Ronan Thomas Fluid Earth 4/28/08 Hw #4 1A-B) on another sheet C.) the North Atlantic subtropical gyre is driven by the wind. The important theory to understand behind this phenomenon is Ekman theory. He proposed that Net water movement is at 90 degrees to the right to the direction of the wind force acting on the water, and that the main surface current is at 45 degrees to the wind force. This net direction of water 90 degrees to the right (right in northern hemisphere due to Coriolis effect, and to the left in southern hemisphere due to the Coriolis effect there) of the wind force is compiled by something known as the Ekman spiral, which is a mass of water that goes down to approximately 100 meters that with each layer farther down rotates slightly more to the right of the layer above it and due to loss of energy from friction also moves relatively slower than the layer above it. When all these vectors of water transport are added a resulting net transport of water is to the right of the acting wind force. This is one key part that combines to form the North Atlantic subtropical gyre, I will discuss more thoroughly later the effect that this net movement of water has and how it affects the Gyre as a whole. Now that Ekman theory is laid out it must be applied to the North Atlantic. The trade winds which blow at 15 degrees North latitude move from East to West and power the North Equatorial Current. Trade winds move from a general East to West direction with flow towards the Equator. Thus the main surface current flows nearly perfectly west, due to Coriolis Effect as stated by Ekman. Now as this North Equatorial Current encounters the western boundary of the Atlantic which is made up of North American Continent the Flow of the water is deflected northward in the Golf Stream, which brings warm water north from the warm waters of the Caribbean, which is responsible for the temperate climate of Western Europe. As the Gulf Stream moves out of the Caribbean it begins moving out into the Atlantic away from the Coast of North America off the coast of North Carolina and continues to move up the western boundary of the Atlantic basin until about 45 degrees north latitude where winds known as the Westerlies, which blow from the West to the East with partial flow to the north, change the flow of the Sub-tropical Gyre with its northern boundary current, known as the North Atlantic Current. As the North Atlantic Current moves across the Atlantic it loses cohesiveness of the Gulf Stream and widens out and travels eastward due to the combined effects of the Westerlies and the Coriolis Effect. When the North Atlantic Current reaches the western boundary of Europe it flows southward and becomes what is known as the Canary Current, which brings down the cooler water from around Europe southward to the coast of Africa where it is reheated and begins its journey again in the North Equatorial Current. The Canary current differs from the Gulf Stream because it is much wider and less strong in its flow, where the Gulf Stream is very cohesive and has a much larger flow relative to the Canary current per unit area of ocean. These four currents and the wind action that helps to make them up are what drive the North Atlantic subtropical gyre. These four currents form a circular flow around the Atlantic basin and in so doing create the gyre, which due to the flow of the currents creates a bulge in the center of the Atlantic due to the net water movement caused by the winds. This bulge is kept up due to the negating effect of gravity which tries to pull down the water, and the Coriolis effect which
pushes it up. This bulge is also the location of Ekman Pumping which is caused by the confluence of the net movement of the water from all around the Atlantic ocean boundaries. This water is pushed together raised into the hill which pushes back against the outward currents which helps to keep the currents from spiraling inward, and then the sinks water from the surface down to the lower layers of the ocean. d.) The Caribbean presents interesting aspects that can't be explained with this explanation with its bubbles of higher sea surface levels caused by warmer water that breaks off of the Gulf Stream. The North Atlantic Ocean Circulation is only being explained by surface movement of water and currents. This only effects a very small portion of the ocean's water. One needs to study deep ocean currents as well to fully understand the Atlantic Ocean's real circulation. Deep ocean circulation is affected by salinity and temperature gradients that cause larger scale water movement. 2.) There is a large concentration of chlorophyll along the equator in the Pacific due to something called Ekman Suction. This is caused because of the location of the Trade winds which blow from West to East on both sides of the equator. What is important about this is the Coriolis Effect, which in the Northern Hemisphere moves at 90 degrees to the right of any moving mass and in the Southern Hemisphere This direction is 90 degrees to the left of any moving mass. This causes Ekman Suction because this causes water on both sides of the equator to move away from the center of the equator which drops the height level of the ocean a bit and causes water to rise from the deep ocean. This water rising from the deep ocean is nutrient rich because it has picked up many of the nutrients and minerals necessary for life from the bottom of the ocean where they accumulate. This very nutrient rich water being raised to the surface into the Euphotic zone where plants can grow due to there being enough sun light. This combination of enough light and the proper nutrients causes a massive eruption in algae and other little plants that can grow in massive quantities due to the good conditions thus causing the surge of chlorophyll.
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4.) El-Nino Southern Oscillation (ENSO) is a shift that naturally occurs in the temperature of the southern pacific. Normally the western pacific is warmer than the Eastern pacific by a few degrees Celsius. These are normal conditions and La Nina one of the two conditions the pacific can be in resembles these normal conditions. Normal conditions can be shown by warmer water in the western pacific and cooler water in the Eastern Pacific. This temperature gradient causes low pressure above the western Pacific, and High Pressure near the Eastern Pacific. This causes something known as Walker circulation with prevailing easterly winds near the surface and higher westerly winds in the upper atmosphere. These conditions cause storms to form over the western pacific very often making Indonesia a very wet area. When it switches to El Nino changes are drastic. The water temperature becomes nearly constant across the whole southern pacific throwing off walker citculation and causing rain to form in the middle of the pacific and causing lower level winds in the western pacific to travel from the west to the east causing the Islands of Indonesia and Papa New Guinea to be relatively dryer. This will have intense sociopolitical problems if it were to persist for too long since much of these western pacific islands live off of rice as a main staple which requires lots of water to grow, and any sudden change in the amount of rain being received can cause pointed food shortages, and lack of fresh water for drinking. This puts large stress on millions of people, which in the past when large groups of people became stressed to the point of breaking revolutions and revolts happened, so any long pronounced El Nino could cause serious ramifications in South East Asia. Currently the World is experiencing a moderate strength La Nina according to the National Ocean and Atmospheric Association (NOAA). This means that everything is in the standard conditions explained above.
Sorucehttp://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf