This oil spill in the Gulf of Mexico has been in the news a lot lately. Recently, I came across this satellite photograph from NASA, which shows the oil heading southeast in a long stream. There’s a lot of speculation on what will happen in the days to come if the spill isn’t capped soon. Where will the oil go? Which areas will be affected?
Here’s a map showing the topology of the sea floorwhere the Deepwater Horizon was located. As you can see, the site is near a steep incline, where the continental shelf drops off sharply towards the sea floor. The depth of the sea floor was around 5000 feet.
I’ve come across a bunch of comments at various websites, with some people saying “oh well, the oceans are huge, so what if we have a spill? If you consider the amount of oil compared to thevast volume of the oceans, it is so tiny it doesn’t matter”. This kind of talk seems singularly uninformed at best and deliberate distortion of the facts at worst to me. It’s clearly obvious if you follow the news at all, that the spill isn’t being distributed evenly among the oceans. Tar balls and oil are appearing on the Louisiana coast. And as the satellite picture from NASA shows, the oil isdefinitely being channeled in a very specific direction. So I thought to write this note to explain what is happening, and what we might expect in the days to come.
In order to understand what’s happening to the oil as it leaks out, we need to understand water currents and wind directions in the Gulf of Mexico. These can be summarized as follows:
There is a deep water current that entersthe Gulf of Mexico from the south, loops through the Gulf, and then exits through the Florida Strait and curves northwards along the east coast of the US, as the Gulf Stream.
The prevailing winds in this area are the Westerlies, or anti-trade winds, which blow from south west to north east. Winds do not affect deep waters, but they do in fact produce currents in surface waters. It might seemintuitively right that the surface water current should be in the same direction as the wind, but this is not so. In fact, surface water currents flow in a direction 90 degrees to the right of the wind direction, because of a phenomenon known as Eckman Transport. Since the prevailing winds in this region are southwest to northeast, surface currents induced by the winds flow northwest to southeast.
Themovement of the oil itself is subject to the Coriolis Effect. This is an effect which happens due to the west to east rotation of the Earth. Air or water or oil flowing in a fluid medium is affected in that its path curves westwards if the flow is towards the equator, and curves eastwards if the flow is away from the equator.
The oil leak is occurring at the bottom of the ocean. The oil rigcollapsed to the sea floor. The leaks are in the valve at the sea bed, and also in the riser pipe which is now lying collapsed on the sea floor. So the oil is being released at the sea floor, and then rising slowly to the surface. Therefore, since this oil is traversing the entire depth of the sea, from the sea floor to the surface, it is affected by both deep water and surface currents.
You can seethese things in this diagram I made:
Notice the deep water current displayed in tan. This current enters the Gulf of Mexico from the south, loops around the Gulf, and then exits around Florida, only to swing back north and proceed along the eastern seaboard as the Gulf Stream.
Next, consider the winds, shown in yellow on the map. These are the Westerlies, or anti-trade winds. At theselatitudes, they blow from southwest to northeast (as an aside, this is why “weather” in the form of storms tends to move from southwest to northeast in the continental US). Because of these winds, the movement of surface water is from northwest to southeast, as shown by the orange arrow. This is because of Eckman Transport, which tends to push water 90 degrees to the right of the wind direction....
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