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The North Pacific Gyre

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The North Pacific Gyre The North Pacific Gyre New Mexico Supercomputing Challenge Final Report April 6, 2011 Team 36 Desert Academy Team Members Sean Colin-Ellerin Sara Hartse Mentors John Paterson Jocelyne Comstock Contents 1. Executive Summary………………………………………………………… 3 2. Introduction ………………………………………………………………... 4 2.1 Goal 2.2 Background 2.2.1 Oceanography 2.2.2 Plastics 2.2.3 Significance 3. Model ……………………………………………………………………….. 11 3.1 Overview 3.2 Ocean Circulation 3.2.1 Geometry 3.2.2 Circulation and Conservation 3.3 Cities and Population 3.3.1 Mathematical Model and Data 3.4 Waste Propagation 3.4.1 Mathematical Model and Data 4. Data and Results…………………………………………………………… 17 4.1 Two Forms of Visualization 4.2 Screenshots of Model 4.3 Contour Maps of Model 5. Analysis…………………………………………………………………….. 28 5.1 Screenshots 2 5.2 Contour Maps 6. Conclusions………………………………………………………………….. 31 Appendix A: Works Cited and Acknowledgements Appendix B: Netlogo Code 3 1. Executive Summary Our project is focused on the phenomenon of garbage patches, particularly in the North Pacific Ocean, which form when municipal solid waste (referred to as MSW in general terms, trash, when applied to water agents and garbage, when applied to cities), particularly plastic, is transported into the center of oceanic gyres. The majority of the programming was done using Netlogo, with other work done in Matlab and Microsoft Excel. The simulation of ocean movement and MSW transport was completed using an agent based method; we succeeded in creating an advection diffusion simulation which takes place in an approximation of the North Pacific Basin geometry. The advection is driven by established ocean currents and the diffusion takes place as MSW is released into the ocean by specific cities on the Pacific Rim. These parameters resulted, as we had hoped, in the formation of highly concentrated areas of MSW in the center of the North Pacific Ocean. Various forms of data were collected from tests of the model, these include screenshots of the interface at certain timesteps and the contour mapping of MSW concentration data generated by agents over a time step of ten years. From this model, we were also able to gather information about the behavior of particles once they enter the North Pacific Gyre. We were also able to conclude that, based on model parameters such as current speed and location, population growth, diffusion rates and MSW production rates; the North Pacific Garbage Patch is a real and growing feature of the Pacific Ocean. 4 2. Introduction 2.1 Goal The goal of this project is to create an ocean circulation model which can accurately simulate the characteristics of the North Pacific Gyre based on ocean surface currents and can model the formation of a garbage patch. Once the gyre is modeled, the formation of the garbage patch will be dependent on the size and source of the garbage inflow. We wish to be able to predict the growth of the garbage patch over the next decades, based on predicted population growth rates for several major cities on the Pacific Rim. The garbage inflow will also be based on the approximated MSW production rates per person of these cities and a general likelihood of MSW making it into the ocean. We hope that by studying the garbage patch in the North Pacific Gyre we will be able to model the growth of the patch and begin to understand the problem of ocean pollution as a whole. 2.2 Background 2.2.1 Oceanography The Pacific is the largest and the oldest of earth’s oceans; being the single ocean which surrounded the continent of Pangaea millions of years ago. The Pacific Ocean is now home to several garbage patches, which are located in the rotating expanses of ocean known as gyres. The largest and most well known garbage patch is located in the North Pacific gyre and is the subject of our project. 5 The most important subject in creating a model of the garbage patch is being able to accurately simulate the movement of the ocean. The planet’s oceans are continuously moving and their motion can be attributed to several factors. Ocean tides are controlled by the gravitational pull of astronomical bodies like the sun and moon. Because the earth is a sphere, the effect of these gravitation pulls varies as the circumference changes between locations. At Earth’s equator and assuming the ocean was limitless in depth and extension, the speed of tidal propagation or tidal celerity, could be up to 1600 km/h. However, the oceans are broken by continents and they have finite depths. The speed of water is very dependent on its depth, moving a much Fig. 1 The Coriolis Effect greater rate in deeper water. The average depth of global oceans is about 4 km, and taking into account the factors of landmass, longitude and depth, the tidal celerity at the equator is actually about 5.08 m/sec (12). Ocean circulation as a whole is caused by many different factors. In addition to astronomical tides mentioned above, it is determined by surface currents, which are affected by occurrences such as evaporation, precipitation, temperature, salinity, density and kinetic energy applied by wind. Another important force to be taken into account in computing ocean circulation is the Coriolis Effect, which results in what is known as the Ekman transport system. The Coriolis Effect exists because the earth is a sphere and its continuous rotation means that objects are deflected (to the right in the Northern 6 Hemisphere and to the left in the Southern Hemisphere) from their original paths (6). This applies to wind, the primary driver of surface currents. Wind blows across water and causes it to move as a result of the frictional drag on the surface. In ideal conditions, a wind blowing at an average rate for twelve hours produces a current about 2% of its velocity. If it were not for the Coriolis Effect, the wind would push the water in a single direction. Each successive layer of water would follow the path of the one above it, although at a slower rate. However, because of the Coriolis Effect, each layer of water is deflected slightly, the direction depending on which hemisphere it is in. In ideal conditions, wind driven currents would vary 45º in each successive layer (see Figure 1.) (12). Surface currents are one of the most important ocean characteristics to account for in our gyre model. A gyre is a large, circular system of rotating ocean waters, determined by long term behavior of surface currents; they are essentially eddies (12,13). There are four primary oceanic gyres which are located in the North Pacific, the South Pacific, the North Atlantic and the South Atlantic oceans. The physical characteristics, like the surface currents of the gyres, vary, but they all share their becalmed central section of water that has historically been avoided by both humans and many animals. The slow moving waters make sailing in gyres very difficult and the desert-like ecosystem makes them poor habitats for many predators and large fish (3). There are four main currents forming the North Pacific Gyre that dictate its size and location. The North Equatorial Current runs parallel to the equator, approximately in the area of the northeast trade winds at speeds of about 0.5 km/h. It then passes the Philippines and Taiwan, curving northwest. As it continues north it becomes part of the 7 Kuroshio Current, sometimes known as the Japan Current, which widens and slows as it passes Japan. The Kuroshio Current is the fastest current in the gyre; its average velocity is 3 to 4 km/h. It behaves in approximately the same way as the Gulf Stream in the Fig. 2 Gyre with location of North Pacific Subtropical High indicated Atlantic; transporting large amounts of warm, tropical water to higher latitudes. As it reaches increasingly arctic latitudes, the water in the Kuroshio Current cools, widens, slows and curves eastward, going between the Aleutians and Hawaiian Islands, where it has now become the North Pacific Current. As it approaches the North American continent, it then becomes the California Current. The California Current moves at a rate of around 1 km/h and is generally quite wide and slow and near the southern end of Baja California it curves to the west, rejoining the North Equatorial Current and completing the general clockwise rotation of the North Pacific Ocean (2). 8 The exact size and location of gyres are indeterminable due to the fact that they are constantly changing based on pressure and currents. Nevertheless, oceanographers can approximate the gyres’ properties by their driving currents. The North Pacific gyre is estimated to cover about 17 million square kilometers. The general location of this gyre is in the Northern Pacific Ocean and the concentration of the garbage patch is roughly pinpointed halfway between the western coast of the mainland of the United States of America and Hawaii. The oceanic gyres all possess garbage patches of their own, but this particular garbage patch is the best documented and is believed to have the greatest amount of waste. Most of this waste is believed to gather in a high pressure, sub-tropical convergence zone, located in the central to eastern portion of the gyre (8). 2.2.2 Plastics It is estimated that a large country such as the United States generates more than 400 million tons of trash each year. Some of this trash is recyclable, and some biodegrades, but most of it simply remains for hundreds of years with nowhere to go. The definition of biodegradability is a substance that can decompose without causing harm to its environment, which is why plastic is considered non-biodegradable (7).
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