Ocean surface currents are the movement of water at the surface of the ocean. These currents are driven by a variety of factors, including wind, tides, and the Earth’s rotation. They can be divided into two main categories: wind-driven currents and thermohaline currents. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content.
Wind-driven currents are caused by the friction between the wind and the surface of the water, which creates a horizontal movement of water. This movement is driven by the prevailing winds and the Coriolis effect, the latter is a phenomenon caused by the rotation of the Earth and is responsible for the rotation of the wind patterns and ocean currents, as well as the direction of storms and low-pressure systems. Wind currents can be further divided into two types: Ekman currents and geostrophic currents. Ekman currents are caused by the friction between the wind and the surface of the water, while geostrophic currents are caused by the balance of the horizontal pressure gradient, which is the change in atmospheric pressure over a horizontal distance and is responsible for the formation of winds, and the Coriolis effect.
On the other hand, thermohaline currents are driven by the density of the water, which is affected by temperature and salinity. These currents are caused by the sinking of cold, dense water and the rise of warm, less dense water. The most notable example of thermohaline current is the ocean conveyor belt, also known as the thermohaline circulation, which is responsible for the global ocean circulation and has a significant impact on the Earth’s climate.
Ocean surface currents play an important role in the global climate and weather patterns, as well as in marine ecosystem, shipping and ocean-based industries.
However, ocean surface currents are not to be confused with tidal currents. Tidal currents are the horizontal movement of water caused by the rise and fall of tides. They are caused by the gravitational pull of the moon and the sun on the Earth’s oceans, and can be observed in the form of ebbing and flowing of water in coastal areas. Tidal currents can be quite strong in certain locations, and are an important consideration for navigation and marine activities such as fishing and boating but do not affect navigation at all once skippers in the Global Solo Challenge will be offshore.
The only areas where tidal currents may be relevant during the Global Solo Challenge are the very first section from A Coruña to Finisterre and, much later, after Cape Horn and near Tierra del Fuego. Skippers who decide to take a shortcut can sail between Tierra del Fuego and Staten Island through the strait of Le Maire. Here currents can exceed five knots and the sea state can become extremely dangerous especially in situations of wind against tide. Skippers will therefore have to carefully consider their options. In heavy weather boats will typically sail past Cape Horn and east of the small Argentinian island avoiding the strait of Le Maire.
What are the main ocean surface currents?
There are several main sea surface currents that are important for global ocean circulation and weather patterns. Some of the most notable include:
- The Gulf Stream: This warm ocean current originates in the Gulf of Mexico and flows along the eastern coast of the United States before crossing the Atlantic Ocean towards Europe. It plays an important role in the climate of the northeastern United States and Western Europe, as it helps to moderate the temperatures and bring warm water from the tropics to higher latitudes.
- The North Atlantic Drift: Also known as the North Atlantic Current, this current is the extension of the Gulf Stream that flows northward along the western coast of Europe. It also helps to moderate the climate of Western Europe, making it milder than other regions at similar latitudes.
- The California Current: This cold ocean current flows southward along the western coast of North America, from British Columbia to Baja California. It brings cold water from the poles to the tropics, and plays an important role in the marine ecosystem of the region.
- The Kuroshio Current: This warm ocean current flows northward along the eastern coast of Taiwan and Japan. It plays a similar role as the Gulf Stream but in the western Pacific, bringing warm water from the tropics to higher latitudes.
- The Agulhas Current: This warm ocean current flows southward along the eastern coast of South Africa, before turning eastward and eventually retroflecting and becoming the Agulhas Return Current. It also plays a key role in the global ocean circulation and climate of the region.
- The Antarctic Circumpolar Current: This cold ocean current circles the Antarctic continent, flowing from west to east. It is the largest ocean current and the only current that flows uninterrupted around the globe and helps to isolate the Antarctic from the rest of the world.
These are just a few examples of the many sea surface currents that exist around the world. Each current plays an important role in the global ocean circulation and weather patterns, and together they help to distribute heat and nutrients around the planet.
Clearly only some of these are relevant to competitors in the Global Solo Challenge. Notably the Agulhas current is certainly the one that requires the closest look at as it affects sea state conditions after rounding Cape of Good Hope.
The Agulhas Current is the most voluminous western boundary current in the southern hemisphere, and it is driven by the strong south-easterly winds that blow over the Indian Ocean and the south-westerly winds that blow over the south-western tip of Africa. It brings warm water from the Indian Ocean to the south-western tip of Africa, where it cools and sinks. This creates a thermal low-pressure system over the south-western tip of Africa, which causes the south-easterly winds to blow stronger.
Referred locally to as the Southestern, they prevail over all other wind directions around the tip of South Africa. When rounding Cape of Good Hope, in fact, Global Solo Challenge skippers will probably choose to sail several hundred miles further south where they won’t face the worst of the Agulhas current or the unfavourable south-easterly winds.
The Agulhas Current is also known for its strong eddies, which are large circular ocean currents that can be up to 500km in diameter and can last for over a year. These eddies can transport warm water, nutrients and plankton from the Indian Ocean to the Atlantic Ocean, and play an important role in the marine ecosystem of the region. They generate confused seas in the region south of South Africa and south of Madagascar, especially when the wind blows in the opposite direction of the surface current. For this reason the navigation in the early part of the Indian Ocean can be very uncomfortable for competitors in the Global Solo Challenge. Choppy seas and steep waves cause frequent slamming and can be stressful for a boat’s structure and a sailor’s mind alike.
The other very important current for the Global Solo Challenge is the Antarctic Circumpolar Current, a drift to the east favourable to skippers caused by the relentlessness of the prevailing westerly winds found in the Roaring Forties and Screaming Fifties.