An Overview of Motional Electromagnetic Induction

By contrast to our knowledge of depth-dependent (baroclinic) motions, only limited information is available about the distribution and statistics of the subinertial depth-independent (barotropic) velocity field in the world ocean. This talk focusses on Eulerian measurement of the latter using inexpensive free-fall electric field recorders. Theory predicts a relation between the horizontal electric field and the seawater conductivity-weighted, vertically-integrated (CWVI) water velocity. This connection will be validated by comparing a six element current meter mooring to nearby electric field data collected in 1986-1987 during BEMPEX. It will also be shown that low coherence between the electric field and the mooring-derived CWVI water velocity occurs only where the coherence between individual current meters is itself low, suggesting the presence of short vertical scale components that are not resolvable by the mooring but integrated out in the electric field. The superior ability of the electric field to sense the vertically-integrated velocity will be demonstrated by showing better coherence between the electric field and the FNOC surface winds at periods of 2-7 days than between the mooring-derived CWVI water velocity and the winds. The effect of ionospheric noise on the seafloor electric field will be quantified using the EMSLAB array, showing negligible effects at periods longer than four days. Evidence for a wave motion near the Juan de Fuca Ridge with a period near four days that is consistent with the dispersion relation for topgraphically-trapped Rossby waves will also be presented.