🌊 Marine Science Study Notes: Tides and Ocean Currents (AS Level 2.3)

Welcome to the chapter on Tides and Ocean Currents! These processes are much more than just the rise and fall of the water level—they are the engine of the ocean, responsible for mixing water, distributing heat globally, and supporting marine ecosystems. Understanding them is key to mastering Earth Processes in marine science. Don't worry if the physics seems tricky; we’ll use clear analogies to break it down!

Key Learning Outcomes Covered:

  • How tides are produced by astronomical alignment (Sun, Moon, Earth).
  • The difference between Spring and Neap tides.
  • The factors driving ocean currents (wind, density, Coriolis).
  • The function of the Global Ocean Conveyor Belt.
  • The causes and effects of El Niño and La Niña.

1. Tides: The Rhythmic Movement of the Ocean

1.1 How Tides Are Produced (Gravitational Pull)

Tides are the periodic rise and fall of sea level, primarily caused by the gravitational pull exerted by the Moon and, to a lesser extent, the Sun, acting upon the Earth's oceans.

The Moon’s Influence:

The Moon is the biggest driver of our tides because, even though it's smaller than the Sun, it is much closer.

  • Tidal Bulge 1 (Direct Tide): The gravitational pull of the Moon draws the ocean water directly facing it outwards, creating a bulge. This area experiences a High Tide.
  • Tidal Bulge 2 (Opposite Tide): On the side of the Earth opposite the Moon, water also bulges out. This happens because the Moon is pulling the solid Earth mass slightly away from the water on the far side (think of inertia or centrifugal force). This also creates a High Tide.

As the Earth rotates beneath these two bulges, most locations experience two high tides and two low tides roughly every 24 hours and 50 minutes (the time it takes for the Moon to return to the same position).

Quick Review: What causes a Low Tide?

Low tides occur in the areas between the two major tidal bulges, where the gravitational forces are pulling the water away towards the high tide zones.

1.2 Spring Tides and Neap Tides (Alignment)

The height of the tides (the Tidal Range) changes over the course of a lunar month due to the changing alignment of the Earth, Moon, and Sun.

1. Spring Tides (Highest Range):

  • Alignment: The Earth, Moon, and Sun are aligned in a straight line (syzygy).
  • Occurrence: During the New Moon and the Full Moon.
  • Effect: The gravitational pull of the Moon and Sun combine, resulting in the maximum possible gravitational force. This creates extremely high high tides and very low low tides.

Memory Aid: Spring tides "spring up" high and "spring down" low!

2. Neap Tides (Lowest Range):

  • Alignment: The Earth, Moon, and Sun form a right angle (90°) with the Earth at the vertex.
  • Occurrence: During the First Quarter Moon and the Third Quarter Moon.
  • Effect: The gravitational pulls of the Sun and Moon work against each other, partially cancelling out the effect. This results in a smaller tidal range, meaning lower high tides and higher low tides.

1.3 Factors Affecting Tidal Range

The actual difference in height between high and low tide (the tidal range) is not solely dependent on astronomical alignment. Several environmental factors play a role:

  1. Coastal Geomorphology: The shape of the coast is critical.
    • In wide, open ocean basins, the tidal range is small.
    • In narrow, funnel-shaped bays or estuaries (like the Bay of Fundy), water is forced into a smaller area, dramatically increasing the tidal height and range.
  2. Size of Water Body: Larger, open oceans have smaller ranges. Semi-enclosed seas (like the Mediterranean) have very small ranges because the volume of water is restricted from forming proper bulges.
  3. Wind: Strong onshore winds can push water toward the coast, increasing the actual high tide height (a wind set-up effect).
  4. Air Pressure: Low atmospheric pressure above the water allows the sea surface to bulge slightly higher, increasing the tidal range. High atmospheric pressure pushes down on the water, slightly suppressing the tidal height.
Interpreting Tide Tables and Graphs (2.3.3)

Tide tables and graphs summarize tidal information for a specific location. You must be able to:

  • Identify Tidal Height (the depth of the water at a specific time).
  • Calculate the Tidal Range (Tidal Range = Highest High Tide Height – Lowest Low Tide Height).
  • Recognise periods of Spring Tides (largest ranges) and Neap Tides (smallest ranges).

Key Takeaway for Tides: Tides are caused by gravity (mostly the Moon). The highest tidal ranges (Spring) occur when the Moon and Sun line up; the smallest ranges (Neap) occur when they are at right angles.


2. Ocean Currents: The Flowing Rivers of the Sea

2.1 Drivers of Ocean Currents (2.3.4)

Ocean currents are continuous, directed movements of seawater. They are crucial for moving heat and distributing marine organisms and plankton worldwide. They are driven by several factors:

1. Wind:

  • Wind provides friction on the ocean surface, dragging the top layer of water along with it.
  • Sustained, strong winds (like the trade winds or westerlies) create vast, circular surface currents called gyres.

2. Temperature and Density:

  • Density is affected by temperature (cold water is denser) and salinity (saltier water is denser).
  • Differences in density create Density Currents. These are key drivers of deep ocean circulation (discussed in Section 3).

3. The Coriolis Effect:

Because the Earth is spinning, moving objects (like air and water currents) appear to be deflected from a straight path. This apparent deflection is the Coriolis Effect.

  • Northern Hemisphere: Currents are deflected to the right (resulting in clockwise circulation).
  • Southern Hemisphere: Currents are deflected to the left (resulting in anti-clockwise circulation).

Did you know? The Coriolis Effect is strongest at the poles and virtually non-existent at the equator.

4. Shape of the Seabed:

Just like coastal geomorphology affects tides, the shape of the ocean floor affects currents. Ridges, trenches, and continental shelves can deflect, slow down, or channel currents, influencing their path and speed.

2.2 Upwelling: Bringing Life to the Surface

Upwelling is a vital process where deep, cold, nutrient-rich water rises to the surface, replacing warmer surface water that has been pushed away.

The Process:

  1. Strong winds blow parallel to the coast (or offshore).
  2. The Coriolis Effect deflects the surface water offshore (e.g., to the right in the NH).
  3. This creates a 'void' at the surface near the coast.
  4. To fill this void, cold water from the deep ocean rises up.

Importance: This deep water is rich in dissolved nutrients (nitrates and phosphates), which act as fertilizer for phytoplankton. Upwelling zones are therefore areas of very high primary productivity and support huge populations of fish (e.g., off the coast of Peru or California).


3. The Global Ocean Conveyor Belt (Thermohaline Circulation) (2.3.5)

The Global Ocean Conveyor Belt is the continuous movement of water that connects all the world's oceans, mixing them thoroughly over centuries. This circulation is primarily driven by density, linking temperature (thermo) and salinity (haline), hence the name Thermohaline Circulation.

Formation and Process:

  1. Water near the poles (especially the North Atlantic and Southern Ocean) cools significantly, becoming very dense.
  2. Ice formation (freezing) extracts fresh water, leaving behind saltier water, which further increases the density.
  3. This extremely cold, salty, dense water sinks to the deep ocean floor.
  4. This dense water mass flows slowly along the ocean bottom, spreading across the globe.
  5. Eventually, this deep water returns to the surface (often through upwelling in other regions) to complete the slow cycle.

Importance of the Conveyor Belt:

The Conveyor Belt is crucial for global climate and marine life because it ensures efficient distribution of:

  • Heat: It transfers heat from the tropics towards the poles (e.g., the Gulf Stream keeping northern Europe warm).
  • Nutrients: It moves deep, nutrient-rich water up to the surface in upwelling areas.
  • Oxygen: It carries dissolved oxygen (from the surface) to the deep ocean, allowing deep-sea organisms to respire.

Key Takeaway for Currents: Surface currents are driven by wind and Coriolis deflection (right in N, left in S). Deep currents are driven by density (cold, salty water sinks) in the Thermohaline Circulation, mixing the oceans globally.


4. The El Niño Southern Oscillation (ENSO) (2.3.6)

The El Niño Southern Oscillation (ENSO) is a large-scale climate pattern observed in the tropical Pacific Ocean that has significant impacts on marine ecosystems and global weather. It involves fluctuations in sea surface temperature and air pressure.

4.1 Normal Conditions (or La Niña)

Normally, strong trade winds blow from East to West across the Pacific, resulting in:

  • Warm surface water piling up in the Western Pacific (near Indonesia/Australia).
  • In the Eastern Pacific (near South America/Peru), surface water is pushed away, causing strong upwelling of cold, nutrient-rich water.
  • This supports extremely high primary productivity and strong fisheries (e.g., anchoveta).
  • The weather pattern features low pressure and heavy rain in the West and high pressure and dry conditions in the East.

La Niña is an extreme version of these normal conditions, characterized by unusually cold sea surface temperatures in the Eastern Pacific and even stronger trade winds/upwelling.

4.2 El Niño Event

An El Niño event (typically occurring every 2–7 years) involves the breakdown or reversal of these normal conditions:

Causes of El Niño:

  • The normal trade winds weaken or cease.
  • The pool of warm surface water begins to move eastward back across the Pacific.
  • Atmospheric pressure systems reverse (low pressure shifts eastward).

Effects of El Niño:

  • Eastern Pacific (Peru/Ecuador): The arrival of warm water suppresses upwelling. This cuts off the supply of deep nutrients.
    • Marine Impact: Primary productivity crashes, leading to starvation and collapse of fish populations (e.g., the Peruvian anchoveta fishery suffers greatly).
    • Climate Impact: Heavy rainfall and flooding.
  • Western Pacific (Australia/Indonesia): The departure of warm water causes high pressure and drought conditions.
  • Global Impact: Major disruption of global climate patterns, affecting rainfall and temperatures far beyond the Pacific.

📝 Chapter 2.3 Comprehensive Review

Here are the absolute key points you must remember for your exam:

TIDES:

  • Caused by the gravity of the Moon (and Sun).
  • Spring Tides: Maximum range; Sun, Earth, Moon are straight (New/Full Moon).
  • Neap Tides: Minimum range; Sun, Earth, Moon are at 90° (Quarter Moons).

CURRENTS:

  • Surface currents driven by wind and the Coriolis Effect (deflection Right in N, Left in S).
  • Deep currents are Thermohaline (driven by density: cold + salty = sink).
  • Upwelling: Wind-driven process bringing cold, nutrient-rich water to the surface, boosting productivity.

ENSO:

  • Normal/La Niña: Strong trade winds, strong upwelling in the East Pacific (good fishing).
  • El Niño: Weakened trade winds, warm water moves East, suppresses upwelling (poor fishing, climate disruption).