Hydrography and circulation near the crest of the East Pacific Rise between 9° and 10°N

Topography has a strong effect on the physical oceanography over the flanks and crests of the global mid-ocean ridge system. Here, we present an analysis of the hydrography and circulation near the crest of the East Pacific Rise (EPR) between 9° and 10°N, which coincides with an integrated study site (ISS) of the RIDGE2000 program. The analysis is based primarily on survey and mooring data collected during the LADDER project, which aimed to investigate oceanographic and topographic influences on larval retention and dispersal in hydrothermal vent communities. Results indicate that the yearly averaged regional mean circulation is characterized by a westward drift of 0.5–1 cm s−1 across the EPR axis and by north- and southward flows along the western and eastern upper ridge flanks, respectively. The westward drift is part of a basin-scale zonal flow that extends across most of the Pacific ocean near 10°N, whereas the meridional currents near the ridge crest are a topographic effect. In spite of considerable mesoscale variability, which dominates the regional circulation and dispersal on weekly to monthly time scales, quasi-synoptic surveys carried out during the mooring deployment and recovery cruises indicate sub-inertial circulations that are qualitatively similar to the yearly averaged flow but associated with significantly stronger velocities. Weekly averaged mooring data indicate that the anticyclonically sheared along-flank flows are associated with core speeds as high as 10 cm s−1 and extend ≈10km off axis and 200 m above the ridge-crest topography. Near the northern limit of the study region, the Lamont Seamount Chain rises from the western ridge flank and restricts along-EPR flow to five narrow passages, where peak velocities in excess of 20 cm s−1 were observed. Outside the region of the ridge-crest boundary currents the density field over the EPR near 10°N is characterized by isopycnals dipping into the ridge flanks. Directly above the EPR axis the ridge-crest boundary currents give rise to an isopycnal dome. During times of strong westward cross-EPR flow isopycnal uplift over the eastern flank causes the cross-ridge density field below the doming isopycnals to be asymmetric, with higher densities over the eastern than over the western flank. The data collected during the LADDER project indicate that dispersal of hydrothermal products from the EPR ISS on long time scales is predominantly to the west, whereas mesoscale variability dominates dispersal on weekly to monthly time scales, which are particularly important in the context of larval dispersal.

Research highlights
► Circulation near the EPR crest is characterized by westward cross-ridge drift and narrow along-crest boundary currents.
► Westward drift is part of a basin-scale flow spanning the entire Pacific ocean near 10N.
► Boundary currents are a topographic effect and associated with doming of isopycnals over the ridge crest.
► Lamont seamount chain has strong influence on dispersal at EPR RIDGE2000 integrated study site.