Physical evolution of the IronEx-I open ocean tracer patch

An 8×8 km tracer-enriched patch was successfully formed in an open-ocean mixed layer in the equatorial Pacific during the first uncontained test of the iron hypothesis. To minimize the effects of horizontal advection, the patch formation and subsequent rapid underway sampling of the patch properties were performed in a Lagrangian reference frame centered on a navigated, drogued buoy that tracked the mixed layer movements on O(1 d) timescales. Daily maps of the evolving patch shape, and corrections for the buoy drift relative to the patch center, were based on objectively analyzed surface concentration maps of an inert tracer, SF6, which had been injected into the ocean surface with trace quantities of iron during the patch formation. The Lagrangian reference frame significantly reduced large scale circular and lateral advection errors in maps of the surface patch shape. A strong pycnocline at approximately 35 m depth and very constant 6 m s-1 wind forcing greatly 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 wind horizontal diffusivity using a Fickian model were 600±100 m2 s-1, with a mean cross wind value of 200±30 m2 s-1 over a 4 d period. On the fifth day an intruding low salinity surface front effectively capped the patch between a 10–20 m thick fresh surface layer and the pycnocline. This experiment demonstrated that an O(100 km2) open ocean mixed layer tracer tagged patch could be formed, and its evolution mapped the presence of advection over 5–10 d periods.