Dr. Sö­ren Thom­sen: Phy­si­cal-bio­geo­che­mi­cal cou­pling wi­t­hin the Pe­ru­vi­an up­wel­ling re­gime

3:00 p.m. Lec­tu­re hall (4th floor), MPI

Host: Dr. Gau­te La­vik, MPI

Up­wel­ling sys­tems are one of the most pro­duc­tive are­as in the world oce­an. Clas­si­cal­ly the­se sys­tems are of­ten de­scri­bed in terms of wind‐dri­ven sim­ple Ek­man off­shore trans­port. Howe­ver, the­re is lar­ge high tem­po­ral and spa­ti­al va­ria­bi­li­ty e.g. ocea­nic

me­so­s­ca­le ed­dies and so cal­led sub­me­so­s­ca­le fil­aments. The­se pro­ces­ses trans­port pro­duc­tive wa­ters off­shore and down­wards and are om­ni­pre­sent in sa­tel­li­te pic­tu­res and high re­so­lu­ti­on oce­an mo­dels. New ob­ser­va­tio­nal me­thods such as gli­ders al­low us now to stu­dy the­se high fre­quent va­ria­bi­li­ty in com­bi­na­ti­on it with high‐re­so­lu­ti­on bio­geo­che­mi­cal sam­ples.

The role of ed­dies and fi­ma­l­ents in the Pe­ru­vi­an up­wel­ling re­gime for the cir­cu­la­ti­on, phy­si­cal / bio­geo­che­mi­cal tra­cer dis­tri­bu­ti­ons and oxy­gen mi­ni­mum zone ven­ti­la­ti­on is in­ves­ti­ga­ted. The stu­dy is ba­sed on a mul­ti‐plat­form four‐ di­men­sio­nal ob­ser­va­tio­nal ex­pe­ri­ment car­ri­ed out in ear­ly 2013 near 13°S. The data set con­sists of >15,000 pro­files from 7 Slo­cum gli­ders and re­pea­ted ves­sel‐ ba­sed ve­lo­ci­ty, hy­dro­gra­phy and nut­ri­ent tran­sects. Forther­mo­re the out­put of a sub­me­so­s­ca­le per­mit­ting phy­si­cal cir­cu­la­ti­on mo­del is used.

The for­ma­ti­on of a sub­sur­face an­ti­cy­clo­nic eddy and its im­pact on the near‐ co­as­tal oxy­gen and nut­ri­ent dis­tri­bu­ti­ons was cap­tu­red by the ob­ser­va­tions. The eddy de­ve­l­o­ped in the Peru‐Chi­le Un­der­cur­rent down­stream of a to­po­gra­phic bend, sug­gesting flow se­pa­ra­ti­on as the eddy for­ma­ti­on me­cha­nism. The core wa­ters ori­gi­na­ted from the bot­tom boun­da­ry lay­er and were cha­rac­te­ri­zed by low po­ten­ti­al vor­ti­ci­ty and an en­han­ced ni­tro­gen‐de­fi­cit.

The sub­duc­tion of high­ly oxy­ge­na­ted sur­face wa­ter in a sub­me­so­s­ca­le cold fil­ament was ob­ser­ved by gli­der‐ba­sed mea­su­re­ments. The sub­duc­tion of new­ly up­wel­led wa­ters ven­ti­la­tes the up­per oxy­cli­ne is sug­gested by the sum­mer ob­ser­va­tions. La­gran­gi­an dia­gnostics in the mo­del sup­ports this fin­dings and sug­gest that in win­ter it might be pos­si­ble that sub­me­so­s­ca­le pro­ces­ses ven­ti­la­te the oxy­gen mi­ni­mum zone core. In the mo­del about 50 % of the new­ly up­wel­led floats lea­ve the mi­xed lay­er wi­t­hin 5 days both in sum­mer and win­ter em­pha­zi­sing a hi­t­her­to un­re­co­gni­zed im­port­an­ce of sub­duc­tion for the ven­ti­la­ti­on of the up­per boun­da­ry of the Pe­ru­vi­an oxy­gen mi­ni­mum zone.