In 1856, a group of sailors who encountered misfortune on a sandbank in the Atlantic Ocean discovered a ball of tar, which was tar encasing a coconut shell. Inside was a letter made of goat skin with strange symbols, sent from a very distant shore. After a long journey, it was hoped that the letter would reach a beautiful island, but the storms instead carried it to a desolate place.
What did people think about using the sea to send letters? For a long time, humans have observed that both on the surface of the sea and deep below, seawater flows in a systematic direction, similar to rivers on land, known as ocean currents.
There are many complex reasons that form ocean currents. Predominant winds, such as the Trade Winds and Westerlies, are the primary causes. Additionally, the Earth’s rotation, the coastal configuration, the distribution of islands, sea level, water temperature, and salinity all have certain impacts on ocean currents.
The ocean currents in the Northern Hemisphere and Southern Hemisphere are essentially symmetrically distributed. In the subtropical waters of the Northern Hemisphere, currents generally flow clockwise around subtropical high-pressure systems, known as the internal circulation. Along the coast, currents flow counterclockwise following the coastline, referred to as the external circulation. In waters north of 40 degrees North latitude, currents flow counterclockwise around subpolar low-pressure systems. In the Southern Hemisphere, the direction of ocean currents flows in the opposite direction.
Wind direction map (Image: maps.weather.com)
Ocean currents have a significant impact on the climate of various regions around the world. Firstly, seawater transmits heat much more efficiently than air, thus ocean currents play a crucial role in heat transfer between low and high latitudes, regulating the temperature differences across latitudes. Secondly, due to the temperature differences of warm and cold currents between the East and West coasts, under the influence of prevailing winds, there are noticeable air temperature differences at the same latitude along the East and West coasts, breaking the rule that temperatures decrease as latitude increases. Additionally, warm coastal currents lead to increased rainfall, while cold coastal currents result in more fog.
Oceanographers rely on various data, such as the paths of floating objects in the sea, the trajectories of sea foam, wreckage of ships, varying sea temperatures, drift records of vessels, current direction and speed, and ocean current maps. In modern times, with the aid of accurate measuring instruments and satellites, our understanding of ocean currents has expanded, revealing the existence of 12 major ocean currents and dozens of smaller ones around the globe.
ocean current
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