Deep Ocean Circulation and Transport Where the East Pacific Rise (9-10 N) Meets the Lamont Seamount Chain

Numerical modeling studies along with current and hydrographic measurements and a SF6 tracer experiment during LADDER (LArval Dispersal along the Deep East Pacific Rise) I and II cruises in November and December 2006 have yielded new insight into ocean flow and transport at the northern end of 9-10 N EPR segment. Long period flows tending westward over the ridge result in uplifted isopycnals over the ridge crest. The consequence is narrow geostrophic (plus rectified flow) 'jets' on each side of the ridge which can transport material north on the western and south on the eastern flanks of the ridge. When the flow reaches the gap between the ridge and the first Lamont seamount (Sasha) to the west of the ridge, flow moves north through the gap and/or along the southern side of the seamounts. A model SF6 tracer dispersion simulation meant to parallel the SF6 field experiment suggest that in late November, 2006, at the time when the tracer would have likely reached the gap in the northward flank flow, mass flux through the gap was atypically southward, partially stalling northward SF6 transport and sending the bulk of the SF6 westward. Measurements of actual SF6 dispersion initially suggested the westward transport just south of the seamounts. The model also indicates long term mean flow in an anticyclonic sense around the entire seamount chain and transport primarily north through the gaps between individual seamounts. If larvae were to disperse passively like the tracer in these experiments, only a small fraction would be retained along the ridge, with most transported to the Lamont Seamount chain where conditions are not suitable for larval settlement. The adjacent poster by Thurnherr et al. presents some of the LADDER field measurements underlying this modeling effort.