ELECTROMAGNETIC-FIELDS GENERATED BY A 3-DIMENSIONAL GLOBAL OCEAN CIRCULATION

A simplified form of the motional induction equation is used to calcul ate the dominant three-dimensional (3-D) electromagnetic (EM) fields g enerated by a specified steady 3-D global ocean circulation. The EM ca lculations require, at most, vertical integrations and do not require running a global 3-D model. Two cases for ocean bottom conductivity ar e considered: an electrically insulating ocean bottom and a high-condu ctance sediment layer. The approximations are discussed, and the solut ions are plotted for various depth levels. Many aspects of the dominan t ocean-generated EM fields (particularly the electric currents near t he sea surface and the magnetic fields) are shown to be insensitive to ocean bottom conductivity. Other aspects (particularly the horizontal electric held in shallow water) are very sensitive: We perform a glob al integration to estimate the role of the ''nonlocal'' electric curre nts. We find that the importance in including these nonlocal currents when making EM field estimates is the same or less than that for inclu ding a model. for the bottom conductance. Hence the simple EM estimate s from one-dimensional integrations are not improved in globally integ rated models until these models include a realistic model for bottom c onductivity.