The dynamics of sea ice spring melt: Ramifications for high latitude marine ecosystems

Oscillatory motion of sea ice nearly matching the local inertial period (i.e., near-inertial) is examined using two ice drifting buoys separated by one degree latitude around 66S in the Marguerite Bay region west of the Antarctic Peninsula. The oscillations are examined with respect to the kinematics involved in the breakup process of sea ice. These include hourly resolved manifestation of circular trajectories, semi-circular oscillations with compressed trajectory cusps, and "accordion-like" compressions along straight line trajectories. Oscillations are found in all trajectory types over the lifetime of both buoys (several months). Traditional circular and semi-circular oscillations are particularly prominent during two episodes, one of which is preceded by strong wind events and a substantial decrease in ice thickness and concentration. These episodes combine with seasonally warming temperatures to break up and melt the sea ice cover. We discuss potential relationships between the degradation of the ice pack during spring breakup and the increase in energy at near-inertial frequencies including a non-linear cascade of energy within the ice from the low frequencies (commensurate with storms and fortnightly tides) to semi-diurnal frequencies. We find these near- inertial oscillations are at their peak during the final decay of sea ice when springtime primary productivity begins. We further comment on the implications this type of high-frequency motion has on local biological ecosystems. Specifically, the oscillatory motion of sea ice not only serves as an effective mixing agent within the ice-ocean mixed layer, but also serves as an effective seeding platform for distributing phyto- and zooplankton that have over-wintered within and around the ice floes. This type of bio-physical coupling is very important from the modeling perspective of bio-physical processes and biomass productivity. These processes are currently in a dynamic-thermodynamic balance with air temperatures, light, nutrients, and the frequency of spring storms. Alternations of the climate may resort in differing phase shifts of the ice melt onset and storm frequency thereby creating new stresses to the local ecosystem.