Ocean Currents

Thermohaline Circulation

Ocean currents, like wind currents, distribute heat throughout the planet, having large impacts on climate. This thermohaline circulation is the reason areas of Europe are warmer than areas of North America at similar latitudes.

Driven by wind currents and the location of continents, ocean currents flow in distinct circulation patterns known as gyres. These patterns are almost circular and rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Warm water is less dense than cold, resulting in it being on the surface. When warm water is blown away, cold water moves up to the surface at upwelling zones, where the cooler, more nutrient and oxygen-rich water (as cold water holds more oxygen than warm water does) rises to replace the warm water that has left. This results in upwelling zones off of the coast being extremely productive for fishing.

While "thermo" refers to the temperature of the water and how that affects the currents, "haline" refers to the salinity. The saltier water is, the denser it is. When warm water evaporates, it leaves behind salt, increasing salinity. Eventually, warm water begins to sink, cooling down and spreading out on the ocean floor. This helps to mix the salt and sediments, preventing settling, and aiding in their mixing throughout the oceans.

ENSO

Let's take a look at the Pacific Ocean. To orient you, Australia and Asia will be west (or, on the left) and the Americas will be east (or, on the right). Normally, the winds blow from east to west on the equator, meaning they are blowing from the Americas toward Australia. This results in the ocean current following the same pattern.

As a result of this circulation pattern, Australia and Southeast Asia are warmer and rainier, while the west coast of South America is drier and cooler. In addition, there is strong upwelling off the coast of South America, resulting in highly productive fisheries.

This wind pattern is not constant, however, and is disrupted every few years. El Niño-Southern Oscillation (ESNO) is the name for this change in wind patterns, resulting in either an El Niño or La Niña year. They don't occur on a regular schedule, but usually happen every 2-7 years or so, with El Niño typically occurring more frequently.

El Niño

During an El Niño year, the equatorial surface water of the Pacific becomes warmer and the trade winds weaken and reverse. This results in warm water moving towards the Americas, resulting in warmer and wetter weather there, while Australia becomes colder and drier. This suppresses upwelling off the South American coast, either weakening it or preventing it altogether. This results in fewer nutrients coming from the deeper water, which causes fewer phytoplankton to be found, which means less food for fish.

This can have wide effects on worldwide climate that are not limited solely to the Pacific Ocean.

  • Fewer hurricanes in the Atlantic Ocean

  • Weakened monsoons in India and Southeast Asia

  • Increased rainfall and flooding in the Southern US Warmer and drier winters in the Northern US

La Niña

La Niña is essentially the reverse of El Niño; it results in cooler surface water and stronger winds in their normal direction. This causes even warmer and wetter weather in Australia and Southeast Asia, even cooler and drier weather in the western South American coast, and stronger upwelling and therefore more productive fisheries.

This can have wide effects on worldwide climate that are not limited solely to the Pacific Ocean. However, they tend to be the opposite effects that are seen when compared to El Niño

  • More hurricanes in the Atlantic Ocean

  • Stronger monsoons in India and Southeast Asia

  • Drier weather and more droughts in the Southern US

  • Colder and wetter winters in the Northern US

A former student of mine, Emily, used to remember the difference with the following phrase: "boys flip flop and girls exaggerate."

El Niño means "the boy" and results in opposite conditions compared to normal. La Niña means "the girl" and results in the same pattern, but more extreme conditions compared to normal.

El Niñito?

Interestingly, there is actually a similar process that occurs in the Atlantic Ocean - Atlantic Niño and Atlantic Niña are similar deviations to normal circulation patterns seen in the Atlantic Ocean. Not as much is known about them as their "family" in the Pacific, however.