2/15/2012. Earth System Science II EES 717 Spring 2012

Transcription

1 Earth System Science II EES 717 Spring The Earth Interior Mantle Convection & Plate Tectonics 2. The Atmosphere - Climate Models, Climate Change and Feedback Processes 3. The Oceans Circulation; climate and the oceans Key Points to Remember 1. The global wind system acts to redistribute heat between lows and high latitudes. 2. Coriolis force influences the direction of winds as they move from regions of high pressure to regions of low pressure. 3. Differential heating between land and water greatly influences global wind patterns. 4. Mid-latitude weather is systems are cyclones and anticyclones. Low latitude circulation is characterized by spiraling Hadley cells. 5. Heat is transported to higher latitudes through convective motions and latent heat transfer. 6. Ocean-Atmosphere interactions are most intense (strong coupling) in the tropics. The Tropics Pressure and Winds Extratropical Circulation Planetary Waves Circulation around Highs and Lows & Redistribution of Heat 1

2 Ocean-Surface Conditions Depend on Latitude, Temperature, and Salinity Extropical Dynamics Frontogenesis Tropic of Cancer Equator de North Latitud Temperature Tropic of Cancer Equator Tropic of Capricorn Tropic of Capricorn South Salinity Temperature Salinity Mid Ocean Average Surface Salinity Table 6-3, p. 166 Sea-surface temperatures during Northern Hemisphere summer Sea-surface average salinities in parts per thousand ( ). The Ocean Is Stratified by Density two samples of water can have the same density at different combinations of temperature and salinity! Fig. 6-16, p. 168 Fig. 6-17, p

3 The Ocean Is Stratified into Three Density Zones by Temperature and Salinity a.the surface zone or surface layer or mixed layer b.the pycnocline, or thermocline or halocline c.the deep ocean (~ 80% of the ocean is below the surface zone h (m) Depth 1,000 2,000 Temperature ( C) Polar Tropical 2,000 Temperate 4,000 6,000 8,000 3,000 10, Temperature ( F) h (ft) Dept Typical temperature profiles at polar, tropical, and middle (temperate) latitudes. Note that polar waters lack a thermocline. Water Transmits Blue Light More Efficiently Than Red most of the ocean lies in complete blackness Dissolved salts Major constituents and trace elements Conservative/nonconservative constituents Major Constituents = [] > 1 part per million Na + Sodium Cl - Chloride SO 4- Sulfate Mg 2+ Magnesium Ca 2+ Calcium K + Potassium 86 % 99 % Trace Elements = [] < 1 part per million Gases Distribution with depth Photosynthesis removes CO 2 and produces O 2 at the surface Respiration produces CO 2 and removes O 2 at all depths Compensation depth (Photosynthesis = Respiration) photosynthesis Oxygen and CO 2 profiles O 2 Concentrations Photosynthesis Bottom water enrichment CO 2 O 2 respiration oxygen minimum CO 2 Concentrations Direct solution of gas from the atmosphere Respiration of marine organisms Oxidation (decomposition) of organic matter 3

4 Surface Currents Are Driven by the Winds Surface Currents Flow around the Periphery of Ocean Basins A combination of four forces surface winds, the sun s heat, the Coriolis effect, and gravity circulates the ocean surface clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, forming gyres. The North Atlantic gyre, a series of four interconnecting currents with different flow characteristics and temperatures. The Ekman spiral and the mechanism by which it operates. Surface Currents Flow around the Periphery of Ocean Basins Consider the North Atlantic. Hydrostatic Pressure: p = ρgz z = depth g = grav. Acc. ρ = density of seawater The surface is raised through wind motion and Ekman transport to form a low hill. The westward-moving water at B feels a balanced pull from two forces: the one due to Coriolis effect (which would turn the water to the right) and the one due to the pressure gradient, driven by gravity (which would turn it to the left). The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline. PGF per Unit Mass: 1/ρ x dp/dx =g x tan(θ) In oceanography, dynamic topography refers to the topography of the sea surface related to the dynamics of its own flow. In hydrostatic equilibrium, the surface of the ocean would have no topography, but due the ocean currents, its maximum dynamic topography is on the order of two meters Seawater Flows in Six Great Surface Circuits Dynamics: The forces and motions that characterize a system Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect. Of the six great currents in the world s ocean, five are geostrophic gyres. Note the western boundary currents in this map. 4

5 Boundary Currents Have Different Characteristics Western boundary currents These are narrow, deep, warm, fast currents found at the western boundaries of ocean basins. The Gulf Stream The Japan Current The Brazil Current The Agulhas Current The Eastern Australian Current Eastern boundary currents These currents are cold, shallow and broad, and their boundaries are not well defined. The Canary Current The Benguela Current The California Current The West Australian Current The Peru Current Boundary Currents Have Different Characteristics Unit for measuring flow rates (or volume transported by ocean currents): sverdrups 1 sv = 1 million cubic meters of water per second The general surface circulation of the North Atlantic. Boundary Currents Have Different Characteristics Vertical Movement of Water Wind induced vertical circulation is vertical movement induced by wind-driven horizontal movement of water. Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface. Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases. Eddy formation The western boundary of the Gulf Stream is usually distinct, marked by abrupt changes in water temperature, speed, and direction. (a) Meanders (eddies) form at this boundary as the Gulf Stream leaves the U.S. coast at Cape Hatteras. The meanders can pinch off (b) and eventually become isolated cells of warm water between the Gulf Stream and the coast (c). Likewise, cold cells can pinch off and become entrained in the Gulf Stream itself (d). (C = cold water, W = warm water; blue = cold, red = warm.) Consider: West and East Winds Nutrient-Rich Water Rises near the Equator Equatorial upwelling. The South Equatorial Current, especially in the Pacific, straddles the geographical equator. Water north of the equator veers to the right (northward), and water to the south veers to the left (southward). Surface water therefore diverges, causing upwelling. Most of the upwelled water comes from the area above the equatorial undercurrent, at depths of 100 meters or less. 29 5

6 Thermohaline Flow and Surface Flow: The Global Heat Connection The global pattern of deep circulation resembles a vast conveyor belt that carries surface water to the depths and back again. Begin with the formation of North Atlantic Deep Water north of Iceland, which flows south through the Atlantic and then flows over (and mixes with) deep water formed near Antarctica. The combined mass circumnavigates Antarctica and then moves north into the Indian and Pacific ocean basins. Diffuse upwelling in all of the ocean returns some of this water to the surface. Water in the conveyor gradually warms and mixes upward to be returned to the North Atlantic by surface circulation. Water Masses May Converge, Fall, Travel across the Seabed, and Slowly Rise A model of thermocline circulation caused by heating in lower latitudes and cooling in higher latitudes. The thermocline at middle and low latitudes is held up by the slow upward movement of cold water. The water layers and deep circulation of the Atlantic Ocean. Arrows indicate the direction of water movement. Convergence zones are areas where water masses approach one another. The Arctic Sea: Relatively enclosed basin (connection to the Pacific through the Bering Strait and to the Atlantic through the Greenland and Norwegian Seas) Enclosed nature influences ice cover Circulation: was originally deduced from ice flows and drifting ships, supplemented with direct current measurements and geostrophic calculations 6

7 Other Major Current Systems Keep In Mind: Coriolis Force on the Equator is zero Coriolis Force by 0.5 o influences flow of water Currents To Know: North Equatorial Current (NEC) South Equatorial Current (SEC) Equatorial Counter Current (ECC) Equatorial Under Current (EUC) Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient Wind driven water from the surface mixed layer piled on the western side of the basin Wind stress balances the pressure gradient (Coriolis Force ~= 0 at equator) Adjustment (depression) of the thermocline on the western end Baroclinic conditions at depth drive a jet-like current eastward eventually balance by friction (eddy viscosity) Waves: the ocean can respond to the winds in distant places by means of large-scale disturbances that travel as waves. Barotropic: surface waves Baroclinic: density surface (thermocline) Rossby (Planetary Waves) Kelvin 7

8 Examples of barotropic and baroclinic waves propagating through the ocean Most tides are barotropic Kelvin waves Think about what would happen if the wind stress was dramatically reduced or changed directions in the case of the Asian Monsoon Kelvin Waves Travel eastward along the equator as a double wave equatorial wave guide Travel along coasts (coast on right in the NH and on the left in the SH) Balance between pressure gradient force and coriolis force. Kelvin Waves Surface equatorial kelvin waves travel ~200 m/s Rossby radius of deformation L = c/f High latitudes smaller eddies closely trapped to the coast (increase in planetary vorticity) Low latitudes larger radius Kelvin waves in the thermocline can have dramatic effects, particularly in low latitudes where the mixed surface layer is thin. Northward migration of ITCZ in western Atlantic generates disturbance that propagates eastward Splits into two coastal Kelvin waves when hits the eastern boundary The region of the disturbance where the thermocline bulges upward cold nutrient rich sub-thermocline water can reach the surface 4-6 week travel time Rossby Waves: Propagate from east to west across basin Travel along lines of latitude Move slower than Kelvin waves Conservation of Potential Vorticity ENSO: El Nino Southern Oscillation Interest in the phenomenon of El Nino goes back to the mid-19 th century but it was the El Nino event of that stimulated large-scale research into climatic fluctuations, which began to be seen as a result of the interaction between atmosphere and ocean. Example: Waves in the jet-stream 8

9 El Nino events are perturbations of the ocean-atmosphere system Disturbance a depression in the thermocline accompanied by a slight rise in sea-level propagates eastwards along the Equator as a pulse or series of pulses (Kelvin Waves) 9