoon, westerly winds previl ner the surfe in the summer sutropis. Strong esterlies previl t upper levels throughout the tropis (Fig. 4d); y grdient-wind lne, the ner-surfe temperture nd MSE grdients re reversed (Fig. 4f), 516 nture geosiene VOL 1 AUGUST Mmilln Pulishers Limited. All rights reserved.

3 S 3 S d S 3 S e S f S Figure 2 Oserved monsoon onset over Asi. Zonl- nd temporl-men irultion in the Asin monsoon setor t two 2-dy periods efore (left pnels, Julin Dy 81 1) nd fter (right pnels, Julin Dy ) monsoon onset.,, Stremfuntion of meridionl overturning irultion (lk ontours, ontour intervl kg s 1, with solid ontours for ntilokwise rottion nd dshed ontours for lokwise rottion), ngulr momentum per unit mss (grey ontours, ontour intervl Ω 2 /15 with Erth s rottion rte Ω nd rdius ) nd trnsient eddy momentum flu divergene div([u v ]osφ), with horizontl veloity vetor v = (u, v ) (olour ontours, ontour intervl m s 2 in nd m s 2 in, with red tones for positive nd lue tones for negtive vlues). Here, ( ) denotes temporl men over the 2-dy period nd over ll yers of dt, primes denote devitions from this men nd [ ] denotes zonl men over the monsoon setor.,d, Zonl wind (lk ontours, ontour intervl 6 m s 1 ) nd eddy momentum flu divergene (olour ontours) s in,. e,f, Preipittion P (lue) nd ner-surfe (85 hp) MSE h (red). Eept for preipittion, ll quntities re otined from the ERA-4 renlysis 3 nd re verged over the yers In the ltitude zones of the tropil overturning irultion, the horizontl eddy momentum flu divergene shown in the figure is the dominnt term lning the Coriolis fore on the men meridionl flow nd the men meridionl momentum dvetion in the zonl momentum udget; other terms, suh s the sttionry eddy momentum flu divergene nd the zonl geopotentil grdient ross the monsoon setor, re smller. primrily s result of MSE dvetion (whih domintes the MSE udget in the oundry lyer). The trnsition etween the equino nd monsoon regimes in the simultion ours rpidly ompred with vritions in rditive heting. This is mnifest in the rpid intensifition nd relotion of the ITCZ into the summer sutropis (Fig. 1) nd the rpid hnges in sutropil ner-surfe winds (Fig. 3). Overll, the trnsition in the simultion resemles the onset of the Asin monsoon. A reverse trnsition ours t the end of the wrm seson; this trnsition ours more rpidly thn in the Asin monsoon, suggesting tht proesses not ptured y our idelized GCM modulte the Asin monsoon retret. For the feedk mehnisms disussed ove to e le to medite rpid trnsitions of the overturning irultion, the surfe therml inerti must e suffiiently low for the ner-surfe MSE to e le to djust rpidly. Only then n irultion hnges our rpidly euse the ner-surfe MSE ontrols the lotion of the sending rnh of the ross-equtoril ell (nd hene of the min preipittion zone) nd, y grdient-wind lne, the upper-level zonl wind, oth of whih must e le to djust u (m s 1 ) Figure 3 Rpid hnges in ner-surfe zonl wind t 15 N. Sesonl yle of zonl- nd pentd-men ner-surfe zonl wind t 15 N from oservtions in the Asin monsoon setor (green) nd from quplnet simultions with oen mied-lyer thikness 1 m (yellow) nd 1 m (red). The ner-surfe zonl wind is evluted t 85 hp in the oservtions nd t σ =.85 in the simultions, where σ = p/p s is pressure p normlized y surfe pressure p s. nture geosiene VOL 1 AUGUST Mmilln Pulishers Limited. All rights reserved.

4 S 3 S d S 3 S e S 2 1 f S 2 1 Figure 4 Simulted monsoon onset. Zonl- nd temporl-men irultion t two 2-dy periods efore (left pnels, Julin Dy 11 ) nd fter (right pnels, Julin Dy ) monsoon onset in simultion with 1-m oen mied lyer. Quntities shown nd ontour intervls re s in Fig. 2, eept for the ontour intervl for eddy momentum flu divergene (3 1 6 m s 2 in, nd m s 2 in,d). Men fields re zonl nd temporl mens over the 2-dy period nd over the lst 2 yers of 25-yer simultion; (trnsient) eddy fields re devitions from this men. The vertil oordinte of the GCM is σ, nd eddy nd men fields in the figure re σ-oordinte nlogues 15 of the pressure-oordinte fields in Fig. 2. The vertil σ-oordinte is epressed s n equivlent pressure y multiplition of σ with the glol-men surfe pressure. rpidly for the feedks to t rpidly. Indeed, in the simultion with thik oen mied lyer, preipittion is onentrted in tropil doule ITCZ (Fig. 1), the intensity nd lotion of whih vry grdully; sutropil ner-surfe zonl winds do not ehiit rpid hnges in strength nd diretion (Fig. 3). Tht simultions with n quplnet GCM with surfe of suffiiently low therml inerti ehiit rpid trnsitions of the tropil irultion, with ll defining hrteristis 1,1 of monsoon trnsitions, shows tht monsoons n our even in the sene of the surfe inhomogeneities tht re trditionlly deemed neessry 1,2. Sutropil ontinents re still needed for monsoons, ut in the view proposed here, their primry role is not to rete therml ontrsts through lnd se ontrsts in surfe properties ut rther to provide surfe of suffiiently low therml inerti for the outlined mehnisms to e le to t on intrsesonl timesles. Topogrphy 2, lnd se ontrsts 1,2 nd other surfe inhomogeneities 2,21,22 modify these mehnisms nd ount for the omple morphology of the Erth s monsoons, inluding the onset/end symmetry of the Asin monsoon; however, monsoons n our without them, for instne, over homogeneous swmp surfe. Consistent with the view of monsoons proposed here, the oserved hnges in preipittion (Fig. 1) nd zonl wind (Fig. 3) in the Asin monsoon setor seem to e superposition of the orresponding hnges in the simultions with thin nd thik mied lyer, the first more losely pturing the dynmis in lnd-overed prts nd the seond in oen-overed prts. Beuse the ner-surfe monsoon flow is direted towrds the summer pole ross the equtor nd turns estwrd further polewrd, where the ner-surfe meridionl momentum lne eomes pproimtely geostrophi 5, monsoon preipittion requires moisture supply equtorwrd nd westwrd of surfe of low therml inerti. This seems to ount for the presene of monsoon preipittion over western ut not entrl Afri. Aquplnet monsoon trnsitions hve een otined in previous simultion studies. In those studies, however, se surfe tempertures were presried, preventing the mehnisms we desried from ting, nd either inhomogeneities in lower oundry onditions were imposed in lieu of lnd msses 23, or wind-indued surfe het ehnge 24,25 (WISHE) ws posited s responsile for the trnsitions 26,27. In simultions in whih we suppressed WISHE y presriing time-invrint ner-surfe wind speed in the evportion prmeteriztion, the monsoon trnsitions do not differ sustntilly from the ones reported here, showing tht WISHE is not neessry for monsoons in our GCM. In ddition, wves nd instilities eited t monsoon onset y the rpid rerrngement of the tropil irultion my ompny ut do not neessrily use monsoon trnsitions 22,28. On the sis of these results, we propose view of monsoons s eddy-medited trnsitions in the tropil overturning irultion etween regimes tht re distint in the degree to whih eddy momentum flues ontrol the strength of the irultion. This view ounts for the hnges in tmospheri irultion nd preipittion t the onset nd end of monsoons s well s for the rpidity of the trnsitions. De-emphsizing the role of surfe 518 nture geosiene VOL 1 AUGUST Mmilln Pulishers Limited. All rights reserved.

5 inhomogeneities, it posits intertions etween etrtropil eddies nd the tropil meridionl overturning irultion s essentil for monsoons. Reeived 6 Ferury 28; epted 4 June 28; pulished 6 July 28. Referenes 1. Wester, P. J. in Monsoons (eds Fein, J. S. & Stephens, P. L.) 3 32 (Wiley, New York, 1987). 2. Wester, P. J. & Fsullo, J. Enylopedi of Atmospheri Sienes (Ademi, New York, 23). 3. Yin, M. T. A synopti-erologi study of the onset of the summer monsoon over Indi nd Burm. J. Meteorol. 6, (1949). 4. Lu, K.-M. & Yng, S. Sesonl vrition, rupt trnsition, nd intrsesonl vriility ssoited with the Asin summer monsoon in the GLA GCM. J. Clim. 9, (1996). 5. Shneider, T. & Bordoni, S. Eddy-medited regime trnsitions in the sesonl yle of Hdley irultion nd implitions for monsoon dynmis. J. Atmos. Si. 65, (28). 6. Plum, R. A. & Hou, A. Y. The response of zonlly symmetri tmosphere to sutropil therml foring: Threshold ehvior. J. Atmos. Si. 49, (1992). 7. Emnuel, K. A. On thermlly diret irultions in moist tmospheres. J. Atmos. Si. 52, (1995). 8. Plum, R. A. in The Glol Cirultion of the Atmosphere (eds Shneider, T. & Soel, A. H.) (Prineton UP, Prineton nd Oford, 27). 9. Privé, N. C. & Plum, R. A. Monsoon dynmis with intertive foring. Prt I: Aisymmetri studies. J. Atmos. Si. 64, (27). 1. Gdgil, S. The Indin monsoon nd its vriility. Annu. Rev. Erth Plnet. Si. 31, (23). 11. Li, C. & Yni, M. The onset nd internnul vriility of the Asin summer monsoon in reltion to lnd-se therml ontrst. J. Clim. 9, (1996).. Held, I. M. & Hou, A. Y. Nonliner illy symmetri irultions in nerly invisid tmosphere. J. Atmos. Si. 37, (198). 13. Lindzen, S. R. & Hou, A. Y. Hdley irultions for zonlly verged heting entered off the equtor. J. Atmos. Si. 45, (1988). 14. Shneider, T. The generl irultion of the tmosphere. Annu. Rev. Erth Plnet. Si. 34, (26). 15. Wlker, C. C. & Shneider, T. Eddy influenes on Hdley irultions: Simultions with n idelized GCM. J. Atmos. Si. 63, (26). 16. Wester, P. J. & Holton, J. R. Cross-equtoril response to middle-ltitude foring in zonlly vrying si stte. J. Atmos. Si. 39, (1982). 17. O Gormn, P. A. & Shneider, T. The hydrologil yle over wide rnge of limtes simulted with n idelized GCM. J. Clim. 21, (28). 18. Trenerth, K. E. & Cron, J. M. Estimtes of meridionl tmosphere nd oen het trnsports. J. Clim. 14, (21). 19. Bordoni, S. On the Role of Eddies in Monsoonl Cirultions: Oservtions nd Theory. PhD thesis, Univ. Cliforni, Los Angeles (27). 2. Fennessy, M. J. et l. The simulted Indin monsoon: A GCM sensitivity study. J. Clim. 7, (1994). 21. Chou, C., Neelin, J. D. & Su, H. Oen-tmosphere-lnd feedks in n idelized monsoon. Q. J. R. Meteorol. So. 7, (21). 22. Privé, N. C. & Plum, R. A. Monsoon dynmis with intertive foring. Prt II: Impt of eddies nd symmetri geometries. J. Atmos. Si. 64, (27). 23. Cho, W. C. & Chen, B. The origin of monsoons. J. Atmos. Si. 58, (21). 24. Neelin, J. D., Held, I. M. & Cook, K. H. Evportion-wind feedk nd low-frequeny vriility in the tropil tmosphere. J. Atmos. Si. 44, (1987). 25. Emnuel, K. A. An ir se intertion model of intrsesonl osilltions in the tropis. J. Atmos. Si. 44, (1987). 26. Numguti, A. Dynmis nd energy lne of the Hdley irultion nd the tropil preipittion zones. Prt II: Sensitivity to meridionl SST distriution. J. Atmos. Si. 52, (1995). 27. Yno, J.-I. & MBride, J. L. An quplnet monsoon. J. Atmos. Si. 55, (1998). 28. Xie, S.-P. & Siki, N. Arupt onset nd slow sesonl evolution of summer monsoon in n idelized GCM simultion. J. Meteorol. So. Jpn 77, (1999). 29. Huffmn, G. J. et l. Glol preipittion t one-degree dily resolution from multistellite oservtions. J. Hydrometeorol. 2, 36 5 (21). 3. Uppl, S. M. et l. The ERA-4 renlysis. Q. J. R. Meteorol. So. 131, (25). Aknowledgements This work ws supported y the Dvidow Disovery Fund, Dvid nd Luile Pkrd Fellowship, Moore Postdotorl Fellowship nd the Ntionl Siene Foundtion (grnt no. ATM-4559). The simultions were rried out on Clteh s Geologil nd Plnetry Siene Dell Cluster, nd the renlysis dt were provided y the Ntionl Center for Atmospheri Reserh (whih is sponsored y the Ntionl Siene Foundtion). Prt of the reserh ws rried while S.B. ws with the Deprtment of Atmospheri nd Oeni Sienes t UCLA (supported y UCLA Disserttion Yer Fellowship). We thnk B. Stevens for omments on drfts of this pper. Author informtion Reprints nd permission informtion is ville online t Correspondene nd requests for mterils should e ddressed to S.B. nture geosiene VOL 1 AUGUST Mmilln Pulishers Limited. All rights reserved.