Ocean and the Atmosphere Notes for Oceanography Wnek
The role of the atmosphere • The atmosphere influences the ocean, especially through wind-driven and density driven circulation • Wind-driven circulation can cause a deflection of water off continents • The wind-driven pattern can create coastal upwelling and downwelling patterns (discussed later)
North Atlantic Gyre The Sargasso Sea Monterey Aquarium Research Institute Sargassum weed (Bermuda.com)
How About the Pacific? Nature-Education.org The North Pacific Gyre is a result of major oceanic currents including the Pacific counterpart to the Gulf Stream called the Kuroshio Current
Earth’s Rotation • Coriolis Effect • Ekman Spiral • The Motion of the earth can be up to 45 degrees to the right of the wind direction in the northern hemisphere and left in the southern hemisphere <>
Upwelling • Movement of water away from land (coasts) where it is replaced by colder water (nutrient rich) from deeper areas • The movement of water away from the coast is called “divergence” • In the northern hemisphere on the east coast of continents a southerly wind causes upwelling…
Along the east coast of the United States Along the west coast of the US
Upwelling Schematic http://oceanservice.noaa.gov/education/kits/currents/03coastal4.html
The role of the wind • Unequal heating of the atmosphere by the land and ocean surfaces • Friction between the moving air and the ocean surface create surface currents • Negating the rotation of the earth, the wind would cause surface currents to move, thus layers move below it causing a net movement
Pacific Decadal Oscillation considered a climate index (Mantua et al. 1997; NOAA 2011)
Increase in carbon dioxide levels between 1960 and 1990. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com).
Are Ice Cores Accurate? • It is difficult to believe that a chemically active gas such as carbon dioxide can remain unaffected by burial in ice for as long as 160,000 years. There are questions whether the carbon dioxide can diffuse through the snow, react with dust particles, or form clathrate compounds under pressure The technique of drilling, removal and preservation of the ice core is very difficult, and there are many things that can go wrong (Jaworski, 1996, 1997).
Findings • The period since 1972, when the increase has been linear despite an increase of over 45% in emissions, suggests that there are new carbon sinks being established in the ocean and in the terrestrial biosphere to absorb the increases (Gray, 1998).
Sea Level Rise • Global mean sea level has been rising at an average rate of 1 to 2 mm/year over the past 100 years, which is significantly larger than the rate averaged over the last several thousand years. Projected increase from 1990-2100 is anywhere from 0.09-0.88 meters (depending on the severity of the warming)
Ocean Currents? Illustration by Jack Cook, Woods Hole Oceanographic Institution This is now being reconsidered in terms of the rate of slowing
Coupled ocean-atmospheric models • Rising carbon dioxide indicate upper-ocean stratification • Stability will increase & mixing will decrease the next 50 years • Net primary productivity (NPP) of microplankton will increase and megaplankton will decrease (Barber 2007)
The Future • Precipitation is also expected to increase over the 21st century, particularly at northern mid-high latitudes, though the trends may be more variable in the tropics. Snow extent and sea-ice are also projected to decrease further in the northern hemisphere, and glaciers and ice-caps are expected to continue to retreat.
Pressure Gradients Air is constantly moving to seek an equilibrium between areas of more air molecules (higher pressure) and those with less (lower pressure). Wind is nothing more than the movement of air molecules from one place to the next. The direction and speed of the wind represents the balance between three basic forces acting on it: the pressure gradient, the Coriolis force, and surface friction. Pressures are trying to stay in balance. Pressures are reported in Isobars
Tropical Systems Defined as low pressure systems that have a counterclockwise rotation in the northern hemisphere & the opposite in the southern hemisphere The opposite rotation in each hemisphere is due to the Coriolis Effect They are all CYCLONES; however they are called HURRICANES in the western hemisphere and strong storms are TYPHOONS in the eastern hemisphere Formed by storms, Intertropical Convergent Zone areas
Tropical Cyclones For a cyclone to form, the ocean waters need to be warm, at least 26°C. Above the warm ocean, water evaporates and form clouds. If there is low air pressure where the clouds are formed, it pulls them in and they begin to rotate. If a storm achieves wind speeds of 38 miles (61 kilometers) an hour, it becomes known as a tropical depression. Hurricanes spin around a low-pressure center known as the “eye.” Sinking air makes this 20- to 30-mile-wide (32- to 48-kilometer-wide) area notoriously calm. But the eye is surrounded by a circular “eye wall” that hosts the storm’s strongest winds and rain.
Scale to determine the strength of tropical cyclones 1 knot = 1.15 miles/hour
Storm Designations The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time.
Where do tropical cyclones come from and when? Most hurricanes begin in the Atlantic as a result of tropical waves that move westward off the African coast. Official Hurricane season is June 1 – November 30 each year. Many early and late Atlantic Ocean Hurricanes originate in the Gulf of Mexico or Caribbean
Tropical Activity as of September 4, 2011 (Weather Underground)
Problems Associated with Tropical Cyclones Winds Storm Surge Flooding