Oceans and Tides

Learning Objectives

Describe the primary cause of ocean tides.

Explain the difference between a spring tide and a neap tide.

Identify the main factors that affect the salinity of ocean water.

Understand that ocean currents act as a global heat transport system.

The Restless Ocean

Earth's oceans are a vast, dynamic system covering over 70% of our planet's surface. They are constantly in motion, driven by forces that create currents and tides.

The Pull of the Tides

Tides are the regular rise and fall of the ocean's surface. If you've been to a beach, you've seen the water level change throughout the day. This phenomenon is caused primarily by the gravitational pull of the Moon.

The Moon's gravity pulls on the Earth and the water on its surface. The water on the side of the Earth closest to the Moon is pulled the hardest, creating a bulge, or high tide.

At the same time, the Earth itself is pulled toward the Moon, leaving the water on the opposite side of the Earth behind. This creates a second high tide bulge on the far side of the Earth.

The areas between these two bulges experience low tide.

As the Earth rotates on its axis once a day, most coastal locations cycle through two high tides and two low tides.

The Sun's gravity also affects the tides, but because the Sun is so much farther away, its effect is less than half that of the Moon.

Spring Tides and Neap Tides

The alignment of the Sun, Earth, and Moon creates two special types of tides:

Spring Tides: These occur during the new moon and full moon, when the Sun, Earth, and Moon are all in a straight line. During these times, the gravitational pulls of the Sun and Moon combine, producing the highest high tides and the lowest low tides. The tidal range is at its maximum.

Neap Tides: These occur during the first and third quarter moons, when the Sun and Moon are at a right angle to each other relative to the Earth. The Sun's gravitational pull works against the Moon's pull. This produces the lowest high tides and the highest low tides. The tidal range is at its minimum.

Salinity: How Salty is the Ocean?

Salinity is the measure of the total amount of dissolved salts in water. On average, the ocean's salinity is about 3.5%. This means that in every 1 kilogram of seawater, there are about 35 grams of salt. Salinity is not the same everywhere and can be affected by:

Factors that Increase Salinity: Evaporation (removes fresh water, leaves salt) and the formation of sea ice.

Factors that Decrease Salinity: Precipitation (rain/snow adds fresh water), runoff from rivers, and the melting of glaciers.

Ocean Currents

Ocean water is also constantly moving in large 'rivers' called currents. Surface currents are driven by winds, while deep ocean currents are driven by differences in water density (caused by temperature and salinity). These currents are incredibly important because they transport huge amounts of heat around the globe, from the equator to the poles, which helps to regulate Earth's climate.

Key Terms

**Tide: The periodic rise and fall of the level of water in the ocean.
**Gravity: The force that attracts a body toward the center of the earth, or toward any other physical body having mass.
**Spring Tide: A tide just after a new or full moon, when there is the greatest difference between high and low water.
**Neap Tide: A tide just after the first or third quarters of the moon when there is the least difference between high and low water.
**Salinity: The measure of the amount of dissolved salts in a given amount of liquid.
**Ocean Current: A continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, and density differences.