MEV 012: Unit 10 - Ocean Currents

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 UNIT 10: OCEAN CURRENTS

10.0 Introduction

Ocean currents are continuous, directed movements of seawater driven by various forces acting upon the water, including wind, temperature, salinity differences, and Earth's rotation. Currents significantly influence global climate, weather patterns, and marine ecosystems.

10.1 Objectives

After studying this unit, learners will be able to:

  • Understand types and causes of ocean currents
  • Differentiate between upwelling and downwelling
  • Describe properties of sea waves and tides
  • Comprehend air-sea interaction and exchange processes
  • Explore ocean general circulation models (OGCMs)

10.2 Ocean Currents

Ocean currents can be surface currents (driven by wind) or deep-water currents (driven by density and salinity).

  • Surface currents: Generally confined to the upper 400 meters of the ocean.
  • Deep-water currents: Driven by thermohaline circulation (temperature and salinity).

Major current systems include:

  • Gulf Stream
  • Kuroshio Current
  • Humboldt Current
  • Canary Current
  • West Wind Drift

10.2.1 Upwelling

Upwelling is the process where deep, cold, nutrient-rich water rises to the surface, replacing warmer surface waters. This occurs along coasts and is crucial for marine productivity (e.g., fisheries).

Causes:

  • Wind-driven Ekman transport
  • Divergence of surface water masses

10.2.2 Downwelling

Downwelling is the sinking of surface water, often warm and oxygen-rich, into deeper layers. It helps in distributing oxygen to deep ocean environments and is critical for thermohaline circulation.

10.3 Waves: Properties and Motion

Waves are oscillations of the ocean surface, primarily caused by wind.

10.3.1 Classification of Waves

  • Capillary Waves: Very small waves influenced by surface tension
  • Gravity Waves: Larger waves driven by gravity as a restoring force
  • Tsunamis: Caused by undersea disturbances like earthquakes or landslides

10.3.2 Sea Waves and Swells

  • Sea Waves: Generated locally by wind, irregular and chaotic
  • Swells: Regular, long-period waves that travel long distances after being generated

10.3.3 Disintegration of Waves

Occurs near the coast when wave energy dissipates due to:

  • Friction with seabed
  • Breaking in shallow waters
  • Formation of surf and backwash

10.3.4 Littoral Current

Littoral (or longshore) currents are generated by waves hitting the shore at an angle, transporting sand and sediments along the coast.

10.3.5 Tsunami

A tsunami is a series of long-wavelength waves caused by seismic activities beneath the ocean. They travel at high speeds and can cause massive coastal destruction.

10.4 Tides

Tides are periodic rise and fall of sea levels caused primarily by the gravitational interaction between the Earth, Moon, and Sun.

10.4.1 Classification of Tides

  • Spring Tides: Occur during full/new moon, higher range
  • Neap Tides: Occur during quarter moon phases, lower range
  • Diurnal Tides: One high and one low tide per day
  • Semidiurnal Tides: Two high and two low tides per day
  • Mixed Tides: Unequal high and low tides

10.5 Air-Sea Exchange

Air-sea exchange involves transfer of energy, gases, and momentum between the ocean and atmosphere.

10.5.1 Solar Radiation: Absorption, Reflection, and Scattering

Solar energy is absorbed by ocean surfaces, influencing sea surface temperature (SST) and thermal structure.

10.5.2 Infrared Radiation: Emission, Reflection, and Absorption

Oceans emit long-wave infrared radiation, contributing to Earth's heat balance.

10.5.3 Turbulent Heat Transfer

Heat is transferred between air and water through turbulence, especially under windy conditions.

10.5.4 Evaporation

Evaporation from the sea surface contributes to atmospheric humidity and cloud formation.

10.5.5 Precipitation

Condensed water vapor returns as rain, affecting salinity and ocean stratification.

10.5.6 Buoyancy Flux at Sea Surface

Combines heat and freshwater fluxes, altering density and mixing in upper ocean layers.

10.5.7 Ocean Surface Wave Generation and Decay

Wind generates surface waves; they decay with distance or when energy dissipates near coastlines.

10.6 Ion Exchange

Ion exchange involves exchange of dissolved ions between seawater and marine sediments or suspended particles. It affects nutrient cycling, chemical composition, and metal availability in marine systems.

10.7 Gas Exchange at Sea-Water Interface

The ocean surface acts as an interface for gas exchange, including:

  • Oxygen (O₂) – enters through photosynthesis and surface diffusion
  • Carbon dioxide (CO₂) – absorbed from the atmosphere
  • Methane (CH₄) – released from sediments

The balance impacts marine chemistry and global carbon cycling.

10.8 Ocean General Circulation Models (OGCMs)

OGCMs are numerical models that simulate the ocean’s circulation using equations for fluid motion, heat, salt transport, and atmospheric forcing.

Applications:

  • Climate prediction (e.g., El Niño–Southern Oscillation)
  • Studying thermohaline circulation
  • Forecasting marine ecosystems’ response to climate change

10.9 Let Us Sum Up

This unit introduced the complex dynamics of ocean currents, waves, tides, and air-sea interactions. We also explored critical processes such as upwelling, gas and ion exchanges, and how these influence marine systems. Ocean General Circulation Models provide essential tools to predict and understand global climate phenomena.

10.10 Keywords

  • Ocean Currents-Continuous flow of ocean water driven by various physical factors
  • Upwelling-Rising of deep, nutrient-rich water to the surface
  • Downwelling-Sinking of surface water into deeper ocean layers
  • Swells-Long-period, regular waves generated far from the shore
  • Littoral Current-Shore-parallel current caused by angled wave approach
  • Tsunami-Giant sea waves caused by seismic activities
  • Spring Tide-High tide with the greatest difference between high and low water
  • Neap Tide-Tides with the least difference in height
  • Air-Sea Exchange-Movement of heat, gases, and momentum between the ocean and atmosphere
  • Buoyancy Flux-Measure of vertical density change caused by heat and freshwater fluxes
  • OGCMs-Ocean General Circulation Models that simulate large-scale oceanic patterns
  • Ion Exchange-Chemical process of exchanging ions in seawater and marine particles
  • Gas Exchange-Transfer of gases like O₂ and CO₂ across the air-sea interface
  • Wave Disintegration-Breakdown of wave energy as it approaches the coast
  • Thermohaline Circulation-Global ocean circulation driven by differences in temperature and salinity

 

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