Analysis of Ocean Tide‐Induced Magnetic Fields Derived From Oceanic In Situ Observations: Climate Trends and the Remarkable Sensitivity of Shelf Regions

Abstract

Tidal motion of oceanic salt water through the ambient geomagnetic field induces periodic electromagnetic field signals. Amplitudes of the induced signals are sensitive to variations in electrical seawater conductivity and, consequently, to changes in oceanic temperature and salinity. In this paper, we computed and analyzed time series of global ocean tide-induced magnetic field amplitudes. For this purpose, we combined data of global in situ observations of oceanic temperature and salinity fields from 1990-2016 with data of oceanic tidal flow, the geomagnetic field, mantle conductivity, and sediment conductance to derive ocean tide-induced magnetic field amplitudes. The results were used to compare present day developments in the oceanic climate with two existing climate model scenarios, namely, global oceanic warming and Greenland glacial melting. Model fits of linear and quadratic long-term trends of the derived magnetic field amplitudes show indications for both scenarios. Also, we find that magnetic field amplitude anomalies caused by oceanic seasonal variability and oceanic climate variations are 10 times larger in shallow ocean regions than in the open ocean. Consequently, changes in the oceanic and therefore the Earth's climate system will be observed first in shelf regions. In other words, climate variations of ocean tide-induced magnetic field amplitudes are best observed in shallow ocean regions using targeted monitoring techniques.

Phase Changes of Electromagnetic Oceanic Tidal Signals

“…It is noteworthy, that the plotted pattern of σ oc induced EMOTS phase changes do not exactly match the position and shape of the published pattern for σ oc induced seasonal EMOTS amplitude changes (Petereit et al, , Figure 2) nor the published pattern for climate change induced EMOTS amplitude changes (Saynisch et al, , Figure 1). For example, EMOTS amplitudes around New Zealand are strongly sensitive to the seasonal cycle as well as oceanic climate change.…”

“…Phases which are influenced by oceanic conductivity instead show monthly varying anomalies and a seasonal cycle (Figure , dashed lines). The oceanic conductivity itself has annual variations and respective sensitivities are recently reported for the annual cycle and EMOTS amplitudes (Petereit et al, ).…”

“…The phase shifts in the lower dashed line show a small decadal transient behavior, too (other areas show much larger σ oc induced phase shift trends). The respective trends must be attributed to oceanic climate related processes (Saynisch et al, ; Petereit et al, ).…”

“…The main question is hereby, are the EMOTS phases equally sensitive to oceanic conditions and to secular variation as the EMOTS amplitudes? (Petereit et al, , ; Saynisch et al, , ).…”

“…Especially since the launch of the Swarm satellite magnetometers (Friis‐Christensen et al, ), the number of EMOTS‐related studies is ever growing. Alone in the last few years, many papers studying or using EMOTS were published (Grayver & Olsen, ; Guzavina et al, , ; Petereit et al, , ; Šachl et al, ; Schnepf et al, ; Saynisch et al, ; Velimsky et al, ). EMOTS are used to infer information about the Earth's lithosphere and mantle (Guzavina et al, ; Grayver et al, ; Kuvshinov et al, ; Schnepf et al, ).…”