The Motion of the Ocean: Part I

The oceans aren’t static. They’re riddled with currents that cover the entire globe and are one of the most important features in heat distribution across the entire planet. There is more intense heating at the equator than at the poles, as solar radiation is more concentrated here. Water has a higher specific heat capacity than land, and so serves as a major heat sink in the oceans, but also serves as a huge stabiliser for Earth’s climate. 

The general pattern of ocean currents today is shown below:

The modern pattern of ocean circulation

The Physics (no groaning) of this is somewhat mathsy and I’m not going to go into that (but if you do, be prepared for lots of diagrams, differentiation, integration and fairly cool greek letters like ρ, Ω, and η). In short, there are 4 forces that affect the movement of water in the oceans:

• Gravity
• Wind
• Coriolis “force”
• Friction

Wind only affects the surface waters, pushing it around much like when you blow a pile of sand on the beach, but over large scales of many kilometers. This doesn’t so much affect ocean circulation as it just pushes the skin of the ocean around a bit. 

The modern pattern of deep ocean circulation

The more influential aspects of the deep ocean circulation are friction and gravity. Together they act to generate the mixing within the ocean. By density differences in the ocean, horizontal pressure gradient forces move water horizontally in the ocean, as well as vertically where denser water sinks through less dense. This factor is particularly important in the North Atlantic where the Gulf Stream, a warm tropical current that bathes Europe in warm water, sinks and is deflected back south as a cold deep water current. 

Equatorial water is more saline, due to higher evaporation rates in the tropics, than the polar waters which are frequently exposed to meltwater and icebergs. As such, the salty dense Gulf Stream is forced under the less dense North Atlantic as it approaches. The oceanic crustal topography deflects the cool current southward where it eventually becomes the Labrador Current, giving the eastern coasts of Canada and USA bitterly cold winters - even though they lie at the same latitude as Europe and North Africa.  Today, Boston, Massachusetts is at the same latitude as Florence, Italy, but have highs today of 3°C and 14°C respectively. 

This little animation gives a brief overview of the global deepwater circulation pattern, but also focuses on the Gulf Stream in the North Atlantic, so watch it and see what happens.

Next time - The Coriolis Effect and why it's so much more important that you thought.