Generation of Rossby waves off the Cape Verde Peninsula: the role of the coastline
<p>In December 2002 and January 2003 satellite observations of chlorophyll showed a strong coastal signal along the west African coast between <span class="inline-formula">10</span> and <span class="inline-formula">22<sup>∘</sup></span>&a...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-12-01
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| Series: | Ocean Science |
| Online Access: | https://www.ocean-sci.net/15/1667/2019/os-15-1667-2019.pdf |
| Summary: | <p>In December 2002 and January 2003 satellite observations
of chlorophyll showed a strong coastal signal along the west African coast
between <span class="inline-formula">10</span> and <span class="inline-formula">22<sup>∘</sup></span> N. In addition,
a wavelike pattern with a wavelength of about 750 km
was observed from 20 December 2002 and was detectable
for 1 month in the open sea, south-west of the Cape Verde Peninsula.
Such a pattern suggests the existence of a locally generated
Rossby wave which slowly propagated westward during this period. This hypothesis
was confirmed by analysing sea surface height provided by satellite altimeter during
this period. To decipher the mechanisms at play, a numerical study
based on a reduced-gravity shallow-water model has first been conducted. A wind burst,
broadly extending over the region where the offshore oceanic signal is observed,
is applied for 5 d. A Kelvin wave quickly develops along the northern edge
of the cape, then propagates and leaves the area in a few days.
Simultaneously, a Rossby wave whose characteristics seem similar
to the observed pattern forms and slowly propagates westward.
The existence of the peninsula limits the extent of the wave
to the north. The spatial extent of the wind burst
determines the extent of the response and correspondingly the timescale
of the phenomenon (about 100 d in the present case). When the wind burst
has a large zonal and small meridional extent, the behaviour
of a wave to the north of the peninsula differs
from that to the south.
These results are corroborated and completed by an analytical study of a linear
reduced-gravity model using a non-Cartesian coordinate system. This system is
introduced to evaluate the potential impact of the coastline shape.
The analytical computations confirm that a period of around 100 d
can be associated with the observed wave considering the value of the wavelength;
they also show that the role of the coastline remains moderate at such timescales.
By contrast, when the period becomes shorter (smaller than 20–30 d),
the behaviour of the waves is modified because of the shape of the coast.
South of the peninsula, a narrow band of sea isolated from the rest
of the ocean by two critical lines appears. Its meridional extent is about 100 km
and Rossby waves could propagate there towards the coast.</p> |
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| ISSN: | 1812-0784 1812-0792 |