SPRINGTIME ICE MOTION IN THE WESTERN ANTARCTIC PENINSULA REGION
Cathleen Geiger and Don Perovich
Abstract
Oscillatory motion of sea ice is examined using two ice
drifting buoys separated by one degree latitude near 66°S during the winter to
spring transition in the Marguerite Bay
region west of the Antarctic Peninsula. The buoys’
motion exhibits spectrally distinct periods (12.87 ± 0.04 and 13.03 ± 0.04
hours, respectively) despite highly correlated motion between them (r2 is 0.62 and 0.81 for u and v, respectively). The periods shift with latitude and nearly match
the local inertial periods (13.00 and 13.10, respectively). The oscillations
are further examined with respect to the kinematics involved in the breakup
process of sea ice. These include hourly resolved manifestations 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 the appearance of a non-linear cascade of energy within
the ice from the low frequencies (commensurate with storms and fortnightly
tides) to semi-diurnal frequencies. We further comment on the implications this type
of high-frequency motion has on local biological ecosystems. Specifically we
find that sea ice semi-diurnal oscillations are at their peak during the final
decay of sea ice just before springtime primary productivity begins. Hence 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 overwintering
within and around the ice floes.
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