Evaluation of Inter-Hemispheric Characteristics of the Tropopause–Stratopause–Mesopause Over Sub-Tropical Regions

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Evaluation of Inter-Hemispheric Characteristics of the Tropopause–Stratopause–Mesopause Over Sub-Tropical

Regions

Som Sharma, Prashant Kumar, Rajesh Vaishnav, Chintan Jethva, Hassan Bencherif

To cite this version:

Som Sharma, Prashant Kumar, Rajesh Vaishnav, Chintan Jethva, Hassan Bencherif. Evaluation

of Inter-Hemispheric Characteristics of the Tropopause–Stratopause–Mesopause Over Sub-Tropical

Regions. Pure and Applied Geophysics, Springer Verlag, In press, �10.1007/s00024-017-1706-8�. �hal-

01634456�

Eva lua t ion

Eva lua t ion of of In In ter-Hem ter-Hem ispher ispher ic ic Charac Charac ter ter is is t t ics ics of of the Tropopause–S tra topause–Mesopause

the Tropopause–S tra topause–Mesopause Over Over Sub-Trop Sub-Trop ica ica l l Reg Reg ions ions

S ^OM S ^HARMA ,

P ^RASHANT K ^UMAR ,

R ^AJESH V ^AISHNAV ,

C ^HINTAN J ^ETHVA ,

and H ^ASSAN B ^ENCHERIF

Abs trac t

Abs trac t —The trans i t ion reg ions in therma l s truc ture v iz . Tropopause , s tra topause and mesopause p lay a v i ta l ro le in the ver t ica l coup l ing of the Ear th’s a tmosphere . For the firs t t ime , in ter-hem ispher ic charac ter is t ics of the trans i t ion reg ions over two sub trop ica l reg ions are s tud ied us ing tempera ture observa t ions from the SABER onboard TIMED sa te l l i te and the ERA In ter im reana lys is dur ing year 2002 to 2015 . Resu l ts show tha t tropopause he igh t is h igher over Reun ion Is land (21 .11S , 55 .53E) in the Sou thern Hem isphere (SH) as compared to M t . Abu reg ion (24 .59N , 72 .70E) in the Nor thern Hem isphere (NH) . Tempora l var ia t ion of tropopause tempera ture revea ls a decreas ing (* 4K) trend from year 2002 to 2008 and beyond th is , an increas ing (* 1 .5 K) trend is found in tropopause tempera ture . These fea tures are re inforc ing for Mesopause as compared to tropopause tem-pera ture . The SH shows s tronger var ia t ions in Mesopause tempera ture (* 7 K) compared to NH dur ing year 2002 to 2008 . The occurrence frequency of mesopause and s tra topause he igh t shows tha t the max imum occurrence frequency (* 60%) of mesopause a t * 100 km in NH , wh i le frequency is found to be * 55% in the SH . Resu l ts show tha t s tra topause (mesopause) is coo ler (warmer) in NH as compared SH . Moreover , Lomb Scarg le Per iodogram and wave le t transform techn iques are used to inves- t iga te the per iod ic i ty of mesopause , s tra topause and tropopause tempera tures and he igh ts . Inves t iga t ions revea led prom inen t annua l osc i l la t ions in the tropopause and s tra topause tempera tures in bo th hem ispheres . These find ings w i l l be of immense use for the ver t ica l and in ter- hem ispher ic a tmospher ic coup l ing s tud ies .

1 .In troduc t ion 1 .In troduc t ion

The Ear th’s a tmosphere is c lass ified in terms of reg ions , mak ing up the ver t ica l s truc ture of the tem- pera ture fie ld . These reg ions are termed the troposphere , s tra tosphere , mesosphere and thermo- sphere , wh i le the boundar ies be tween them are ca l led the tropopause (abou t* 16 km a t low- la t i- tudes ,* 12 km a t m id- la t i tudes and* 8km a t h igh- la t i tudes) , s tra topause (* 48 km) and meso- pause (* 85 km) , respec t ive ly . The tropopause is the boundary be tween convec t ive ( turbu len t) and non- convec t ive (s tab le) reg ions , s tra topause is jo in ing s tab le to turbu len t reg ion and Mesopause is a tran- s i t ion reg ion be tween the homosphere and he terosphere . Coup l ing be tween d ifferen t reg ions of the a tmosphere is very impor tan t to s tudy as each and every reg ion is in tr ica te ly coup led w i th d ifferen t reg ions , and th is coup l ing is very comp lex (Ge t te l- man and Fors ter2002) .

The tropopause , s tra topause and mesopause p lay a v i ta l ro le in the ver t ica l coup l ing of the Ear th’s a tmosphere . He igh t and tempera ture of these pauses is cruc ia l in inves t iga t ion of var ious processes tak ing p lace in the a tmosphere (Sharma e t a l .2006,2012) . A tmospher ic waves tha t propaga te upward are work ing as a coup ler in d ifferen t a tmospher ic reg ions by in terac t ing w i th mean flow . Dunker ton (1997) showed tha t the trop ica l Quas i-B ienn ia l Osc i l la t ion (QBO) is de term ined by jo in t forc ing of grav i ty waves , Ke lv in waves and by m ixed Rossby-grav i ty waves . P lane tary waves (PW) are very cruc ia l in dec id ing dynam ica l s ta te of the a tmosphere . In the Nor thern hem isphere (NH) w in ter , PW are influenced by a ir flow ing over the moun ta in r idges and over con t inen ta l land masses . PW propaga tes upwards and

1

Phys ica l Research Labora tory , Ahmedabad 380009 , Ind ia . E-ma i l : somkumar@pr l .res . in ; sharmasomkumar@gma i l .com

2

Space App l ica t ions Cen ter (ISRO) , Ahmedabad 380015 , Ind ia .

3

Depar tmen t of Phys ics , Saurash tra Un ivers i ty , Ra jko t , Ind ia .

4

Labora tory of the A tmosphere and Cyc lones (LACY) ,

Un ivers i ty of Re´un ion Is land , Reun ion , France .

are affec t ing s tra tosphere , mesosphere and /or may break in these reg ions (e .g . , McIn tyre and Pa lmer 1983) . In con tras t to the NH , Sou thern Hem isphere (SH) is hav ing lesser land mass and re la t ive ly less number of moun ta in ranges . Therefore , PW are weaker in SH , and hence weak ly modu la t ing s tra to- sphere–mesosphere sys tem than in the NH ; consequen t ly sou thern po lar vor tex is re la t ive ly dor- man t (Thompson e t a l .2002) .

The tropopause is a changeover reg ion be tween the convec t ive ly dom ina ted troposphere and the rad ia t ive ly con tro l led s tra tosphere (Ge t te lman and Fors ter2002) . Recen t ly , the tropopause layer has rece ived large a t ten t ion due to s tra tospher ic– tropo- spher ic exchange of mass , wa ter and chem ica l cons t i tuen ts be tween the two layers (Ho inka1998) . Rande l e t a l . (2004) s tud ied the in terannua l var ia t ions of s tra tospher ic wa ter vapor and i ts corre la t ions w i th trop ica l tropopause tempera tures over 1992–2003 us ing Ha logen Occu l ta t ion Exper imen t (HALOE) sa te l l i te measuremen ts . Au thors repor ted tha t QBO and E l N ino Sou thern Osc i l la t ions (ENSO) have the ir influence on tropopause tempera ture . Ho inka ( 1998) s tud ied g loba l s ta t is t ics of tropopause he igh t , tem- pera ture , po ten t ia l tempera ture , m ix ing ra t io of wa ter vapor , and zona l , mer id iona l and ver t ica l w ind parame ters for 15 years per iod (1979–93) der ived from European Cen ter for Med ium range Wea ther Forecas t ing (ECMWF) reana lys is In ter im (ERA-In- ter im) da ta by app ly ing the therma l and dynam ica l defin i t ions of the tropopause .

Tropopause he igh t and i ts tempera ture p lay an impor tan t ro le in the transpor t be tween the s tra to- sphere and the troposphere . I t is cruc ia l to proper ly quan t ify the transpor t be tween the trop ica l , sub- trop ica l , m idd le- la t i tude and over the Po lar Reg ions wh ich are hav ing d ifferen t processes and cons t i tuen t charac ter is t ics . Dan ie lsen (1968) s tud ied upper tro- pospher ic fron ts and assoc ia ted tropopause fo lds . Fur thermore , d ifferen t synop t ic even ts , wh ich are vary ing w ide ly , and con tro l the processes assoc ia ted w i th irrevers ib le transpor t , need fur ther inves t iga- t ions . These are hav ing influence on var ious processes such as wave trans ience , turbu lence and convec t ive transpor t (Sharma e t a l .2012) .

The s tra tospher ic– tropospher ic in terac t ions and exchanges have been s tud ied ex tens ive ly in las t

decades (Appenze l ler and Dav ies1992; Baray e t a l . 1999; Me loen2003; Hock ing e t a l .2007; Das2009, S ˇkerlak et al.2014and reference therein). Holton e t a l . (1995) presen ted a very exhaus t ive rev iew on the fea tures of dynam ica l , chem ica l and rad ia t ive coup l ing in the troposphere and s tra tosphere . Over the m id la t i tude reg ions , var ious s tud ies were carr ied ou t on s tra tosphere– troposphere in terac t ions and exchanges processes (e .g . , Narayana Rao e t a l .2008 and references there in) . The trop ica l upwe l l ing is an impor tan t fea ture of s tra tosphere– troposphere cou- p l ing as a ir en ters the s tra tosphere pr imar i ly in the trop ics . Trop ica l and sub- trop ica l tropopause and the chem ica l compos i t ion of a ir in these he igh t ranges lay down the boundary cond i t ion for the g loba l s tra to- spher ic compos i t ion . The s tra tospher ic c ircu la t ion is a lso s ign ifican t ly influenced by the trop ica l upwe l l ing (Rosen lof1995) , and i t can be inferred from the in terpre ta t ions of m inor cons t i tuen ts (Ha l l and Waugh 1997; N iwano e t a l .2003and references there in) . Dur ing NH w in ter the trop ica l upwe l l ing imposes compara t ive ly larger year ly cyc le in the lowers tra tosphere and is seen in form of a large annua l cyc le in the tempera ture of the tropopause reg ion , wh ich fur ther influences seasona l cyc le of s tra tospher ic wa ter vapour (e .g . , Mo te e t a l .1996) . Seasona l i ty in the trop ica l upwe l l ing cou ld a lso resu l t in s tra tospher ic forc ing induced by PW . The max i- mum forc ing was found dur ing Sep tember to November in the SH and dur ing November to March in the NH (e .g . , Yu laeva e t a l .1994) . Tropopause fea tures have been s tud ied ex tens ive ly over the g lobe as i t is ra ther easy to exp lore troposphere us ing ground-based ins trumen ts v iz . , MST radars , L idars , Ba l loons , e tc . Exp lora t ion of s tra topause reg ion is ra ther d ifficu l t from ground-based observa t ions as ba l loons are l im i ted by he igh t coverage and MST radars are b l ind in th is reg ion and Rocke ts are l im i ted to coas ta l coverage and are very sparse . From ground , Ray le igh l idars are very good bu t these are a lso l im i ted to n igh t- t ime c lear wea ther cond i t ion obser- va t ions . Space-borne ins trumen ts are ab le to prov ide g loba l observa t ions in these a l t i tudes reg ions .

S tra tosphere is one the impor tan t reg ions of the

Ear th’s a tmosphere and hav ing very weak and /or no

turbu lence due to s tab le tempera ture s truc ture .

Ramaswamy e t a l . (2001) s tud ied tempera ture trends

in the s tra tosphere us ing l idar and rocke t observa t ions and showed the upper s tra tosphere is coo l ing a t the ra te of* 1–2 K /decade and they fur ther repor ted tha t the larges t coo l ing of* 3 K /decade occurred in the s tra topause reg ion . H is find ing paves the pa th to fur ther exp lore and unders tand the processes tha t modu la te s tra topause reg ion and to quan t ify var i- ab i l i t ies assoc ia ted w i th i t . S tra topause is var iab le la t i tud ina l ly and seasona l ly due to d ifferen t phys ica l processes . Trop ica l and sub- trop ica l la t i tude reg ions rece ive max imum energy from the Sun . Due to absorp t ion of shor twave rad ia t ions by ozone , the s tra topause is the warmes t and found a t he igh t of* 50 km ; in con tras t to i t over h igh la t i tude /po lar reg ions there is poor /no so lar inso la t ion and , there- fore , po lar w in ter s tra topause is sus ta ined by grav i ty wave-dr iven d iaba t ic descen t a t h igh la t i tudes (H i tchman e t a l .1989and reference there in) . Kan- zawa (1989) repor ted tha t in the und is turbed s i tua t ions the s tra topause in the po lar reg ion is a t h igher he igh ts and re la t ive ly warmer than in m id- la t i tudes . Dur ing sudden s tra tospher ic warm ing even ts , PW amp l i tudes are large in the NH and s tra topause is warm by up to 50 K (Lab i tzke 1977,1981; Lab i tzke and Nau joka t2000) . The tem- pera ture and he igh t of the s tra topause in the Po lar Reg ions are ma in ta ined by GW and PW ac t iv i ty and h igh- la t i tude var iab i l i ty is governed by an t icyc lones . In ter-annua l var iab i l i ty and compar ison be tween the two hem ispheres of the po lar vor t ices was presen ted by Waugh e t a l . (1999) . So far , there are no sys- tema t ic long- term inves t iga t ions on the sub- trop ica l s tra topause reg ion over bo th the hem ispheres .

To unders tand the deve lopmen ts assoc ia ted w i th neu tra l and e lec tro-dynam ica l coup l ing , in dep th inves t iga t ion of the ver t ica l s truc ture of tempera ture in the mesosphere and lower thermosphere reg ion is impor tan t bu t ra ther d ifficu l t . Exp lora t ions of the Mesopause reg ion are ra ther more d ifficu l t and s tud ies on i t are repor ted us ing op t ica l , rad io , L idar and sa te l l i te observa t ions (e .g . , Lu ¨bken and von Zahn 1991; She e t a l .1993; Berger and von Zahn1999;

Wa l tersche id e t a l . 1999; She e t a l .2004; L i e t a l . 2005a,b; Venka t Ra tnam e t a l .2010; Tang e t a l . 2014, and references there in) . There are s tud ies based on the observa t ions of ins tab i l i ty s truc tures and GW charac ter iza t ion in the mesosphere , lower

thermosphere and in the Mesopause reg ion (cover ing the he igh t range of* 80–105 km) (L i e t a l . 2005a,b) . T iny r ipp les are though t to be represen ta- t ive of ins tab i l i t ies in th is reg ion and have been repor ted in the Mesopause reg ion us ing a irg low observa t ions (Tay lor e t a l .1997; Nakamura e t a l . 1999; Hech t2004and reference there in) and severa l Na l idar-based s tud ies of Mesopause reg ion have been repor ted (e .g . , W i l l iams e t a l .2002; L iu e t a l . 2004; Prasan th e t a l .2009;Sarkhe l e t a l .2009a,b;

Sharma e t a l .2006)) . Venka t Ra tnam e t a l . (2010) repor ted charac ter is t ics of Mesopause us ing Sound- ing of the A tmosphere us ing Broadband Em iss ion Rad iome try (SABER) onboard Thermosphere Iono- sphere Mesosphere Energe t ics Dynam ics (TIMED) sa te l l i te observa t ions over a trop ica l reg ion dur ing 2002–2008 on ly and found tha t Mesopause is a t* 100 km a l t i tude in the trop ica l reg ion . Fur ther , they found tha t the Mesopause tempera ture is decreas ing w i th a ra te of* 0 .72±0 .05 K /year by ana lyz ing SABER da ta dur ing 2002-2008 .

L im i ted s tud ies are ava i lab le over sub- trop ica l reg ions and on charac ter is t ics of s tra tosphere meso- sphere in terac t ions and exchange . Th is s tudy is firs t of i ts k ind us ing long t ime per iod sa te l l i te observa- t ions , to unders tand the fea tures and charac ter is t ics of a l l the three impor tan t reg ions v iz . Tropopause , s tra topause and Mesopause cover ing bo th the hem i- spheres . The ob jec t ive of th is s tudy was to fu l ly scru t in ize he igh t and tempera ture s truc tures in the tropo-s tra to-meso pause in the sub- trop ica l reg ions in bo th the hem ispheres us ing* 14 years of SABER observa t ions and ERA-In ter im mode l reana lys is .

2 .Da ta 2 .Da ta Used Used 2 .1 . SABER Onboard TIMED

In th is s tudy we have used SABER onboard

TIMED sa te l l i te observed tempera ture from the year

2002 to 2015 in the a l t i tude range of 20–110 km over

two sub- trop ica l reg ions , M t . Abu (24 .59N , 72 .70E)

and Reun ion (21 .11S , 55 .53E) in the NH and in the

SH , respec t ive ly . SABER was launched dur ing

December 2001 and qua l i ty da ta are ava i lab le s ince

January 2002 . The SABER measures k ine t ic

tempera ture a long w i th o ther parame ters from* 20 to* 150 km a l t i tude range w i th very good la t i tud i- na l coverage from* 80Nto* 80S (e .g . , Russe l l e t a l .1999; Yee e t a l .1999; Xu e t a l .2006, and references there in) . SABER has orb i ta l inc l ina t ion of 73 and looks the a tmosphere perpend icu lar to the sa te l l i te ve loc i ty vec tor . SABER g loba l v iew ing geome try prov ides a un ique oppor tun i ty for the s tudy of var ious processes and the ir ro le in coup l ing the troposphere-s tra tosphere-mesosphere- thermosphere sys tem . Mer tens e t a l . (2001) presen ted a me thod for ex trac t ing tempera tures from CO

em iss ions under cond i t ions of non loca l thermodynam ic equ i l ibr ium , wh ich is of concern above* 70 km a l t i tude reg ion . A de ta i led descr ip t ion of the errors in SABER- observed tempera ture was prov ided by Remsberg e t a l . (2008) . Four teen years (2002–2015) of obser- va t ions from SABER sa te l l i te overpasses over M t Abu , in the NH , and over Reun ion in the SH , w i th in±5 in la t i tud ina l and long i tud ina l gr id have been emp loyed in the presen t s tudy . From the mon th ly tempera ture profi les s tra topause and Meso- pause tempera ture and he igh t have been ex trac ted . Fur thermore , we have app l ied Lomb Scarg le Per i- odogram (LSP) (Lomb1976; Scarg le1982) . Th is me thod can be used for uneven ly spaced da ta se ts (Press e t a l .1992) a lso . The LSP me thod we igh ts the da ta on a per-po in t bas is , ins tead of per- t ime bas is (Luo e t a l .2002) . Fur thermore , to have be t ter ins igh t of per iod ic i t ies w i th s imu l taneous loca l iza t ion in t ime and frequency doma in , we have app l ied wave le t ana lys is techn iques to inves t iga te fea tures in the tempera tures and he igh ts of the tropo-s tra to-meso- pauses over bo th the reg ions .

2 .2 . ERA-In ter im Reana lys is

As SABER observa t ions are very good above* 20 km , for the s tudy of Tropopause char- ac ter is t ics we have used ERA In ter im Reana lys is da ta se t over bo th the reg ions , M t . Abu and Reun ion . ERA-In ter im is a th ird-genera t ion reana lys is and prov ides much improved a tmospher ic da ta se t as compared to ERA-40 (Dee e t a l .2011) . A vas t amoun t of qua l i ty da ta ob ta ined from surface mea- suremen ts , rad iosonde observa t ions a long w i th a ircraf t and sa te l l i te observa t ions are in troduced in

da ta ass im i la t ion sys tem of ERA-In ter im reana lys is . S ix hour ly ERA-In ter im da ta are ava i lab le a t 00 , 06 , 12 and 18 UTC . The hor izon ta l reso lu t ion of the ERA-In ter im fie lds is 0 .70 w i th to ta l 37 ver t ica l hybr id leve ls . Kumar e t a l . (2015) compared three g loba l mode l ana lyses over Ind ian reg ions .

3 .Resu l ts

3 .Resu l ts and and D D iscuss iscuss ions ions

The ob jec t ive of th is s tudy was to unders tand the var iab i l i ty of Tropopause , s tra topause and Mesopause he igh t and tempera ture in bo th hem ispheres a t M t . Abu and Reun ion Is land over sub- trop ica l reg ions . For th is purpose , tempora l var ia t ions of SABER observa t ion dur ing January 2002 to Sep tember 2015 are s tud ied ex tens ive ly . The a l t i tude- tempera ture profi le from ERA In ter im reana lyses (be low 20 km) and SABER observa t ions (above 20 km) are shown in F ig .1, wh ich demons tra tes mean mon th ly var ia- t ions of tempera ture from January to December w i th 10 K sh if t a longx-ax is for be t ter c lar i ty . The mean he igh t of Tropopause , s tra topause and Mesopause are* 17 ,* 46 and 98 km , respec t ive ly , over M t . Abu reg ion in the NH ; a t these he igh ts of pauses , va lue of tempera ture is around 195 , 260 and 180 K , respec t ive ly . Chandra e t a l . (2005) and Sharma e t a l . (2006,2012) a lso repor ted mean s tra topause he igh t and tempera ture ,* 48 km and 271 K , respec t ive ly , us ing Ray le igh L idar observa t ions over M t . Abu . Over Reun ion Is land the mean he igh t of Tropopause , S tra topause and Mesopause are* 16 ,* 47 and* 98km , respec t ive ly . Ray le igh L idar-based s tudy by S ivakumar e t a l . (2011) revea led mean S tra topause he igh t and tempera ture in the range of* 44–52 km and* 265–270 K , respec t ive ly , over Reun ion Is land . As compared to NH , Tropo- pause he igh t is s l igh t ly h igher in the SH over Reun ion Is land wh i le no no tewor thy changes are found in s tra topause and mesopause he igh ts in these two hem ispher ic reg ions . Coo ler ( * 3 K) Mesopause is no t iced over M t . Abu in the mon th of February as compared to Reun ion Is land .

Tempora l var ia t ions of he igh t and tempera ture

dur ing the per iod 2002–2015 are shown in F ig .2.A

very d im inu t ive increas ing trend in mesopause and

s tra topause he igh t is seen in se lec ted t ime per iod ,

wh ich is* 1 and 0 .5 km for mesopause and S tra to- pause , respec t ive ly , in the pas t 14 years . No s ign ifican t changes are found in Tropopause he igh t dur ing th is per iod (figure no t shown) . Somewha t more var ia t ions are seen in Mesopause he igh t in the NH as compared to SH , wh ich are* 1 km and 0 .3 km over M t . Abu and Reun ion Is land , respec- t ive ly . Tempora l var ia t ion of tempera ture shows a very in teres t ing trend , wh ich is decreas ing from year 2002 to 2008 and beyond th is per iod , an increas ing trend is found in tempera ture a t d ifferen t a l t i tudes . Venka t Ra tnam e t a l . (2010) have a lso repor ted a decreas ing trend in the Mesopause he igh t dur ing 2002–2008 us ing SABER observa t ions . These fea- tures are re inforc ing for mesopause and s tra topause as compared to Tropopause tempera ture . The d iffer- ence be tween s tra topause and mesopause tempera ture for years 2002 and 2008 are* 4 K , wh i le increas ing trend ( * 1 .5 K) is observed dur ing year 2009 to 2015 over M t . Abu reg ion (Tab le1) . As compared to NH , Reun ion Is land in SH shows s tronger var ia t ions in Mesopause tempera ture ( * 7 K) dur ing years 2002 to 2008 , wh i le changes in Tropopause are no t subs tan t ia l in th is per iod . In teres t ing ly , resu l ts infer

tha t M t . Abu and Reun ion Is land show a coo l ing trend in Mesopause ; th is coo l ing trend is s tronger in the SH ( -0 .14 K /year) as compared to NH (-0 .07 K /year) . Moreover , we found tha t Mesopause coo l ing over Reun ion Is land is near ly 50% t imes ex tra than M t . Abu dur ing year 2002 to 2008 , and s l igh t ly more warm ing trend (0 .1 K /year) is seen in year 2009 to 2015 . In the s tra tosphere , NH shows a coo l ing trend (* -0 .5 K) in pas t 14 years . In teres t- ing ly , Reun ion Is land con t inuous ly shows a coo l ing trend wh i le M t . Abu shows coo l ing in years 2002 to 2008 and very sma l l warm ing in years 2009 to 2015 . Overa l l , we found tha t SH has a more coo l ing ten- dency in mesopause and s tra topause as compared to NH in the pas t decade .

F igure3shows the occurrence frequency of the mesopause and s tra topause he igh t and tempera ture dur ing year January 2002 to Sep tember 2015 . I t shows tha t the max imum occurrence frequency (* 60%) of the Mesopause he igh t is* 100 km in bo th the hem ispheres , wh i le frequency is h igher in NH as compared to SH a t th is a l t i tude . Bu t a t 95 km , occurrence frequency is more in Reun ion Is land as compared to M t . Abu . These resu l ts revea l tha t

F igure 1

Mon th ly tempera ture profi les from , (1) ERA-In ter im reana lyses from surface to 20 km and (2) SABER observa t ions from 20 to 110 km over

M t . Abu and Reun ion Is land reg ions . The success ive profi les are offse t by 10 K

Mesopause he igh t is grea ter in NH compared to SH . The Mesopause occurrence frequency is less than 5%

be low a l t i tude of 95 km in bo th the hem ispheres . S tra topause he igh t occurrence frequency l ies w i th in 44–50 km ; in wh ich s tra topause mos t ly ex is t be tween the a l t i tude range of 46 and 48 km . S ivakumar e t a l . (2011) have a lso found s tra topause he igh t be tween 44 and 52 km us ing ground-based Ray le igh L idar observa t ions . In add i t ion to Mesopause and s tra to- pause he igh t , tempera ture va lues a t these he igh ts are a lso shown in F ig .3. A t the Mesopause , mos t ly

tempera ture l ies be tween 175 and 185 K . I t is in ter- es t ing to no t ice here tha t Mesopause occurrence are less in the NH for low- tempera ture range (165–175 K) and more for h igh- tempera ture range (180–190 K) , wh ich shows tha t Mesopause temper- a ture is warmer in the NH as compared to the SH . However , s tra topause occurrences are more a t low- tempera ture range (257–261 K) in the NH and less a t h igh- tempera ture range as compared to the SH . These pa t terns show tha t the s tra topause is coo ler ; Meso- pause is warmer , in the NH as compared to the SH .

F igure 2

Tempora l var ia t ion of :aMesopause and bs tra topause he igh t , andcmesopause ,ds tra topause , andetropopause tempera ture re tr ieved from SABER sa te l l i te over M t . Abu reg ion dur ing January 2002 to Sep tember 2015 . Tempora l var ia t ion offmesopause andgs tra topause he igh t , andhmesopause ,is tra topause , andjtropopause tempera tu re over Reun ion Is land for same per iod . Orange and b lack l ine shows bes t curve

fi t t ing us ing l inear regress ion

Tab le 1

Tempera ture d i f ference be tween SABER observed and a tmospher ic mode ls s imu la ted tempera ture over M t . Abu and Reun ion Is land Mon th M t . Abu Reun ion Is land Tempera ture D ifference (K)

Mesopause S tra topause Tropopause Mesopause S tra topause Tropopause M t . Abu—Reun ion Is land S-M (K) S-C

(K) S-M (K) S-C

(K) S-M (K) S-C

(K) S-M (K) S-C

(K) S-M (K) S-C

(K) S-M (K) S-C

(K) Mesopause S tra topause Tropopause Jan 11 .7 -9 .3 -3 .1 -2 .6 -5 .0 -6 .4 0 .9 -7 .9 -3 .1 -4 .0 -7 .5 -7 .4 2 .0 -0 .9 4 .2 Feb 11 .3 -8 .6 -5 .9 -6 .0 -6 .7 -8 .0 5 .7 -4 .8 -5 .4 -4 .9 -8 .3 -7 .9 -1 .9 -1 .0 3 .2 Mar 11 .2 -5 .7 -8 .9 -6 .8 -6 .8 -8 .6 5 .1 -7 .1 -7 .5 -4 .8 -7 .4 -7 .2 0 .9 -1 .5 2 .2 Apr 10 .3 -3 .9 -9 .5 -6 .8 -5 .8 -8 .4 5 .5 -9 .5 -8 .2 -4 .0 -5 .6 -6 .4 2 .6 -0 .8 1 .3 May 16 .5 3 .1 -5 .1 -3 .9 -7 .1 -10 .0 15 .0 -3 .4 -4 .7 -1 .1 -3 .8 -5 .5 1 .5 0 .5 -1 .8 Jun 8 .7 -4 .4 -3 .4 -4 .1 -8 .6 -11 .0 6 .8 -12 .9 -3 .3 -2 .4 -4 .9 -6 .9 2 .8 0 .9 -2 .4 Ju l 11 .7 -1 .1 -4 .1 -4-.5 -9 .6 -11 .5 12 .2 -6 .8 -3 .1 -2 .7 -5 .2 -6 .7 0 .1 -0 .4 -3 .2 Aug 8 .5 -3 .6 -5 .0 -4 .7 -10 .6 -11 .6 4 .3 -13 .3 -3 .6 -5 .8 -5 .5 -6 .3 5 .2 -1 .6 -4 .0 Sep 15 .4 2 .5 -3 .1 -3 .6 -10 .9 -10 .9 12 .8 -1 .4 -3 .0 -3 .8 -4 .8 -5 .7 1 .7 -0 .8 -4 .8 Oc t 7 .4 -7 .5 -4 .3 -4 .5 -8 .7 -8 .8 2 .8 -7 .9 -4 .7 -5 .9 -4 .3 -5 .5 0 .8 -0 .7 -3 .0 Nov 12 .6 -6 .0 -3 .6 -3 .1 -6 .6 -7 .2 6 .4 -2 .7 -3 .0 -6 .0 -4 .6 -5 .5 -0 .8 -1 .6 -0 .5 Dec 15 .3 -5 .2 -3 .7 -4 .4 -5 .4 -6 .4 2 .7 -5 .5 -4 .7 -7 .6 -6 .0 -6 .5 4 .0 0 .1 2 .3

F igure 3

Occurrence frequency of :amesopause andbs tra topause tempera ture from SABER sa te l l i te , andctropopause tempera ture from ERA- in te r im .

dmesopause andes tra topause he igh t from SABER sa te l l i te over M t . Abu and Reun ion Is land

Mon th ly s ta t is t ics of the Mesopause and s tra to- pause tempera ture and he igh t for M t . Abu and Reun ion Is land are shown in F ig .4. The mean Mesopause tempera ture is * 179 .7 and 178 .1 K over M t . Abu and Reun ion Is land , respec t ive ly . The

max imum and m in imum va lues of the Mesopause tempera ture are near ly 190 .7 (188 .7) K in Sep tember and* 167 .4 (165 .1) K in Oc tober (Apr i l) over M t . Abu (Reun ion Is land) . Va lue of med ian is h igher (* 2 K) over the NH as compared to the SH . Bo th

F igure 4

Box p lo t shows :aMesopause and bs tra topause tempera tu re from SABER sa te l l i te ,ctropopause tempera ture from ERA- in ter im , and dmesopause andes tra topause he igh t from SABER sa te l l i te over M t . Abu reg ion dur ing January 2002 to Sep tember 2015 .fMesopause and gs tra topause tempera tu re from SABER sa te l l i te ,htropopause tempera ture from ERA- in ter im , andimesopause andjs tra topause he igh t from

SABER sa te l l i te over Reun ion Is land dur ing same per iod . Exp lana t ion of symbo ls and marks of box p lo t are a lso shown here

the hem ispheres show the h igh va lue of mean tem- pera ture in the mon ths of May and Sep tember . Mean s tra topause tempera ture is a lso the max imum in the mon th of May in bo th hem ispheres . Year ly mean s tra topause tempera ture is somewha t more (* 1K) in the SH aga ins t NH ; moreover , the Tropopause tempera ture is a lso h igher in Reun ion Is land . The Tropopause tempera ture is less in February over NH and Augus t–Sep tember in the SH . The max imum and m in imum va lues of Mesopause he igh t are* 102 (* 101) km in January (March) and* 79 (* 77) km in May , respec t ive ly , over M t . Abu (Reun ion Is land) in the NH (SH) . Mean va lue of the Mesopause he igh t reaches up to* 100 km in the mon th of Apr i l and m in imum in the mon th of Sep tember (* 90 km) over M t . Abu reg ion , wh i le max imum va lue of mean is* 98 km over Reun ion Is land . Med ian va lue of the Mesopause he igh t is l i t t le h igher in the NH as

compared to the SH . No ma jor var ia t ions are seen in the s tra topause he igh t in bo th the hem ispheres .

Moreover , we a lso inves t iga te the per iod ic i ty of the mesopause , s tra topause and tropopause tempera- tures and he igh ts us ing Lomb Scarg le Per iodogram (LSP) and wave le t transform techn iques . F igure5 shows annua l osc i l la t ions in tropopause and s tra to- pause tempera ture , bu t these osc i l la t ions do no t ex is t in mesopause over bo th hem ispheres . Moreover , s tra topause tempera ture shows quar ter ly osc i l la t ions , s tronger in NH , are a lso observe in Mesopause . Quar ter ly osc i l la t ions in Mesopause tempera ture are s ign ifican t over M t . Abu . F igure shows tha t a t h igher a l t i tude osc i l la t ions are of lower per iod . Mesopause and s tra topause he igh t show s trong annua l osc i l la t ion in bo th hem ispheres . I t is in teres t ing to no te here tha t sma l l per iod osc i l la t ions are more s ign ifican t in Mesopause he igh t as compared to S tra topause he igh t .

F igure 5

Lomb Scarg le per iodograms for :amesopause andbs tra topause from SABER sa te l l i te andctropopause tempera ture from ERA- in ter im over M t . Abu and Reun ion Is land reg ion .dmesopause andes tra topause he igh t from SABER sa te l l i te . The b lack do t ted l ine shows s ign ifican t leve l

a t 95% confidence in terva l

Wave le t transform me thod sugges ts tha t quar ter ly per iod ic i ty of Mesopause and S tra topause is more s ign ifican t over M t . Abu as compared to Reun ion Is land (F ig .6) . Bo th hem ispheres show s trong annua l osc i l la t ions for Tropopause tempera ture . Dur ing year 2010 , annua l osc i l la t ions are very s trong over Reu- n ion Is land . S im i lar to S tra topause and Mesopause tempera ture wave le t-based osc i l la t ions , S tra topause and Mesopause he igh t show quar ter ly osc i l la t ions .

F igure7shows the Tropopause , S tra topause and Mesopause mon th ly tempera ture var ia t ions over M t . Abu reg ion and Reun ion Is land . F igure7a shows tha t the Mass Spec trome ter and Incoheren t Sca t ter Radar Ex tended (MSISE-90) tempera ture is lower than the COSPAR In terna t iona l Reference A tmosphere

(CIRA-86) and SABER-observed Mesopause tem- pera ture . Dur ing the mon ths of May and Sep tember Mesopause tempera ture observed from the SABER is more than the CIRA-86 tempera ture over M t . Abu reg ion . Mesopause tempera ture ex trac ted from CIRA-86 revea ls decreas ing tendency from January to Ju ly and la ter s l igh t ly increases t i l l December mon th . F igure7d shows Mesopause var ia t ions ex trac ted from CIRA-86 and MSISE-90 over Reu- n ion Is land ; and shows s im i lar var ia t ions as observed over M t . Abu . I t shou ld be no ted tha t the Mesopause tempera ture (CIRA-86) is h igher than Mesopause tempera ture from SABER . MSISE-90 and CIRA-86 revea l a weak an t i-corre la t ion over M t . Abu and Reun ion Is land . The var ia t ions of s tra topause tem- pera ture over M t . Abu reg ion are shown in F ig .7b . We found tha t the mode l-der ived S tra topause tem- pera ture is h igher than SABER-observed tempera ture in the S tra topause . S im i lar charac ter is t ic are found over Reun ion Is land and g iven in F ig .7e . The d if- ference in CIRA-86 and MSISE-90 S tra topause tempera ture is h igh over Reun ion Is land as compared to M t . Abu . A max imum S tra topause tempera ture ia observed from MSISE-90 dur ing mon th of Apr i l . S tra topause tempera ture from CIRA-86 is h igher

bF igure 6

A con t inuous Wave le t transform of :amesopause andbs tra topause tempera ture from SABER sa te l l i te ,ctropopause tempera tu re from ERA-In ter im , and dmesopause and es tra topause he igh t from SABER sa te l l i te over M t . Abu reg ion dur ing January 2002 to Sep tember 2015 .fmesopause andgs tra topause tempera tu re from SABER sa te l l i te ,htropopause tempera ture from ERA- in ter im , and imesopause and js tra topause he igh t from SABER sa te l l i te over Reun ion Is land dur ing same per iod . B lack l ine in the con tour p lo t

shows cone of confidence

F igure 7

Tempora l var ia t ion of mean :amesopause andbs tra topause tempera ture re tr ieved from SABER sa te l l i te , andctropopause tempera ture from ERA-In ter im reana lys is over M t . Abu reg ion . Tempora l var ia t ions of mean ,dmesopause and es tra topause tempera tu re re tr ieved from

SABER sa te l l i te , andftropopause tempera tu re from ERA- in ter im reana lys is over Reun ion Is land reg ion

dur ing February to May over M t . Abu reg ion than Reun ion Is land , and from Ju ly to December , i t is h igher over Reun ion Is land as compared to M t . Abu reg ion . The s tra topause tempera ture observed from the SABER shows a s im i lar pa t tern over bo th the reg ions and the coo les t S tra topause is found in the mon th of Augus t over M t . Abu reg ion . F igure7c shows tempera ture var ia t ion in Tropopause reg ion . The Tropopause tempera ture is ex trac ted from the ERA-In ter im a tmospher ic mode l and found to be lower than the CIRA-86 and MSISE-90 mode l tro- popause tempera tures . S im i lar k ind of trend has been observed over Reun ion Is land and shown in F ig .7f . Tropopause tempera ture is the max imum dur ing Sep tember mon th and we no ted reverse trend as compared to M t . Abu reg ion .

To have fur ther quan t i ta t ive ins igh t of pauses charac ter is t ics , mon th ly mean tempera ture d iffer- ences in tropopause , s tra topause and mesopause among SABER (S) , CIRA-86 (C) and MSISE-90 (M) have been g iven in Tab le1. In the Mesopause reg ion over M t . Abu the tempera ture d ifference be tween SABER and MSISE-90 is much h igher and found to be max imum dur ing the mon th of May (* 16 .5 K) , whereas CIRA-86 shows h igher than SABER observa t ions and the max imum d ifference observed dur ing January (* 9 .3 K) . In the S tra to- pause reg ion , i t is seen tha t the tempera ture from bo th the mode l is h igher and max imum d ifference be tween SABER and CIRA-86 is* 9 .5 K dur ing the mon th of Apr i l . S im i lar ly , Tropopause tempera ture is lower than the CIRA-86 and MSISE-90 and max imum d ifference is found to be* 10 .9 K from MSISE-90 dur ing Sep tember , and* 11 .6 from the CIRA-86 dur ing the mon th of Augus t .

Over Reun ion Is land , Mesopause tempera ture revea led m in imum d ifference be tween SABER and MSISE-90 (* 0 .9 K) dur ing January and the max i- mum d ifference (* 15 K) is observed in the mon th of May . The h igher tempera ture observed from CIRA-86 as compared to SABER in th is reg ion and the max imum d ifference in CIRA-86 and SABER is* 13 .3 K in the mon th of Augus t . In the S tra to- pause reg ion , bo th the mode ls show h igher tempera ture than the SABER tempera ture . The max imum tempera ture d ifference in MSISE-90 and SABER is observed (* 8 .2 K) in Apr i l . The

tempera ture observed in M t . Abu reg ion is h igher than Reun ion Is land in Mesopause reg ion dur ing mos t of the mon ths excep t dur ing February and November , whereas in the S tra topause reg ion i t is lower excep t dur ing May , June and December mon ths . In the Tropopause reg ion the ERA-In ter im tempera ture is h igher dur ing January to Apr i l and in December and lower dur ing res t of the mon ths .

4 .Conc lus ion 4 .Conc lus ion

In th is s tudy , Tropopause , S tra topause and

Mesopause he igh t and tempera ture are s tud ied a t two

sub- trop ica l reg ions in bo th hem ispheres . Tempora l

var ia t ions of SABER observa t ions dur ing January

2002 to Sep tember 2015 are s tud ied ex tens ive ly . The

a l t i tude- tempera ture profi le comb ined from ERA

In ter im re-ana lyses and SABER observa t ions shows

mean mon th ly var ia t ion of tempera ture for d ifferen t

mon ths . As compared to NH , Tropopause he igh t is

s l igh t ly h igher in the SH over Reun ion Is land . Very

sma l l increas ing trend in Mesopause and S tra topause

he igh t are seen dur ing the s tudy per iod , wh ich

are* 1 and* 0 .7 km for Mesopause and S tra to-

pause , respec t ive ly , in the pas t 14 years . Though ,

quan t i ta t ive ly the trends may no t be very s ign ifican t

in v iew of l im i ted ver t ica l reso lu t ion , tendency of

trends is c lear from th is ex tens ive s tudy . Tempora l

var ia t ion of tempera ture shows a very in teres t ing

trend , wh ich is decreas ing from year 2002 to 2008

(repor ted by Venka t Ra tnam e t a l .2010) and beyond

th isper iod , an increas ing trend is found in tempera-

ture a t d ifferen t a l t i tudes . In teres t ing ly , resu l ts infer

tha t M t . Abu and Reun ion Is land show a coo l ing

trend in Mesopause and tha t th is coo l ing trend is

s tronger in the SH (-0 .14 K /year) as compared to

NH (-0 .07 K /year) . Resu l ts sugges t tha t SH has a

more coo l ing tendency in Mesopause and S tra topause

as compared to NH in pas t decade . Resu l ts from

occurrence frequency pa t terns sugges t tha t S tra to-

pause is coo ler , Mesopause is warmer , in NH as

compared to SH . Moreover , we a lso used LSP and

wave le t transform to find the per iod ic i ty of Meso-

pause , S tra topause and tropopause tempera tures and

he igh ts . Annua l osc i l la t ions ex is t in Tropopause and

S tra topause tempera ture , bu t these osc i l la t ions are no t

observed in Mesopause over bo th the hem ispheres . I t is in teres t ing to no te here tha t sma l l per iod osc i l la- t ions are more s ign ifican t in Mesopause he igh t as compared to S tra topause he igh t . H i ther to find ings presen ted in th is work on the Tropopause , S tra to- pause and Mesopause charac ter is t ics w i l l s ign ifican t ly con tr ibu te to fur ther g loba l inves t iga- t ions on the ver t ica l and in ter-hem ispher ic coup l ing processes .

Acknow ledgemen ts Acknow ledgemen ts

Au thors are thankfu l to the sc ien t ific , techn ica l and manager ia l team members of SABER onboard TIMED for prov id ing such a qua l i ty g loba l observa- t ions of the Era th’s m idd le a tmosphere . Our thanks are due to the ERA-In ter im reana lys is team members (h t tp : / /apps .ecmwf . in t /) . Th is work is financ ia l ly suppor ted by Depar tmen t of Space , Gov t . of Ind ia .

R

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