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RESEARCH

Multi-platform, multi-disciplinary approach to study island wakes

Madeira bathymedry 3D
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 !

Madeira numerical wakes modeling 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 + ...).

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).
 
Madeira atmospheric wakesBarkley (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.

sat images wakes 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 + ...) lab studies images
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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