Motional induction effect on the planetary‐scale geoelectric potential in the eastern North Pacific

Abstract

Geoelectric potentials induced by seawater motion on a planetary scale were studied using voltage differences measured on the Hawaii‐1 (HAW‐1) submarine cable between Hanauma Bay, Oahu, Hawaii, and Point Arena, California. The motionally induced field was extracted from a 3.6‐year segment of the HAW‐1 voltage by cancelling the component that is coherent with the externally produced geomagnetic field at Honolulu. The corrected voltage shows significant coherence with atmospheric variables (wind stress curl, wind stress, and surface pressure) estimated from the European Centre for Medium‐Range Weather Forecasts product at points throughout the Pacific basin at periods from 5 to 133 days. This suggests that the voltage reflects regional to basin‐wide depth‐averaged (barotropic) water currents that are directly wind driven. Contour maps of the squared coherence reveal that (1) peak coherence as large as 0.6–0.8 is seen at points both near the cable and nonlocally, (2) higher coherence is seen at periods where the cable voltage power spectrum peaks, and (3) spatial patterns of the coherence vary with period. Comparison of power spectra or coherence maps for individual years shows that the wind‐driven flow fluctuates temporally owing to long‐term variations of the wind field. The main features of these observations are consistent with those from point electric field measurements, suggesting that planetary‐scale voltages measured with cables can serve as a useful tool to monitor large‐scale water currents at long periods.