Unit 11

HYDROSPHERE: CIRCULATION OF THE WORLD OCEAN

Unit Overview

This unit examines the critical role of the world ocean in the transfer of energy and associated atmospheric processes. The main sections are

  • Surface currents
  • Generation of ocean currents
  • Flow behaviour of ocean currents
  • Deep-sea currents
  • The coupled ocean atmosphere system
  • El Nino - Southern Oscillation

Surface ocean currents are related to atmospheric circulation features because the surface currents are created by subtropical gyres. The gyres, in turn, are generated by subtropical high-pressure cells situated over the oceans. The resulting warm and cold ocean currents redistribute heat across the ocean basins.

Besides horizontal ocean currents, there can be substantial vertical movements of ocean water. An example is upwelling, which is the upward movement of cold water along the eastern side of ocean basins. Unlike the surface current system, the deep-sea system is a thermohaline circulation. This circulation is controlled by differences in density (caused by temperature and salinity) between water masses. The deep-sea system involves predominantly the movement of cold-water masses from high to low latitudes.

Associated with changes in ocean currents is the well-publicized phenomenon known as the El Nino - Southern Oscillation. El Nino is concurrent with a weakening of the subtropical high pressure cell in the South Pacific Ocean; consequently, warm water that had been piled up on the western side of the basin "sloshes" back to the east (i.e. South America). The redistribution of warm water causes an eastward shift in convective activity in the South Pacific, thereby brining excessive precipitation to the usually dry coastal areas of Peru and Ecuador.

Unit Objectives

  • To relate the surface oceanic circulation to the general circulation of the atmosphere
  • To describe the major currents that constitute the oceanic circulation
  • To demonstrate the role of oceanic circulation in the transport of heat at the Earth's surface


Glossary of Key Terms

Drift A term often used as a synonym for an ocean current, whose rate of movement usually lags well behind the average speeds of surface winds blowing in the same direction. These currents are characterized by a slow and steady movement that very rarely exceeds 8 kph (5 mph).
El Nino A periodic, large-scale, abnormal warming of the sea surface in the low latitudes of the eastern Pacific Ocean that produces a (temporary) reversal of surface ocean currents and airflows throughout the equatorial Pacific; these regional events have global implications, disturbing normal weather patterns in many parts of the world.
ENSO Acronym for El Nino - Southern Oscillation; the reversal of the flow of ocean currents and prevailing winds in the equatorial Pacific Ocean that disturbs global weather patterns.
Global conveyor belt Ocean circulation that moves masses of cold, deep-sea water from high to lower latitudes (see Fig. 11.7).
Gyre The cell-like circulation of surface currents that often encompasses an entire ocean basin; for example, the subtropical gyre of the North Atlantic Ocean consists of the huge loop formed by four individual, continuous legs - the North Equatorial, Gulf Stream, North Atlantic Drift, and Canaries currents.
La Nina The lull or cool ebb in low-latitude Pacific Ocean surface temperatures that occurs between El Nino peaks of anomalous sea-surface warming.
North Atlantic Oscillation (NAO) The alternating pressure gradient between the Icelandic Low and the Bermuda High, especially its eastern segment, the Azores High. See Perspectives: The North Atlantic Oscillation.
Ocean current Large-scale movements of ocean water.
Southern Oscillation The periodic, anomalous reversal of the pressure zones in the atmosphere overlying the equatorial Pacific; associated with the occurrence of the El Nino phenomenon. As the sea-surface temperatures change and water currents reverse, corresponding shifts occur in the windflows above.
Subpolar gyre Oceanic circulation loop found only in the Northern Hemisphere; its southern limb is a warm current steered by prevailing westerly winds, but the complex, cold returning flows to the north are complicated by sea-ice blockages and the configuration of landmasses vis-à-vis outlets for the introduction of frigid Arctic waters.
Subtropical gyre Circulates around the Subtropical High that is located above the center of the ocean basin; dominates the oceanic circulation of both hemispheres, flowing clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Thermohaline circulation Describes the deep-sea system of oceanic circulation, which is controlled by differences in the temperature and salinity of subsurface water masses.
Tropical gyre Narrow, low-latitude oceanic circulation loop, in both the Northern and Southern Hemisphere, comprised of the equatorial currents and returning countercurrents; reinforced by the converging winds of the Northeast and Southeast Trades.
Upwelling The rising of cold water from the ocean depths to the surface; affects the local climatic environment because cold water lowers air temperatures and the rate of evaporation.


Unit Outline

  • Surface currents
    • Ocean currents are large-scale water movements, the counterparts of wind belts and pressure cells
    • A vital role of ocean currents is to adjust Earth's surface heat imbalance
    • "Warm" currents travel from tropics to poles
    • "Cold" currents travel from poles to tropics
    • Most currents referred to as drifts because they move slowly and steadily
  • Generation of ocean currents
    • Main trigger is the frictional drag of winds that blow across the ocean surface
    • Water piles up along up along coastlines, creating higher sea levels than surrounding water
    • Variation in density of seawater; cool water sinks and spreads toward the Equator
    • Variation in salinity of seawater; saltier/denser water sinks and spreads toward the Equator
  • Flow behavior of ocean currents
    • Meanders of currents become such pronounced curves, they can detach into eddies
    • Gyre circulations
      • gyres are continuously moving loops that can be clockwise or counterclockwise
      • created by prevailing winds, the Coriolis force, and landmasses
      • subtropical gyres in both hemispheres around subtropical highs
      • tropical gyres from equatorial currents
      • subpolar gyres in northern hemisphere
      • upwelling in subtropical gyres is the rising of deep, cold water at coastlines in the eastern parts of ocean basins
    • The geography of ocean currents
      • Pacific Ocean currents dominated by subtropical gyres
      • Atlantic Ocean currents dominated by subtropical gyres
      • Indian Ocean currents controlled by both subtropical and equatorial gyres
  • Deep-sea currents
    • Deep-sea movement is thermohaline circulation
      • dependent upon temperature and salinity
      • dominant global movement is flow from higher to lower latitudes
  • The coupled ocean - atmosphere system
    • Larger amounts of heat moved by ocean at lower and middle latitudes
    • Sea is a vast heat reservoir
    • Latent heat from evaporation of oceans powers the general circulation of the atmosphere
  • El Nino - Southern Oscillation
    • Temperature anomalies in eastern equatorial waters of the Pacific Ocean
    • Reversal of ocean currents and atmospheric conditions
    • Disturbance of weather patterns, local fishing industries


Review Questions

  1. Describe the general features of the North Atlantic Oscillation.
  2. What is meant by the term global conveyor belt?
  3. Describe upwelling and list some its consequences on weather patterns.