Depth of origin of ocean-circulation-induced magnetic signals

Abstract

Abstract. As the world ocean moves through the ambient geomagnetic core field, electric currents are generated in the entire ocean basin. These oceanic electric currents induce weak magnetic signals that are principally observable outside of the ocean and allow inferences about large-scale oceanic transports of water, heat, and salinity. The ocean-induced magnetic field is an integral quantity and, to first order, it is proportional to depth-integrated and conductivity-weighted ocean currents. However, the specific contribution of oceanic transports at different depths to the motional induction process remains unclear and is examined in this study. We show that large-scale motional induction due to the general ocean circulation is dominantly generated by ocean currents in the upper 2000 m of the ocean basin. In particular, our findings allow relating regional patterns of the oceanic magnetic field to corresponding oceanic transports at different depths. Ocean currents below 3000 m, in contrast, only contribute a small fraction to the ocean-induced magnetic signal strength with values up to 0.2 nT at sea surface and less than 0.1 nT at the Swarm satellite altitude. Thereby, potential satellite observations of ocean-circulation-induced magnetic signals are found to be likely insensitive to deep ocean currents. Furthermore, it is shown that annual temporal variations of the ocean-induced magnetic field in the region of the Antarctic Circumpolar Current contain information about sub-surface ocean currents below 1000 m with intra-annual periods. Specifically, ocean currents with sub-monthly periods dominate the annual temporal variability of the oceaninduced magnetic field.

Predictability of Ocean Heat Content From Electrical Conductance

“…Indeed, ocean tides may supply the most predictable naturally occurring, global‐scale electromagnetic field source on Earth and the predictable phase‐locked astronomical periodicity has been a key element allowing both the successful theoretical modeling of these fields as well as their extraction from satellite magnetic records (Sabaka et al, , ; Tyler et al, ). The study of ocean tidal electromagnetic fields and their use as a sounding source in oceanographic and geophysical studies is currently receiving much attention (Grayver et al, , ; Irrgang et al, , , , ; Sabaka et al, , ; Saynisch et al, , , ; Schnepf, , ; Tyler, ; Velimsky et al, ) due to the unprecedented geomagnetic field coverage coming from the ESA Swarm mission (e.g., Friis‐Christensen et al, ) consisting of three satellites launched in 2013 and expected to continue until 2030.…”

“…A recent study of ocean climatology data (Tyler et al, ) suggests that over much of the global ocean, observed conductivity tracks observed temperature and over much of the remainder of the ocean, observed conductivity tracks observed salinity. However, time variations and depth integrals (e.g., Irrgang et al, ; Saynisch et al, ) combined with the nonlinearity of the equation for conductivity complicates the question of whether a linear relationship holds between conductance and OHC. Because of aliasing and sparse sampling of some regions of the ocean, it is possible that the relationship between conductance and OHC is not simply linear.…”

Electromagnetic characteristics of ENSO

“…Compared to the signal strength at satellite altitude, the signal is approximately 30 % stronger at sea level. Additionally, substantial research has already investigated the relationship between oceanic-induced magnetic field variations, especially their radial component, and their oceanic causes (Saynisch et al, 2016Irrgang et al, 2016aIrrgang et al, , 2017Irrgang et al, , 2018. We add to this canon by investigating the impact of ENSO, the biggest interannual climate signal, on the oceanic tidally induced radial magnetic field.…”

“…Hz (Jager et al, 2010) are able to resolve the global structure of the oceanic tidally induced radial magnetic field. To be able to observe the presented variations an increase magnetometer precision of several orders of magnitude down to the femtotesla scale is necessary.…”

Answer to review #2

“…Hz (Jager et al, 2010) magnetic field. To be able to observe the presented variations an increase magnetometer precision of several orders of magnitude down to the femtotesla scale is necessary.…”