REVIE W ARTICLE Lagra ngia n Vie ws of t he Pat h ways of t he Atla ntic 10.1029/2019J C015014 Meridio nal Overt ur ni ng Circ ulatio n S pecial Sectio n: A. Bo wer 1 , S. L ozier 2, 3 , A. Biastoc h 4 , K. Dro ui n 2 , N. Fo u kal 1 , H. F urey 1 , Atla ntic Meridio nal 5 6 2, 7 Overt ur ni ng Circ ulatio n: M. La n k horst , S. R ü hs , a n d S. Zo u Revie ws of Observational and 1 2 Modeling Advances Woods Hole, Oceanographic Institution, Woods Hole, M A, US A, Depart ment of Earth and Ocean Sciences, Duke University, Durha m, N C, US A, 3 No w at Depart ment of Earth and At mospheric Sciences, Georgia Institute of Technology, Atla nta, G A, US A, 4 G E O M A R Hel mholtz Centre for Ocean Research Kiel, Kiel, Ger many, 5 Scri p ps I nstit utio n of Key Poi nts: Ocea nograp hy, U niversity of Califor nia Sa n Diego, La Jolla, C A, US A, 6 Ocea n Fro ntier I nstit ute Dal ho usie U niversity, • Lagra ngia n met hods are critical to Halifax, Nova Scotia, Ca nada, 7 No w at Depart ment of Physical Oceanography, Woods Hole Oceanographic Institution, describi ng t he t hree ‐di m e nsi o n al str uct ure of t he Atla ntic Meridio nal Woods Hole, M A, US A Overt ur ni ng Circ ulatio n • Fl ui d parcel trajectories reveal t he lack of meridio nal co n nectivity i n A bstract The Lagrangian method — w here c urre nt locatio n a nd i ntensity are deter mi ned by tracki ng t he bot h t he s hallo w a nd deep li mbs of move ment of fl o w alo ng its pat h — is t he oldest tec h niq ue for measuri ng t he ocea n circ ulatio n. For t he overt ur ni ng circ ulatio n ce nturies, mari ners used co mpilatio ns of s hip drift data to map o ut t he locatio n a nd i nte nsity of s urface • Hi g h s p ati al ‐te mporal resolutio n trajectories ide ntify regio ns w here c urre nts alo ng major s hippi ng routes of t he global ocea n. I n t he mid ‐20t h ce ntury, tec h nological adva nces i n co here nt eddies co ntrib ute to t he electro nic navigation allo wed oceanographers to conti nuo usly track freely drifting surface b uoys t hroug hout overt ur ni ng circ ulatio n t h e i c e ‐free ocea ns a nd begi n to co nstruct basi n ‐scale, a nd eve nt ually global ‐scale, maps of t he s urface circ ulatio n. At about t he sa me ti me, develop me nt of acoustic met hods to track neutrally b uoya nt fl oats belo w t he s urface led to i mportant ne w discoveries regardi ng t he deep circulatio n. Since t he n, Lagra ngian observi ng Correspo nde nce to: a nd modeli ng tec hniq ues have bee n used to explore t he struct ure of t he ge neral circulatio n a nd its A. B o w er, variability t hro ug ho ut t he global ocea n, b ut especially i n t he Atla ntic Ocea n. I n t his revie w, Lagra ngia n st udies abo wer @ whoi.edu t hat focus on pat h ways of t he upper a nd lo wer li mbs of the Atlantic Meridional Overtur ni ng Circulation ( A M O C), both observational and nu merical, have been gathered together to illustrate aspects of the A M O C Cit ati o n: t hat are u niquely capt ured by t his tech nique. These i ncl ude t he i mporta nce of horizo ntal recirculatio n gyres B o w er, A., L o zi er, S., Bi ast o c h, A., Dr o ui n, K., F o u k al, N., F ur e y, H., et al. and i nterior (as opposed to bou ndary) path ways, t he con nectivity (or lack t hereof) of t he A M O C across (2019). Lagra ngia n vie ws of t he latitudes, and t he role of mesoscale eddies i n so me regions as t he pri mary A M O C transport mec hanis m. There pat h ways of t he Atla ntic Meridio nal re mai n vast areas of t he deep ocea n w here t here are no direct observations of the path ways of t he A M O C. Overt ur ni ng Circ ulatio n. Jo ur n al of Geophysical Research: Oceans , 1 2 4 , Plain Language Su m mary Measuri ng ocea n curre nts by follo wi ng t heir pat hs — rat her t ha n by 5313 – 5335. https://doi.org/10.1029/ 2019J C015014 observi ng t he m at a fi xe d locatio n — is t he oldest met hod for explori ng ocea n c urre nts. For ce nt uries, mari ners used co mpilatio ns of s hip drift data to map o ut t he locatio n a nd i nte nsity of s urface c urre nts alo ng Received 30 J A N 2019 major s hippi ng ro utes of t he global ocea n. I n t he mid ‐20t h ce nt ury, tec h nological adva nces i n electro nic Accepted 8 J U L 2019 navigatio n allo wed ocea nograp hers to track freely drifti ng surface buoys co nti nuously t hroug hout t he Accepted article o nli ne 19 J U L 2019 Published online 15 A U G 2019 i c e ‐free ocea ns a nd begi n to co nstruct global maps of t he surface curre nts. At about t he sa me ti me, t he develop ment of methods using sound to track fl oats belo w t he surface led to i mporta nt ne w discoveries regardi ng t he curre nts bar belo w t he surface. Si nce t he n, t hese tec h niques have bee n used to explore t he location and strength of currents, and ho w they change in ti me, throughout the world's oceans, but especially i n t he Atla ntic Ocea n. Co mp uter si m ulatio ns of t he ocea ns are also used to calc ulate t he pat h ways of virt ual s urface drifters a n d s ubs urface fl o ats. I n t his r e vi e w, st u di es t h at us e t h es e fl o w ‐follo wing methods to measure t he pat h ways of t he nort h ‐sout h s hallo w a nd deep curre nts t hat make up t he Atla ntic Meridio nal Overturning Circulation (also so meti mes referred to as the Great Ocean Conveyor) have been gathered toget her to ill ustrate aspects of t he co nveyor syste m t hat are u niq uely capt ured by t his tec h niq ue. T hese i ncl ude t he i mporta nce of large recirc ulatio ns, or “ waiti ng areas, ” w here water circulates arou nd a nd arou nd before moving onto the next seg ment of the conveyor. Also, these techniques for observing ocean currents ©2019. T he Aut hors. illustrate ho w disco n nected so me parts of t he co nveyor are, a nd ho w s wirli ng, tra nslati ng pools of water T his is a n ope n access article u nder t he ~100 k m i n dia meter, a nd not co nti nuous curre nts, help to move water alo ng t he co nveyor's pat h. Vast areas ter ms of the Creative Co m mons of t he deep ocea n re mai n w here t here are no direct observatio ns of t he pat h ways of t he co nveyor. Attri b uti o n ‐Non Co m mercial ‐No Derivs Lice nse, w hic h per mits use a nd distri- bution in any mediu m, provided the 1. I ntrod uctio n origi nal wor k is properly cited, t he use is no n ‐co m mercial and no modi fi c a- T he Atla ntic Meridio nal Overtur ni ng Circulatio n ( A M O C) refers to t he net nort h ward tra nsport of relatively tio ns or adaptatio ns are made. w ar m, l o w ‐de nsity upper ‐ocean waters fro m the South Atlantic (S A) to the high latitudes of the North B O WER ET AL. 5313 Jour nal of Geop hysical Researc h: Ocea ns 10.1029/2019J C015014 Atla ntic a nd Nordic Seas ( Nor wegia n, Gree nla nd, a nd Icela nd Seas), w here t hey lose heat to t he at mosp here a nd beco me colder a nd de nser.