Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Oceanography 10, T. James Noyes, El Camino College 8B-1 Hurricanes Hurricanes are especially huge and powerful tropical storms. They have wind speeds of 75 mph or more and cover well over 100 miles. Fueled by warm ocean water during summer and fall, they are enormously destructive, flooding the nearby land and drowning people. Different parts of the world have different names for hurricanes. In East Asia they are called “typhoons.” In the Indian Ocean, they are called “cyclones.” (To some Australians, a hurricane is a “willy willy.”) Our word “hurricane” comes from Hurican, a meso-american god of the wind. How the Ocean Fuels Hurricanes Warm water fuels hurricanes by warming the air above in two ways. The air is warmed by contact with the surface of the ocean and – more importantly – by evaporation of warm ocean water. The warmed air rises quickly, and lots of cooler air rushes in from the sides to replace it. As the air comes in from the sides, it too warms 1 and rises as part of the giant convection cells that make up hurricanes. The warmer the air is, the faster the air rises, and the faster the air rises, the faster air from the sides moves in to replace the rising air, resulting in the strong winds of the hurricane. (The winds are the air that moves in from the sides to replace the rising air.) As the warm, moist air rises upward, it cools and its water vapor condenses into clouds and rain. Since lots of warm water evaporates from the surface, the rising air contains an enormous amount of water; it eventually falls back into the ocean as rain. LOTS of air rises, LOTS of air comes in from the side = Strong Winds 1 Records show an indisputable increase in hurricane activity since the 1990s (the IPCC recognizes a correlation going back to at least the 1970s) that is probably attributable to global warming. For example, waves created by hurricane winds are larger than in the past, consistent the idea that storm winds have gotten stronger. Research has shown that small changes in ocean temperature can have a big impact on a storm: a 1oC increase can raise a hurricane’s strength by 1 category (e.g., 3 to 4) and will increase the storm’s rainfall by about 20%. Winds Ocean Hurricanes and Global Warming While it is fair to say that global warming will increase storms’ rainfall, it is not clear that global warming will produce more storms. Hurricanes are fueled by sucking heat out of the ocean, and one big storm is more effective at removing heat than 2 small ones. Thus, global warming may lead to larger hurricanes each year, not more each year. If we observe fewer (and larger) storms, then would support the claim that global warming is responsible, not undermine it. Winds carrying lots of cool, dry air into the hurricane enhance evaporation, because there is lots of air with plenty of “room” available in the air for more water molecules. So think of a hurricane this way: the dry winds help the water heat the air, which makes the winds stronger, which helps the water heat the air even more, which makes the winds even stronger, which…you get the idea. This is a positive feedback loop which helps a hurricane become more and more powerful so long as it has a source of warm water. Oceanography 10, T. James Noyes, El Camino College 8B-2 Structure of a Hurricane The center of a hurricane is primarily a place where air rises (except for the small “eye” at the very center). Air outside the hurricane tries to move towards the center to replace the rising air, but it turns away from the center because of the Coriolis effect; it cannot go directly in. Air keeps trying to move towards the center and bending away, resulting in a slow, inward spiral 2. As the air spirals in, it may warm, rise, cool, and sink several times. Thus, a hurricane is made up of bands of clouds where the air is rising and gaps between them where air is sinking. Costliest Hurricane: Katrina (Gulf Coast, 2005) $100 billion+ Hurricanes and El Niño Upper and lower levels winds change in the Pacific Ocean during an El Niño, and the effects ripple outward, altering the jet stream and increasing the wind shear (difference in the direction and speed of upper and lower level winds) in the subtropical Atlantic. This wind shear tends to break up hurricanes’ convection cells as they are trying to form, so during El Niño years (e.g., 2006) fewer and weaker hurricanes hit the eastern and southern coasts of the United States. Hurricane Katrina (above) and hurricane structure (below). National Oceanic and Atmospheric Administration. 2 Without the Coriolis effect, the hurricane cannot “wrap up” into its familiar circular, self-contained shape and move as an independent unit. Oceanography 10, T. James Noyes, El Camino College 8B-3 Death and Destruction Caused by Hurricanes Hurricanes strike the east and southern coasts of the United States each summer and fall. Hurricanes cause damage in a variety of ways. Obviously, the strong winds 3 can fling objects around and tear down buildings. In addition, hurricanes produce an enormous amount of rain. However, the most devastating aspect of a hurricane by far is the hurricane’s storm surge 4. Sea level rises beneath hurricanes, more than 3 feet in the open ocean. This may not seem like much, but the strong winds of the hurricane push the mound of water against the land, causing more and more water to pile up: sea level can go up 20 feet or more! So when hurricanes hit the coast, they flood the shoreline. (This is how Hurricane Katrina overtopped the levees, pouring water into New Orleans.) In addition, huge waves created by the strong winds can reach much farther up the shoreline thanks to the storm surge. These waves cause much of the damage. Deadliest Hurricane: Bhola (Ganges Delta, 1970) 300,000+ dead (comparable to the Indian Ocean Tsunami of 2004) Storm Surge and its effects. NOAA (top and bottom) and FEMA (middle). 3 Hurricanes are low pressure zones, and the strong winds blowing past buildings further reduces the air pressure via a phenomenon called the “Bernoulli effect.” (Have you ever felt “pulled” towards a fast-moving 18-wheeler going by as you’re walking along the side of the road? That’s the Bernoulli effect.) The air pressure inside a sealed house can become so much greater than air pressure outside that it literally helps push the roof off the building. This is why homeowners need to leave their windows open a little bit (just a crack) in spite of all the rain. 4 About 90% of hurricane deaths are caused by drowning. Oceanography 10, T. James Noyes, El Camino College Why does the ocean rise beneath a hurricane? The ocean is pushed down strongly beneath the higher air pressure outside the hurricane, so the ocean rises beneath the hurricane where the downward push is weaker. (The water is that is pushed down has to go somewhere. This is somewhat like a “see saw” or “teeter-totter”: if one end goes down, the other must come up.) Hurricanes are associated with low atmospheric pressure, because the air in a storm system is warm, rising air. Warm air expands (get “bigger”), so there is less air above each location (it has spread off to the side), and therefore less weight above (less pressure) as well. Effects of Storm Surge. National Oceanic and Atmospheric Administration. Air Molecules Hurricanes cause much more property damage by flooding the shoreline than with their winds, which is why insurance companies will not insure against flooding. You have to buy flood protection from the national flood insurance program run by the federal government. (Similarly, the state government has an earthquake insurance program in California, because no private insurer wants to insure against earthquake damage.) However, Land even though many people who buy the flood insurance consider it too expensive, the “I lost my property in program typically pays out more each Carla in ’61, in ’83 – year to hurricane victims than it takes in what was that, Alicia? – and then again this time from policies, so other people’s taxes [Hurricane Ike, 2008].” have to make up the difference. Many James Clark, a resident people don’t buy the more expensive of Galveston, Texas flood insurance 5, and thus can lose everything if they are unlucky enough to be hit by a hurricane. We have similar problems with earthquakes and earthquake insurance here in California. As we all know, living in “paradise” is expensive, one way or the other. 5 8B-4 Ocean This division between the insurance companies paying for “wind damage” and the NFIP paying for “flood damage” can make it very difficult to file a claim, because it is not always clear what caused the damage. The judgments of homeowners, the NFIP, and the insurance company could all be different (and biased depending upon what they stand to gain or lose). Moreover, the ambiguity can greatly delay payouts for claims, which may have to be settled in court.