The variation of climate sensitive tidal ocean-dynamo signals on sub-decadal and seasonal time scales

Abstract

Motional induction describes the induction of electric currents through charged particles moving perpendicular to an ambient magnetic field. A well-known device that uses motional induction to induce electric currents is the bicycle dynamo. The induction of electric currents in the ocean due to the motion of saltwater within the ambient geomagnetic field is, by contrast, lesser-known; this phenomenon is called the ocean-dynamo effect which indicates the similarity of both phenomena. The electromagnetic field signals emitted by ocean-dynamo induced electric currents are primarily sensitive to three factors: 1. the number of moving charged particles, 2. the magnetic field strength of the ambient field, and 3. the velocity with which the particles move perpendicular to said magnetic field.

The amount of electrically charged particles in the seawater, a saline solution, is measured with the electrical seawater conductivity σ. σ is determined by the saline solution's chemical equilibrium, which in return is predominantly defined by the physical properties of seawater temperature and salinity. Thus, changes in the spatial distribution of seawater temperature and salinity cause changes in the spatial distribution of electrical seawater conductivity, which in return affect the ocean-dynamo signals. In theory, ocean-dynamo signals are therefore suitable for ocean observation applications.

Out of all ocean-induced electromagnetic signals, signals induced by ocean tides play a unique role. The signatures of the periodic tidal flow are the only ocean-dynamo signals that have been successfully observed in magnetometer observations, space-borne and land-based. In addition to the proven measurability, the signals are also modelled with sufficient accuracy so that, on a global scale, observed tidal ocean-dynamo signatures agree well with model predictions. These two preconditions allow for an investigation of the relationship between ocean dynamics and tidal ocean-dynamo signals, a much-needed advancement towards practical ocean observation applications. In the past, sensitivity studies of tidal ocean-dynamo signals have focused mainly on changes on long time scales. By contrast, the present cumulative thesis examines the influence of ocean dynamics on tidal ocean-dynamo signals on short and intermediate time scales. In particular, it investigates the mechanisms and effects of ocean dynamics and recent seawater temperature and salinity changes on tidal ocean-dynamo signals. Furthermore, it investigates the detectability and measurability of short-term variations of said signals in magnetometer observations.

Out of the presented three research studies, the first is a model-based characterization of tidal ocean-dynamo amplitude variations attributed to the El Niño/Southern Oscillation (ENSO). The study shows that tidal ocean-dynamo signal changes precede the onset of warm and cold ENSO phases and attributes these findings to the underlying oceanic processes. Furthermore, the study provides an assessment of the measurability of ENSO-induced tidal ocean-dynamo amplitude variations. The second study covers a time series analysis of modeled tidal ocean-dynamo amplitudes on a global scale. Here, the amplitudes were modeled based on existing oceanic seawater temperature and salinity observations. Based on the analysis of the underlying in-situ data, the study assesses recent developments in signal amplitudes to resolve a conflict between existing model-based sensitivity studies. Furthermore, the study identifies the heightened sensitivity of coastal tidal ocean-dynamo signals and provides a physical explanation for this fact. The third study focuses on local ocean phenomena and analyses time series of coastal island magnetometer observations. It presents evidence for seasonal amplitude variations and trends in amplitudes and phases of tidal ocean-dynamo signals.

The advancements in the field contribute to the transition from retrospective or model-based analysis to an actual inference of the oceanic temperature and salinity dynamics from magnetometer observations.