Blue-shifted Deep Ocean Currents in the Equatorial Indian Ocean

Spectra from two decades of zonal current data at ∼ 4000 m in the central and western equatorial Indian Ocean show a shift in the dominant frequencies from the west to the east. The 120–180-day period is stronger at 77ºE , the 60–120-day period at 83ºE, and the 30–90-day period at 93ºE. The weakening of lower frequencies near the eastern boundary can be explained using theoretical ray paths of Kelvin waves and reflected Rossby waves. The equatorial Kelvin wave forced by winds reflects from the eastern boundary as Rossby waves with different meridional modes. After reflection, the low (high) frequency Rossby beams travel a larger (shorter) distance before reaching the bottom, thereby creating a shadow zone, a region with low wave energy, between the ray path and the eastern boundary. The shift in frequency with longitude is not evident in the top 1000 m, where the current is dominated by the semi-annual cycle.

Generation of Hydroacoustic Waves by an Oscillating Ice Block in Arctic Zones

The time harmonic problem of propagating hydroacoustic waves generated in the ocean by a vertically oscillating ice block in arctic zones is discussed. The generated acoustic modes can result in orbital displacements of fluid parcels sufficiently high that may contribute to deep ocean currents and circulation. This mechanism adds to current efforts for explaining ocean circulation from a snowball earth Neoproterozoic Era to greenhouse earth arctic conditions and raises a challenge as the extent of ice blocks shrinks towards an ice-free sea. Surprisingly, unlike the free-surface setting, here it is found that the higher acoustic modes exhibit a larger contribution.

Retention of ice-associated amphipods: possible consequences for an ice-free Arctic Ocean

Recent studies predict that the Arctic Ocean will have ice-free summers within the next 30 years. This poses a significant challenge for the marine organisms associated with the Arctic sea ice, such as marine mammals and, not least, the ice-associated crustaceans generally considered to spend their entire life on the underside of the Arctic sea ice. Based upon unique samples collected within the Arctic Ocean during the polar night, we provide a new conceptual understanding of an intimate connection between these under-ice crustaceans and the deep Arctic Ocean currents. We suggest that downwards vertical migrations, followed by polewards transport in deep ocean currents, are an adaptive trait of ice fauna that both increases survival during ice-free periods of the year and enables re-colonization of sea ice when they ascend within the Arctic Ocean. From an evolutionary perspective, this may have been an adaptation allowing success in a seasonally ice-covered Arctic. Our findings may ultimately change the perception of ice fauna as a biota imminently threatened by the predicted disappearance of perennial sea ice.