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.