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Multi-platform,
multi-disciplinary approach to study island wakes
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The
bathymetric configuration of Madeira Archipelago is complex. There are
three main island groups (left-right) 'Porto Santo', 'Madeira and
Desertas', and 'Selvagens' (cone-like islands, furthest to the right).
A shallow ridge connects Madeira to Desertas Islands. Depths drop very
abruptly after the small island's shelf reaching 4000m in the deepest
surroundings. A typical deep-sea island ! |
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Idealized
numerical modeling studies will help determine the 'continuum' of
regimes under which island wakes are formed. A methodology similar to
that discussed by Dong et al (2007), was used to study the numerical
regimes of the Madeira Island case. In geophysical island wake studies,
the relative importance of stratification and planetary rotation must
be considered. In that regard, dimensional parameters such as Reynolds
(Re), Burger (Bu) and Rossby (Ro) numbers, play a vital role in
positioning the various wake scenarios in a continuum of regimes. For
instance, in Dong et al (2007), small values of Ro induced symmetric
anticyclonic and cyclonic eddies. Nevertheless, in the Madeira Island
case, the 'obstacle' (Madeira) is asymmetric, and small Ro values are
not expected to induced symmetry in the wake formation. Furthermore,
the presence of the SE Desertas Islands play a significant role in the
eddy shedding. The figure (left) shows the Sea Surface Temperature
(SST) field after a 150 days, Regional
Ocean Modeling System (ROMS) study. Warmer colors (reds')
represent higher temperatures, whereas colder colors (blues') colder
temperatures. It is clearly visible the lack of symmetry between
cyclonic and anticyclonic eddies as well as the influence of the small
islands (Desertas) in the Madeira leeward wake. (more + ...).
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A
collaborative effort between members of this project, the physical
oceanographic group at the University of Las Palmas, Gran Canarias
(ULPGC) lead by Pablo Sangrá, and the UCLA research group
lead by James McWilliams, will also result in a comparative
(semi-realistic) numerical study of island wakes of all the
Macaronesian Archipelagos (Azores, Madeira and Canaries).
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 Barkley
(1972) compared
Johnston Atoll’s deep-ocean wake, with the atmospheric wake
system observed leeward of Madeira Island. The two were comparable in
size and produced quite similar wakes since the current flow field of
the North Equatorial Pacific and the Trade Winds over the North
Atlantic Ocean produced similar Reynold’s number (Re = 90);
the higher atmospheric eddy viscosity compensated the much higher wind
velocity. This similarity suggested that the lengths of both wakes were
on the order of 600 km offshore. Barkley also predicted that it would
take about 7.2 h for the atmospheric wake to be formed off Madeira
Island. Nevertheless, the relationship between atmospheric and oceanic
wakes leeward of Madeira and other islands, is not yet fully
understood. (more + ...)
Scorer (1986) used the images of Guadeloupe and Madeira Island
atmospheric wakes, to illustrate the formation of Von Karman Vortex
Streets in his book on cloud investigation.
The 'biological pumps' that often result from the eddy activity in the
oceanic wakes of these islands, leave a very distinguishable signature
in optical-based, satellite sensors. Various physical processes such as
filaments, small-scale upwelling cells or three-dimensional
instabilities, enhance horizontal transport and vertical mixing of
nutrient-rich deep water, when an upper surface current encounters
oceanic islands. The results is an 'oasis' of life in the newly
fertilized, nutrient-rich, regions. These 'oasis' are often known, in
the scientific literature, as the 'island mass effect' phenomenon which
was focus of Caldeira et al (2002), study that lead to the formulation
of this project proposal. |
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The image (left)
shows a patch of sea surface chlorophyll acquired on the 28 August
2006, by MODIS (Moderate
Resolution Imaging spectroradiometer), which is a key instrument aboard
the Terra (EOS AM) and Aqua (EOS PM) satellites. Cyclonic eddy activity
often detected in the western flank of Madeira Island, induces frequent
upwelling of, nutrient rich, deep-water to the surface. Since at these
latitudes solar radiation is not a limiting factor, phytoplankton growth
follows these nutrient enrichment (oceanic) events, eventually leading
to built of a small 'eddy-ecosystem'. Birds, fish and marine mammals
are expected to take advantage of such a system. Current work, in
collaboration with the whale museum
through an INTERREG IIIB supported project EMECETUS,
will help start to understand the role of these oceanographic features
as 'generators' / 'aggregators' of marine life 'i.e. biological pumps' |
| In order to
expand our understanding of the Geophysical Fluid Dynamic aspects of
island wakes, members of this project, in collaboration with
international colleagues from France , Spain, Germany and the UK, were
awarded financial support to participated in an european integrated
infrastructure initiative: HYDRALAB III, in order to study a
'continuum of wake regimes' in a laboratory setting. The experiments
will take place during November 2008 in the rotating tank for
geophysical studies, located in Grenoble, France.
The 'Coriolis
rotating platform' due to its large size, provide access
to a range of Ro and Re numbers, for density stratified flows, with no
equivalent world wide. In other words, dynamical regimes with low
viscosity and high Coriolis effects can be reached, while the
centrifugal force remains negligible. Such regimes are directly
relevant for the study of oceanic mesoscale processes (1-100 km
horizontal scale). The high Re, high Ro island wake experiments will
also include studies around a Madeira Island-like obstacle. Results
will serve as benchmarks for numerical experiments as well as reference
to interpret observations. (more + ...) |
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References:
- Barkley, R. A. (1972). Johnston Atoll's
wake. Journal of Marine Research, 30: 201-216.
- Caldeira, R.M.A., S. Groom, P. Miller,
and N. Nezlin. (2002).
Sea-surface signatures of the island mass effect phenomena around
Madeira Island, Northeast Atlantic. Remote Sensing of the Environment,
80: 336-360.
- Dong, C., J.C. McWilliams, and A.
Shchepetkin. (2007). Island Wakes in Deep Water. Journal of Physical
Oceanography, 37: 962-981
- Scorer, R. S. (1986). Cloud investigation
by satellite. Ellis Horwood. Chichester.
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