Nick Drake

24-Apr-02

 

 

1. What is the average width of the Gulf Stream and how much does it vary over time?

 

The gulf stream is a body of warm water flowing from Florida / Caribbean origin, mixing as it encounters colder water from the East Coast of Northern USA and Canada. The process of warm and cold mixture produces pockets of warmer and colder water, and as the warmer water continues to move faster than colder water continually slowly down at boundary lines due to liquid thermofraction the Gulf Stream fans out or narrows according to the surrounding water temperature.

The slope water provides a natural thermal barrier to the Gulf Stream’s ability to progress Northwards. Despite the inability for the Gulf Stream to counter-affect the cooling of its water, from entering the coastal slope water (32.5Nlat-80Wlong) the Gulf Stream peels away from the coastline assuming after a distance further North of average 100-200 miles off-shore.

 

At entry, on average the Gulf Stream is around 450-500 miles across, it then spreads out, either creating warm core or cold core rings spreading the width from no less than 470 miles to up to 1000 miles across before it tapers to an end due to overcooling.

 

The following was created using an AVHRR image layered with a longitude and latitude grid. Width was calculated by measuring the approximate co-ordinates of the Gulf Stream and calculating using a formula (http://www.nau.edu/~cvm/latlongdist.html) the distance in latitude and longitude from significant widths in the Gulf Stream:

 

 

 

The width of the Gulf Stream is very dependant on the temperature of the surrounding waters, the Sargasso Sea which provides a cold flow of water from the North, the temperature of the Gulf Stream itself. It does not appear that the Gulf Stream is increasing over any yearly time scale, unlike the El Nino and La Nina phenomenon which themselves are more regulated by salinity than temperature.

 

What is the average size of warm core and cold core rings, how much do they vary in size and how persistent are they, and what direction do they flow in?

 

Gulf Stream rings up to 200 kilometres in diameter form as cold, cyclonic eddies south of the current, and warm, anticyclonic eddies routinely occur on the north side of this North Atlantic current.

(http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/shuttle_oceanography_web/oss_134.html)

 

The limits of variations in size of the warm and cold core rings is infinite. If there was still enough warm volumous water being fed in, followed by a continual current from the North, a ring could grow to any size. However due to fluid frictional dynamics, supply and seasonal temperature variation, the size of warm and cold core rings can range from meters to many hundreds of miles across.

 

Cold and Warm core rings form regularly on average ring generation between 11 and 14 months time period is common, with lifespan dependent on fluid dynamics, temperature, volume and turbulence.

 

The warm and cold core rings form once the ocean water has enough geographical area to disperse, from 35N lat., 75W long. A cold core ring which forms as cold water spins in on itself in a counter-clockwise direction, delivering cold water to the Sargasso Sea. A warm core ring will spin clockwise, mixing with cold slope water and delivering warmer water to the coast of Northern USA. Both core rings move in the general direction of the Gulf Stream - Westward, typically moving 0.5-2.5 miles a day for a period of a few months after which the ring looses it’s dominant temperature, the spin reduces and the ring breaks up.

Cyclonic Gulf Stream rings have been tracked by ships, aircraft, satellites, and drifting buoys for as long as three years. Most, though, have a much shorter life cycle.

 

 

What is the average location of the Gulf Stream and how much does it vary over time?

 

The average location of the Gulf Stream is:

85°W to 55°W, 20°N to 45°N

(http://users.erols.com/gulfstrm/)

 

Exhaust Turbulence stream

The above picture is to illustrate the process of turbulence, laminar flows and diffusion of particle or liquid physics.

 

Turbulence is an important concept when determining the size of the Gulf Stream as the size of the Gulf Stream is dependant on two things:

 

1)     Temperature which defines the operation of…

2)     Turbulence which determines the position of and location of the Gulf Stream

 

The study of turbulence is a major component of the larger field of fluid dynamics, which deals with the motion of all liquids and gases. Similarly, the application of powerful computers to simulate and study fluid flows that happen to be turbulent is a large part of the burgeoning field of computational fluid dynamics (CFD).

(http://www.sciam.com/0197issue/0197moin.html)

 

What are limitations of remote sensing the oceans?

 

The limitations of satellite altimetry are as follows:

a)      Accuracy of Instruments – A satellite on average is around 1000km above the Earth, for accurate measurements, we should be achieving an accuracy of around 1cm. This means that a satellite has to measure at an accuracy level of 1 part to 100,000,000. Therefore we need to know the exact travelling radius of the satellite, before we can accurately calculate the distance to the surface.

b)      Shape of Earth and Ocean surface – The Earth is not elliptical, if it was we would be able to calculate the sea levels on the ground and subtract them from whatever values we have before and after an event. A resting ocean surface produces a Geoid which is a skin for the oceans IF ocean circulation did not take place. We can still however model the Geoid but use it apprehensively due to obvious inaccuracy's.

c)      Gravitational field variations – Earth is not a solid resting body, it is not spherical exactly, but it is the underlying dynamics of internal structure that give us a complicated varying value for gravity. This variance in gravitational field could result in an inaccuracy of vertically 100m.

d)      Atmospheric interference – Satellite measurements are distorted by the actions of the atmosphere which lie in-between the oceans and the satellite. Distortion of results can be from most frequently water vapour in the air, sunlight reflection below or above expected, atmospheric particulates blocking or distorting the reflected radar beam and how much atmospheric drag it encounters

e)      Coriolis force – A very important part of the Ocean circulation systems is the Coriolis force, as we use mathematical modelling we cannot be achieving 100% accuracy and so the affect of this upon the Gulf Stream affects our modelling of other sea features such as Warm core ring sizes, force and volumes.

 

References:

 

Websites

http://dcz.gso.uri.edu/amy/avhrr.html

http://www.sciam.com/0197issue/0197moin.html

http://users.erols.com/gulfstrm/

http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/OCDST/shuttle_oceanography_web/oss_134.html

http://fermi.jhuapl.edu/avhrr/

http://www.nau.edu/~cvm/latlongdist.html

Class Notes