Electromagnetic fields generated by a three dimensional global ocean circulation

Abstract

A simplified form of the motional induction equation is used to calculate the dominant three-dimensional (3-D) electromagnetic (EM) fields generated by a specified steady 3-D global ocean circulation. The EM calculations require, at most, vertical integrations and do not require running a global 3-D model. Two cases for ocean bottom conductivity are considered: an electrically insulating ocean bottom and a high-conductance sediment layer. The approximations are discussed, and the solutions are plotted for various depth levels. Many aspects of the dominant ocean-generated EM fields (particularly the electric currents near the sea surface and the magnetic fields) are shown to be insensitive to ocean bottom conductivity. Other aspects (particularly the horizontal electric field in shallow water) are very sensitive. We perform a global integration to estimate the role of the “nonlocal” electric currents. We find that the importance in including these nonlocal currents when making EM field estimates is the same or less than that for including a model for the bottom conductance. Hence the simple EM estimates from one-dimensional integrations are not improved in globally integrated models until these models include a realistic model for bottom conductivity.