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11.
Synoptic scale variability of the Southern Ocean wind field in the high-frequency range of barotropic Rossby waves results
in transport variations of the Antarctic Circumpolar Current (ACC), which are highly coherent with the bottom pressure field
all around the Antarctic continent. The coherence pattern, in contrast to the steady state ACC, is steered by the geostrophic
f/h contours passing through Drake Passage and circling closely around the continent. At lower frequencies, with interannual
and decadal periods, the correlation with the bottom pressure continues, but baroclinic processes gain importance. For periods
exceeding a few years, variations of the ACC transport are in geostrophic balance with the pressure field associated with
the baroclinic potential energy stored in the stratification, whereas bottom pressure plays a minor role. The low-frequency
variability of the ACC transport is correlated with the baroclinic state variable in the entire Southern Ocean, mediated by
baroclinic topographic–planetary Rossby waves that are not bound to f/h contours. To clarify the processes of wave dynamics and pattern correlation, we apply a circulation model with simplified
physics (the barotropic–baroclinic-interaction model BARBI) and use two types of wind forcing: the National Centers for Environmental
Prediction (NCEP) wind field with integrations spanning three decades and an artificial wind field constructed from the first
three empirical orthogonal functions of NCEP combined with a temporal variability according to an autoregressive process.
Experiments with this Southern Annular Mode type forcing have been performed for 1,800 years. We analyze the spin-up, trends,
and variability of the model runs. Particular emphasis is placed on coherence and correlation patterns between the ACC transport,
the wind forcing, the bottom pressure field and the pressure associated with the baroclinic potential energy. A stochastic
dynamical model is developed that describes the dominant barotropic and baroclinic processes and represents the spectral properties
for a wide range of frequencies, from monthly periods to hundreds of years. 相似文献
12.
Modeling the impact of wind and waves on suspended particulate matter fluxes in the East Frisian Wadden Sea (southern North Sea) 总被引:3,自引:1,他引:2
Suspended particulate matter (SPM) fluxes and dynamics are investigated in the East Frisian Wadden Sea using a coupled modeling
system based on a hydrodynamical model [the General Estuarine Transport Model (GETM)], a third-generation wave model [Simulating
Waves Nearshore (SWAN)], and a SPM module attached to GETM. Sedimentological observations document that, over longer time
periods, finer sediment fractions disappear from the Wadden Sea Region. In order to understand this phenomenon, a series of
numerical scenarios were formulated to discriminate possible influences such as tidal currents, wind-enhanced currents, and
wind-generated surface waves. Starting with a simple tidal forcing, the considered scenarios are designed to increase the
realism step by step to include moderate and strong winds and waves and, finally, to encompass the full effects of one of
the strongest storm surges affecting the region in the last hundred years (Storm Britta in November 2006). The results presented here indicate that moderate weather conditions with wind speeds up to 7.5 m/s and
small waves lead to a net import of SPM into the East Frisian Wadden Sea. Waves play only a negligible role during these conditions.
However, for stronger wind conditions with speeds above 13 m/s, wind-generated surface waves have a significant impact on
SPM dynamics. Under storm conditions, the numerical results demonstrate that sediments are eroded in front of the barrier
islands by enhanced wave action and are transported into the back-barrier basins by the currents. Furthermore, sediment erosion
due to waves is significantly enhanced on the tidal flats. Finally, fine sediments are flushed out of the tidal basins due
to the combined effect of strong erosion by wind-generated waves and a longer residence time in the water column because of
their smaller settling velocities compared to coarser sediments.
相似文献
Karsten A. LettmannEmail: |