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Current Affairs: Oceans in Motion
By Wayne Belding
Nov 14, 2017
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One important factor for many of the world’s famous winegrowing regions is the influence of ocean currents.  The movement of warm or cold waters off the coasts of continents can profoundly affect the local climates and growing conditions.

Some of the World’s Oceanic Currents

Ocean currents are the vertical or horizontal movement of both surface and deep water throughout the world’s oceans.  Currents normally move in a specific direction and aid significantly in the balancing of the earth’s moisture and temperature equilibrium.

Oceanic currents are found all over the globe and vary in size, importance, and strength, and some are key to understanding the distinction of winegrowing regions. There are seventeen major surface currents flowing through the oceans of the world.  As students of wine, we focus on the cold ocean currents that affect the growing conditions of coastal vineyards. 

The Humboldt Current off the west coast of South America brings cold water near the Chilean Coast.  In the westerly wind belt, moisture-laden air is pushed toward the Chilean coast.  As the air passes over the Humboldt Current, however, it is cooled.  Since cold air cannot hold as much water vapor as warm air, it drops some of its moisture load over the Pacific.  Then the already dried air is pushed over the Chilean Coast Range and dries out even more as it rises.  By the time the air gets to the winegrowing regions of the Valle Central, it is dry enough that the vineyards require irrigation.  The dry air also allows nighttime temperatures to drop, creating the diurnal shift that benefits flavor development in wine grapes.  Additionally, dry air inhibits the development of fungal diseases, so the vines and grapes are healthier as well.

Marine Air-Induced Fog in the Napa Valley

We find the phenomenon repeated in other famous winegrowing regions.  The California Current that flows off the west coast of North America is another noted Pacific Ocean cold-water current.  The Atlantic Ocean has the Benguela Current flowing off the west coast of South Africa and the Canary Current off the coast of North Africa as cold-water currents.  In Australia, the West Australian Current is a cold current off that continent’s west coast.  Similar to the Chilean example, the cold currents wring the moisture out of the marine air before it moves onto the continents.

How do these currents come to be?  The answer is quite complex.  Currents differ in size and strength and can be generally categorized as either surface or deep-water currents.  Surface currents are those found in the upper levels of the ocean and are mostly caused by winds that create friction as they move over the water’s surface. The friction, combined with the Coriolis Force caused by the rotation of the earth, forces the water to move in a spiral pattern in the currents.  In the northern hemisphere, the circulation moves clockwise while in the southern hemisphere, the motion is counterclockwise.

Differences in water density, resulting from the variability of water temperature and salinity causes convection that results in the formation of the deep ocean currents.  In cold regions like the North Atlantic Ocean, water loses heat to the atmosphere and becomes cold and dense.  When ocean water freezes into sea ice, salt is left behind causing surrounding seawater to become saltier and denser.  The dense, cold and salty water sinks and surface water flows in to replace it.  The convective process is known as thermohaline circulation, a connected system of deep and surface currents that circulate around the globe.  It takes a thousand years to complete a circulation.  Additionally, seafloor topography, the position of continents and the shape of the ocean’s basins impact both surface and deep water currents as they restrict areas where water can move and divert it to a less restricted area.

The Earth’s Thermohaline Circulation Pattern

There is one place on earth, however, where water can flow around the earth unimpeded by continental masses.  Between about 40 and 60 degrees of south latitude, the Antarctic Circumpolar Current does indeed circumnavigate the earth, although it takes eight years or so for a complete journey.  Due to the earths rotation and the prevailing winds, the current flow is predominantly from west to east.  Sailors, beginning with James Cook in 1775, have found that the Southern Ocean was a hostile environment with continuous storm-force westerly winds, mountainous seas and huge expanses of ice.

The Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) has a strong influence on the world’s climate.  It connects the three great ocean basins, Atlantic, Indian and Pacific, and allows not only water, but also heat, salinity and other properties to flow from one to the other.  As the ACC moves, there are waves and eddies that break off the main flow and transport cold waters to the north.  The Antarctic cold is thus transported by what we recognize as the Humboldt, Benguela and West Australian currents.

Not all ocean currents that influence winegrowing regions are cold.  Warm water currents are important as well.  The Gulf Stream is a famously warm current that originates in the Gulf of Mexico and moves north toward Europe where it is named the North Atlantic Drift.  The warm flow has a very significant moderating influence on the climate of northern Europe.  The warm current arguably raises the latitude of viticultural viability in France and Germany by three degrees when compared to similar latitudes on other continents.

The Gulf Stream as It Flows Off of North America

As you sample your favorite wines, give a thought and perhaps thanks to the ocean currents that have influenced their delicious characteristics.  Those waters may literally have come from halfway around the world to create an optimal environment for growing wine grapes.