4c_AtmosOceanicCirculation3_Slides

Ocean Water Circulation

• Ocean circulation is driven by temperature, density, and composition contrasts.

• Involves both shallow and deep water currents.

• Pathways influenced by:

  • Earth’s spin.

  • Positions of continents.

Layers of the Ocean

• Surface Mixing Zone:

  • Varies in temperature with latitude and seasons.

• Thermocline Transition Zone:

  • Temperature dramatically decreases with depth.

• Deep Cold Zone:

  • Relatively uniform in temperature (near freezing).

Formation of Deep Water

• Near the equator, heat warms cold deeper water.

• Warmed deep water becomes less dense and rises towards the surface, joining warm shallow current.

• Warm, shallow current flows northward toward the pole.

• As warm currents move north, they cool due to heat loss from the ocean.

Deep Water Dynamics

• Ocean water near poles sinks when its density exceeds that of surrounding water.

• Higher density arises from:

  • Colder temperatures.

  • Higher salinity.

• Cold bottom water flows southward.

Deep Ocean Water Migration

• Slow migration of deep ocean water (averages 5 km or 3 miles a day).

• Two principal deep water flows:

  • North Atlantic Deep Water.

  • Antarctic Bottom Water (deepest).

Thermohaline Circulation

• Driven by temperature and salinity differences.

• Warm, shallow Gulf Stream flows toward North Atlantic and cools.

• Cool water subsides in Labrador and Greenland Seas to form North Atlantic Deep Water.

• Cold surface water subsides off Antarctica to form Antarctic Bottom Water.

Ocean Currents

• Narrow channels of swift-moving surface water, typical velocities of 10 km/day, and locally up to 160 km/day.

• Currents move in definite, predictable directions, distributing heat and nutrients.

• Driven by winds and influenced by the Coriolis Effect.

• Land masses redirect currents.

Gyres Formation

• Surface currents deflected towards the right in the Northern Hemisphere (left in Southern Hemisphere) by the Coriolis Force.

• Surface currents flow in clockwise loops in the Northern Hemisphere (counterclockwise in Southern Hemisphere) to form gyres:

  • Examples: North Atlantic Gyre, Pacific Gyre.

Multiyear Oscillations in Global Circulation

• Fluctuations due to changes in strength/location of primary high and low pressures:

  • North Atlantic Oscillation: Alternating pressure differences between Icelandic low and Azores high.

  • Pacific Decadal Oscillation: 20-30 year temperature/pressure fluctuations between northern and tropical Pacific.

  • Southern Oscillation: Originates in the tropical Pacific; relates to El Niño.

North Atlantic Oscillation

Positive Phase:

• Colder stratosphere leads to strong low pressure over Iceland and strong high pressure over Azores.

  • Effects:

    • Strong polar jet stream confines cold air to northern Europe and the Arctic.

    • Mild weather conditions in the U.S. and Europe.

Negative Phase:

• Weakened stratosphere results in weakened Icelandic low and Azores high.

  • Effects:

    • Cold air spills southward leading to colder winters in U.S. and Europe.

Pacific Decadal Oscillation

• Positive Phase:

  • Warmer equatorial surface waters, cooler North Pacific waters.

• Negative Phase:

  • Cooler equatorial surface waters, warmer North Pacific waters.

El Niño – Southern Oscillation

• Involves circulation of winds and ocean currents around a subtropical high-pressure cell in the southern Pacific.

Normal Conditions:

• Low pressure in the western Pacific, high pressure in eastern Pacific:

  • Drought in Australia and SE Asia.

La Niña:

• Normal conditions intensify.

  • Stronger Pacific trade winds push warm surface water further west.

  • Strong upwelling results in cooler equatorial waters.

Climatic Effects of La Niña in North America

• High atmospheric pressure moves from one side of the Pacific to the other:

  • High pressure in western Pacific diminishes trade winds.

  • Results in drought in Australia and Southeast Asia.

Climatic Effects of El Niño in North America

• El Niño typically lasts 9 to 12 months, prolonged events may last for years.

Comparison with Pacific Decadal Oscillation (PDO):

• Cold Mode (Negative): Cooler equatorial Pacific, mostly La Niñas.

• Warm Mode (Positive): Warmer equatorial Pacific, mostly El Niños.