Movements of Ocean Water

MOVEMENTS OF OCEAN WATER

Ocean water is dynamic and constantly in motion. The **horizontal motions** include ocean currents and waves, while **vertical motions** include tides and upwelling/sinking of water. Ocean currents are continuous flows of huge amounts of water in definite directions, while waves are surface energy transfers where water particles move in circles but don't travel with the wave. Tides are periodic rises and falls of sea level due to gravitational attraction of the sun and moon. All these movements are influenced by physical characteristics like temperature, salinity, density, and external forces like solar radiation, wind, and gravitational attraction.

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WAVES

**Definition:** Waves are forms of energy, not actual water movement, that travel across the ocean surface. Water particles only move in small circles as a wave passes; the energy propagates forward while individual water molecules remain relatively stationary.

**Source of Wave Energy:** Wind provides the primary energy for waves. When wind blows over water, friction causes the water surface to rise and fall, creating waves. Waves continue to grow larger as they move and absorb energy from the wind.

**Wave Formation Process:**

When a breeze of 2 knots or less blows over calm water, small ripples form

As wind speed increases, ripples grow until white caps appear in breaking waves

Waves may travel thousands of kilometers before reaching shore

**Wave Motion Mechanics:**

Wind pushes water horizontally while gravity pulls wave crests downward

Falling water pushes former troughs upward

The wave moves to a new position through this cyclical process

**Actual water motion beneath waves is circular:** water moves up and forward as the wave approaches, then down and back as it passes

**Wave Characteristics and Definitions:**

**Wave Crest:** The highest point of a wave

**Wave Trough:** The lowest point of a wave

**Wave Height:** Vertical distance from the bottom of a trough to the top of a crest

**Wave Amplitude:** One-half of the wave height

**Wavelength:** Horizontal distance between two successive crests (or troughs)

**Wave Period:** Time interval between two successive wave crests or troughs passing a fixed point

**Wave Speed:** Rate at which the wave moves through water, measured in knots

**Wave Frequency:** Number of waves passing a given point during a one-second time interval

**Important Wave Properties:**

The largest waves are found in the open oceans

As waves approach the beach, they slow down due to friction between dynamic water and sea floor

When water depth is less than half the wavelength, the wave breaks

**Wave size and shape reveal origin:** Steep, young waves indicate local wind formation; slow, steady waves originate from far away and may come from another hemisphere

Maximum wave height is determined by: (1) wind strength, (2) duration of wind blowing, (3) area over which it blows in a single direction

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TIDES

**Definition:** The **periodical rise and fall of sea level**, occurring once or twice a day, mainly due to the gravitational attraction of the sun and moon.

**Distinction from Surges:** Movements of water caused by meteorological effects (winds and atmospheric pressure changes) are called **surges**, which are irregular unlike tides. The study of tides is complex due to great spatial and temporal variations in frequency, magnitude, and height.

**Causes of Tides:**

The **moon's gravitational pull** is the major cause, with the **sun's gravitational pull** as a secondary cause. The **centrifugal force** (force that counterbalances gravity) works alongside gravitational attraction to create tidal bulges.

**Tide-Generating Force:**

Defined as the difference between gravitational attraction of the moon and centrifugal force

On the side of Earth facing the moon: gravitational pull > centrifugal force = net force bulge towards moon

On the opposite side: centrifugal force > gravitational pull = net force bulge away from moon

Creates two major tidal bulges on Earth, located on opposite sides

**Critical Factor:** **Horizontal tide-generating forces** are more important than vertical forces in generating tidal bulges.

**Factors Affecting Tidal Characteristics:**

**Continental shelves:** Tidal bulges on wide continental shelves have greater height

**Islands:** Tidal bulges hitting mid-oceanic islands become low

**Bays and estuaries:** Shape can magnify tidal intensity; funnel-shaped bays greatly change tidal magnitudes

**Tidal currents:** When tide is channeled between islands or into bays and estuaries

**Case Study — Bay of Fundy, Canada:** The world's highest tides occur here (15-16 meters). With two high and two low tides daily in roughly 24 hours, tide rises about 240 cm per hour. This creates hazardous conditions for beach walkers who may find water over their heads within an hour of the tide starting to come in.

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TYPES OF TIDES

**Tides Based on Frequency of Occurrence:**

**Semi-diurnal Tide:** Most common tidal pattern globally. Features **two high tides and two low tides each day**. Successive high tides or low tides are approximately the same height.

**Diurnal Tide:** Only **one high tide and one low tide during each day**. Successive high and low tides are approximately the same height.

**Mixed Tide:** Tides having **variations in height**. Generally occur along the west coast of North America and on many Pacific Ocean islands.

**Tides Based on Sun-Moon-Earth Positions:**

**Spring Tides:**

Occur when **sun, moon, and earth are in a straight line**

Result in **higher high tides and lower low tides**

**Occur twice a month:** once during full moon period and once during new moon period

Maximum tidal range occurs

The moon's attraction (2.17 times stronger than sun's) combines with sun's gravitational pull in same direction

**Neap Tides:**

Occur approximately **seven days after spring tides**

Occur when **sun and moon are at right angles (90°) to each other**

Forces of sun and moon tend to **counteract one another**

The moon's stronger attraction is **diminished by counteracting force of sun's gravitational pull**

Result in **lower high tides and higher low tides**

**Minimum tidal range occurs**

**Additional Tidal Variations Based on Moon-Earth Distance:**

**Perigee (closest moon position):** Occurs once monthly; creates unusually high and low tides with greater tidal range than normal

**Apogee (farthest moon position):** Occurs two weeks after perigee; moon's gravitational force is limited, creating less than average tidal ranges

**Additional Tidal Variations Based on Earth-Sun Distance:**

**Perihelion (closest to sun):** Occurs around January 3rd annually; tidal ranges are much greater with unusually high and low tides

**Aphelion (farthest from sun):** Occurs around July 4th annually; tidal ranges are much less than average

**Tidal Terminology:**

**Ebb:** Time period between high tide and low tide when water level is falling

**Flow (Flood):** Time period between low tide and high tide when water level is rising

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IMPORTANCE AND APPLICATIONS OF TIDES

**Predictability:** Since tides are caused by precisely known earth-moon-sun positions, **tides can be predicted well in advance**, enabling proper planning by navigators and fishermen.

**Navigation Applications:**

**Tidal flows** are of great importance in navigation planning

**Tidal heights** are crucial for harbors near rivers and within estuaries with shallow 'bars' at entrance that prevent ship entry

Predictability allows safe passage planning for vessels

**Environmental and Sanitation Benefits:**

Help in **desilting sediments** from estuaries

Aid in **removing polluted water** from river estuaries

Natural flushing mechanism for water bodies

**Power Generation:**

**Tidal energy** is used to generate electrical power in several countries

Examples: Canada, France, Russia, China

**Indian Project:** 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is under way

Sustainable, renewable energy source based on predictable phenomena

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OCEAN CURRENTS

**Definition:** Ocean currents are like **river flows in oceans**, representing **regular volumes of water in definite paths and directions**. They constitute continuous masses of ocean water moving from one location to another.

**Types of Forces Influencing Currents:**

**Primary Forces (Initiate Movement):**

1. **Heating by solar energy:** Causes water to expand; near equator ocean water is about 8 cm higher than in middle latitudes, creating slight gradient for water to flow downslope

2. **Wind:** Blowing on ocean surface pushes water to move; friction between wind and water surface affects water movement

3. **Gravity:** Pulls water down slopes and creates gradient variations

4. **Coriolis Force:** Causes water to move to the right in Northern Hemisphere and to the left in Southern Hemisphere

**Formation of Gyres:** Large accumulations of water and flows around them called **Gyres** produce large circular currents in all ocean basins.

**Secondary Forces (Influence Current Flow):**

**Density Differences:** Affect vertical mobility of ocean currents.

**High salinity water** is denser than low salinity water

**Cold water** is denser than warm water

**Denser water tends to sink**; lighter water tends to rise

**Cold-water currents:** Form when cold water at poles sinks and slowly moves toward equator

**Warm-water currents:** Travel from equator along surface, flowing toward poles to replace sinking cold water

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CLASSIFICATION OF OCEAN CURRENTS

**Based on Depth:**

**Surface Currents:**

Constitute approximately **10% of all ocean water**

Found in upper **400 meters** of the ocean

More visible and directly influenced by wind

**Deep Water Currents:**

Make up the other **90% of ocean water**

Move around ocean basins due to **variations in density and gravity**

Deep waters sink into deep ocean basins at **high latitudes** where temperatures are cold enough to increase density

Much slower than surface currents

**Based on Temperature:**

**Cold Currents:**

**Bring cold water into warm water areas**

Usually found on **west coasts of continents** in low and middle latitudes (both hemispheres)

Found on **east coasts in higher latitudes of Northern Hemisphere**

Example: Benguela Current (west coast of Africa), California Current (west coast of North America)

**Warm Currents:**

**Bring warm water into cold water areas**

Usually observed on **east coasts of continents** in low and middle latitudes (both hemispheres)

In Northern Hemisphere, found on **west coasts of continents** in high latitudes

Example: Gulf Stream (east coast of North America), Kuroshio Current (east coast of Asia)

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CHARACTERISTICS OF OCEAN CURRENTS

**Current Strength and Speed:**

**Drift:** Term used to refer to current speed

Currents are **strongest near surface** and may attain speeds **over 5 knots**

At depths, currents are generally **slow with speeds less than 0.5 knots**

**Strength of current** refers to speed; fast current is considered strong

**Current strength decreases with depth**

Most currents have speeds **less than or equal to 5 knots**

Measured in terms of **knots** (nautical miles per hour)

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MAJOR OCEAN CURRENTS

**Influencing Factors:**

**Stresses exerted by prevailing winds**

**Coriolis force**

**Oceanic circulation pattern** roughly corresponds to **earth's atmospheric circulation pattern**

**Oceanic Circulation Patterns:**

**Middle latitudes:** Air circulation is mainly **anticyclonic** (more pronounced in Southern Hemisphere than Northern Hemisphere); oceanic circulation follows this pattern

**Higher latitudes:** Wind flow is mostly **cyclonic**; oceanic circulation follows this pattern

**Monsoonal regions:** **Monsoon winds** influence current movements

**Heat Distribution Function:**

The oceanic circulation **transports heat from one latitude belt to another** similar to atmospheric general circulation:

**Cold waters** of Arctic and Antarctic circles move toward warmer tropical and equatorial regions

**Warm waters** of lower latitudes move toward poles

This creates temperature balance across latitudes

**Coriolis Force Effects:**

**Warm currents** from low latitudes tend to move to the **right in Northern Hemisphere**

**Warm currents** tend to move to the **left in Southern Hemisphere**

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EFFECTS OF OCEAN CURRENTS ON CLIMATE AND HUMAN ACTIVITIES

**West Coasts in Tropical and Subtropical Latitudes (Except Equatorial):**

Bordered by **cool/cold waters**

**Relatively low average temperatures** with narrow diurnal and annual ranges

**Foggy conditions** with generally arid climates

Examples: West coasts of South America, South Africa, Australia

**West Coasts in Middle and Higher Latitudes:**

Bordered by **warm waters**

Cause **distinct marine climate**

Characterized by **cool summers** and **relatively mild winters**

**Narrow annual temperature range**

Examples: West coasts of Europe and North America

**East Coasts in Tropical and Subtropical Latitudes:**

**Warm currents** flow parallel to coasts

Result in **warm and rainy climates**

Located in **western margins of subtropical anticyclones**

Examples: East coasts of South America, Asia, Australia

**Fishing and Marine Biology:**

**Mixing zones of warm and cold currents** are crucial for marine productivity

**Replenish oxygen levels** in water

**Favor growth of planktons** (primary food for fish populations)

**Best fishing grounds of the world** exist mainly in these mixing zones

Example: Grand Banks off Newfoundland (mixing of Gulf Stream and Labrador Current), Peruvian Current regions

**Indian Example:** The coastal waters of India are influenced by warm ocean currents that bring moisture-laden winds, contributing to the monsoon climate and supporting rich fishing grounds along both coasts.

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KEY EXAMINATION POINTS

**To Remember:**

Distinguish between **waves** (energy transfer), **currents** (continuous water flow), and **tides** (vertical rise and fall)

**Moon** has greater tidal influence (2.17 times) than sun

**Spring tides** occur at full and new moon; **neap tides** at quarter moons

**Coriolis force** direction differs between hemispheres

**Cold currents on west coasts** of tropical latitudes; **warm currents on east coasts** of tropical latitudes

**Ocean current mixing zones** support world's best fishing grounds

Tides are **predictable**; surges are **not predictable**