Introduction to Oceanography Dynamic Oceanography: Atmospheric circulation and Currents

6-1 Atmospheric Processes 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/cm2 (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.

Atmospheric Processes 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.

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 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?

Uneven Solar Heating and Latitude Earth as a whole is in thermal equilibrium, but different latitudes are not. Moving masses of air and ocean currents transport energy from locations with a surplus to those with a deficit. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Rotation of the Earth strongly influences winds. 6-1 Atmospheric Processes 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.

Three major convection cells are present in each hemisphere. 6-1 Atmospheric Processes Three major convection cells are present in each hemisphere. The Hadley cell extends from the Equator to about 30o latitude. The Ferrel Cell extends from 30 o to about 50 o latitude. The Polar Cell extends from 90 o to about 50o latitude.

The Coriolis Effect Air moves in cells, influenced by the Coriolis effect. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Wind Patterns © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Monsoons, Sea Breezes and Land Breezes Monsoons are patterns of wind circulation that change with the season. Areas with monsoons generally have dry winters and wet summers. Sea breeze is cool air from over the water moving toward land. Sea breezes occur after sunrise. Land breezes occur after sunset when air warmed by the land blows toward the water. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Sea Breezes And Land Breezes Top: Sea breeze Bottom: Land breeze © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Storms The orange shaded areas and arrows designate the location and movement of most hurricanes. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Surface Currents Surface currents are driven by wind... Most of Earth’s surface wind energy is concentrated in the easterlies and westerlies. Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often moves in a circular pattern called a gyre. What are some effects of ocean currents? transfer heat from tropical to polar regions influence weather and climate distribute nutrients and scatter organisms © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

6-2 Surface Ocean Currents 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.

Surface Currents Water moves clockwise in the Northern Hemisphere gyres and counterclockwise in the Southern Hemisphere gyres.

Flow Within a Gyre Gyres can be further subdivided into distinct currents. The four interconnected currents in the North Atlantic Gyre have different flow characteristics and temperatures. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

6-2 Surface Ocean Currents 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 90o 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.

Flow Within a Gyre The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

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.

Flow Within a Gyre The surface of the North Atlantic is raised through wind motion and Ekman transport to form a low hill. Can you determine why the center of the hill is offset to the west? © 2002 Brooks/Cole, a division of Thomson Learning, Inc.