River bulge evolution and dynamics in a non-tidal sea – Daugava River plume in the Gulf of Riga, Baltic Sea
Satellite remote sensing imagery and numerical modelling were used for the study of river bulge evolution and dynamics in a non-tidal sea, the Gulf of Riga (GoR) in the Baltic Sea. Total suspended matter (TSM) images showed a clearly formed anti-cyclonically rotating river bulge from Daugava River d...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-03-01
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| Series: | Ocean Science |
| Online Access: | http://www.ocean-sci.net/12/417/2016/os-12-417-2016.pdf |
| Summary: | Satellite remote sensing imagery and numerical modelling were used for the
study of river bulge evolution and dynamics in a non-tidal sea, the Gulf of
Riga (GoR) in the Baltic Sea. Total suspended matter (TSM) images showed a
clearly formed anti-cyclonically rotating river bulge from Daugava River
discharge during the studied low wind period. In about 7–8 days the bulge
grew up to 20 km in diameter, before being diluted. A high-resolution
(horizontal grid step of 125 m) General Estuarine Transport Model (GETM) was
used for detailed description of the development of the river plume in the
southern GoR over the period when satellite images were acquired. In the
model simulation, the bulge growth rate was estimated as <i>r</i><sub>b</sub> ∼ <i>t</i><sup>0.5 ± 0.04</sup> (<i>R</i><sup>2</sup> = 0.90). Both the model simulation and the satellite
images showed that river water was mainly contained in the bulge and there
were numerous intrusions at the outer perimeter of the bulge. We performed
numerical sensitivity tests with actual bathymetry and measured river runoff
without wind forcing (1) having an initial three-dimensional density
distribution, and (2) using initially a homogeneous ambient density field. In
the first case, the anti-cyclonic bulge did not develop within the course of
the model simulation and the coastal current was kept offshore due to ambient
density-driven circulation. In the second case, the river plume developed
steadily into an anti-cyclonically recirculating bulge, with <i>r</i><sub>b</sub> ∼ <i>t</i><sup>0.28 ± 0.01</sup> (<i>R</i><sup>2</sup> = 0.98), and a coastal current. Additional
simulations with constant cross-shore and alongshore winds showed a
significant effect of the wind in the evolution of the river bulge, even if
the wind speed was moderate (3–4 m s<sup>−1</sup>). While previous studies
conclude that the mid-field bulge region is governed by a balance between
centrifugal, Coriolis and pressure gradient terms, our study showed that
geostrophic balance is valid for the entire mid-field of the bulge, except
during the 1–1.5 rotation period at the beginning of the bulge formation. In
addition, while there is discharge into the homogenous GoR in the case of a
high inflow Rossby number, the river inflow might split into two jets, with
strong mixing zone in-between, in the plume near-field region. |
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| ISSN: | 1812-0784 1812-0792 |