Gyres, jets and waves in the Earth’s core

Turbulent motions of liquid metal in Earth’s outer core generate the geomagnetic field. Magnetic field observations from low-Earth-orbit satellites, together with advanced numerical simulations, indicate that present-day core motions are dominated by a planetary-scale gyre, a jet in the northern polar region and waves involving the magnetic field. In this Review, we explore the dynamics of core gyres, jets and waves and discuss their impact on the magnetism and rotation of the Earth. The planetary gyre is anticyclonic, offset from the rotation axis towards low latitudes under the Atlantic hemisphere and involves flow speeds of 15–50 km yr−1 that are fastest in a focused westward jet under the Bering Strait. A quasi-geostrophic, Magnetic–Archimedes–Coriolis force balance is thought to control the dynamics of the planetary gyre and high latitude jet. Waves in the core flow with periods ~7 years have been detected at low latitudes, that are consistent with an interplay among magnetic, Coriolis and inertial effects. The arrival of wave energy at the core surface accounts for many of the characteristics of interannual geomagnetic field variations. Fluctuations in outer core flow patterns, including the planetary gyre, account for decadal changes in Earth’s length of day, while interannual changes are well explained by wave processes. Systematic investigations of core–mantle coupling mechanisms in models that include wave dynamics promise new insights on poorly constrained physical properties, including deep mantle conductivity. Long-term satellite monitoring of changes in the Earth’s magnetic field is essential if further progress is to be made in understanding core dynamics, as the high-resolution magnetic record remains short compared with the timescales of waves and convection in the core.