Presentation on theme: "Introduction to Oceanography Dynamic Oceanography: Atmospheric circulation and Currents."— Presentation transcript:
Introduction to Oceanography Dynamic Oceanography: Atmospheric circulation and Currents
Density of air is controlled by temperature, pressure and moisture content. Warm air is less dense than cold air and moist air is less dense than dry air. Air pressure is the weight of the air from Earth’s surface to the top of the atmosphere and equals 1.04kg/cm 2 (standard air pressure, one atmosphere) at sea level. Low pressure zone is where air density is lower than in surrounding areas because the air is warmer or has a higher moisture content. High pressure zone is where air pressure is higher than in surrounding area because of cooling or lower moisture content. 6-1 Atmospheric Processes
Fluids (air and water) flow from areas of high pressure to areas of low pressure. Change in pressure across a horizontal distance is a pressure gradient. Greater the difference in pressure and the shorter the distance between them, the steeper the pressure gradient and the stronger the wind. Movement of air across a pressure gradient parallel to Earth’s surface is called a wind and winds are named for the direction from which they come. In contrast, ocean currents are named for the direction towards which they travel. 6-1 Atmospheric Processes
Composition and Properties of the Atmosphere Ascending air cools as it expands. Cooler air can hold less water, so water vapor condenses into clouds. Descending air warms as it compresses and the clouds evaporate.
Atmospheric circulation is powered by sunlight. Since Earth is in thermal equilibrium, what assumption can be made about the input and output of heat on Earth? Atmospheric Circulation
Rotation of the Earth strongly influences winds. Global winds blow in response to variation in pressure related to uneven solar heating (insolation) of Earth’s surface. Coriolis deflection is the apparent deflection of objects moving across Earth’s surface to the right of direction of travel in the northern hemisphere and to the left of direction of travel in the southern hemisphere. 6-1 Atmospheric Processes
Three major convection cells are present in each hemisphere. The Hadley cell extends from the Equator to about 30 o latitude. The Ferrel Cell extends from 30 o to about 50 o latitude. The Polar Cell extends from 90 o to about 50 o latitude. 6-1 Atmospheric Processes
Wind-driven currents are produced by the interaction between the wind and the water. As wind moves across the water, collision of air molecules with water molecules inefficiently transfers energy from the air to the water. Water moves at about 3-4% of the wind speed. Zonal wind flow is wind moving nearly parallel to latitude as a result of Coriolis deflection. Westerly-driven ocean currents in the trade winds, easterly- driven ocean currents in the Westerlies and deflection of the ocean currents by the continents results in gyres, which occupy most of the ocean basin in each hemisphere. 6-2 Surface Ocean Currents
Surface Currents Water moves clockwise in the Northern Hemisphere gyres and counterclockwise in the Southern Hemisphere gyres.
With time, wind-driven surface water motion extends downward into the water column, but speed decreases and direction changes because of Coriolis deflection. Eckman Spiral is the spiraling pattern described by changes in water direction and speed with depth. Eckman transport is the net transport of water by wind-induced motion. Net transport of the water in an Eckman spiral has a Coriolis deflection of 90 o to the direction of the wind. Along coastal areas Eckman transport can induce downwelling or upwelling by driving water towards or away from the coast, respectively. 6-2 Surface Ocean Currents
Pressure gradients develop in the ocean because the sea surface is warped into broad mounds and depressions with a relief of about one meter. Mounds are caused by convergences, places where water flows together and sinks. Depressions are caused by divergences, places from where water rises to the surface and flows outward. Water flowing down pressure gradients on the ocean’s irregular surface are deflected by Coriolis and the amount of deflection is a function of location and speed. 6-2 Surface Ocean Currents