Research
Posted by Zo
Welcome to my website, my name is Gonçalo and I am a PhD student working in the Environmental and Fluid Mechanics Laboratory (EFML) at Stanford University. This website contains information related to my research at Stanford. I am interested in learning more about physical processes in the coastal zone, in particular, I hope to understand how internal waves affect cross-shelf transport. The focus of my research is the transport of contaminants due to internal waves in Huntington Beach, California.
Education
Posted by Zo
- PhD Candidate, Environmental Fluid Mechanics, Stanford University 07-present
- M.Sc. Aero/Astronautical Engr., Stanford University
01-03
- B.Sc. Engr. Mechanics and Astronautics,
University of Wisconsin-Madison
98-01
- B.Sc. Aerospace Engr., Universidade Técnica de Lisboa 95-97
Teaching
Posted by Zo
Spring 2010
Teaching assistant for:
Atmosphere, Ocean, and Climate Dynamics: the Ocean Circulation EESS 146B/246B (visit course website)
Introduction to the physics governing the circulation of the atmosphere and ocean and their control on climate with emphasis on the large-scale ocean circulation. This course will give an overview of the structure and dynamics of the major ocean current systems that contribute to the meridional overturning circulation, the transport of heat, salt, and biogeochemical tracers, and the regulation of climate. Topics include the tropical ocean circulation, the wind-driven gyres and western boundary currents, the thermohaline circulation, the Antarctic Circumpolar Current, water mass formation, atmosphere-ocean coupling, and climate variability.
Instructor: Prof. Leif Thomas (visit homepage)
Internal Waves
Posted by Zo
Introduction
Internal waves are ubiquitous in the interior of the oceans and have been the focus of much attention in recent years due to their role in oceanic mixing and mass transport. In general, gravity waves can form at the interface between fluids of different densities, that is, the boundary separating the two fluid masses. Surface waves, propagate at the intersection of two fluids of different density, in this case, air and water. Very much like surface waves, internal waves appear as undulating forms travelling between two fluids of different density, where here, instead of air overlying water, we have less dense water overlying more dense water.
This particular arranjement can be achieved in a variety of ways, one is by having a layer of fresh water over a layer of salt water such as in a Fjord where the fresh snow water melts into the ocean. This is shown in the figure to the right depicting the Puget Sound Estuary.
Another density boundary propitious to the formation of internal waves occurs at the thermocline. If you are a diver you have certainly felt and maybe seen the thermocline which is characterized by an abrupt change in temperature over a fraction of the depth. The interface separates an overlying mass of warm water and an underlying mass of cool deep oceanic water, allowing for internal waves to be generated.
A sudden boundary is not obligatory for the formation of waves, a stably stratified configuration also allows for internal waves. Internal waves can arise in a variety of ways, by the action of moving weather fronts, tides, wind stress (surface waves), boats, currents moving over topography.
Gravity waves are driven by gravity in a restless attempt to bring a particle fluid that has been removed from it's equilibrium position back to rest. For this reason, the density difference between the two layers is related to the restoring force, in the general, the larger the density difference is, the stronger the restoring force is. Hence, since the density difference between air and water is several orders of magnitude larger than the density difference between layers in the ocean's interior, the restoring forces in internal waves are much smaller than restoring forces in surface waves. This smaller density difference translates to waves with much longer wave periods, lengths and amplitudes than surface waves (wave heights up to 20 meters or more). In addition, internal waves tend to contain relatively small energy as compared to surface waves and travel at much smaller speeds averaging 20 cm/s.
Transport
Non-linear effects associated with internal waves include the transport of water along with suspended mass such as sediment, nutrients, larvae, as well as contaminants. The details of such effects are crucial to the understanding of a wide array of physical situations where transport by internal waves is inherent. Internal tides have been shown to assume an important function in the transport of larvae and other organisms to the near shore and play a role in the development of the benthic communities [1]. Moreover, high frequency internal waves have been shown to influence the spatial distribution of plankton, effectively controlling nutrient dispersal in coastal regions [1].
The transport of contaminants by Internal waves at Huntington Beach is of particular interest as it can contribute to a significant decline in coastal water quality which in turn can directly affect human health. At Huntington Beach, there is high potential for internal tidal waves to advect waste water from the Orange County Sanitation District ocean outfall toward Huntington Beach and surrounding areas at both diurnal and semi-diurnal frequencies [2]. Resulting in a significant health risk for bathers and beach goers and challenging the efficacy of ocean outfalls. It highlights a need for a comprehensive understanding of the mechanics of internal tides and how they affect the water quality of coastal regions. At Huntington Beach it has been shown that lunar variability in coastal water quality is related to the nearshore transport of contaminants by tides [3]. Several factors influence this variability including the horizontal and vertical advection of offshore contaminants towards the shore due to internal tides [3].
References
[1] Leichter, J. J.; Wing, S. R.; Miller, S. L.; Denny, M. W., Pulsed Delivery of
Subthermocline Water to Conch Reef (Florida Keys) by Internal Tidal Bores.
Limnology and Oceanography 1996, 41, (7), 1490-1501.
[2] Boehm, A. B.; Sanders, B. F.; Winant, C. D., Cross-Shelf Transport at Huntington Beach. Implications for the Fate of Sewage Discharged through an Offshore Ocean Outfall. Environ. Sci. Technol. 2002, 36, 1899-1906.
[3] Boehm, A. B.; Grant, S. B.; Kim, J. H.; Mowbray, S. L.; McGee, C. D.; Clark, C. D.; Foley, D. M.; Wellman, D. E., Decadal and Shorter Period Variability of Surf Zone Water Quality at Huntington Beach, California. Environ. Sci. Technol. 2002, 36, (18), 3885-3892.
Contact Information
Posted by Zo
Department of Civil and Environmental Engineering
The Jerry Yang and Akiko Yamazaki Environment and Energy Building
473 Via Ortega, Office M25
Stanford, CA 94305
email: gilg{@}stanford.edu