PHYSICAL EVOLUTION OF THE IRONEX-I OPEN-OCEAN TRACER PATCH

An 8 x 8 km tracer-enriched patch was successfully formed in an open-o cean mixed layer in the equatorial Pacific during the first uncontaine d test of the iron hypothesis. To minimize the effects of horizontal a dvection, the patch formation and subsequent rapid underway sampling o f the patch properties were performed in a Lagrangian reference frame centered on a navigated, drogued buoy that tracked the mixed layer mov ements on O(1 d) timescales. Daily maps of the evolving patch shape, a nd corrections for the buoy drift relative to the patch center, were b ased on objectively analyzed surface concentration maps of an inert tr acer, SF6, which had been injected into the ocean surface with trace q uantities of iron during the patch formation. The Lagrangian reference frame significantly reduced large scale circular and lateral advectio n errors in maps of the surface patch shape. A strong pycnocline at ap proximately 35 m depth and very constant 6 m s(-1) wind forcing greatl y limited turbulent diffusion below the mixed layer. Rapid small scale mixing over the first 24 h was followed by a four day period of slow spreading, primarily in the along-wind direction. Estimates of along w ind horizontal diffusivity using a Fickian model were 600 +/- 100 m(2) s(-1), with a mean cross wind value of 200 +/- 30 m(2) s(-1) over a 4 d period. On the fifth day an intruding low salinity surface front ef fectively capped the patch between a 10-20 m thick fresh surface layer and the pycnocline. This experiment demonstrated that an O(100 km(2)) open ocean mixed layer tracer tagged patch could be formed, and its e volution mapped the presence of advection over 5-10 d periods. (C) 199 8 Elsevier Science Ltd. All rights reserved.